S5 Thermal Model; Motor Thermal Limits - GE 469 Instruction Manual

4.6 S5 THERMAL MODEL

4.6S5 THERMAL MODEL
One of the principle enemies of motor life is heat. When a motor is specified, the purchaser communicates to the manufac-
turer what the loading conditions and duty cycle will be, as well as, environment and other pertinent information about the
driven load such as starting torque, etc. The manufacturer then provides a stock motor or builds a motor that should have a
reasonable life under those conditions.
Motor thermal limits are dictated by the design of both the stator and the rotor. Motors have three modes of operation:
locked rotor or stall (when the rotor is not turning), acceleration (when the rotor is coming up to speed), and running (when
the rotor turns at near synchronous speed). Heating occurs in the motor during each of these conditions in very distinct
ways. Typically, during motor starting, locked rotor and acceleration conditions, the motor is rotor limited. That is to say that
the rotor will approach its thermal limit before the stator. Under locked rotor conditions, voltage is induced in the rotor at line
frequency, 50 or 60 Hz. This voltage causes a current to flow in the rotor, also at line frequency, and the heat generated
2
(I R) is a function of the effective rotor resistance. At 50 or 60 Hz, the reactance of the rotor cage causes the current to flow
at the outer edges of the rotor bars. The effective resistance of the rotor is therefore at a maximum during a locked rotor
condition as is rotor heating. When the motor is running at rated speed, the voltage induced in the rotor is at a low fre-
quency (approximately 1 Hz) and therefore, the effective resistance of the rotor is reduced quite dramatically. During run-
ning overloads, the motor thermal limit is typically dictated by stator parameters. Some special motors might be all stator or
all rotor limited. During acceleration, the dynamic nature of the motor slip dictates that rotor impedance is also dynamic,
and a third overload thermal limit characteristic is necessary.
4
The figure below illustrates typical thermal limit curves. The motor starting characteristic is shown for a high inertia load at
80% voltage. If the motor started quicker, the distinct characteristics of the thermal limit curves would not be required and
the running overload curve would be joined with locked rotor safe stall times to produce a single overload curve.
The motor manufacturer should provide a safe stall time or thermal limit curves for any motor they sell. To program the 469
for maximum protection, it is necessary to ask for these items when the motor is out for bid. These thermal limits are
intended to be used as guidelines and their definition is not always precise. When operation of the motor exceeds the ther-
mal limit, the motor insulation does not immediately melt. Rather, the rate of insulation degradation has reached a point that
motor life will be significantly reduced if it is run any longer in that condition.
Figure 4–5: TYPICAL TIME-CURRENT AND THERMAL LIMIT CURVES (ANSI/IEEE C37.96)
4-28
Courtesy of NationalSwitchgear.com
400
HIGH
300
INERTIA
RUNNING OVERLOAD
MOTOR
200
100
A,B,AND C ARE THE
80 ACCELERATION THERMAL LIMIT
CURVES AT 100%, 90%, AND
60
80%VOLTAGE, REPECTIVELY
40
20
10
8
6
4
E,F, AND G ARE THE
SAFE STALL THERMAL LIMIT
TIMES AT 100%, 90%, AND
2
80%VOLTAGE, REPECTIVELY
1
0 100 200 300
469 Motor Management Relay

4.6.1 MOTOR THERMAL LIMITS

C
B
A
G
F
E
400 500 600 % CURRENT
806827A1.CDR
4 SETPOINTS
GE Multilin