DC Electric Motors

DC Electric Motors

When it comes to the history of electric motors, DC motors were the first form of motor widely used, as they could be powered from existing direct-current lighting power distribution systems. The first commutator DC electric motor capable of turning machinery was invented by British scientist William Sturgeon in 1832, while the AC motor didn't come along until the late 1800's, around 1887.

DC motors were originally designed as two-winding devices. One winding was in the "frame" or stationary portion of the motor and the second winding was in the rotating (armature) portion. The interaction of the magnetic fields formed by these two windings, caused the rotation of the armature and thus the ability to power a load.

Construction

One of the main components of the DC motor armature is the "commutator". This device is a mechanism used to switch the input of most DC machines. It consists of conductive segments insulated from each other and from the shaft. The segments of the commutator (the "bars") are connected to the ends of the armature winding coils. A current is supplied through stationary brushes that are in contact with the revolving commutator. These brushes are of a specific width consistent with the width of the commutator segment, so that the applied current energizes ONE coil of the armature. When that coil is energized, it sets up a magnetic field that aligns itself with the stationary field in the frame, and the armature is rotated by that small segment width increment. As the armature turns, this process is repeated during the full 360° rotation. Since the motor is connected to the machine (the load), the power is transmitted in an optimal manner as the armature rotates from pole to pole. In light of improved technologies in electronic-controllers, sensor-less controls, induction-motors, and permanent-magnet-motor fields, the externally-commutated induction and permanent-magnet motors are displacing electromechanically-commutated motors in numerous applications and projects.

Enclosures

Various enclosure types exist for DC motors, the most common being the ODP (Open Drip-Proof). There are also TEFC (Totally Enclosed Fan Cooled), TENV (Totally Enclosed Non-Ventilated, and TEBC (Totally Enclosed Blower Cooled). This latter type, the TEBC, is used in most of the higher horsepower designs, and motors in applications that may be subject to operational conditions where they are run at SLOW SPEEDS for extended periods. In such an application, the slow rotational speed of the armature does not allow the internal cooling fan to produce sufficient air to reduce the heat generated by the required power. As with other electric motors, application conditions also have a significant bearing on the type of enclosure selected.

We will explore some additional details of a number of these DC motor types in the topics shown below.

Shunt Wound Motor

This photo shows the frame of a 4-pole DC motor with "shunt" (field) coils {the 4 larger coils}, and "interpoles" (compensating) coils {the 4 smaller/narrower coils}. The "shunt" field coils are made of many turns of small diameter wire; i.e. 1500 turns of #24 wire, while the "series" coils are made of only a few turns of a large wire; i.e. 20 turns of #8 wire (or a rectangular wire). The series coils are subjected to the same current that passes through the armature circuit (which could be hundreds of amps), so the wire must be large enough to carry the current without overheating. The shunt windings can be used and/or wired in different configurations to change the operating characteristics of the motor.

For instance, they can be wired to a "separate", external power supply to offer them a fully controllable, changeable, and modifiable voltage source. This power source would normally come from the DC drive that is powering the DC motor, or possibly a "storage battery system" or maybe a generator. This motor is commonly referred to as a "Separately Excited Shunt Wound Motor". This type of connection also allows the motor "field" to be "weakened", which will cause the motor to run faster. Sometimes, in a particular application, it is desirable to "overspeed" the motor during a particular part of the operation. For instance, you may want a powered "planer bed" to operate slowly in the "cutting" direction, but move more rapidly on the "return" stroke of the bed. Field weakening can help us achieve that action.

They can also be connected in "parallel" (shunted) with the armature supply. In this connection, a change in the armature voltage and current will also effect the voltage in the field coils. Connected in this manner is a standard connection for a motor that is normally operated at a single, constant speed.

The "interpoles" or "compensating" windings were introduced into the DC motor to improve the "commutation" process between the brushes and the commutator. Without the interpole windings, it was found that the brushes would "arc" or "spark" as the motor operated. The improvement by introducing these coils to the armature circuit was significant with respect to the maintenance of the DC motor.

These interpoles assist in the "speed" control and "torque compensating" action as they are applied to the armature circuit of the motor. This action takes us into the "Stabilized Shunt Wound Motor" discussed in the next topic.

Stabilized Shunt Wound Motor

A stabilized shunt-wound motor is a direct-current motor in which the shunt field circuit is connected either in parallel with the armature circuit or to a separate source of excitation voltage, and which also has a light series winding added to prevent a rise in speed or to obtain a slight reduction in speed with increase in load.

