however is not critical the rotor resistance values as illustrated in fig5 must also be known to allow adjustment of the resistors 20 and 22 for the purpose of satisfying equation 2 the transistor 40 is selected to have a high u03b2 so that the collector and emitter currents of this transistor are substantially the same with the emitter resistor 42 the emitter current of transistor 40 is substantially proportional to the error voltage at the output of the amplifier 26 and therefore the driving current applied to the motor is made proportional to this error voltage the u03b2 of transistor 16 divided by the value of resistor 42 is the coefficient of proportionality of the error voltage to the motor current to obtain rapid starting and stopping characteristics of the motor it has been found desirable to drive the motor with current control as in the present invention rather than with voltage control the average value of the back emf as provided by the output of amplifier 18 is a true measure of the motor speed in view of the condition that resistors 20 and 22 have been set or adjusted to satisfy equation 2 the output 24 of the amplifier 18 is fed back to the comparator or summing amplifier 26 which compares the output 24 with the command input 38 the error voltage obtained at the.