eBay Motors is currently one of the seven specialty sites of eBay, catering specifically to buyers and sellers interested in automotives and vehicles. First launched by eBay in 2000 as an effort to boost sales in its automotive auction site, the site has undergone many changes to become what it is now. Positioned at first as two separate sites, eBay Motors was combined with the eBay Auto-trader site and launched as one site under its current name. The company also added a slew of services to attract more users. Although the move upset some of the affected sellers, it proved to be a wise strategy as it boosted the sales of the auction site.

eBay Motors is currently one of the most popular online destinations for buyers and seller of automotives, parts and accessories. eBay Motors also goes beyond cars and trucks auctions: The complete category list includes motorcycles, powersports, boats and other vehicles such as buses, commercial trucks and even aircrafts. eBay Motors is truly the best one-stop online auction shop for anything and everything about motors.

Like other eBay sites, the eBay Motors' special site is designed to make the buying experience very easy and convenient for the user. eBay has successfully created a simple process that works across all of their specialty sites, and eBay Motors is no exception. Online auction and buying experience doesn't get any easier than the popular Find - Bid/Buy - Pay method.

For the buyers, the Search function and the Category list both serve as an excellent starting point. If the buyer has a specific idea of something that he's looking for, the search function is probably the quickest way of finding it. If not searching for something specific though, browsing through the categories may be the best option for prospective buyers. Browsing also has the added advantage of seeing items or options that the users wouldn't have seen using the specific search function. "Surprising discoveries", as the eBay search help page says.

Then the buyer can proceed to the actual bidding for the item or, if the seller of the item allows it, buy it directly through eBay Motor's Buy It Now feature. Buy it now is an eBay-wide feature that was introduced by the company as an alternative to the normally used bidding process. With Buy It Now, you can buy an item instantly at a seller-defined price without going through the bidding process.

Paying for items you bought can be done through different methods, depending on the seller. Usually Credit card and electronic checks are accepted. You can also use PayPal, eBay's own payment system.

Although in the ideal world the three-step process enumerated above should work flawlessly, actually buying vehicles through eBay Motors entails additional steps for the buyer to avoid frauds. The general rule is that the more the buyer knows about the seller and the items that's being are auctioned, the safer the buyer will be.

Getting to know the vehicle that you're bidding on or buying means an extensive research must be done if you don't know much about it yet. Do research on the particular model that you are interested in and make sure that it meets your needs as well as the requirements that your state has set, such as emission efficiency. It is also imperative that you have a good knowledge of actual value of the vehicle. Beware of vehicles that are listed significantly lower than that of the manufacturer's suggested retail price. As for the actual vehicle being auctioned, it would help a lot if you can personally visit and see the vehicle to ensure it is accurately represented. If this is not possible you can have an inspection service check the vehicle for you.

The next important thing to know is the seller of the vehicle being auctioned. eBay Motors has an excellent feedback rating system that will let you gauge the seller's reputation on eBay. Make sure that you get to know your seller well before bidding or fully engaging into a deal. It is also important to contact the seller and to ask more questions about the auctioned item. Make sure that he replies to your inquiry and questions.

Although care should still be taken by the buyer, eBay Motors has a free Vehicle Purchase Protection Program that protects the buyer's vehicle purchases from fraud and material misrepresentation. This is granted that the transaction is eligible. The vehicle purchase is protected for up to twenty thousand dollars or the vehicle purchase price, whichever is lower.

There are also other services offered by eBay Motors for the buyers. The PayPal buyer protection program gives protection to users who use PayPal for their parts and accessories transactions. As mentioned, there are Vehicle Inspection programs that provide detailed and thorough mechanical and cosmetic inspection on auctioned vehicles. The Vehicles History Report shows facts about a pre-owned car before the buyer buys the item.

Buying vehicles or parts online gives you the advantage of a consolidated data about vehicles that is very helpful in comparing various items or vehicles that you are considering. Make use of this advantage really well. Compare and always find the best price for items that you're bidding on. The excellent and proven auction system can also give you possible big savings when buying vehicles in eBay Motors.

After getting all the relevant information go ahead and place your bid. Remember that when bidding on eBay Motors you only need to enter the maximum amount you are willing to pay for the vehicle you wish to buy. This doesn't automatically mean that that would be the price you are going to pay for the vehicle. eBay Motors will do the bidding for you and stop bidding and notify you through email once your indicated maximum amount is reached. Bidding on eBay Motors is just as easy and hassle free as bidding on any eBay auction site.