This "secondary" coil is no more than a few turns (as few as 4 and maybe as many as 20) of a "large" size wire. This coil is connected in series with the "armature" circuit so it must be large enough to conduct the same current that is passing through the armature. This coil of wire is simply "wrapped" around the circumference of the shunt coil. A layer of insulating paper is usually wrapped first, then the turns of the series coil is wound around the paper. It is held in place with tie cord and then insulated by dipping and baking to solidify the complete assembly into one rigid package. This completed assembly is then placed over the motor "pole piece" (a laminated steel assembly) and connected to the other field coils.

When the motor assembly is completed, the "series" coils will be connected together and wired in series with the armature circuit. When the motor is energized, the shunt field is connected to it's power supply (a steady state voltage supply), and the armature circuit is connected to it's power source (usually a variable power source from a DC drive or something similar). As the motor is operating, the shunt field is supplying a steady amount of magnetic flux used to produce the torque necessary to move the load. As the load increases, the current through the "series" coil increases, because it's in the armature circuit. As the current increases, additional flux is created (since it is wrapped around the shunt field coil) and that flux ADDS to the normal flux created by the shunt field coils. The motor just became MORE powerful, but more importantly, it has become more STABLE when it comes to speed regulation based on increasing load. Thus the name... Stabilized Shunt Wound DC Motor.

The type of motor is used heavily on: Printing Presses, Conveyors, Packaging Equipment, and Plastic Extruders.

Compound Wound Motor

A compound-wound motor is a direct-current motor which as two separate field windings. One, usually the predominating field (and also usually the "shunt" field), is connected in parallel with the armature circuit while the other is connected in series with the armature circuit.

Both sets of field coils combine to provide the required amount of magnetic flux to facilitate armature rotation at the desired speed. A compound wound DC motor is the marriage of a shunt wound DC motor and a series wound DC motor, resulting in the better properties of both these types being featured. The shunt wound DC motor is very efficient at speed regulation, while the DC series motor has high starting torque.

So the Compound Wound DC Motor reaches a compromise of two motor designs, resulting in a good combination of proper speed regulation and high starting torque. And while it's starting torque is not as high as a Series Wound DC Motor, nor is it's speed regulation as good as a Shunt Wound DC Motor, the overall characteristics of a Compound Wound DC Motor falls somewhere in between these 2 extreme limits.

In addition to the definitions mentioned above, we need to also discuss the TWO types of "compound" wound DC motors... "Cumulative" and "Differential". These two types differ based on the manner in which the "compound (series) winding" is connected.

If the "shunt" winding and the "series" winding are connected in a manner that causes the magnetic lines of flux to be "additive", then we say that the motor is connected as a "Cumulative Compound DC Motor. In such a connection, as the motor is loaded, more current passes through the "series" winding. And since the magnetic flux created by the "series" and "shunt" windings is in an "additive" mode, the flux is stronger, the motor has more starting torque, and speed regulation will be better. This is the most desirable, and heavily used type of connection for a "compound wound DC motor". Typical applications for the DC Compound Wound Motor include: Mixers, Rolling Mills, Stamping Presses, Metal Shears, and Hoists.

The second type of connection, is the "Differential" connected compound wound DC motor. In this connection, the "shunt" winding and the "series" winding are connected in such a manner that the magnetic flux created by those windings is actually in "opposition" to each other. In this way, the motor will run at a rather constant speed, irrespective of the load. This type of connection is heavily favored in Elevators and escalators.

Series Wound Motor

DC Series motors are the power behind the trains we see everyday. As the general public looks at the rail industry, the engine of the train is a "diesel" engine. And our general knowledge stops, and says, "... a diesel train is like a diesel "over the road" 18-wheeler". But that's simply not the right answer. The "diesel" portion of the railway industry's train engine, is nothing more that a "DIESEL GENERATOR". The power that drives the wheels of the engine is a "Series Wound DC Traction Motor".

Since the series motor's speed can be dangerously high, series motors are often geared or direct connected to the load. Look closely at the photo above; see that "hole" toward the right side, near the pinion gear? That's the bore through which the axle of the train engine's drive wheels is installed. A spur gear on the engine's axle mates with the pinion gear of the DC motor for an absolute direct connection. There's no transmission slip here!!!

A "Series DC Motor" has the armature and field coils connected in SERIES. That means that the same VOLTAGE is applied to both windings. Furthermore, the current that passes through the armature, passes through the field coils too. We know from other topics that as the current/voltage increase, the field strength increases, which means the OUTPUT TORQUE, increases.

