Why does series dc motors race when unloaded?

Question

explain the function of counter electromotive force

Answer 1

The current through the stator and rotor are the same (in series), so the basic formula for torque is:

T = k * Ia^2

T is the torque

k is the motor constant

Ia is the armature current (squared)

This leads to a torque vs speed curve that looks like the one in this link:

http://www.tpub.com/content/doe/h1011v2/...

As torque (load) goes down, speed goes up, and since there is an inverse proportionality, speed can theoretically be infinite if torque is zero.  In reality speed gets very high when torque is very low.

T = k * Ia^2

When torque is low, Ia is also very low, hence the magnetic energy in the armature coil is low, so the back EMF (counter EMF) will also be low, hence the (almost) full voltage of the source will be applied to motor, thus spinning it up to a rapid speed.

http://www.tpub.com/content/doe/h1011v2/...

.