Motor Types

Motor Types


Industrial motors come in a variety of basic types. These variations are suitable for many different applications. Naturally, some types of motors are more suited for certain applications than other motor types are. This document will hopefully give some guidance in selecting these motors.
• AC Motors
• DC Motors
• Brushless DC Motors
• Servo Motors
• Brushed DC Servo Motors
• Brushless AC Servo Motors
• Stepper Motors
• Linear Motors
AC Motors
The most common and simple industrial motor is the three phase AC induction motor, sometimes known as the "squirrel cage" motor. Substantial information can be found about any motor by checking its (nameplate).
Advantages
• Simple Design
• Low Cost
• Reliable Operation
• Easily Found Replacements
• Variety of Mounting Styles
• Many Different Environmental Enclosures
Simple Design
The simple design of the AC motor -- simply a series of three windings in the exterior (stator) section with a simple rotating section (rotor). The changing field caused by the 50 or 60 Hertz AC line voltage causes the rotor to rotate around the axis of the motor.
The speed of the AC motor depends only on three variables:
1. The fixed number of winding sets (known as poles) built into the motor, which determines the motor's base speed.
2. The frequency of the AC line voltage. Variable speed drives change this frequency to change the speed of the motor.
3. The amount of torque loading on the motor, which causes slip.
Low Cost
The AC motor has the advantage of being the lowest cost motor for applications requiring more than about 1/2 hp (325 watts) of power. This is due to the simple design of the motor. For this reason, AC motors are overwhelmingly preferred for fixed speed applications in industrial applications and for commercial and domestic applications where AC line power can be easily attached. Over 90% of all motors are AC induction motors. They are found in air conditioners, washers, dryers, industrial machinery, fans, blowers, vacuum cleaners, and many, many other applications.
Reliable Operation
The simple design of the AC motor results in extremely reliable, low maintenance operation. Unlike the DC motor, there are no brushes to replace. If run in the appropriate environment for its enclosure, the AC motor can expect to need new bearings after several years of operation. If the application is well designed, an AC motor may not need new bearings for more than a decade.
Easily Found Replacements
The wide use of the AC motor has resulted in easily found replacements. Many manufacturers adhere to either European (metric) or American (NEMA) standards. (For Replacement Motors)
Variety of Mounting Styles
AC Motors are available in many different mounting styles such as:
• Foot Mount
• C-Face
• Large Flange
• Vertical
• Specialty
Many Different Environmental Enclosures
Because of the wide range of environments in which people want to use motors, the AC motor has been adapted by providing a wide range of enclosures:
• ODP - Open Drip Proof
• TEFC - Totally Enclosed Fan Cooled
• TEAO - Totally Enclosed Air Over
• TEBC - Totally Enclosed Blower Cooled
• TENV - Totally Enclosed Non-Ventilated
• TEWC - Totally Enclosed Water Cooled
Disadvantages
• Expensive speed control
• Inability to operate at low speeds
• Poor positioning control
Expensive speed control
Speed control is expensive. The electronics required to handle an AC inverter drive are considerably more expensive than those required to handle a DC motor. However, if performance requirements can be met -- meaning that the required speed range is over 1/3rd of base speed -- AC inverters and AC motors are usually more cost-effective than DC motors and DC drives for applications larger than about 10 horsepower, because of cost savings in the AC motor.
Inability to operate at low speeds
Standard AC motors should not be operated at speeds less than about 1/3rd of base speed. This is due to thermal considerations. A DC motor should be considered for these applications.
Poor positioning control
Positioning control is expensive and crude. Even a vector drive is very crude when controlling a standard AC motor. Servo motors are more appropriate for these applications.
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DC Motors
The brushed DC motor is one of the earliest motor designs. Today, it is the motor of choice in the majority of variable speed and torque control applications.
Advantages
• Easy to understand design
• Easy to control speed
• Easy to control torque
• Simple, cheap drive design
Easy to understand design
The design of the brushed DC motor is quite simple. A permanent magnetic field is created in the stator by either of two means:
• Permanent magnets
• Electro-magnetic windings
If the field is created by permanent magnets, the motor is said to be a "permanent magnet DC motor" (PMDC). If created by electromagnetic windings, the motor is often said to be a "shunt wound DC motor" (SWDC). Today, because of cost-effectiveness and reliability, the PMDC motor is the motor of choice for applications involving fractional horsepower DC motors, as well as most applications up to about three horsepower.
At five horsepower and greater, various forms of the shunt wound DC motor are most commonly used. This is because the electromagnetic windings are more cost effective than permanent magnets in this power range.
Caution: If a DC motor suffers a loss of field (if for example, the field power connections are broken), the DC motor will immediately begin to accelerate to the top speed which the loading will allow. This can result in the motor flying apart if the motor is lightly loaded. The possible loss of field must be accounted for, particularly with shunt wound DC motors.
Opposing the stator field is the armature field, which is generated by a changing electromagnetic flux coming from windings located on the rotor. The magnetic poles of the armature field will attempt to line up with the opposite magnetic poles generated by the stator field. If we stopped the design at this point, the motor would spin until the poles were opposite one another, settle into place, and then stop -- which would make a pretty useless motor!
However, we are smarter than that. The section of the rotor where the electricity enters the rotor windings is called the commutator. The electricity is carried between the rotor and the stator by conductive graphite-copper brushes (mounted on the rotor) which contact rings on stator. Imagine power is supplied:
The motor rotates toward the pole alignment point. Just as the motor would get to this point, the brushes jump across a gap in the stator rings. Momentum carries the motor forward over this gap. When the brushes get to the other side of the gap, they contact the stator rings again and -- the polarity of the voltage is reversed in this set of rings! The motor begins accelerating again, this time trying to get to the opposite set of poles. (The momentum has carried the motor past the original pole alignment point.) This continues as the motor rotates.
In most DC motors, several sets of windings or permanent magnets are present to smooth out the motion.
Easy to control speed
Controlling the speed of a brushed DC motor is simple. The higher the armature voltage, the faster the rotation. This relationship is linear to the motor's maximum speed.
The maximum armature voltage which corresponds to a motor's rated speed (these motors are usually given a rated speed and a maximum speed, such as 1750/2000 rpm) are available in certain standard voltages, which roughly increase in conjuntion with horsepower. Thus, the smallest industrial motors are rated 90 VDC and 180 VDC. Larger units are rated at 250 VDC and sometimes higher.
Specialty motors for use in mobile applications are rated 12, 24, or 48 VDC. Other tiny motors may be rated 5 VDC.
Most industrial DC motors will operate reliably over a speed range of about 20:1 -- down to about 5-7% of base speed. This is much better performance than the comparible AC motor. This is partly due to the simplicity of control, but is also partly due to the fact that most industrial DC motors are designed with variable speed operation in mind, and have added heat dissipation features which allow lower operating speeds.
Easy to control torque
In a brushed DC motor, torque control is also simple, since output torque is proportional to current. If you limit the current, you have just limited the torque which the motor can achieve. This makes this motor ideal for delicate applications such as textile manufacturing.
Simple, cheap drive design
The result of this design is that variable speed or variable torque electronics are easy to design and manufacture. Varying the speed of a brushed DC motor requires little more than a large enough potentiometer. In practice, these have been replaced for all but sub-fractional horsepower applications by the SCR and PWM drives, which offer relatively precisely control voltage and current. Common DC drives are available at the low end (up to 2 horsepower) for under US$100 -- and sometimes under US$50 if precision is not important.
Large DC drives are available up to hundreds of horsepower. However, over about 10 horsepower careful consideration should be given to the price/performance tradeoffs with AC inverter systems, since the AC systems show a price advantage in the larger systems. (But they may not be capable of the application's performance requirments).
Disadvantages
• Expensive to produce
• Can't reliably control at lowest speeds
• Physically larger
• High maintenance
• Dust