For buyers of collector cars eBay Motors has a special feature called Roadside Assistance. Roadside Assistance provides buyers throughout USA and Canada a free 30-day roadside assistance via the Hagerty Collector Network. The free roadside assistance also covers the first $100 worth of other roadside services such as battery jump, fuel delivery, tire change and lockout service. This feature lets buyers enjoy the first 30 days of their collector cars worry free and can then later on sign up with Hagerty Collector Network if they want to continue with the service.

Buying a vehicle in eBay Motors is surely not only very easy and hassle free but offers very good perks as well. If you need to buy a vehicle and choose to do it online, then eBay Motors is the place to do it.



By: Danny Wirken

About the Author:

It's a must for a rider to protect himself in order to secure safety and protection. Moreover, a rider must get hold of high-quality street motor accessories to aid the same. Street motorcycle accessories provide riders the ultimate protection that they need. It is every rider's solution to be safe and secured.

It's hard going to battle without armor. Same thing when you're a rider with no accessories to protect you from untoward incidents and accidents. It's like accepting an accident helplessly. Chances are, you can be a victim of a collision, fracture, injury or fatal consequences. Good thing we have dependable street motor accessories in our market. Its main concern is to provide riders ultimate protection, comfort at the same time style that reflects the rider's personality and preferences. A wide variety of accessories with different designs like the ameritex fork bags, bikepack, ameritex round utility bag, tourbag backpack, ssb and the ameritex tool bags are already made available to guaranty its objectives.

These accessories are excellent protectors and gears. Every part of our body is vital this is the reason why we have to protect each of them. Not only that, our motorcycle must also be safe and sound for riding so as not to invite terrible collisions and accidents. When you are a rider you are bound to give more protection to your head, chest, legs and hands. To protect one of your vital parts, you can use street motorcycles accessories. This is to avoid fracture, limp and other damage. Damage to the brain may mean disaster. It spells coma thus envision a vegetable state or worst - death. Damage to the heart has same fatal effect. Damage to both of your hands may mean saying goodbye to driving. Damage to your feet may mean to walk no more. Consecutively to avoid these circumstances from occurring and for you to be able to enjoy the thrill and excitement of riding protected and secured use street motorcycle accessories!

Street motorcycle accessories are needed safety measure of today's riders that can save them from impending danger and life risks. You can depend on them during your rainy, troublesome and bumpy rides. These accessories can also save you a trip to your doctor at the same time can give you thrilling, hassle-free rides. High-quality street motorcycle accessories will give you the finest and most durable motorcycle accessories that you badly needed in your everyday routine



By: Maricon Williams

About the Author:

There are always thousands of motor boats for sale in the UK so it can be difficult finding the right one. The best place to start is deciding what you are looking for in a motor boat. Do you want an ocean going boat? A yacht? A small motorboat? And then within the type of boats there are different makes of such as the Sunseeker, Princess, Beneteau, Riva, Pearl, Sealine etc. It is vitally important you choose the right boat and decide on a specific boat or at least a few to narrow your search and gain knowledge of what you are looking for. There are so many options and you need to research your needs and budget first. Buying a boat can be very expensive and then there's maintaining a boat; this can be even more expensive. Check into berthing costs, fuel costs and other additional costs that will not be figured into the actual purchase price to make sure it is an affordable option.

Next is the fun part, start shopping around. Once you decide on the type of boat you want, start doing some in-depth research on the motorboat - the different options such as different styles and features, length, conditions, age, interior options, cabin sizes, bathrooms, Engine size and make. Get on the internet and check prices or all the boats you are interested in. Look at all sources from magazines to internet sites to get an idea of what motorboats are selling for. Talk to marina managers and harbour masters. They are likely to know if any boats are for sale in their areas and about the prices you will be looking at.

Once you have found a boat you want to purchase, negotiate the purchase like you would a car. Walk away once, check into insurance, and give it a thorough inspection, if need be take a mechanic with you and check out the engine and condition of the parts. When you buy a motorboat, you have to check the hull integrity, condition of the motor, accessories, and dozens of other criteria. Know what you are looking for, especially if you are buying a used boat. Perhaps write a checklist of things you should check out beforehand.

Make sure that when you put in an offer you indicate that it is subject to a Marine Survey and if necessary, a sea trial and make sure you have a valid and recognized contract. The BMF and RYA provide standard contracts which should be used.

Once you have paid for the motorboat you need to register It is not a legal requirement to register your boat if you if kept and used in UK waters. For overseas use it must be registered. It must also be registered if its company owned or subject to a mortgage.

Although there are hundreds of motor boats for sale in the UK and it's easy to buy one, it is very important you protect yourself and do it properly by doing your research.



