OPTIMAL THEORETICAL OUTPUT TORQUE OF A STEPPER MOTOR BASED ON POWER DISSIPATION CONSTRAINTS
Publication Date : 01/02/2014
Stepper motors are extensively used for a wide range of applications, especially for precise position control. This is due to their advantages of having higher efficiency; maintenance of very high resolution due to their variable step angle, their positional error is non-cumulative and is compatible with modern digital equipment. However, a major limitation in the use of stepper motor is that their output torque is lower than that of a comparable dc motor. Therefore, to maximize their output torque, their phase current must be increased. Though, increase in their phase current will result in increased power dissipation in the phase windings of the motor and consequently reduce the efficiency of the motor. This paper considered the problem of maximizing the output torque of a stepper motor while satisfying power dissipation constraints. Also, the optimal current which maximizes the output torque is derived. Several sub-optimal driver control modes are compared to the optimal one and their relative power output evaluated. When the driver mode is a square current wave, there is a 10% loss in motor output power for the same power dissipation. When the motor is driven by a voltage source it was found that the output power of the motor is nearly the same as when driven from current sources when the motor speed is low. However, a noticeable reduction occurs at increased motor speed. It is shown that the most common driver control mode (i.e. using square wave voltages) is the least efficient from the torque versus power dissipation constraint point of view.
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