So, as we load the Series motor, it slows down and requires MORE current to drive the load. As the CURRENT increase through the armature and field, the field strengthens and the motor becomes more powerful. It seems like a never ending circle. And, in reality, that's pretty much the case. The limiting factor is the VOLTAGE. If we control the voltage, we control the speed. So what do we do with the train? If the engineer wants the train to go faster, or needs more power, he (or she), pushes the throttle of the "diesel generator". The generator speeds up, produces a HIGHER VOLTAGE, and the "series DC motor" goes faster. If the motor is stalled, like the train at a standstill trying to get a "gazillion" freight cars rolling, the current is limited only by the total resistance of the windings and the torque can be very high, but there is a danger of the windings becoming overheated.

A critical problem with the series wound DC motor is what makes it such a powerful device. The motor MUST be connected to a load of some size. If a series wound DC motor is connected to an "infinitely" powerful source and that power is energized BUT the motor is not connected to a load, the motor will accelerate VERY RAPIDLY, and due to the nature of the device, will attempt to CONTINUE to accelerate. It will basically (and literally) fly apart!!! So if you're working around a series DC motor, NEVER power it without having a load connected.

Permanent Magnet Motor

The Permanent Magnet DC Motor has become a major tool in lower horsepower applications. We still have the maintenance issue, like any DC motor, of the brushes and commutator, but the "shunt" field (field winding) has been replaced by a permanent magnet.

Construction

The magnet is made of "sintered" (powdered) metal that is pressed under high pressure and heat into a form that fits inside the shell of the motor frame. These metal forms, after being manufactured, are subjected to a high strength magnetic field that aligns the molecules into magnetic particles. When complete, each piece will have a NORTH and a SOUTH pole. if you break it in half, each piece will have a NORTH and a SOUTH pole. No matter how small the pieces... they will have a NORTH and SOUTH pole. These completed magnets are then "glued" to the inside of the motor shell and it's ready for the "wound armature", end bells, bearings and brushes. When completely assembled the junction box has a mere, 2 wires, that are connected to the brushes. That's it! The motors are compact and less expensive to manufacture. So there's a lot to be happy about.

Magnet Life

The question sometimes comes up about the life of the magnets... I found an internet source from the United Kingdom (www.first4magnets.com) that states: "...Neodymium magnets are permanent magnets, and lose approximately 5% of their performance every 100 years". That seems like pretty good life, from where I sit. There are "3rd party vendors" who specialize in "re-magnetizing" permanent magnets, but under most conditions, due to the economical nature of a PM motor, it's usually better to purchase a complete new motor.

Safety and Maintenance Tips

We'll offer a maintenance and safety note for you here. The strength of a PM is extremely high, and the larger the magnet, the stronger the pull. When you disassemble/reassemble a PM motor, keep your hands and fingers out of the way of the "bore" of the shell, inside the magnets. When you re-insert the armature back into the frame, the pull is extremely strong and if your finger gets in the way, it could have disastrous results.

 Another point is that a PM wants to have something into which it's magnetic field is collected. The armature serves that purpose when the motor is assembled. But when it is NOT, then the flux of the magnetic field from the PM passes through the air gap. This action, if allowed to exist for a long time (don't ask me how long), can "drain" the PM of their power and weaken them. Remember the 5% number over 100 years? Don't shorten that life by draining the magnet. Simply place a steel bar (shaft stock), key stock, something steel, into the bore to allow the magnets to have something to absorb the flux.

Power Output Available

The following paragraph was copied from an the 1994 proceeding of an IEEE committee:

Permanent magnet motors are now practical up to thousands of horsepower. The relative advantages of disc, rotating cylinder, and cup rotor geometries are dependent on shaft speed and power level. Brushless disc motors with pulse width modulated drives have been demonstrated over a range of 200 to 20000 RPM and 10 to 700 horsepower.

And while that statement is most likely true from a research and theoretical standpoint, I'm not certain it is practical to our general industrial climate today. And while I've seen some "on-line" ads for PM motors to 10 HP, I believe that the reality of things are that most industrial electric motor manufacturers are going to top out their offerings in the 2 to 3 HP range. Fractional to 2 HP is really where the industry seems to be today. And most PM DC motors are either TENV or TEFC in enclosure design.

Summary

Another note of caution for you... as you're looking to apply a DC motor to your project, I'll make the assumption that you will be powering that motor with a DC Drive of some modern design. This is as it should be. However, use caution if you are going to apply a DC Drive to an "older" DC motor. Depending on the age of the motor, it may NOT be suitable to be run with today's SCR controllers. Just use caution.

If your interested in reading additional information about DC motors, check out this link on Wikipedia.org©. It's a really detailed and well written article.