CTV-PRB004-EN
15
Motor Protection
Overload situations, left unchecked by protection, can cause excessive motor heat, that can
permanently damage the windings and lead to motor failure. The time until motor damage
depends mainly on the magnitude of the overcurrent and has an inverse time versus current
relationship. The greater the overcurrent, the less time it takes to cause motor damage.
Overcurrent can be the result of motor overload, low line voltage, unbalanced line voltage, blocked
load (rotor cannot freely rotate), single phasing, bad connections, broken leads, or other causes.
It can occur in any one winding, a set of windings, or in all the motor windings.
The threshold of overcurrent is generally the primary RLA, which may be raised for service factor
or lowered due to any derating factor, such as ambient temperature or line-voltage imbalance.
Overload protection is bypassed during a start due to the high currents associated with locked rotor
and motor acceleration. Maximum allowed acceleration times per the AdaptiView unit controller
are listed in
Table 4
.
Motor overheat protection
The unit controller monitors the motor winding temperatures in each phase and terminates chiller
operation when the temperature is excessive. This feature also prevents the chiller from starting
if the motor temperature is too high.
Momentary power loss protection (distribution fault)
Momentary power losses longer than two or three line cycles will be detected and cause the chiller
to shut down, typically within six cycles.The chiller can also shut down due to excessive or rapid
voltage sags. Shutting down the chiller prevents power from being reapplied with different motor
phasing.
Figure 5.
Tracer AdaptiView chiller controller overload time-to-trip curves
Table 4.
Long acceleration protection
Starting Method
(starter type)
Maximum Setting for the
Acceleration Timer (sec)
Wye-Delta
27
Solid-State
27
Variable-Frequency Drive
12
Across-the-Line
6
Primary Reactor
16
Autotransformer
16
0
5
10
15
20
25
102
108
114
120
126
132 138
144
150
Nominal
Minimum
Maximum
% Run-Load Amps
Overload Trip
Time (sec)
CTV-PRB004.book Page 15 Sunday, December 18, 2011 6:39 PM
