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Page 8-1
Effective 10/02 IL17562BH04
SECTION 8—MOTOR THERMAL PROTECTION
BASICS
8.0 General
This section describes how the MP-3000 hardware and software
function together to control, monitor, and protect the motor.
8.1 Sensing Inputs
The MP-3000 receives motor phase currents from 3 main motor phase
current transformers. See Figure 8-1. If an optional ground fault
transformer is used, the MP-3000 monitors ground leakage or fault
current as well.
The MP-3000 takes 36 samples per power cycle of the input current
signals, digitizes each sample, and stores it in the microprocessor
memory. From these samples, the MP-3000 computes rms currents
and average currents; plus phasor calculations leading to direct and
precise measurement of positive and negative sequence currents—see
below. The high sampling rate, plus a unique patented sample-shifting
technique, allows the MP-3000 to properly measure and account for
the impact of harmonics in heating the motor.
If the optional URTD module is used, the MP-3000 gathers winding
temperature data from up to six resistance temperature detectors (RTDs)
embedded in the stator windings of the motor. It can monitor four RTDs
associated with the motor bearings and load bearings. It can also monitor
one auxiliary RTD, such as motor case temperature.
8.2 Protective Functions
Protective functions continuously monitor motor operating conditions
such as current history and temperature. When measured or derived
measurements exceed user-selected levels, an alarm condition is
initiated, and then, if necessary, a trip output opens the motor
contactor or trips a breaker.
The MP-3000 can protect the motor, starter, and load in the following
ways:
• Stator and rotor thermal protection by modeling of heating and
cooling effects, including heating by negative sequence currents.
• Stator overtemperature protection by direct measurement (with
optional URTD module).
• Instantaneous overcurrent protection for faults.
• Ground fault protection.
• Phase reversal protection.
• Phase unbalance protection.
• Motor bearing, load bearing, and auxiliary RTD temperature
protection (with optional URTD module).
• Jam protection.
• Underload protection.
• Transition trip for abnormal starting time-versus-current behavior.
• Incomplete sequence protection (missing status feedback from load
or starter).
• Trip-bypass output for failure of contactor to interrupt current after a
trip command.
• Zero-speed switch stalled-motor trip protection.
• Process load shedding function to forestall impending jam or thermal
trips.
• Jogging protection—minimum time between starts, maximum
number of starts per set time, maximum number of consecutive cold
starts, and minimum time between stop and start (antibackspin
protection).
Many of these functions also have separate alarm thresholds to warn
the user, who may be able to act before a trip occurs.
The MP-3000 has four output relays. The Trip relay is connected in
series with the motor contactor, and de-energizes the contactor or
blocks starting for any MP-3000 trip condition. All trips are steered to
this relay.
The three other relays are designated as Alarm, Auxiliary 1, and
Auxiliary 2. Normally, all alarm and warning conditions are steered to
the Alarm relay. However, the Alarm relay and the two Auxiliary relays
are all fully programmable. They can be set by the user to operate for
a designated list of internal MP-3000 measured or calculated condi-
tions.
8.2.1 Direct Load-Based Protection
The monitored level of actual motor current is used to determine when
the instantaneous overcurrent trip, jam trip, load shedding, underload
trip, transition trip, and load-shedding settings have been reached.
Also, direct temperature feedback from the stator, load bearing, motor
bearing, and auxiliary RTDs are compared with respective settings. If
necessary, the relay gives alarm and/or trip outputs.
8.2.2 Thermal-Model and Rotor Temperature Protection
Each motor has a specific damage curve. Usually it is called the I
2
t
(current squared multiplied by time) curve. With larger horsepower
motors, the thermal capability is usually rotor-limited, so it is important
to track the total heating of the rotor. In ac motors, the current balance
between phases is of major concern due to the additional rotor heating
associated with the negative sequence component of an unbalanced
phase current condition. Current unbalance is usually caused by
voltage unbalance, the result of single-phase loads on a 3-phase
system, and/or motor winding unbalance.
Any unbalanced set of three-phase currents or voltages can be
mathematically transformed into a linear combination of positive,
negative, and zero sequence components. The measured current
phasor in each phase is the sum of the three sequence component
phasors in that phase. The zero-sequence component is a common-
mode component which is equal in the three phases, and requires a
neutral or ground path for return. So, in a motor without a neutral
return, we see no zero-sequence current unless there is a ground fault.
Thus we focus on the positive and negative sequence components
which can routinely be present.
For analysis and understanding, consider the motor to have two
tandem virtual rotors as shown in Figure 8.2. One is driven only by the
positive-sequence current I
1
, which is symmetrical and balanced. The
other is driven only by counterrotating negative-sequence current I
2
,
directly related to unbalanced current; and produces a proportional
torque in the reverse direction. If perfect current balance and phase-
angle symmetry exists among the three phases, then I
1
would be the
only component of line current squared, with no effect from the second
rotor. This positive sequence component of current produces the motor
output torque and work.
The negative sequence current I
2
is a 3-phase current component with
a reverse phase rotation compared to that of the ac source. This
current generates a reverse torque in the second rotor, and works
against the main action of the motor, doing negative work. Because the
negative work caused by I
2
stays within the rotor, it is completely
absorbed as heat and therefore has a far more significant effect on the
rotor heating than the balanced l
1
.
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