By: Ian Morris

About the Author:

The Motor Control Center

The MCC enclosure protects personnel from contact with current carrying devices, and it protects the components from various environmental conditions. It is important that the enclosure is mounted to assure accessibility so that qualified personnel (such as a trained thermographer) can open the panel under load. There are different classes and types of MCCs, but generally speaking, an MCC looks like a row of file cabinets with each cabinet representing an MCC section. The drawers of the file cabinet represent the plug-in units that contain the motor control components. Three phase power is distributed within the MCC by bus bars, large metal current carrying bars. The horizontal bus provides three-phase power distribution from the main power supply. Vertical bus in each section is connected from it to individual MCCs. Bracing and isolation barriers are provided to protect against fault conditions. The plug-in units of an MCC have power stabs on the back to allow it to be plugged into the vertical power bus bars of the structure.

Beginning Your MCC Infrared Inspection

Before opening the panel or door on a motor controller, prescan the enclosure to assure a safe opening condition. If excessive heat appears on the surface of the door, extra care should be taken when opening it. The thermographer or escort may decide to note the condition as unacceptable and not take a chance on opening it under load. Once the unit is open, begin with both an infrared and a visual inspection to assure no dangerous conditions exist. Be systematic while conducting the infrared inspection. Remember the system must be under load to conduct the inspection. Work from left to right or follow the circuit through carefully, inspecting all of the components. Look for abnormal thermal patterns caused by high-resistance connections, overloads, or load imbalances. In three-phase systems this can be accomplished by comparing phases. Adjust the level and span on the infrared system to optimize the image. Proper adjustment will identify primary and secondary anomalies. The bus stabs and the connections to the main are important inspection points that are often overlooked or misdiagnosed. The incoming connection to the main horizontal bus is usually located behind a cover or panel that is not hinged. These are typically bolted connections and may have parallel feeders. The bus stab connections on the back of the plug-in units are more difficult to inspect. The thermographer does not have direct view of the connection, and the first indication of a problem can be seen on the incoming conductors feeding the breaker or fused disconnect. Remember, even small temperature rises identified at this point could mean serious problems.

Motor Starters and Motor Controllers

The purpose of the motor starter is to protect the motor, personnel, and associated equipment. Over 90% of the motors used are AC induction motors, and motor starters are used to start and stop them. A more generic term would identify this piece of equipment as a motor controller. A controller may include several functions, such as starting, stopping, overcurrent protection, overload protection, reversing, and braking. The motor starter is selected to match the voltage and horsepower of the system. Other factors used to select the starter include: motor speed, torque, full load current (FLC), service factor (SF), and time rating (10 or 20 seconds).

Understanding the thermal patterns of this equipment is critical to a successful inspection. Also correctly identifying the source of the anomaly can make recommendations more valuable.

Motors may be damaged or their life significantly reduced if they operate continuously at a current above full load current. Motors are designed to handle in-rush or locked rotor currents without much temperature increase, providing there is a limited duration and a limited number of starts. Overcurrents up to locked rotor current are generally caused by mechanical overloading of the motor. The National Electric Code (NEC) describes overcurrent protection for this situation as "motor running overcurrent (overload) protection." This can be shortened to overload protection. Overcurrents caused by short circuits or ground faults are dramatically higher than those caused by mechanical overloads or excessive starts. The NEC describes this type of overcurrent protection as "motor branch-circuit short-circuit and ground-fault protection." This can be shortened to overcurrent protection. The four common varieties of motor starters are: across-the-line, the reversing starter, the multispeed starter, and the reduced voltage starter. Motor starters are generally comprised of the same types of components. These include a breaker or fused disconnect, contactor and overloads. There may also be additional components, including control circuitry and a transformer. Understanding the thermal patterns of this equipment is critical to a successful inspection. Also correctly identifying the source of the anomaly can make recommendations more valuable.

Overcurrent Protection

NEC requires overcurrent protection and a means to disconnect the motor and controller from line voltage. Fused disconnects or thermal magnetic circuit breakers are typically used for overcurrent protection and to provide a disconnect for the circuit. A circuit breaker is defined in NEMA standards as a device designed to open and close a circuit by non-automatic means and to open the circuit automatically on a predetermined overcurrent without injury to itself when properly applied within its rating. If we look at a cutaway of a breaker, we can identify potential connection problems. The line side and load side lugs are the most common source of abnormal heating, but many breakers have a second set of bolted connections on the back of the breaker. Heat from this connection can be misdiagnosed as the main lug. There are also internal contacts where current flow is interrupted by exercising the component. These contacts experience arcing each time the breaker is opened. An arc is a discharge of electric current jumping across an air gap between two contacts. Arcs are formed when the contacts of a circuit breaker are opened under a load. Arcing under normal loading is very small compared to an arc formed from a short circuit interruption. Arcing produces additional heat and can damage the contact surfaces. Damaged contacts can cause resistive heating. Thermal patterns from these poor connections appear as diffuse heating on the surface of the breaker. In addition, there are several types of breakers that have internal coils used for circuit protection. These coils have heat associated with them and can appear to be an internal heating problem, when in fact, it is a normal condition.

