Page 213 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

VARIABLE-SPEED DRIVES

10.12 CHAPTER TEN

Comments about Failure Rates
1. The lifetime of all semiconductors can be calculated.
2. The thermal fatigue life decreases when
● The temperature changes increase
● The size of the device is larger
● The device is soldered rather than press-packed
3. Damage to the device would occur at a fault current, resulting in 300°C change.
4. Fault coordination for each application should be done properly in the design phase to
prevent explosion of the devices.
5. The failure rate depends on
● Temperature
● PIV
● Bridge failure mode (catastrophic or not)
6. Failed devices are normally shorted (less than 10 ).
7. There is no need to routinely test the devices if the drive is operating properly.

AC DRIVE APPLICATION ISSUES

Introduction
The ac input characteristics are
1. Diode source current unbalance
2. AC power factor
3. AC input power change with AC input voltage
The ac output characteristics cover
1. IGBT switching transients
2. Cabling details for ac drives
3. Motor, cable, and power system grounding
4. Motor bearing currents

Diode Source Current Unbalance
Any unbalance in the ac source voltage will cause very large ac unbalances to flow in non-
regenerative ac drives. This is due to the very low impedance of the dc link energy storage
capacitor. In general, the ac current unbalance is about 10 to 20 times larger than the ac volt-
age unbalance (depending on whether there is a dc link inductor and what its size is). Since
most ac power systems have an unbalance of 1 to 2 percent, these problems can occur:
● The rating of the ac cables must increase.
● The rating of the supply and any ac switchgear must also increase.

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.