This power loss is proportional to speed and must be subtracted from the ideal, flat output power curve past the corner speed. In other words, the faster the motor turns the greater the detent torque contributes power loss at the motor’s output shaft. It is always a loss when the motor is turning and the power consumed to overcome it is proportional to speed. Detent torque is usually specified in the motor datasheet. The most important effect is the contribution of detent torque. This is illustrated in Figure 2.Ī real stepper motor has losses that modify the ideal speed-torque curve. The result now is a two-part speed-torque curve that features constant torque from zero speed until it intersects the motor’s natural load line, called the corner speed, beyond which the motor is in the constant power region. Above the corner speed, motor current is limited by the motor’s inductive reactance. Because torque is proportional to current, motor torque is constant from zero speed to the corner speed. To prevent this, the drive must be set to limit the motor current to its rated value. Below a certain speed, called the corner speed, the current would rise above the motor’s rated current, ultimately to destructive levels as the motor’s speed is reduced further. This then is the motor’s natural speed-torque curve. In the previous section, it was shown that motor torque varies inversely with speed. Magnetic saturation sets a limit on the current to torque proportionally while eddy current and hysteresis (iron losses) along with winding resistance (copper losses) cause motor heating. Also, the iron in the motor is subject to magnetic saturation, as well as having eddy current and hysteresis losses. Because current is proportional to torque, the motor current would be infinite at zero as well.Įlectrically, a real motor differs from an ideal one primarily by having a non-zero winding resistance. In an ideal stepper motor, as speed approaches zero, its torque would approach infinity while at infinite speed torque would be zero. Since torque is proportional to ampere-turns (current times the number of turns of wire in the winding), and current is the inverse of speed, torque also has to be the inverse of speed.
In this case, we substitute inductive reactance for resistance in Ohm’s law and conclude motor current is the inverse of motor speed. Inductance (L) has a property called inductive reactance, which for the purposes of this discussion may be thought of as a resistance proportional to frequency and therefore motor speed.Īccording to Ohm’s law, the current is equal to voltage divided by resistance. Inductance describes the energy stored in a magnetic field anytime current passes through this coil of wire. Ampere-turns simply means that torque is proportional to the number of turns of wire in the motor’s stator multiplied by the current passing through those turns of wire.Īnytime there are turns of wire surrounding a magnetic material such as the iron in the motor’s stator, it will have an electrical property called inductance. To help understand why a step motor’s power is independent of speed, we need to construct (figuratively) an ideal step motor.Īn ideal stepper motor would have zero mechanical friction, its torque would be proportional to ampere-turns and its only electrical characteristic would be inductance. This means motor torque is the inverse of motor speed. Section 1: Motor TheoryĪ stepper motor is a constant output power transducer, where power is defined as torque multiplied by speed. You can read our famous Stepper Motor Basics Guide all on one page here or skip to any chapter by clicking the links in the drop-down menu to the right.