14
CTV-PRB004-EN
Motor Protection
Overload protection
Overload or overcurrent protection shields the motor from small levels of overcurrent ranging from
107 to 140 percent of the primary RLA of the chiller. In contrast, fuses and circuit breakers are used
to protect against short-circuit currents which may range to well over 100,000 amps.
Inductive loads, such as a chiller motor, behave differently than resistive loads such as electric
heaters. Their current draw is greatest at startup and corresponds to the existing load when
running. In other words, a motor operating normally draws rated amps (RLA) at rated load, fewer
amps at less-than-rated load and more amps at greater-than-rated load. It is the latter condition that
requires overload protection.
Adding an overload protection device prevents the motor from drawing more than its rated
amperage for an extended period. Basic overload devices simply open the circuit when current
draw reaches the “trip” point. More sophisticated devices attempt to restore normal motor
operating conditions by reducing the load, but will disconnect the motor if overloading persists.
As with most overload devices, the chiller controller determines the “trip time” by measuring the
magnitude of the overload. It then compares the overload to the programmed RLA “time-to-trip”
curve. At startup, the standard overload protection is bypassed for the starter’s acceleration time,
or until the motor is up to speed. Refer to
Figure 5, p. 15
for the chiller controller’s overload time-
to-trip curve.
Table 3.
Starter ONLY: metering functions and accuracies
Metering Functions
Tracer AdaptiView
MP 3000
(a)
IQ 150
IQ DP 4130
Ampere demand
N/A
N/A
Standard (±0.25%)
Standard (±0.3%)
Current (%RLA)
Standard (±3% to ±7%) Standard
N/A
N/A
Current (3-phase)
Standard (±3%)
Standard
Standard (±0.25%)
Standard (±0.3%)
Voltage (3-phase)
Standard
(b)
(±2%)
N/A
Standard (±0.25%)
Standard (±0.3%)
Frequency
N/A
N/A
Standard
Standard
Harmonic distortion current
N/A
N/A
N/A
Standard (31
st
)
Harmonic distortion voltage
N/A
N/A
N/A
Standard (31
st
)
Kilowatt
Standard
(b)
(±5%)
N/A
N/A
N/A
Power factor
Standard
(b)
(±5%)
N/A
Standard
Standard
VA (volt-amperes)
N/A
N/A
Standard (±0.5%)
Standard (±0.6%)
VA demand
N/A
N/A
Standard (±0.5%)
Standard (±0.6%)
VA hours
N/A
N/A
Standard (±0.5%)
Standard
VARs (volt-amperes reactive)
N/A
N/A
Standard (±0.5%)
Standard (±0.6%)
VAR demand
N/A
N/A
Standard (±0.5%)
Standard (±0.6%)
VAR-hours
N/A
N/A
Standard (±0.5%)
Standard
Watt, see Kilowatt
Standard
(b)
N/A
Standard (±0.5%)
Standard (±0.6%)
Watt demand
Standard
N/A
Standard (±0.5%)
Standard (±0.6%)
Watt-hours
Standard
N/A
Standard (±0.5%)
Standard
(a) The MP 3000 features Intel-I-Trip overload protection, enhanced custom trip curve development, UL 1053 ground fault, and advanced data logging and
diagnostics.
(b) Under/over phase-voltage sensors include volts per phase, kW, power factor, kWh, and under/overvoltage. A required pick on medium-voltage starters.
CTV-PRB004.book Page 14 Sunday, December 18, 2011 6:39 PM
