The pulse width modulation uses a carrier frequency which may be 2,000 to 15,000 Hz. The lower frequencies are easier to produce and transmit over cables to the motor, but result in the very annoying buzz or hum associated with VSDs. The higher frequencies produce more accurate ac signals, more accurate speed and torque regulation and the buzz cannot be heard by half deaf industrial workers.
For each pulse, the average voltage is the on time of the pulse divided by the period of the pulse times the DC voltage. If 75% voltage is required, the pulse is on (pulse width) for 75% of the pulse period. Another word for this is 75% duty cycle. The drive calculates the frequency to the motor, calculates a sine wave based on that frequency. At low frequency, the Volts/Hertz computation reduces the ac voltage. The PWM circuit chops the DC into pulses of varying width to give a pulse with the same average voltage as the desired ac signal. When the sine wave should have low volts, the pulses are very short. When the sine wave is at its peak, the pulses are very wide.
PWM is hard on motor insulation because the volts change very quickly with each pulse. Motors should be rated for inverter duty. Some of your companies produce webs with dielectric properties. PWM is a great way to test these films.
PWM is also used in other areas of web handling, notably the heaters for extruders and dies. The electrical heaters are switched on and off using PWM. For heater circuits, the input is the 50 or 60 Hz ac mains and the carrier signal is typically 2 to 5 seconds between pulses. One might almost say that the resin delivery systems use PWM, although these systems do not use a fixed carrier frequency.
For Advanced Readers
The spectral analysis of PWM shows that the harmonic content is significantly higher than that of a pure sine wave. We consider the output frequency to the motor, not the carrier frequency, as the fundamental frequency. Both even and odd harmonics are included in PWM. It turns out the harmonic content is at its minimum when the pulses have a 50% duty cycle (because only odd harmonics are present with a 50% duty cycle square wave). The harmonics peak at roughly 20% and 80% duty cycles. In general, a computer program is required to perform the spectral analysis of a VSD.
VSDs also reflect harmonics back the AC mains. All odd harmonics are produced, but the 5th and 7th harmonics pre-dominate. IEEE 519-1992 addresses the recommended maximum harmonics to be reflected back to the utility. Generally Total Current Demand Distortion (TDD) should be less than 5%. This recommendation may require isolating transformers, and active or passive filtering. Other methods are also available for reducing reflected harmonics.