58 Frequency Converters The output voltage of the FC cannot equal the motor voltage if full sinusoidal wave form is needed, as the output voltage would be roughly 13 % too low. However, the extra voltage needed can be obtained by reducing the number of pulses when the frequency exceeds approximately 45 Hz. The disadvantage of using this method is that it makes the voltage alternate step-wise and the motor current becomes unstable. If the number of pulses is reduced, the harmonic content at the FC output increases. This results in higher losses in the motor. Another way of dealing with the problem involves using other reference voltages instead of the three sine references.
These voltages could have any shape of waveform, for example, trapezoidal or step-shaped. For example, one common reference voltage uses the third harmonic of the sine reference. By increasing the amplitude of the sine reference by 15.5 % and adding the third harmonic, a switching pattern for the inverter semiconductors can be obtained which increases the output voltage of the FC. All control values of the inverter are transmitted from the control card, and the various reference signals for determining the switching times are stored in a table in memory and are then read out and processed according to the reference value.
There are other ways of determining and optimising the on and off switching times of the semiconductors. The Danfoss VVC and VVCplus control principles are based on microprocessor calculations which identify the optimum switching times for the inverter semiconductors. The specifi cations for the software involved in calculating the switching times are manufacturer-specifi c and will not be covered here. If more stringent requirements are imposed on the FC speed setting range and smooth running characteristics, then the PWM switching times need to be determined by an additional digital IC rather than the microprocessor.
For example, an ASIC (Application Specifi c Integrated Circuit) can determine the PWM switching times. This component incorporates the manufacturer’s proven knowledge. Meanwhile, the microprocessors are responsible for handling other control tasks.