Fused Disconnects

Fused disconnects are used to provide over-current protection for motor in the same manner as a breaker. Instead of opening contacts, fuses fail opening the circuit. When overcurrent protection is provided by fuses, a disconnect switch is required for manual opening of the circuit. The disconnect switch and fuse block are typically one assembly. The hinge and blade connections on the switch are a typical source of overheating. High resistance from overuse or underuse is usually the cause. Fuse clips are also a weak connection point for some disconnect designs. Different types or manufacturers of fuses of the same amperage may produce different thermal signatures. While different size or amperage fuses will also have a different thermal pattern, fuse bodies may appear warmer than the rest of the circuit due to conductor size.

Contactors

Starters are made from two building blocks, contactors and overload protection. Contactors control the electric current flow to the motor. Their function is to repeatedly establish and interrupt an electrical power circuit. A contactor can stand on its own as a power control device, or as part of a starter. Contactors operate electromechanically and use a small control current to open and close the circuit. The electromechanical components do the work, not the human hand, as is the case with a knife blade switch or a manual controller. The sequence of operation of a contactor is as follows: first, a control current is applied to the coil; next, current flow into the coil creates a magnetic field which magnetizes the E-frame making it an electromagnet; finally, the electromagnet draws the armature towards it, closing the contacts. A contactor has a life expectancy. If the contactor contacts are frequently opened and closed, it will shorten the life of the unit. As the contacts are exercised, an electrical arc is created between the contacts. Arcs produce heat, which can damage the contacts. Contacts eventually become oxidized with a black deposit. This black deposit may actually improve the electrical connection between the contacts by improving the seat, but burn marks, pitting, and corrosion indicate it is time to replace the contacts. The following thermal patterns are associated with contactors. The coil of the contactor is usually the warmest part of the unit. High temperatures may indicate a breakdown of the coil. Line side and load side lug connections may show high resistance heating from poor connections. Heating from burned and pitted contacts may be thermally "visible" on the body of the contactor.

Overload Protection

The ideal motor overload protection is a unit with current sensing capabilities similar to the heating curve of the motor. It would open the motor circuit when full load current is exceeded. Operation of this device would allow the motor to operate with harmless temporary overloads, but open up when an overload lasts too long.

Typical thermal problems in overloads are found in the connections to the contactor, overload relay, or motor.

This protection can be provided by the use of an overload relay. The overload relay limits the amount of current drawn to protect the motor from overheating. It consists of a current sensing unit and a mechanism to open the circuit. An overload relay is renewable and can work for repeated trip and reset cycles. Overloads, however, do not provide short circuit protection. The melting alloy (or eutectic) overload relay consists of a heater coil, a eutectic alloy, and a mechanical mechanism to activate a tripping device when an overload occurs. The relay measures the temperature of the motor by monitoring the amount of current being drawn. This is done indirectly through a heater coil, which under overload conditions, melts a special solder allowing a ratchet wheel to spin free and open the contact. A bimetallic thermal overload uses a U-shaped bimetal strip. In an overload condition heat will cause the bimetal to deflect and open a contact. The solid state overload relay does not generate heat to cause a trip. Instead, it measures current or a change in resistance. The advantage of this method is that the overload relay doesn't waste energy generating heat and doesn't add to the cooling requirements of the panel. Normal heating for an overload may look like a thermal anomaly. Heat generated in the coil or bimetal may look like a connection problem. Typical thermal problems in overloads are found in the connections to the contactor, overload relay, or motor.

Starters

Starters are the combination of a controller, usually a contactor and an overload relay. The above descriptions of the individual components apply to the starter systems. Reduced voltage starters are used in applications that involve large horsepower motors. They are used to reduce the in-rush current and limit the torque, and thus the mechanical stress on the load. The components of this type of starter should be inspected as the motor steps up to speed. A separate low-voltage starter circuit is used to step the motor up to speed. Once at operating speed, these components are de-energized.

Completing Inspections

Remember that primary anomalies are the problems that readily stand out while secondary anomalies may require that primary anomalies be adjusted into saturation to allow for the identification of a secondary anomaly. For example, different fuse types and sizes will cause different thermal signatures as will overload relays that are sized differently within the same circuit. Anomalies like this should be identified and reported. Also note that when evaluating the severity of a problem, temperature is just one variable. All of the parameters involved with the severity of the anomaly should be considered. To improve temperature measurements, avoid low emissive surfaces. Look for cavity radiators or highly emissive insulation on conductors. Measure loads where component sizing, overloading, or load imbalances are observed. Beware of the effects of wind or convection on components. Note ambient temperatures, large thermal gradients, and the source of heating. Safety should be the top consideration.

