Quote:
Originally Posted by Roger Heuckeroth
Steve,
The field windings on these series motors are made up of a few turns of big wire. The field windings on the SepEx motors have many more turns of smaller wire.
Roger
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Thanx, that's what I expected. So, assuming rewinding the field is undesirable, we do a buck conversion to produce a low voltage/ high current for the low-z field winding. Since we want a field controller anyway, it comes for ~free. Next question: how bad are the field core AC losses? ie do we need an external choke in series? Probably a good idea for EMC compliance anyway?
Intrigued: By reducing the field, the armature has to spin faster to generate a back EMF that matches the input. Some big AC motors use this for speed control, but if the field supply fails the motor goes crazy. Likewise, an automotive alternator is controlled by controlling the field current.
A basic concept is that the motor current is the difference between the input voltage and back EMF divided by the coil resistance, so as the torque load increases, the motor slows down a tiny bit, reduces the back EMF and the current increases, pulling harder. So the motor speed is a function of input voltage divided by the field current. Of course, there are practical limits like the field current must not be too low.
Steve