Electric motor speed and back E.M.F.

AIUI (someone please correct me), that's right. In steady state the motor is spinning at a constant angular velocity and, if there was no friction, it's doing no work so no power is drawn and no current does flow.

A real motor is not frictionless so it's doing some work to overcome the friction losses. That means the back EMF does not quite match the supply voltage and the difference is the work done to overcome those losses. The result is that a small current flows even when the motor is spinning at a constant rpm.

When you load the motor, or you ask it to change its speed, the motor has to do work to overcome the momentum, and at that point the back EMF drops and a more substantial current flows.

Theo

The more 'real' model of a motors is a generator in series with a resistor and in p;arelel with another one. The series one represent resistance in the windings and the parallel one represents lumped together more or less constant losses. Like friction.

Which is a neater way of saying what you just said.:)

I like the way he said it :)

I like the way he said it :)

As a youngster who frequently visited west London I used to look for the striped “flag” in the drivers window of the Underground Trains. This was part of a switch that when operated weakened the field of the DC motors they were using then lowering the back E.M.F which allowed an increase in speed on lines where it was permitted such as west of Turnham Green. The “Flag” was only visible in that setting. All those types of train have long been replaced *with more modern types some with AC motors so it will have all changed now.

• One example left as a museum operation.

GH

Thank you very much!