Conclusion

Knowing the equipment under inspection allows for the correct identification of problems that could be misdiagnosed or overlooked. Analyzing unfamiliar thermal patterns on a component is easier when equipment design is reviewed. More precise repair recommendations can also be made. Locating temperature differences qualitatively or quantitatively is the real benefit of infrared thermography. Knowing where to look for these temperature differences comes from knowledge of the equipment, and knowledge of the equipment will make a better thermographer.

Please visit us at www.electrophysics.com/snellmcab

For more comprehensive White Papers visit our online Knowledge Center. www.electrophysics.com/thermal-imaging

Electrophysics - IR Cameras for Thermography Professionals

373 Route 46, Fairfield, NJ 07004  Phone: 973-882-0211   Fax: 973-882-0997



By: Josh White

About the Author:

This section covers all types of motors, from the elementary circuitry needed to control a variable reluctance motor, to the H-bridge circuitry needed to control a bipolar permanent magnet motor. Each class of drive circuit is illustrated with practical examples, but these examples are not intended as an exhaustive catalog of the commercially available control circuits, nor is the information given here intended to substitute for the information found on the manufacturer's component data sheets for the parts mentioned.

 

This section only covers the most elementary control circuitry for each class of motor. All of these circuits assume that the motor power supply provides a drive voltage no greater than the motor's rated voltage, and this significantly limits motor performance. The next section, on current limited drive circuitry, covers practical high-performance drive circuits.

 

 

Variable Reluctance Motors

Typical controllers for variable reluctance stepping motors are variations on the outline shown in Figure 3.1:

 

Figure 3.1

In Figure 3.1, boxes are used to represent switches; a control unit, not shown, is responsible for providing the control signals to open and close the switches at the appropriate times in order to spin the motors. In many cases, the control unit will be a computer or programmable interface controller, with software directly generating the outputs needed to control the switches, but in other cases, additional control circuitry is introduced, sometimes gratuitously!

 

Motor windings, solenoids and similar devices are all inductive loads. As such, the current through the motor winding cannot be turned on or off instantaneously without involving infinite voltages! When the switch controlling a motor winding is closed, allowing current to flow, the result of this is a slow rise in current. When the switch controlling a motor winding is opened, the result of this is a voltage spike that can seriously damage the switch unless care is taken to deal with it appropriately.

 

There are two basic ways of dealing with this voltage spike. One is to bridge the motor winding with a diode, and the other is to bridge the motor winding with a capacitor. Figure 3.2 illustrates both approaches:

 

Figure 3.2

 

The diode shown in Figure 3.2 must be able to conduct the full current through the motor winding, but it will only conduct briefly each time the switch is turned off, as the current through the winding decays. If relatively slow diodes such as the common 1N400X family are used together with a fast switch, it may be necessary to add a small capacitor in parallel with the diode.

 

The capacitor shown in Figure 3.2 poses more complex design problems! When the switch is closed, the capacitor will discharge through the switch to ground, and the switch must be able to handle this brief spike of discharge current. A resistor in series with the capacitor or in series with the power supply will limit this current. When the switch is opened, the stored energy in the motor winding will charge the capacitor up to a voltage significantly above the supply voltage, and the switch must be able to tolerate this voltage. To solve for the size of the capacitor, we equate the two formulas for the stored energy in a resonant circuit:

 

P = C V2 / 2

P = L I2 / 2

Where:

P -- stored energy, in watt seconds or coulomb volts

C -- capacity, in farads

V -- voltage across capacitor

L -- inductance of motor winding, in henrys

I -- current through motor winding

Solving for the minimum size of capacitor required to prevent overvoltage on the switch is fairly easy:

C > L I2 / (Vb - Vs)2

Where:

Vb -- the breakdown voltage of the switch

Vs -- the supply voltage

Variable reluctance motors have variable inductance that depends on the shaft angle. Therefore, worst-case design must be used to select the capacitor. Furthermore, motor inductances are frequently poorly documented, if at all.

The capacitor and motor winding, in combination, form a resonant circuit. If the control system drives the motor at frequencies near the resonant frequency of this circuit, the motor current through the motor windings, and therefore, the torque exerted by the motor, will be quite different from the steady state torque at the nominal operating voltage! The resonant frequency is:

 

f = 1 / ( 2 (L C)0.5 )

Again, the electrical resonant frequency for a variable reluctance motor will depend on shaft angle! When a variable reluctance motors is operated with the exciting pulses near resonance, the oscillating current in the motor winding will lead to a magnetic field that goes to zero at twice the resonant frequency, and this can severely reduce the available torque!

