I am trying to learn more about universal motors (like postwar Lionel or American Flyer) and even bought an American Flyer motor from a train-meet "junk" box to help me in the process. I now have the motor in operating condition and have run it with either AC or DC power. My question for the present is this: Are two of the armature segments of one polarity and the third opposite or is one of the segments magnetically neutral (as I once read in a Model Builder magazine)? Does operating on AC vs. DC produce different results as to the polarity of the three segments? Thanks in advance for your input.
Universal motors are wired with the field and armature in series. That way, current enters one brush, passes through the arature, then out the other brush through the field coil. To reverse the direction of rotation of the armature, interchange the field and brush connections. The motor will operate the same way with AC or DC.
There are three windings, one on each of the three poles. The windings are connected in a ring; and the junction between each pair of windings is connected to one of the commutator plates. At any time, two of the windings are in series and connected between the two brushes and the third winding is connected directly, by itself, between the brushes.
The magnetic polarity of the two poles whose windings are in series is opposite to that of the third winding, because the current is flowing clockwise through one path and counter-clockwise through the other. This makes an electromagnet of the entire armature, with one of the three armature poles serving as one of the magnetic poles and the other two together as the other magnetic pole.
As the armature turns, the commutator plates move under the brushes; and, every 60 degrees, one of the windings from the pair of windings in series switches to the other group and the other polarity. This keeps the electromagnet that is the armature aligned roughly at right angles to the stationary electromagnet that is the "field" and produces the torque that moves the motor, by continually trying to align with each other but never succeeding.
On top of the polarity changes I just described, the polarity of everything, armature and field both, are flipped every 1/120 of a second as the track voltage alternates. But this has no effect on the motor operation, since the attraction of opposite poles of the armature and field windings is the same as long as they both change at the same time.
A universal motor is very similar to a series DC motor. The main difference is that the magnetic circuits are made out of thin iron laminations instead of solid iron. This keeps them from acting as electrical conductors, which solid iron would be, by blocking electrical current at the surfaces between the plates. The same trick is used in transformers, so that the iron remains a magnetic conductor but not an electrical one, which would act like a short-circuited secondary winding.
Except for some of the very new ones, prototype traction motors are also series motors.
Bob Nelson
Thanks Bob. I'm not sure why the article mentioned above stated one segment is north, one south and one neutral. A second question: is it correct to say that when running a universal motor on DC it is basically the same as a DC permanent magnet motor?
There is an alternate way to wire a 3-pole armature, which actually came up on the forum not long ago. In this scheme, the coils are organized not in a circle but a star, with all three connected to a common point and with their other ends connected to the commutator segments. This is not quite as good as other arrangement, since one of the coils is always idle. If you increase the number of poles above three, the fraction of the armature wiring in use at any one time gets lower and lower. So you are unlikely to encounter this wiring method.
No, a universal motor is a series motor and behaves rather differently from a permanent-magnet motor, whether run on AC or DC.
An ideal permanent-magnet motor appears at its terminals like a voltage source. The voltage is proportional to the motor speed; and the current is proportional to the torque. So it wants naturally to run at a speed set by the track voltage. This can cause a problem with motors fighting each other in a two-motor locomotive or among multiple locomotives at the point of the same train, unless the motors are carefully matched to run at the same speed when given the same voltage.
In an ideal series motor, the motor appears at its terminals like a resistance. The resistance is proportional to the motor speed; and the current is proportional to the square-root of the torque. So the motor can run at different speeds. If you apply a voltage to a stopped ideal series motor with no load on it, an infinite current will flow briefly, creating infinite torque, which will accelerate the motor quickly. As it speeds up, the motor resistance will increase and the current will drop. But, with no load, it will continue to accelerate indefinitely. In the real world, with a load on the motor, it will accelerate until the torque is just what is necessary to move the load. You perhaps can see that this behavior is almost ideal for starting and pulling a train, that is, very high torque at first, dropping as the train accelerates, to the level needed to keep the train moving. It is a simple electrical automatic transmission!
Thanks again, Bob, for your explanations. They certainly help increase my understanding.
It just occurred to me that, in tribute to their long history in toy trains, one might label a factory on a layout "Universal Motors".
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