Hello,
you have two possibilities.
A. To control rotor or stator power supply
B. To control motor power supply
In first case you will have a much better result, in the second case your motor will be with more soft torque/rpm ratio. For both cases i will recommended to use feedback, you can use optical, hal sensors or tacho sensor, if you have a time and experience you can get feedback from the impulse over power supply, generate from collector.
For supply regulation you have many different ways. You can use a very simple, tiristor regulator with phase control, you will regulate the openning moment toward the begin of the period. Other way is using of a PWM control, if your source is a DC. If the motor have a small power then you can use regular linear regulator.