PWM RATE AND CURRENT RIPPLE
Motor power dissipation (motor heating) is often a concern for precision mechanisms. Depending on the application, current ripple created from PWM drives and low inductance motors can be a major contributor to motor heating. In the 50% duty cycle case shown in Figure 2 above, the peak to peak ripple is almost as large as the average current. For a motor with 1 amp of average current, and 0.6 amps of peak to peak (proportionally similar to the case shown in Figure 2), the RMS of the total current is 1 + 1/sqrt(3)*0.3 = 1.17 amps. This is a 17% increase over the average current. In motor power dissipation, the current is squared, so 1.17 amps squared becomes 1.37 amps and power dissipation increases by 37%. Thermal resistance is the measure of temperature rise per watt, so a 37% increase in dissipated power will equate to a 37% increase in temperature of the motor coil.
A simple way to reduce the dissipated power is to increase the PWM rate. Not all drive manufacturers will suggest doing this, as it puts more thermal load on the drive devices, which has likely not been evaluated at the proposed PWM rate and motor load. It is always recommend discussing these issues with the drive company’s Applications Engineers and inform them of any motor power dissipation concerns when driving a low inductance motor.
The formula for voltage drop across an inductor (described earlier) can be used to calculate the maximum amount of current ripple in a motor. In this calculation, care must be taken in using the correct voltage. For example, when evaluating current ripple at very low speeds, we can typically ignore Vbemf.
When the back emf becomes larger than 10% of the bus voltage, it must be subtracted out to get an accurate current ripple calculation. = ( − − ∗ ) ∗ 1�2 ∗
Equation 1. Current Ripple from PWM Rate and Vbus
In this section, current ripple versus PWM rate is compared using a low inductance motor (0.1mH). Figure 5 below shows how doubling the PWM rate will half the peak to peak current ripple. The reduction in ripple is coming from the reduced time available for current to rise and fall based on the RL time constant.