In the previous section it was shown that motor torque varies inversely with speed. This then is the motor’s natural speed-torque curve. Below a certain speed, called the corner speed, current would rise above the motor’s rated current, ultimately to destructive levels as the motor’s speed is reduced further.
A real step motor has losses that modify the ideal speed-torque curve. The most important effect is the contribution of detent torque. Detent torque is usually specified in the motor datasheet. It is always a loss when the motor is turning and the power consumed to overcome it is proportional to speed. In other words, the faster the motor turns the greater the detent torque contributes power loss at the motor’s output shaft. This power loss is proportional to speed and must be subtracted from the ideal, flat output power curve past the corner speed. This now constitutes a practical speed-torque curve.
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