Electromagnetic Problems of Stepping Motor

In a permanent magnet or hybrid stepping motor, the magnetic field of the motor rotor changes with the change of the axis angle. The result is that rotating the motor rotors will induce AC voltage in each motor winding. This is called inverse EMF because the induced voltage in each motor winding is always in the same phase and reverse phase as the ideal waveform for the motor to rotate in the same direction. The frequency and amplitude of the back-EMF increase with the increase of the speed of the rotor, so the back-EMF decreases the torque with the increase of the step rate.

Variable reluctance stepping motors also generate back-EMF! This is because the magnetic resistance of the magnetic circuit decreases as the stator winding pulls the teeth of the rotor toward their balanced position. This drop increases the inductance of the stator winding, and this change in inductance requires that the current through the winding be reduced to save energy. This reduction has proved to be anti-EMF.

The reactance (inductance and resistance) of the motor windings limits the current flowing through them. Therefore, according to Ohm's law, increasing the voltage increases the current, thereby increasing the available torque. The increased voltage is also used to overcome the induced back-EMF in the motor windings, but the voltage cannot be increased arbitrarily! Thermal, magnetic, and electronic considerations all limit the useful torque that motors can generate.

The heat emitted by the motor windings is caused by simple loss of resistance, eddy current and hysteresis. If the heat is not fully transmitted from the motor, the motor windings will overheat. This may lead to a simple failure of insulation breakdown, but it also heats the permanent magnet rotor above its Curie temperature, which is the temperature at which the permanent magnet loses its magnetization. This is a particular risk for many modern high-strength magnetic alloys.

Increased drive voltage does not necessarily result in increased torque even if the motor is connected to enough radiators. Most motors are designed to have a magnetic circuit close to saturation when rated current flows through the windings. Increasing the current will not cause a significant increase in the magnetic field in this motor!

Given a drive system, the current of each motor winding will be limited to the rated value of the winding, but there are other limitations at work that use high voltage to achieve higher cut-off torques and higher cut-off above torques. At high speeds, the motor windings must carry high frequency AC signals. This leads to eddy current loss in the motor magnetic circuit and skin effect loss in the motor windings.

Therefore, motors designed for high-speed operation should have a magnetic structure using very thin laminates or even non-conductive ferrite materials, and their windings should use fine-sized wires to reduce skin effect loss. Common motors with large torque have large winding sizes and thick core laminations. When operating at high speed, the motor is easy to overheat, so it should be used at low speed accordingly!