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Re: (ET) ET - DC Motors 101
Chris,
A pure shunt motor would have no field except for some residual. As
you state, a compound motor should produce flux in the main field
because current flows thru the series winding if armature current
applied at standstill with no field.
The E12/15/20 motors are stabilized shunt which is technically a very
light series or compound winding.
E15 - 466 turns in main field x 1.85 amps = 862 amp turns of flux.
- 1 turn x 25 rated armature amps in stabilized shunt (compound)
winding = 25 amp turns of flux or more but only a small % or rated
field.
Your E20 probably turned due to residual magnetic field plus they
usually shifted these brushes off neutral for best commutation which
gives the motor a "series" affect and can cause it to turn. Don't
expect any torque though. One test for larger DC (400HP +) is to apply
400 amps to the armature and see if the rotor turns without field...
little dangerous but checks symmetry and neutral. If it is off much, it
will try to take off.
Would not recommend trying to run these motors without field... the
repair outweighs the thrill of seeing a motor overspeed for sure!
..Walt
-----Original Message-----
From: Christopher Zach [mailto:cz alembic crystel com]
Sent: Monday, September 17, 2012 10:03 PM
To: elec-trak cosmos phy tufts edu
Subject: Re: (ET) ET - DC Motors 101
On 9/17/2012 4:07 PM, Konstanty, Walter (GE Energy) wrote:
> If you have no field and apply voltage to the armature, the
> armature windings in series are very low resistance and it will cause
> high current to flow (like when the meter pegs and the tractor doesn't
> move if the field relay is open, fields are open or grounded).
This is where I am hanging up here: Technically if you have a compound
motor you have two sources of field flux: The first is the normal shunt
field, which provides most of the flux strength. The second is the field
that is in series with the armature. It provides flux in proportion to
the current going through the armature.
If it's a pure shunt motor with the field at zero then the armature will
act as a dead short and pull a lot of amps without moving. Because you
have nothing but a magnet sitting in a metal tube (the motor housing).
That is what the E12 and E15 motor seem to do.
The E20 was different. When I applied 12 volts on the bench to it with
no field the motor would turn. Oddly, which made me wonder till I hooked
up the field and it spun normally. Since there had to be a source of
flux for the field my conclusion was that it's more of a compound motor,
and the current through the armature as a short induced a field through
the series windings which turned the motor.
I'll fiddle with this more over the weekend. What I do not want to do is
run either motor without field for any length of time to avoid blowing
something up.
Chris
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