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

5.6

S5 Thermal Model

5.6.1

Motor Thermal Limits

5–36
One of the principle enemies of motor life is heat. When a motor is specified, the purchaser
communicates to the manufacturer 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 (I
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 frequency (approximately 1 Hz) and therefore, the
effective resistance of the rotor is reduced quite dramatically. During running 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.
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 thermal 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.
2
R) is a function of the effective rotor resistance. At 50
469 MOTOR MANAGEMENT RELAY – INSTRUCTION MANUAL
CHAPTER 5: SETTINGS