Application Guide

63-706253

Closing a Switch on a Running VFD Output

There are no electrical defects in the circuit. It is caused by incorrect programming and application. If the VFD is running and a

switch is closed in the output cables from the VFD, the motor and VFD will try to effect a

direct on-line (DOL) start

. Typically,

surge currents in excess of 6 times full-load motor current flow during a DOL start. The VFD has a normal rating of 150% of its

nameplate current for one minute and 200% current for 0.5 seconds. Under these conditions the VFD will trip.

Correcting the control wiring and reprogramming the VFD resolves this problem.

Testing Bridge Rectifier and Output Power Transistors

Diode and Power Transistor Test

IMPORTANT

Before undertaking these tests, ensure that main power to the VFD is isolated, additional power supplies are made

safe, and the DC bus voltage has decreased to 0V. Remove any components from the connections shown to ensure

readings are not misleading.

Table 3 and Fig. 57 indicate the various points for testing the power devices on the VFD. For this test, use a standard

multimeter set to read resistance.

Control Computer

AC Choke

Charging Resistor

TR5TR3TR1PD3D2D1

TR2TR6D4 D5 D6 N TR4

Fig. 57. Points for testing VFD power devices.

Table 3. Proper Power Device Test Readings.

Tester Polarity Circuit Tester Polarity

Measurement Symbol

Measurement

D1 L1 P Open D4 L1 N Closed

P L1 Closed N L1 Open

Converter D2 L2 P Open D5 L2 N Closed

Module P L2 Closed N L2 Open

D3 L3 P Open D6 L3 N Closed

P L3 Closed N L3 Open

TR1 U P Open TR4 U N Closed

P U Closed N U Open

VFDTR3VPOpen TR6VNClosed

Module P V Closed N V Open

TR5 W P Open TR2 W N Closed

P W Closed N W Open