REWINDING A DC BRUSH-TYPE MOTOR
by Robert Deppen
This document is written for those who wish to rewire a DC brush type motor. Rewiring a DC motor is not difficult and can be quite rewarding depending on what a person wishes to achieve. Electric motor design itself is a very complex undertaking and is not meant for the novice. This document will not discuss matters of motor design as the contents of such a subject are well beyond the scope of this document or many readers’ ability to comprehend. We will stick to the basics as that is all that is needed in order to successfully complete a motor armature rewind.
Included on this page is a small, easy to use PROGRAM that will assist anyone interested in rewinding motors in choosing the proper "Magnet Wire" for their particular motor. The program is offered free from the manufacturer and is included here as a small download so as to eliminate the necessity of serious web browsing in order to find this information.
WHY REWIND A MOTOR?
Most people might wonder why it is that a motor would need to be rewound. There are two reasons. The motor may have failed due to extreme overheating. If a motor overheats, the windings in the motor may melt the enamel originally coating the windings, causing a short in the coils. A person may wish to rebuild a motor to his or her own specifications. A motor can be rewound to change its performance.
By decreasing the number of windings per coil in a motor, it will turn at higher RPM but will deliver lower output torque. By increasing the number of windings per coil, the motor will rotate at lower RPM, but will deliver higher output torque. Anyone, who creates his or her own scooters or go-carts, will benefit from the information in this document.
STARTING OUT
First a person needs to determine if rewinding a motor is really what they need, or want to do. I cannot stress this enough! Once you begin this process, there is no turning back! You cannot remove the original windings of a motor without damaging them. This is the point of no return.
This tutorial is based on a 280 or 300 watt, 24 Volt DC brush type motor used in scooters. Your motor may be different somewhat but the technique is the same. Use this tutorial as an example only. As with any project, you must pay attention to what you are doing during each step of the process. I would recommend obtaining a pad and pencil to keep records of anything you believe would not be committed to memory. Do not take chances! There is nothing worse than rushing to remove the original coils, only to face the fact that you didn’t document the pattern the coils were wound in. As part of the document, I have drawn some simple CAD drawings that show the disassembly of a typical motor, as well as the winding pattern of the typical 280-Watt scooter motor.
It is important in any motor work, to have a clean and dry place in which to work. Keep this work area free from metal shavings, as we are dealing with strong magnets that will suck them up and cause problems later on. If a file is to be used at any point in the rewire, use it away from your work surface. Do not bring tools to the work area unless you are sure that they are free of metal particles such as metal dust or shavings. Be clean. If metal debris of any sort is detected in the work area, brush or vacuum them away promptly. If metal debris is already clinging to your magnets upon motor disassembly, use a toothbrush to brush them off the magnets. A quick movement with the brush should provide enough speed to break metal debris from the grip of the magnet. If it does not, then pinch the debris off with a soft cloth. This aside let us continue to step one.

