Page 224 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

VARIABLE-SPEED DRIVES

VARIABLE-SPEED DRIVES 10.23

Paper Machine Winder
This application has a very high load inertia (a few hundred times larger than the rotor iner-
tia). The load must be started and stopped frequently as new rolls of paper are made. Brakes
are needed to prevent the reels from coasting for a long time. In the past, water-cooled
mechanical brakes were used to slow down the winder. Tremendous energy is lost in this
process. The VSD can regenerate this energy back to the ac line. The only losses in this
process are the ones that occur in the drive itself (2 to 3 percent of its rated output).

Dynamic Braking
In a dynamic brake, the mechanical energy in the load is dissipated in a water- or air-cooled
resistor bank as shown:
Dynamic braking power flow:

VSD ← motor ← load
↓
Dynamic braking resistor

The resistor bank is installed across the dc bus inside the VSD. It is switched in when the
link voltage rises above a preset limit. It remains on until the voltage drops below the pre-
set limit. This braking method is used when the amount of energy that can be regenerated
back to the ac line is not very large or the retrieval of energy is done very infrequently. This
option costs about 10 percent of regeneration.

MAINTENANCE

All the components of the VSD have definite lifetimes that decrease significantly when
their operating temperatures increase. These are some typical component lifetimes for a
drive operating well within its design rating and ambient temperature between 0 and 40°C:
Electrolytic capacitors 5–7 years
Fans 3–7 years
IGBTs (assuming no overloads) 10–12 years
Power supplies 5–7 years
Motor bearings 2–7 years
The lifetime of all VSD components as well as bearings and insulation system drops sig-
nificantly when the operating temperature increases. It is essential to maintain the ambient
temperature within the acceptable range to achieve high reliability and longevity of this
equipment.

COMMON FAILURE MODES

These are the most common failure modes of VSDs (most common listed first):

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.