31 Electric Motors As the magnets behave like air in relation to the resulting magnetic fi eld, salient and non-salient fi elds are created. With SPM motors L d and L q have the same value resulting in a non-salient fi eld while the diff erent L d and L q of an IPM creates a salient fi eld which produces an additional torque in fi eld-weakening. 1.4.1.1 Back EMF When the shaft of a PM motor is turned, the motor produces a voltage at its terminals. This voltage is called back EMF (EMF = electromotive force), and describes an important characteristic of the motor. The higher the voltage, the better the motor effi ciency.
Depending on the connection and placement of the windings, the shape of the back EMF can be trapezoidal or sinusoidal. For trapezoidal voltage so-called block commutation is required which is easy to realise in the electronics but has drawbacks like noise and torque ripples. Typically PM motors have sinusoidal back EMF and will be operated via sinusoidal commutation.
Given that the motor actively generates a voltage must be considered, not only during operation but also when the feeding FC is disconnected from mains (power loss, breakdown, switched off ), because the motor can potentially generate suffi cient energy to power up the device while the shaft is rotating (for example, when coasting). The voltage needed for powering the FC depends on the mains voltage the FC is designed for. Example: Required speed of a PM motor with 200 V back EMF to power on a 400 V mains FC (required DC link voltage approx. 320 V).
U DC on 3 2 U BackEMF@1000RPM 320V 3 2 200V n power on = = 1000 RPM = = 1000 RPM = 1134 RPM If the voltage generated by the motor is too high the converter can be destroyed. Practically this can happen when the controlling FC is switched off while the motor is operating at very high speed. During operation the FC limits the voltage coming back from the motor. When the control is suddenly switched off the full back EMF voltage can be seen at the terminals immediately. This critical speed depends on the back EMF of the motor and the voltage the FC is designed for.