 

Unipolar Permanent Magnet and Hybrid Motors

 

Typical controllers for unipolar stepping motors are variations on the outline shown in Figure 3.3:

 

Figure 3.3

In Figure 3.3, as in Figure 3.1, boxes are used to represent switches; a control unit, not shown, is responsible for providing the control signals to open and close the switches at the appropriate times in order to spin the motors. The control unit is commonly a computer or programmable interface controller, with software directly generating the outputs needed to control the switches.

As with drive circuitry for variable reluctance motors, we must deal with the inductive kick produced when each of these switches is turned off. Again, we may shunt the inductive kick using diodes, but now, 4 diodes are required, as shown in Figure 3.4:

 

 

 

 

 

 

 

 

 

Figure 3.4

The extra diodes are required because the motor winding is not two independent inductors, it is a single center-tapped inductor with the center tap at a fixed voltage. This acts as an autotransformer! When one end of the motor winding is pulled down, the other end will fly up, and visa versa. When a switch opens, the inductive kickback will drive that end of the motor winding to the positive supply, where it is clamped by the diode. The opposite end will fly downward, and if it was not floating at the supply voltage at the time, it will fall below ground, reversing the voltage across the switch at that end. Some switches are immune to such reversals, but others can be seriously damaged.

A capacitor may also be used to limit the kickback voltage, as shown in Figure 3.5:

 

Figure 3.5

The rules for sizing the capacitor shown in Figure 3.5 are the same as the rules for sizing the capacitor shown in Figure 3.2, but the effect of resonance is quite different! With a permanent magnet motor, if the capacitor is driven at or near the resonant frequency, the torque will increase to as much as twice the low-speed torque! The resulting torque versus speed curve may be quite complex, as illustrated in Figure 3.6:

Figure 3.6

Figure 3.6 shows a peak in the available torque at the electrical resonant frequency, and a valley at the mechanical resonant frequency. If the electrical resonant frequency is placed appropriately above what would have been the cutoff speed for the motor using a diode-based driver, the effect can be a considerable increase in the effective cutoff speed.

 

The mechanical resonant frequency depends on the torque, so if the mechanical resonant frequency is anywhere near the electrical resonance, it will be shifted by the electrical resonance! Furthermore, the width of the mechanical resonance depends on the local slope of the torque versus speed curve; if the torque drops with speed, the mechanical resonance will be sharper, while if the torque climbs with speed, it will be broader or even split into multiple resonant frequencies.

 

Practical Unipolar and Variable Reluctance Drivers

 

In the above circuits, the details of the necessary switches have been deliberately ignored. Any switching technology, from toggle switches to power MOSFETS will work! Figure 3.7 contains some suggestions for implementing each switch, with a motor winding and protection diode included for orientation purposes:

 

Figure 3.7

 

Each of the switches shown in Figure 3.7 is compatible with a TTL input. The 5 volt supply used for the logic, including the 7407 open-collector driver used in the figure, should be well regulated. The motor power, typically between 5 and 24 volts, needs only minimal regulation. It is worth noting that these power switching circuits are appropriate for driving solenoids, DC motors and other inductive loads as well as for driving stepping motors.

 

The SK3180 transistor shown in Figure 3.7 is a power darlington with a current gain over 1000; thus, the 10 milliamps flowing through the 470 ohm bias resistor is more than enough to allow the transistor to switch a few amps current through the motor winding. The 7407 buffer used to drive the darlington may be replaced with any high-voltage open collector chip that can sink at least 10 milliamps. In the event that the transistor fails, the high-voltage open collector driver serves to protects the rest of the logic circuitry from the motor power supply.

 

The IRC IRL540 shown in Figure 3.7 is a power field effect transistor. This can handle currents of up to about 20 amps, and it breaks down nondestructively at 100 volts; as a result, this chip can absorb inductive spikes without protection diodes if it is attached to a large enough heat sink. This transistor has a very fast switching time, so the protection diodes must be comparably fast or bypassed by small capacitors. This is particularly essential with the diodes used to protect the transistor against reverse bias! In the event that the transistor fails, the zener diode and 100 ohm resistor protect the TTL circuitry. The 100 ohm resistor also acts to somewhat slow the switching times on the transistor.

