Theory on DC motor

Hello everyone.

in this post there will be some general information about DC motor.

Basically, a DC motor is made up of an inductor L, a resistor R and a voltage component

 

figure shows that a loop of wire with some resistance is inserted between the two permanent magnets and the motor turns.

of course, we could have several commutators and loops.

So, the torque is proportional to the current through the windings,

T = kI where T is the torque, I is the current, and k is a constant

The wire coils have both a resistance, R, and an inductance, L. When the motor is turning, the current is switching, causing a voltage,

V = L dI/dt

This voltage is known as the back-emf(electromotive force), e.

If the angular velocuty of the motor is w, then e = kw (like a generator)

This voltage, e, is working against the voltage we apply across the terminals, and so,

(V- kw) = IR where I = T/R

which implies (V-kw) = (T/k) R

The maximum or stall torque is the torque at which w = 0 or T = kV/R, and

The stall or starting current, I = V/R

The no load speed, w = V/k, is the maximum speed the motor can run. Given a constant voltage, the motor will settle at a constant speed, just like a terminal velocity.