Diode speed control and uneven brush wear

Universal motors are designed to operate on AC or DC. Using a rectifier limits power by only supplying half wave DC. Since the motor is designed to run fine on un-rectified AC I doubt there would be any problem. In fact, since it is operated at half power I would expect it to last longer.

The diagram below represents an Universal Motor UM and the waveforms of the Supply voltage (shown in red) and the field coil current and magnetic flux (shown in blue and green).

An UM works similar to a DC motor with the exception that the magnetic field in the stator is generated electrically with a coil called the Field Coil FC rather than a permanent magnet.

First let us look what happens when A is positive with respect to B then the current in the coils is represented by the blue portion of the graph. If you now look at the diagram of the motor the direction of the magnetic fields generated by the rotor and FC is also shown in blue. As you can see the rotor and stator fields oppose each other and thus the rotor will move in a attempt to align itself with the stator. This however can never happen because once the rotor has moved through 90º the other coil will be energized and the process will continue indefinitely.

Now if we look at what happens when B is positive to A the current in the coils would be represented by the green portion to the graph. As you can see the fields in the stator and rotor (shown in green) have both reversed so the same process continues and the rotation continues.

If this were a DC motor the stator field would be generated by a permanent magnet and could not reverse with the reverse in rotor current and hence to would try and spin in the reverse direction negating any rotation generated by the other part of the cycle.

Now since the current through the field coil is oscillating due to the AC supply voltage the strength of the magnetic fields generated by these currents will also fluctuate. This means that the force turning the rotator will also fluctuate and the motor will try and rotate synchronously with the oscillating field strength. This doesn't mean that the motors must spin at a harmonic of the supply but rather that they will generate maximum torque at these synchronous speeds.

Introducing a diode in series will only allow the current to flow in one direction as shown by the blue line in the lower graph. Since the rotor and FC currents can only flow in one direction the rotor and stator magnetic fields can only go one way as well. If the motor is running synchronously then the second coil may never get energized. In the motor above this would result in it only being able to generate torque 50% of the time and may even stall the motor. Another problem that may occur is if the motor has multiple sets of brushes and one of the coils isn't getting any power then that set of brushes won't be carrying current and hence there will be uneven brush wear. This is however only theory but it is the only way I could think of that would result in uneven brush wear.

Using a diode to regulate the speed of a UM isn't the best way to go about it and will result in the torque being generated by the motor only being present 50% of the time. There are much better ways of regulating the speed. Personally I would a triac motor control unit as it would give much greater flexibility and steadier torque.

The wear on brushes depends on the current the brushes carry. For a given surface and a given quality of brush, there is an ideal current -- lets say 4Amp. If the brushes are are larger, they will wear out quicklier. If the brushes are too small, they heat up and same story - they will wear out very quickly. So the current acts sometimes as grease, sometimes as a stove. If you change something on a universal or dc motor, please keep van eye on the brushes!

Regards,

Theo