GE 369, Быстрый старт

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CHAPTER 4: KEYPAD SETPOINTS S3 OVERLOAD PROTECTION
369 MOTOR MANAGEMENT RELAY – QUICKSTART GUIDE 4–35
FIGURE 4–1: Typical Time-current and Thermal Limit Curves (ANSI/IEEE C37.96)
4.4.2 Overload Curves
Standard Overload Curve:
The overload curve accounts for motor heating during stall, acceleration, and running in
both the stator and the rotor. The
OVERLOAD PICKUP
setpoint dictates where the running
overload curve begins as the motor enters an overload condition. This is useful for service
factor motors as it allows the pickup level to be defined. The curve is effectively cut off at
current values below this pickup.
Motor thermal limits consist of three distinct parts based on the three conditions of
operation, locked rotor or stall, acceleration, and running overload. Each of these curves
may be provided for both a hot motor and a cold motor. A hot motor is defined as one that
has been running for a period of time at full load such that the stator and rotor
temperatures have settled at their rated temperature. A cold motor is defined as a motor
that has been stopped for a period of time such that the stator and rotor temperatures
have settled at ambient temperature. For most motors, the distinct characteristics of the
motor thermal limits are formed into one smooth homogeneous curve. Sometimes only a
safe stall time is provided. This is acceptable if the motor has been designed conservatively
and can easily perform its required duty without infringing on the thermal limit. In this
case, the protection can be conservative and process integrity is not compromised. If a
motor has been designed very close to its thermal limits when operated as required, then
the distinct characteristics of the thermal limits become important.