dc motor question

Just a commutator, as far as I know. The armature is separately energized (not by the field windings).

?? My mental image when you say a compound DC machine is an armature which is on a rotor, connected to the outside world through brushes and a commutator, and two stationary field windings, one (shunt field) that's relatively high resistance and in parallel with the brushes going to the armature and one (series field) that's relatively low resistance and in series with the parallel combination of shunt field and armature. Series-connected DC motors have the characteristic that they can provide a lot of torque but do not have very good speed regulation, since the field magnitude depends on load. Increased load causes increased field current, resulting in lower RPM to get the same back EMF, and higher torque. Shunt-connected DC motors hold much more constant speed (assuming constant input voltage) since the field strength is independent of the mechanical load, and the back EMF depends only on the armature speed. A compound wound motor is a hybrid between the two, and the characteristics fall in between. I'd expect commonly the two field windings would be on the same magnetic core. The net field is just the sum of the contributions from those two windings.

But maybe you're talking about a different sort of DC machine than that.

Cheers, Tom

The series field is just wired in series with the armature (connected in series with one of the brushes, usually).

Lots of good motor basics, here:

THe field isn't in series with the armature. You do have a high current carrying series winding that is integrated with in the structure of the field how ever. This is placed in series with the armature leads.. The idea is to either enhance field strength of existing field when armature current increases or weaken the field strength..

Enhancing it will give you more torque for heavy loads at lower RPM's and for starting on heavy loads. Weakening will give you less torque but makes for higher RPM's in the motor.

These days, series wound motors are not so common because drives now have better logic that allows for auto field weakening at higher RPM's when the drive current starts dropping back, increases the field current when excess current is at the armature and RPM's are low.

Of course, you don't have to operate the drive in that state, you still can use the series configuration of the motor if you wish.. It was common years ago to use series motors to help with better self regulation on older drives that didn't offer such dynamic control.

Polarity config is a little tricky how ever..

if you apply + & - to the + & - leads of the field. It's A1, A2+S1, S2 for torque, reverse the S1 & S2 for speed. A1 being the + lead and S2 the - lead from the drive output.

now, if you need to reverse the motor, you can do so by switching the A1 and A2 feeds from the drive or the Field how ever! when you do this, you need to switch the S1 and S2 to maintain torque so that the polarity is correct with the field to maintain the state of how you want the motor to react. This is due to the fact that S1 and S2 are mixed with in the field windings and you must keep the polarity the same for both in torque mode and opposite for weakening (speed).

If you hook up to a drive that already has this field control in it, you don't need to use the series winding..