Re: DC motor field question

Murry,

A compound wound DC motor has multiple field windings which
can be configured to operate some of the field as a shunt
and some of the field as series. This allowed controls that
had variable speed and some degree of feedback regulation
without resorting to complicated electronics. The purpose
of the multiple windings was not related to providing for
changing the winding's inductance.

It is possible to have a compound motor that was not
connected to take advantage of the compound winding though
there were typically two quite different field windings.
Typically there would be one high current winding that was
intended to go in series with the armature, and one low
current winding that was intended to go in parallel with
the armature. Could it be that you have mis-identified the
field winding ends in your measurements? It would seem
likely that you might have a 300 ohm field coil and a
1 ohm field coil in series with each other with only
three leads coming out of the motor. In this case the
300 ohm winding would go across the armature and power
would be applied to the "outboard" end of the 300 ohm
winding and to the "outboard" end of the 1 ohm winding.

WARNING: ascii art below requires a monospaced font

---{1 ohm field}-----+----{300 ohm field}---+-----
| |
| |
+----{ armature }------+

A DC motor's rotation is reversed by changing the direction
of the current in either the field OR the armature. This
is possible on almost all DC motors, but in the case shown
above it must be done on the armature. Being designated as
a reversable motor is more likely related to the design
of the brush holders. Some are designed for rotation in
only one direction and they may have poor performance or
even be mechanically damaged if run in reverse.

Without specific information on the motor's design or an
accurate diagram of the original connections, you should
take the suggestions above cautiously. Running the motor
with a 25 watt light bulb in series might save things if
the connections are wrong.

If you are replacing the variac with an electronic drive,
try hooking the controllers field terminals to the 300
ohm field winding on the motor. You may or may not have
good luck depending on how much of the motor's field was
originally developed by the 1 ohm winding.

Regards,
Steve Stallings


--- In CAD_CAM_EDM_DRO@y..., "Multi-Volti Devices (Murray)" <multi-
volti@s...> wrote:

> Hello:
>
> I'm replacing a Variac speed control (adjustable DC supply) for a

1/3 hp dc

> motor with an electronic drive. I'm puzzled by the motor wiring.
>
> The motor says compound wound elec. rev. which I take to mean

electrical

> revese or electrically reversible. Because that is normally

possible anyway

> simply to reversing the armature, it suggests there is something in

the

> motor's wiring that is uniquely characterized for electrical

reverse.

> Armature is 115 VDC 3.4 A and field is 115 VDC 0.4 A.
>
> With this assumption, I am attempting to interpret a weird field

wiring

> described below.
>
> 2 leads measure 3-10 ohms, rotor position dependent. This suggests

brush

> resistance and low resistance suggests the winding that handles

current.

>
> There are 3 more leads that measure 300 ohms between each pair, and
> essentially a short (1 ohm) across the whole thing...looks like two

windings

> in series, or centertapped, then 'folded over' to connect the upper

and

> lower ends...what I am describing could also suggest that they are
> paralleled. I doubt a short - that is more likely to happen on the

armature,

> and the two windings are identical - how likely is it a short could

develop

> that allows symmetry? Not very in my opinion.
>
> The old wiring diagram doesn't show any of this weirdness, so

perhaps the

> motor has cababilities that were never used, like maybe a dual

voltage

> field. I also wonder if there is an analogy with transformers

wherein

> windings can be paralleled (like dual primary) without changing the

turns

> ratio or flux, but series does affect the turns

ratio...contradicting any

> intuitive analogy between series and parallel inductors (separate

ones not

> sharing the same core, which links the flux).
>
> Anyone have any insight or answers? I'm not sure guesses will be
> helpful..I'm doing enough of that already.
>
> Thanks
>
> Murray