STEP 1
Locate the four motor end plate screws. Be careful with them! They are not the highest quality screws and are installed at the factory using power drivers. They are screwed in pretty tight. The best way to break them free is to first use a screwdriver that fits the screw head the best. Too large or too small will damage the head of the screw making it nearly impossible to further attempt removal. Apply as much downward pressure as possible, to insure that the head of the driver remains in the screw. Apply a continuous turning pressure until you feel the screw crack loose. Do this to all four screws. Once all screws are cracked loose, continue to remove all of them in normal fashion. Place the screws in a plastic bag to prevent them from becoming damaged, lost or dirty.
Next you will need to remove the end plate. It is best to use a thin bladed flat screwdriver. Place the end of the screwdriver in the seam between the end plate and the rest of the motor housing. Gently twist the screwdriver to crack the end plate loose. It may be necessary to do this at several locations along the edge of the end plate. Once the end plate is loose, gently wiggle it back and forth with a slight pulling pressure to slide it off the main shaft bearing. The bearing sits in a recess that is machined into the end plate itself. The bearing fits this recess pretty closely, but should slide out of the end plate with little difficulty. A gentle wiggling movement should cause the end plate to slip off of the bearing. The bearing will remain on the shaft and you should leave it there. There is no reason to remove it.
STEP 2

With the end plate removed, look into the motor. You will see the armature sitting in the center of the permanent magnets. Your next goal is to remove the armature. This is to be done with care. You do not want to damage either the armature or the magnets surrounding it. The armature is made of many steel plates called a lamination. These plates are magnetic and will be attracted to the magnets. The magnetic force keeping the armature within the motor housing must be overcome. The armature will not want to leave the area it resides due to magnetic attraction to the magnets. It will be necessary to hold the motor housing firmly, then push on the shaft until the armature and the other motor end plate can be pushed free of the housing. This is not difficult to do, but if it is your first time you may be surprised at how much force it takes. Be careful, when the armature and end plate are being removed, the magnets will try to pull them back in again! Be aware of this before you start or you will be in for a surprise.
Once you have the armature and end plate removed from the main housing, be careful to remove the main housing from your work area. The magnets are strong and will cause the housing to roll towards any large metal objects nearby.
You will now notice that the armature and its end plate are still stuck together. This is because the second shaft bearing is still seated within the end plate. The bearing is not directly seated within the end plate. The bearing resides within a rubber sleeve. The rubber sleeve is seated in the end plate. Gently pull the shaft and bearing out of the sleeve. You will note a small spring washer fall out of the sleeve. This spring washer is needed when you reassemble the motor. Note its location and put it in a bag for safekeeping.
You will notice that there are four carbon brushes mounted to a plastic plate. These brushes will not fly away when the armature is removed from the end plate. These brushes are wired directly to the main power wires. Be careful to remove the springs behind the brushes by pulling the brushes gently from their brass housings. Keep the springs in your bag. This will prevent them from getting dirty or lost.
When handling the armature from now on, take care not to damage the commutation pads. They are durable, but are made of copper and can be easily scratched or gouged by tools. Following is an exploded view of the motor when fully disassembled. Yes, the clever observer will notice that the screws and the spring washer and rubber bearing sleeve are missing from the drawing. I did not take the time to include them. You will also note that I did not draw the actual windings in any of the assembly drawings. This is because I am "ok" with AutoCAD, but not a master. Honestly, I don’t know how to draw coils. If anyone has this skill, please submit a drawing to this page for inclusion.

