DC Motors

DC Motors were some of the first to be used in industry. We didn't know everything about the electrical sine-wave but we knew that if we made this motor with brushes and a commutator, we could make the motor turn the way we wanted it to.

We also found that if we changed the voltage to those brushes and the commutator, we could change the speed of the motor. So, we had the best of both worlds. A device that could turn the load we wanted and also be able to change the speeds.

We learned rapidly, however, that the brushes and commutator always had to be "looked after" and we didn't need to always change the speed. So AC motors found their place..... but that's another story.

A DC motor has a fixed field in the outside FRAME of the motor and a rotating field on the ROTATING MEMBER (usually referred to as the armature). The fixed field is just for that purpose. It's power supply is normally a fixed voltage such that once it is applied to the field windings, a rather strong magnetic field is set up between the coils of the frame. Because the laws of physics tell us that "like poles repel and unlike poles attract", if we apply a voltage to the armature windings, and that voltage sets up a magnetic field that is opposite to the one in the frame, the two field will be attracted to each other and the rotating member will be "PULLED" along as the two field try to meet!

Well, the fact that the commutator has the brushes attached to it and those brushes are bringing in the armature voltage and current, just about the time the two fields are close to each other the brush slides off of the commutator bar and disconnects the armature coil from the circuit.... BUT at the same time, it connects the NEXT coil to the power supply and this coil sets up a magnetic field that is attracted to the field in the frame and the process begins again. Like a "cat chasing it's tail", the armature turns around and around.

Now... we can control a lot of parameters here and that's exactly what we do. If the FIELD voltage is constant, we can change the voltage on the ARMATURE. With a "weaker" field in the armature the rotor will not move so fast... so we control the strength of the armature field by applying more or less voltage to the armature, thereby controlling the speed that the armature turns.

Another scenario is that we change the voltage in the outside FIELD. If we LOWER the voltage in the FIELD we weaken the magnetic field and the field set up in the armature may be more likely to "catch" the outside field. Basically, that's the way we make the motor rotate faster than it would normally turn. We simply "weaken the field". A number of DC controls do this automatically and thereby give the user a very wide range of speeds with the same motor. This practice is NOT without problems and should not be applied without due caution. A.R.& E. personnel can help you if you think your application requires "over-speeding".

The DC motor can be equipped to supply accurate speed and regulation through the use of "feedback" devices such as tachometers and encoders. These devices allow the DC control to change the voltage to the motor in order to keep the speed very close to the required value.

Today's world also brings us the Vector Drive which offers very accurate speed and regulation. It's being used more widely and the digital circuitry used to control the motor is much more accurate than the analog feedback that was prevalent in earlier designs.

There are a number of DC motor designs in today's world and the ones listed below, with their characteristics, are the major types that most of us will encounter.

• Permanent Magnet

• Series Wound

• Shunt Wound

• Stabilized Shunt Wound

• Compound Wound

And in each of these different designs, there are a number of different enclosure types too. So, selecting a DC motor isn't just opening a catalog and picking out a number. A.R.& E's application personnel can make that job a lot easier though. Give them a call on your next project or send an email. Either way, we think we can make your life a "Whole Lot Easier"!

Permanent Magnet - A DC motor needs a "constant" magnetic field. This design motor has just that. Usually two magnets, made of a special magnetic iron, actually "glued" to the outside frame of the motor. These magnets produce the magnetic field that the armature needs. This is one of the least expensive designs but the horsepower output of such a device is limited by the strength of the magnetic field produced by the magnets. Usually these motors are 5HP and below. There have been a few larger, but you can use 5HP as a practical limit on PM motors.

If you're going to look at one of these motors at your facility, here are a couple hints.....

1. When you take it apart, be careful of armature movement. When you remove the end bell, the armature will be shifted by the strength of the magnets and if you're finger's in the way.... you'll be sorry.
   

2. DON'T remove the armature from inside the magnets without replacing it with some steel object like a steel shaft or a steel pipe that can be magnetised. Theoretically, magnets in close proximity to each other, like the ones in this motor, will try to pull each other together. If there is nowhere for the magnetic lines of flux to go, the power of the magnets will be drained. If you place some pipe in the hole, the magnetism will go through the pipe and the power will NOT be drained.

Series Wound - The most powerful DC motor you can design. A typical application for this motor is the "traction" (or drive) motors on railroad locomotives. We think of those big Diesel engines on the locomotives as the power that pulls the load. But those engines simply turn a generator, that makes electricity, to power these monster motors on the drive wheels. A series motor will basically suck up all the electricity attached to it and the more you feed it the harder it'll work. So there are two VERY important things that MUST be present for a series motor application. First you need a WHOLE BUNCH OF POWER!!!!! .... and secondly, you MUST HAVE A LOAD APPLIED when you energize the motor. If you don't have a load attached, the motor will "RUN AWAY" and blow itself apart.

Remember the paragraph above where we said that if you weaken the field of a DC motor the armature will be more likely to catch up to the other field. Well..... in a SERIES wound DC motor...THERE IS NO OTHER FIELD!!! So, theoretically, the field is ZERO. How fast do you think it'll try to go? Believe me, don't try it. Motors have been known to literally "fly apart". It's an extremely dangerous situation.

Shunt Wound - Work horse of the DC motors. This type of motor has only the ARMATURE and SHUNT fields. The SHUNT field is usually the winding in the external portion of the motor. The shunt winding is most often connected to a separate DC power source from the armature circuit. This source is usually a constant voltage and in this manner, a constant magnetic field is produced, that surrounds the armature. The SHUNT coil windings are wound and connected so that they provide the North and South poles necessary to produce the magnetic field required to properly operate the motor. As mentioned above, with this kind of winding, and the SHUNT field being separately powered, it's a simply matter to strengthen or weaken this stationary field of the motor and thereby affect the speed and power of the DC motor. 

Stabilized Shunt Wound - This is a hybrid of the shunt wound motor. Every motor, as we apply a load to the shaft, will want to slow down. If we apply too large of a load, it may actually STALL the driving motor.

The stabilized shunt motor was designed to assist with this problem. A SERIES field is wrapped around the circumference of the SHUNT field such that as current passes through it, the resulting magnetic field will (normally) be "added to" the motors standard shunt field. This series field is then wired into the armature circuit. Now... as the load is increased, the current in the armature will increase (since the motor will work harder trying to keep the speed up), and all of this current will pass through the auxiliary SERIES field. This extra current will act as a "stabilizing" feature to the speed of the motor because it will make the FIELD stronger and help keep the motor's speed more constant.

Compound Wound - This motor is a motor that has the ARMATURE circuit, the SHUNT field circuit and a "COMPOUNDING" (or SERIES) field. It was found that the compound field, if placed in series with the armature circuit, could be designed with different characteristics depending on the application. If the designer wanted a DC motor with characteristics closely representative of a SHUNT wound motor, then the COMPOUNDING was "light" and had a smaller impact on the total motor output. If however, the designer wanted a motor whose characteristics more closely paralleled the SERIES wound DC motor, the compound winding was made STRONGER and the motors characteristics were more like a SERIES wound DC motor.

Many of the early COMPOUND wound DC motors were manufactured with the field and armature circuits rated at the same voltage. These motors were used in many marine applications and others where there was ONLY DC power available and were NOT specifically designed to be used as a "variable" speed motor, as most DC motors today are used.