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CTV-PRB004-EN
Motor Types and Voltage Classes
Voltage Classes
There are two primary voltage classes typically used in the water-cooled chiller industry: low and
medium. For centrifugal chillers this is generally restricted to three-phase power only.
Low voltage ranges from 208 to 600 volts. Starters and frequency drives in this voltage range
include sizes up to 1,700 amps.
Medium voltage has two main voltage groups. One ranging from 2,300 to 6,600 volts and the other
ranging from 10,000 to 13,800 volts. Starters in this class have sizes up to 360 amps and 94 amps
respectively.
For a given power (kW), the higher the voltage the lower the amperage. The voltage is typically
established prior to creating the job plans and specifications.
Motors
A centrifugal motor is a relatively simple motor. Specifically, it is referred to as a three-phase,
squirrel-cage, 3,600-rpm, alternating-current induction motor with two-pole construction.
The squirrel-cage motor consists of a fixed frame, or stator, carrying the stator windings and a
rotating member called the rotor.
Figure 1
shows a cutaway of a typical low-voltage motor. The
rotor is built by rigidly mounting steel laminations to the motor shaft.The motor winding consists
of aluminum bars that are die-cast into slots in the rotor.The aluminum bars are connected at each
end by a continuous ring.This skeleton of rotor bars with end rings looks like a squirrel cage and
gives the motor its name.
In a three-phase motor, three windings on the stator connect to a motor terminal board, and
ultimately to the power grid via the starter. When the polyphase alternating current flows through
the stator winding it produces a rotating magnetic field.The resulting magnetic forces exerted on
the rotor bars cause the rotor to spin in the direction of the stator field.The motor accelerates until
a speed is reached corresponding to the slip necessary to overcome windage and friction losses.
This speed is referred to as the no-load speed.
Low-voltage motors typically have six motor terminals to electrically connect the motor in a wye
(star) or delta configuration. Most low-voltage motors are random-wound motors, but larger
amperage higher horsepower motors are form wound for better heat dissipation. Connecting links
can be used to convert the six motor electrical connections to three connections.
Medium-voltage motors (2,300–6,600 V) have three motor terminals.
Figure 2
shows a typical
medium-voltage motor minus the rotor shaft. You can visually compare most low- and medium-
Figure 1.
Low-voltage motor
Figure 2.
Medium-voltage motor
Stator
Squirrel-cage
Rotor shaft
Stator
Squirrel-cage
Rotor shaft
Stator
Stator
CTV-PRB004.book Page 8 Sunday, December 18, 2011 6:39 PM
