IMM 8001/8002 – Digital Motor Protection Relay
ICE / CEE
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IMU307-1 GB Ind. B
59
5.8. ROTOR STALLING PROTECTION DURING START [51LR]
Function
Some motors do not withstand rotor stalling conditions during a time equal to the starting time. In
general, the rotor temperature exceeds the allowed limits due to the absence of ventilation.
The amplitude of the currents flowing through the motor generally varies in a noticeable way only at the
end of start beyond the time allowed for rotor stalling. Consequently, current monitoring cannot be a
criterion to detect these conditions. Other methods must be used, such as the detection of the motor
mechanical position (position detector causing actuation of a contact when rotor movement occurs).
The IMM 8000 relays have a digital input with a contact used to start the motor if, after an adjustable
time-delay, the signalling contact has no changed status, and the motor absorbs an Ipos positive phase
sequence current greater than 1.5 In.
Note that rotation sensors at shaft end are sometimes used. They generate a signal proportional to the
speed, and allow its permanent supervision in this way. However, the drawback of this method is shaft
end unbalance liable to produce vibrations, and sometimes installation of a generator in a classified
dangerous area.
From now on, the motor manufacturers provide this protection with a cell (proximity detector)
associated with a pulse frequency counter (NPN or PNP).
Main characteristics
Operating logic
enabling when start begins with analysis
of Ipos or EL1 input contact (see [48]).
disabling by speed display contact
(EL4 input).
settings
operating level 51-tIlr>
fixed operating level equal to 1.5 In.
time-delay 51-tIlr>
1 to 100 s (steps of 0.1s ).