 

For applications where each motor winding draws under 500 milliamps, the ULN200x family of darlington arrays from Allegro Microsystems, also available as the DS200x from National Semiconductor and as the Motorola MC1413 darlington array will drive multiple motor windings or other inductive loads directly from logic inputs. Figure 3.8 shows the pinout of the widely available ULN2003 chip, an array of 7 darlington transistors with TTL compatible inputs:

 

Figure 3.8

 

The base resistor on each darlington transistor is matched to standard bipolar TTL outputs. Each NPN darlington is wired with its emitter connected to pin 8, intended as a ground pin, Each transistor in this package is protected by two diodes, one shorting the emitter to the collector, protecting against reverse voltages across the transistor, and one connecting the collector to pin 9; if pin 9 is wired to the positive motor supply, this diode will protect the transistor against inductive spikes.

The ULN2803 chip is essentially the same as the ULN2003 chip described above, except that it is in an 18-pin package, and contains 8 darlingtons, allowing one chip to be used to drive a pair of common unipolar permanent-magnet or variable-reluctance motors.

 

For motors drawing under 600 milliamps per winding, the UDN2547B quad power driver made by Allegro Microsystems will handle all 4 windings of common unipolar stepping motors. For motors drawing under 300 milliamps per winding, Texas Instruments SN7541, 7542 and 7543 dual power drivers are a good choice; both of these alternatives include some logic with the power drivers.

 

Bipolar Motors and H-Bridges

 

Things are more complex for bipolar permanent magnet stepping motors because these have no center taps on their windings. Therefore, to reverse the direction of the field produced by a motor winding, we need to reverse the current through the winding. We could use a double-pole double throw switch to do this electromechanically; the electronic equivalent of such a switch is called an H-bridge and is outlined in

 

Figure 3.9

As with the unipolar drive circuits discussed previously, the switches used in the H-bridge must be protected from the voltage spikes caused by turning the power off in a motor winding. This is usually done with diodes, as shown in Figure 3.9.

It is worth noting that H-bridges are applicable not only to the control of bipolar stepping motors, but also to the control of DC motors, push-pull solenoids (those with permanent magnet plungers) and many other applications.

 

With 4 switches, the basic H-bridge offers 16 possible operating modes, 7 of which short out the power supply! The following operating modes are of interest:

 

Forward mode, switches A and D closed.

 

Reverse mode, switches B and C closed.

These are the usual operating modes, allowing current to flow from the supply, through the motor winding and onward to ground. Figure 3.10 illustrates forward mode:

Figure 3.10

Fast decay mode or coasting mode, all switches open.

Any current flowing through the motor winding will be working against the full supply voltage, plus two diode drops, so current will decay quickly. This mode provides little or no dynamic braking effect on the motor rotor, so the rotor will coast freely if all motor windings are powered in this mode. Figure 3.11 illustrates the current flow immediately after switching from forward running mode to fast decay mode.

 

 

 

 

Figure 3.11

Slow decay modes or dynamic braking modes.

 

In these modes, current may recirculate through the motor winding with minimum resistance. As a result, if current is flowing in a motor winding when one of these modes is entered, the current will decay slowly, and if the motor rotor is turning, it will induce a current that will act as a brake on the rotor. Figure 3.12 illustrates one of the many useful slow-decay modes, with switch D closed; if the motor winding has recently been in forward running mode, the state of switch B may be either open or closed:

Figure 3.12

Most H-bridges are designed so that the logic necessary to prevent a short circuit is included at a very low level in the design. Figure 3.13 illustrates what is probably the best arrangement:

 

Figure 3.13

Here, the following operating modes are available:

XY  ABCD Mode 

   

00  0000 fast decay 

01  1001 forward 

10  0110 reverse 

11  0101 slow decay 

 

The advantage of this arrangement is that all of the useful operating modes are preserved, and they are encoded with a minimum number of bits; the latter is important when using a microcontroller or computer system to drive the H-bridge because many such systems have only limited numbers of bits available for parallel output. Sadly, few of the integrated H-bridge chips on the market have such a simple control scheme.

 

Practical Bipolar Drive Circuits

 

There are a number of integrated H-bridge drivers on the market, but it is still useful to look at discrete component implementations for an understanding of how an H-bridge works. Antonio Raposo (ajr@cybill.inesc.pt) suggested the H-bridge circuit shown in Figure 3.14;

 

Figure 3.14

The X and Y inputs to this circuit can be driven by open collector TTL outputs as in the darlington-based unipolar drive circuit in Figure 3.7. The motor winding will be energised if exactly one of the X and Y inputs is high and exactly one of them is low. If both are low, both pull-down transistors will be off. If both are high, both pull-up transistors will be off. As a result, this simple circuit puts the motor in dynamic braking mode in both the 11 and 00 states, and does not offer a coasting mode.