STEP 3
Now that you have the motor taken apart, observe the armature and how the windings are attached to the brush pads. You will see that the wire travels from one slot to a small tab on the brush pads, then back into another slot. You must remove the wire from these little tabs before you go cutting and removing the wire from the slots in the armature. You must bend the tabs gently so as not to stress them, yet give you enough space to remove the wire from their grasp. Use the edge of a sturdy knife, or the blade of a thin straight edged screwdriver. Place the edge of the knife or screwdriver under the edge of the tab and gently lift the tab a little higher than the diameter of the wire. Do this to all 16 tabs. Cut the wires that go to the tabs for easy removal.
Next is to start cutting coils. How you do this is completely up to you. I have chosen to do this where the wire jumps from one slot to the other. Cutting a bit at a time, you can cut through this hump of wire until the bridge of wire is completely severed. Make sure you cut the wire leading to the tab on the brush pad and remove it from the tab. Next you should bend the wire bundle upwards so that it will pass through the slot. At this time, one full winding should be cut and its ends jutting upwards making the coil now look like a staple. Remove the entire coil from the armature. Continue to do this until all coils are removed from the armature. It should now look like this:

STEP 4
Now it is time to rewind the armature. In the case of the 280-Watt motor, use 22-gauge magnet wire. Included in this page is a small program that will allow you to view all characteristics, gauges, temperature ratings of various coatings etc. I chose to use a coating of Polyurethane and Nylon. This is because it is capable of withstanding temperatures up to 700 degrees. If a motor is going to overheat due to overcurrent conditions, it is wiser to go with a good magnet wire coating that will accept higher temperatures. This way, if the motor is overheating, it will not destroy your coils so easily. The original coating on the magnet wire was cheap enamel in a single layer. This is not a good choice for replacement windings. As indicated above, it is best to use Polyurethane and Nylon.
The winding pattern for this motor is not very complex. At first glance however, it may appear to be complex or confusing. Observe the winding pattern closely and you will see that each coil follows the same pattern of slot to tab around the entire armature. Numbers 1-4 shows this repeating pattern.
If the cardboard insulation pieces you removed from the armature during the removal of the original windings are not damaged simply put them back into the armature slots as they were before. You must wind the coils with these insulators in place. This will protect the windings from abrasion against the armature laminations. If the cardboard pieces were damaged either by burning during an overheat condition or by any other means, check them to see if they are still usable. If they are not or if you are in question as to their insulating quality, replace them with like material. An insulating tape meant for high temperature applications can also be used. Be sure to allow at least a little bit of insulation material to extend above and below the armature slots. This is to insure that the wire does not abrade against the edge of the lamination slots.
I have included a drawing of the winding pattern. In this drawing, you will see numbers. The numbers refer to the path of the wire in steps. Start with #1. This is the first tab on the commutation pads. Do not crimp the wire in the tab quite yet. Leave enough wire length for the coil to reach tab one and just leave it dangling until the first coil is done. This is because when the very last coil is wound, the end of the wire goes back to this tab. So, leave it open for now. Every other tab you pass a wire across however, should be crimped down again to hold the wire in place. Before you crimp the wire down, you must remove the coating from the wire to enable the wire to make electrical contact with the tab. You may do this by scraping it with a sharp X-acto type knife. You may also choose to use sandpaper, etc. Really the choice is yours, as long as the wire has good electrical contact with the tab. Do NOT remove too much insulation from the wire! Keep the stripped area very close to the tab. If you strip too much, you may cause a wire passing over it to conduct with it. The tab itself is only about .075" across. Strip a maximum of .125" of the wire, or 1/8th of an inch. Remember also that you are not to cut the wire during winding. All 16 coils are made from one long piece of wire!
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