Re: [CAD_CAM_EDM_DRO] Wound vs PM DC Motors

Alison & Jim Gregg wrote:

> John and list.
>
> I don't know if this will work in your application. but we sometimes get
> involved in applications where a speed and direction controler (electronic
> PWM) normally used with permanent magnet type DC motors has to be used with
> various wound field motors. The quick and dirty solution is to bring out
> both ends of the field windings and connect them to the DC terminals of a
> solid state bridge rectifier module while the AC connections of the module
> are connected to wherever the field connections were originally connected.

If you are substituting a wound field motor where a PM was intended, there
will be no "wherever the field connections were originally connected".

> This gives DC excittation of the field windings and the motor behaves like
> a PM type, Note in a series motor this means the bridge rectifier has to
> be able to carry the full motor current.. It is far easier than trying
> to drive the fields with constant current DC separately excited, and tends
> to preserve the motors characteristics more.

This doesn't sound right at all! It will probably work fine in the case of
variable speed drives which don't go very low in speed. The problem is
that RPM will no longer be proportional to armature voltage. The reason
for this is that in the PM motor, field flux is constant. In this shunt configuration,
the field flux will be proportional to armature applied voltage, and the
field will get stronger when voltage is increased, defeating the controller's
attempt to increase speed.

If the writer intended to hook the AC terminals of the rectifier bridge
in SERIES with the armature terminals, that will indeed work, but the
motor will have very poor speed regulation, as all series-wound motors
do. Perhaps the speed controller could compensate for this, but the
characteristic would not duplicate the PM motor's characteristics at
all.

Hooking the rectifier for the shunt field in PARALLEL with the armature in
a PWM servo system will be a disaster, and blown fuses or amplifiers
tripping on overcurrent is guaranteed. The reason is that servo systems
are often required to hold position against loads. In this case, the armature
voltage is almost zero, while substantial currents flow. Here, there would
be no voltage to excite the field, so the motor would be required to
produce torque with no field. No torque means error increases, turn up
the current, still no voltage, no field, no torque, etc. until the current reaches
the limit.

Separately exciting a shunt wound motor's field with a (relatively) constant
voltage most closely duplicates a PM motor's characteristics.

Jon