Application Guide

63-706217

Control Computer

AC Choke

Charging Resistor

Fig. 17. VFD Construction.

Braking Resistor

Under normal circumstances, with the motor under load, the flow of energy through the system is from the commercial power

supply to the VFD and from the VFD to the motor and finally from the motor to the load. There are operating conditions where

the load tries to over run the motor. An example of this would be a high inertia load. For example: a large diameter fan, running

at high speed, where the control system calls for the fan to run at low speed.

The VFD begins to lower its output frequency and the motor follows. However, due to the inertia in the fan, the fan resists the

change in speed, causing the motor to run above the frequency output from the VFD. This situation will cause energy to flow

from the load back through the motor and into the VFD. A general purpose VFD does not normally have the ability to pass this

energy back into the commercial power supply. However, if required, additional equipment can do this.

The result of this regeneration is a build up of energy in the DC bus capacitors, which manifests itself as an increasing voltage.

If this were allowed to occur unchecked, damage would occur to the components in the VFD due to exceeding the operating

voltage limits. To ensure problems do not occur, the VFD has a bus voltage monitoring circuit. This circuit attempts to reduce

regeneration until bus voltage falls to an acceptable level. If however it is important that the motor and load follow the control

signal exactly, it may be necessary to add a braking resistor system to the VFD. (See Fig. 18.)

This would take the form of a power transistor, a power resistor and a control circuit, the layout of which is shown above. In the

event that the DC bus voltage exceeds the threshold of the control circuit, the power transistor switches on. It also connects the

positive and negative sides of the DC bus together, via a large power resistor. This action dissipates the excess energy as heat

from the resistor. This resistor is subjected to high voltages and currents, so it is a highly stressed component and has to be

carefully selected to ensure reliability.