The circuit in Figure 3.14 consists of two identical halves, each of which may be properly described as a push-pull driver. The term half H-bridge is sometimes applied to these circuits! It is also worth noting that a half H-bridge has a circuit quite similar to the output drive circuit used in TTL logic. In fact, TTL tri-state line drivers such as the 74LS125A and the 74LS244 can be used as half H-bridges for small loads, as illustrated in Figure 3.15:

 

 

 

 

 

 

Figure 3.15

This circuit is effective for driving motors with up to about 50 ohms per winding at voltages up to about 4.5 volts using a 5 volt supply. Each tri-state buffer in the LS244 can sink about twice the current it can source, and the internal resistance of the buffers is sufficient, when sourcing current, to evenly divide the current between the drivers that are run in parallel. This motor drive allows for all of the useful states achieved by the driver in Figure 3.13, but these states are not encoded as efficiently:

XYE  Mode 

  

--1  fast decay 

000  slower decay 

010  forward 

100  reverse 

110  slow decay 

 

The second dynamic braking mode, XYE=110, provides a slightly weaker braking effect than the first because of the fact that the LS244 drivers can sink more current than they can source.

The Microchip (formerly Telcom Semiconductor) TC4467 Quad CMOS driver is another example of a general purpose driver that can be used as 4 independent half H-bridges. Unlike earlier drivers, the data sheet for this driver even suggests using it for motor control applications, with supply voltages up to 18 volts and up to 250 milliamps per motor winding.

 

One of the problems with commercially available stepping motor control chips is that many of them have relatively short market lifetimes. For example, the Seagate IPxMxx series of dual H-bridge chips (IP1M10 through IP3M12) were very well thought out, but unfortunately, it appears that Seagate only made these when they used stepping motors for head positioning in Seagate disk drives. The Toshiba TA7279 dual H-bridge driver would be another another excellent choice for motors under 1 amp, but again, it appears to have been made for internal use only.

 

The SGS-Thompson (and others) L293 dual H-bridge is a close competitor for the above chips, but unlike them, it does not include protection diodes. The L293D chip, introduced later, is pin compatible and includes these diodes. If the earlier L293 is used, each motor winding must be set across a bridge rectifier (1N4001 equivalent). The use of external diodes allows a series resistor to be put in the current recirculation path to speed the decay of the current in a motor winding when it is turned off; this may be desirable in some applications. The L293 family offers excellent choices for driving small bipolar steppers drawing up to one amp per motor winding at up to 36 volts. Figure 3.16 shows the pinout common to the L293B and L293D chips:

 

Figure 3.16

This chip may be viewed as 4 independent half H-bridges, enabled in pairs, or as two full H-bridges. This is a power DIP package, with pins 4, 5, 12 and 13 designed to conduct heat to the PC board or to an external heat sink.

 

The SGS-Thompson (and others) L298 dual H-bridge is quite similar to the above, but is able to handle up to 2-amps per channel and is packaged as a power component; as with the LS244, it is safe to wire the two H-bridges in the L298 package into one 4-amp H-bridge (the data sheet for this chip provides specific advice on how to do this). One warning is appropriate concerning the L298; this chip very fast switches, fast enough that commonplace protection diodes (1N400X equivalent) don't work. Instead, use a diode such as the BYV27. The National Semiconductor LMD18200 H-bridge is another good example; this handles up to 3 amps and has integral protection diodes.

 

While integrated H-bridges are not available for very high currents or very high voltages, there are well designed components on the market to simplify the construction of H-bridges from discrete switches. For example, International Rectifier sells a line of half H-bridge drivers; two of these chips plus 4 MOSFET switching transistors suffice to build an H-bridge. The IR2101, IR2102 and IR2103 are basic half H-bridge drivers. Each of these chips has 2 logic inputs to directly control the two switching transistors on one leg of an H-bridge. The IR2104 and IR2111 have similar output-side logic for controlling the switches of an H-bridge, but they also include input-side logic that, in some applications, may reduce the need for external logic. In particular, the 2104 includes an enable input, so that 4 2104 chips plus 8 switching transistors can replace an L293 with no need for additional logic.

 

The data sheet for the Microchip (formerly Telcom Semiconductor) TC4467 family of quad CMOS drivers includes information on how to use drivers in this family to drive the power MOSFETs of H-bridges running at up to 15 volts.

 

A number of manufacturers make complex H-bridge chips that include current limiting circuitry; these are the subject of the next section. It is also worth noting that there are a number of 3-phase bridge drivers on the market, appropriate for driving Y or delta configured 3-phase permanent magnet steppers. Few such motors are available, and these chips were not developed with steppers in mind. Nonetheless, the Toshiba TA7288P, the GL7438, the TA8400 and TA8405 are clean designs, and 2 such chips, with one of the 6 half-bridges ignored, will cleanly control a 5-winding 10 step per revolution motor.

 

 



By: s.sankar

About the Author: