# How can a variable resistor be used for the speed control of a DC shunt motor?

#### JohnA12

Joined Mar 8, 2021
8
How can a variable resistor be used for the speed control of a DC shunt motor?

#### crutschow

Joined Mar 14, 2008
27,756
You generally need some control electronics to go with the variable resistor.
What is the voltage and current rating of the motor?

Joined Jul 18, 2013
23,642
Vary the shunt voltage current is one simple way.
That is if wound field, not PM

#### wayneh

Joined Sep 9, 2010
17,153
What you generally do NOT want to do is throttle the motor power through a pot. The pot would have to be rated to at least the power of the motor and that makes for an expensive pot. It can be done for small-ish motors (like a toy train set) but becomes impractical at larger scales.

Joined Jul 18, 2013
23,642
What you generally do NOT want to do is throttle the motor power through a pot. The pot would have to be rated to at least the power of the motor and that makes for an expensive pot. It can be done for small-ish motors (like a toy train set) but becomes impractical at larger scales.
Not if its wound field, all you need to do is vary the relatively low field current.
Less current, higher RPM

#### JohnA12

Joined Mar 8, 2021
8
Vary the shunt voltage current is one simple way.
That is if wound field, not PM
I had another answer given to me which was adding the variable resistor in the field winding of the dc shunt motor and you can change the speed by varying the flux. Is this correct?

Joined Jul 18, 2013
23,642
That is what I said?
Called the flux control method.
Lower current = higher RPM