Page 222 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

VARIABLE-SPEED DRIVES

VARIABLE-SPEED DRIVES 10.21

7. a. The motor connecting cables (including ground conductors) for PWM inverters having
IGBTs must be symmetric.
b. The best cable for these applications has continuous aluminum sheath, three conduc-
tors, and three grounds.
c. The bearing currents and radiated EMI are reduced by the cable symmetry and con-
tinuous outer sheath.
8. The damage to the motor bearings is reduced by an inverter output filter or an electro-
static shield.

SELECTION CRITERIA OF VSDs

The following are the advantages of VSDs:

Variable Process Speed

Some processes operate at different speeds. For example, an ore grinding will operate at
different speeds depending on the type, consistency, and size of the ore. VSDs are most
suitable for these applications. Other applications requiring variable speed are conveyors
and dynamometers.

Compressors and Pumps

Many applications require pumps and compressors to operate at part load. Conventional
constant-speed motors and control valves have been used for these applications. This alter-
native is more expensive and causes flow-induced vibrations, cavitation, and erosion in the
system. The motor operates at full load continuously regardless of the flow required. For
100-kW motor and 5 percent per kilowatthour utility rate, the operating cost of the motor
will be $125.00 per day.
Typical pump characteristics (Fig. 10.21) confirm that the flow is proportional to the
speed, while the torque varies with the square of the speed and the power with the cube of
the speed. If the system requires 50 percent of the maximum flow rate, $113.00 will be
wasted every day.

Motor Starting
Starting large motors can be very stressful on the windings and insulation systems due to
high currents and torques. The power supply is also affected adversely due to voltage dips.
A much smaller VSD (rated at a few percent of the motor) can be used to increase the speed
of the motor gently, so it can be synchronized.

REGENERATION

During normal drive operation, the power is supplied to the VSD, then to the motor, and
then to the load. Most of the energy is dissipated in the process, and the remainder is stored
in load. A rotating load can be slowed down and energy removed from it and returned to

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