Page 223 - Electrical Equipment Handbook _ Troubleshooting and Maintenance
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VARIABLE-SPEED DRIVES
10.22 CHAPTER TEN
100
80
Torque Power 60
40
20
0
20 40 60 80 100
Speed
FIGURE 10.21 Typical pump power and torque versus speed.
the ac power line (regeneration). This is done by lowering the synchronous speed below the
actual mechanical speed of the load (magnetic break). The amount of magnetic braking and
power generation is proportional to the difference between the synchronous speed and
actual mechanical speed. In the extreme case, the motor can be “plugged” (stopped almost
instantly) by reversing the leads to any two phases; e.g., the synchronous speed of a two-
pole motor is switched from 3600 to 3600 rpm instantly. These are the power flow dia-
grams during normal operation and regeneration:
Power flow in normal operation:
AC utility power → VSD → motor → load
Power flow during regeneration:
AC utility power ← VSD ← motor ← load
If the cost of energy can be tolerated, the machine can coast down, dissipating its energy
to the atmosphere. Mechanical brakes can be used to slow down the machine by convert-
ing the mechanical energy to heat. However, if the machine must be slowed down quickly
or the cost of lost energy is high (e.g., frequent slowdown of large inertia), regeneration is
required. These are some examples:
Dynamometer
These devices are used to test engines and transmissions. The dynamometers are used to
load up the engine or transmission to determine its performance. Since regeneration is used
to feed the mechanical energy in the load back to the power supply, the only power con-
sumed is the losses in the system. In the past, a mechanical brake (water-cooled) was used
for this function. The cost of lost energy in this case would be significant for dynamome-
ters rated more than a few hundred horsepower.
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