GE MM200, Инструкция по эксплуатации

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4–14 MM200 MOTOR MANAGEMENT SYSTEM – INSTRUCTION MANUAL
PROTECTION ELEMENTS CHAPTER 4: SETPOINTS
Protection elements
Thermal protection
The primary protective function of the MM200 is the thermal model. The MM200 integrates
stator and rotor heating into a single model. The rate of motor heating is gauged by
measuring the terminal currents. The present value of the accumulated motor heating is
maintained in the
Thermal Capacity Used actual value register. When the motor is in
overload, the motor temperature and thermal capacity used will rise. A trip occurs when
the thermal capacity used reaches 100%. When the motor is stopped and is cooling to
ambient, the thermal capacity used decays to zero. If the motor is running normally, the
motor temperature will eventually stabilize at some steady state temperature, and the
thermal capacity used increases or decreases to some corresponding intermediate value,
which accounts for the reduced amount of thermal capacity left to accommodate
transient overloads.
The thermal model consists of four key elements.
•
Hot/cold biasing that accounts for normal temperature rise.
• An overload curve that accounts for the rapid heating that occurs during stall,
acceleration, and overload.
• Cooling rate that accounts for heat dissipation.
• Thermal protection reset that controls recovery from thermal trips and lockouts.
Each of these categories are described in the following sub-sections.
Hot/cold biasing
When the motor is running with a constant load below the overload level, the motor will
eventually reach a steady state temperature, which corresponds to a particular steady-
state thermal capacity used. As some thermal capacity is used, there is less thermal
capacity left in the motor to cover transient overloads than is available when the motor is
cold. Typically, the extent of this effect is calculated by taking the ratio of the motor's rated
hot safe stall time to its rated cold safe stall time. The safe stall time (also known as locked
rotor time) is the time taken with the rotor not turning for the motor to heat to a
temperature beyond which motor damage occurs at an unacceptable rate. The term 'cold'
refers to starting off with the motor at ambient temperature, while 'hot' refers to starting
off with the motor at the temperature reached when running at rated load. The method
the thermal model uses to account for the pre-overload state is thus known as hot/cold
biasing.
The MM200 calculates the steady-state thermal capacity used according to the following
equation.
Eq. 1
In the above equation:
•
TCU
SS
represents the steady-state thermal capacity used expressed as a percentage.
• I
eq
represents the equivalent motor heating current in per-unit values on an FLA base.
Refer to unbalance biasing for additional details.
• HCR represents the value of the
Hot/Cold Safe Stall Ratio
setpoint expressed as a
percentage.
If a
Hot/Cold Safe Stall Ratio
value of 100% is entered, the hot/cold biasing is defeated, and
unless RTD biasing is deployed, the thermal model will operate as if the motor was cold
prior to overload.
( )
HCR % 100 TCU
2
SS
- ´ =
eq
I