In our post Step Motor Heating we looked at step motor losses over a wide range of speeds and power supply voltages. Now, we deepen our examination. Step motors waste power in two ways: copper losses that result from the electrical resistance of the stator coils and iron losses from magnetic hysteresis and eddy currents. In both cases, this lost power results in the motor heating. Copper losses are highest at lower speeds where the motor back emf and inductance do not prevent the driver from applying full current to the stator coils. Magnetic losses from hysteresis, on the other hand, increase uniformly with speed. Eddy current losses are the product of current and frequency, so they tend to create a big spike at 5 – 10 rev/sec and then taper off. Typical losses for a popular NEMA size 23 step motor operating at 24V are seen in the chart below.
The next chart illustrates how a step motor’s torque drops off as speed increases. Because output power is the product of speed and torque, it actually increases with speed, reaching a peak of 54 watts at 20 rev/sec.
By comparing input power to output power we can determine step motor efficiency at any given speed. At full load, step motors can achieve efficiency as high as 80%.
Before our next post, we’ll go back to the lab and run some more tests in an effort to further pinpoint the optimal operating conditions of step motors.