Is this explaination of field shunting correct?

[Nukie Poo]

I read the following quote on a electric railroad web site. Is the
explanation of field weakening causing the motor to accelerate correct?

"The DC motor can be made to run faster than the basic "balancing speed"
achieved whilst in the full parallel configuration without any resistance in
circuit. This is done by "field shunting". An additional circuit is
provided in the motor field to weaken the current flowing through the field.
The weakening is achieved by placing a resistance in parallel with the
field. This has the effect of forcing the armature to speed up to restore
the balance between its magnetic filed and that being produced in the field
coils. It makes the train go faster...."

Thanks

 

[daestrom]

I read the following quote on a electric railroad web site. Is the
explanation of field weakening causing the motor to accelerate correct?

"The DC motor can be made to run faster than the basic "balancing speed"
achieved whilst in the full parallel configuration without any resistance in
circuit. This is done by "field shunting". An additional circuit is
provided in the motor field to weaken the current flowing through the field.
The weakening is achieved by placing a resistance in parallel with the
field. This has the effect of forcing the armature to speed up to restore
the balance between its magnetic filed and that being produced in the field
coils. It makes the train go faster...."

I was going to post some things about 'shunt' wound DC motors, but now that
I re-read this and consider that it's a traction motor, it probably is a
series-wound motor. So the armature and field winding are in series, not
parallel across the line. In this setup a *large* wattage resistor across
just the field winding would work (not across the field and armature in
series though). The armature current would be 'split' and some flow through
the resistor (I said it had to be large ;-) and the remainder through the
field winding.

Motors are often built such that a reduction in field flux will speed them
up. The flux going down would normally reduce the available torque, but it
also reduces the induced emf in the armature. The result is armature
current rises *more* than the flux drops, so the overall torque is higher
and the motor speeds up.

Yeah, it's right. But could use a little clearing up.

daestrom

 

[Nukie Poo]

Squared vs double, as I recall. Anyway, re my and DK's
thread - tell me about it (clearing up, I mean)

Bill W.

Thanks guys