56 Frequency Converters Using PAM can result in lower motor noise and very minor effi ciency advantages in special applications like high speed motors (10.000 – 100.000 RPM). However, this often does not overrule the drawbacks like higher costs for the more sophisticated hardware and control issues like higher torque ripples at low speed. 2.6.2 Pulse width Modulation (PWM) PWM is used in FC’s with constant intermediate circuit voltage. This is the most widely-established and best developed method. Compared with PAM, the hardware requirements for this modulation method are lower, control performance at low speed is better and brake resistor operation is always possible.
Some manufacturers dispense with electrolytic capacitors and inductors (chokes) (slim intermediate circuit). The motor voltage can be varied by applying the intermediate circuit (DC) voltage to the motor windings for a certain length of time. The frequency can be varied by shifting the positive and negative voltage pulses for the two half periods along the time axis. Because the technology varies the width of the voltage pulses, it is called ‘”Pulse Width Modulation” or PWM. With conventional PWM techniques, the control circuit determines the on and off times of the semiconductors in a way which makes the motor voltage waveform as sinusoidal as possible.
Thus the losses in the motor winding can be reduced and a smooth motor operation, even at low speed is achieved. The output frequency is varied by connecting the motor to half the intermediate circuit voltage for a specifi c period of time. The output voltage is varied by dividing the voltage pulses of the FC output terminals into a series of narrower individual pulses with pauses in between. The pulse-to-pause ratio can be modifi ed depending on the required voltage level. This means that the amplitude of the negative and positive voltage pulses always corresponds to half the intermediate circuit voltage.