Seeing from the following figure, we can easily get the result that the motor speed varies with its load amount when the input voltage is fixed. The motor rotates at the no-load speed, the fastest possible speed, when there’s no load on the shaft. The speed would reach zero when the It is fully loaded and the motor is producing its stall torque (ST), the maximum possible torque. At that moment, motors could only works for a short time to save on batteries.
The compound motor’s speed is changed with the applied voltage amount. The below chart shows: the higher the armature current is, the higher the compound motor’s speed is. As for the extra current contributes to the weakness of the magnetic field, the increasing of the speed happens. That can be translated into this sentence: the magnetic field is weakened with the speed of the motor.
B. Shunt wound motor speed versus armature current:
The shunt motor’s armature torque is proportionally increased with the speed of the motor increasing for the proportional relationship between the torque and the armature current.
When the motor is starting: Low speed, little torque
When the motor reaches full rpm: Fullest torque
When the motor is at full rpm, shunt field current is reduced slightly:
The rpm slightly increases, the motor's torque will also increase slightly.
C. Series wound motor torque versus speed:
When under no-load, the motor will eventually reach some high speed.
Notes: some series motors can be damaged by excessive speed.