Application Guide

63-7062 18

Control

Braking Resistor

Bus Monitoring Circuit

Fig. 18. VFD braking resistor.

Pulse Width Modulation (PWM) Control Principles

Various methods produce a synthesized three-phase power supply suitable for driving a standard three-phase electric motor.

However, the industry has standardized on the PWM method of control. See Fig. 19.

Fig. 19. Pulse Width Modulation.

VFD Voltage to Frequency Ratio.

When connected to a VFD, motor speed is no longer fixed by supply frequency, since the VFD can vary its output frequency.

Under perfect conditions, at zero speed the terminal voltage would also be zero. Obviously if this was the case then the motor

would produce zero torque and in many cases this would be unacceptable. Also at very low speeds the motor winding appears

more like a resistive load than an inductive load. To overcome this problem with a general purpose VFD, a degree of fixed

voltage boost is applied at zero speed. As the motor accelerates, a proportion of fixed boost is replaced by normal V/F ratio

until, at some speed above zero, governed by the amount of fixed boost applied, all boost is replaced by the normal V/F ratio. If

an excessive amount of fixed boost is applied, the motor can become overheated due to over fluxing. (See Fig. 20.)

It is also possible to program the drive to adjust the V/F ratio automatically according to the load applied to the motor. Within

limits, as the current drawn increases the drive responds to this as an increase in load and to maintain torque and speed, the

drive increases the terminal voltage, within predefined limits.

If the drive has been programmed correctly, the maximum terminal voltage will be reached at maximum speed. However, the

application can require that the motor run over speed. Normally, 20% over speed is acceptable, providing the load can

withstand the stresses caused by this additional speed. Fig. 20 illustrates that at maximum speed terminal voltage is at

maximum. Therefore, there can be no increase in voltage due to speed increases. This point is called the field weakening

point, as a constant motor flux can no longer be maintained. According to the V/F ratio, therefore the motor torque capability

begins to reduce. If the motor continues to increase in speed then it passes into an area of operation called the constant power

area.