AC MOTORS PULL OUT AND STALL PROTECTION RELAYING

Induction-motor stalling
An induction motor stalls when the load torque exceeds the breakdown torque and causes its speed to decrease to zero, or to some stable operating point well below rated speed. This occurs when the applied shaft load is greater than the producing motor torque due to the suppression of the motor terminal voltage.

Also, a stall condition can occur when an excessive mechanical load is applied beyond motor torque capability. This condition will develop motor current equal to or approaching locked-rotor current.

Synchronous motor loss of synchronism (pullout)
When a synchronous motor loses synchronism with respect to the system frequency with which it is connected, it is referred to as “out of step.” This condition occurs when the following actions take place individually or in combination:

a) Excessive load is applied to the shaft
b) The supply voltage is reduced excessively
c) The motor excitation is too low


Torque pulsations applied to the shaft of a synchronous motor are also a possible cause of loss of synchronism, if the pulsations occur at an unfavorable period relative to the natural frequency of the rotor with respect to the power system.

A prevalent cause of loss of synchronism is a fault occurring on the supply system. Fault-clearing time, fault location, fault type, and system configuration are significant factors relating to the stability of the motor. Fast fault clearing, multiple ties, and remoteness of faults favor stability.

Underexcitation of the machine is a common cause of out-of-step operation. This may be caused by incorrect tripping of the rotor field circuit breaker (or contactor), or by opening or short circuiting the field circuit.

When loss of synchronism (pullout) occurs, and the motor is not separated from the system on the first pole slippage, field excitation must be disconnected and the field connected to the discharge resistor immediately. This minimizes the current that flows until the motor can be isolated. The motor should then be isolated as quickly as possible, because this is not an acceptable long-term operating condition.

Electrical quantities change during a stall
For an induction motor to stall during normal operation, the load torque must exceed the breakdown torque as described above. During this process, the motor current will increase rapidly (which is called “inrush current” or “locked rotor”) until the breakdown torque is reached.

Beyond breakdown torque, the motor current continues to increase approaching locked-rotor current. Along with the increase in current, the speed of the motor decreases and the impedance of the motor approaches the locked-rotor impedance. There are two types of stall causes, as follows:

a) Excess shaft load torque prior to a motor startup (e.g., failure to open the pump’s discharge gate)
b) Sudden change of increased shaft load torque during normal operation (e.g., bearing failures)

For a synchronous motor, loss of synchronism is a gradually evolving phenomenon rather than an instantaneous occurrence. During the initial phase of pullout, stator current increases, terminal voltage decreases, and a voltage is induced in the rotor circuit at the slip frequency. Power flow into the motor increases until approximately a 90° angle is reached between the equivalent machine voltage and the system voltage.

At approximately the 180° point, current is maximum and lags the system voltage by the angle of the total impedance between the motor and the system (including the stator resistance and transient reactance of the motor). Also at this point, the direction of power flow reverses, with the motor mass supplying energy to the system.

When resistance is significant, this reversal occurs prior to the 180° point. The reactive power flow for virtually the full slip cycle is into the motor, but it may be provided by the motor for a small part of the slip cycle, depending on the machine excitation.

Protective devices for detecting abnormal motor conditions
Stall detection for an induction motor is usually provided by an overcurrent relay, with an inverse characteristic set to detect current above the breakdown torque level. Since motor starting can result in a stall or locked-rotor condition, this protection is usually covered by setting the motor-starting relays above the motor-starting time-current curves and below the running and accelerating thermal limit for the motor.

In cases where motors are applied to high-inertia loads, overcurrent protection may need to be combined with speed switches, distance relays, or additional rotor thermal protection to fully protect the motor. Out-of-step detection devices for synchronous motors usually operate on the stator power-factor angle.

Impedance-type devices are available for detecting loss of field, and they may also be set to operate on out of-step conditions without field failure, where the motor transient reactance exceeds the system impedance viewed from the motor terminals (the usual case).


For very large synchronous motors or synchronous condensers, a loss-of-field relay is often used to detect VAR flow into the machine. Accidental tripping of the rotor field circuit breaker (or contactor) or loss-offield current can be accurately detected by this device.

There have also been successful applications of rotor field current devices operating from a rotor field current shunt and of notching relays that count pole slips based on power reversals.

A device sensing alternating current in the rotor field circuit may also detect the motor out-of-step condition. These devices used in the rotor field circuit usually consist of a current transformer (CT) with an ac relay on its secondary.

When the machine is operating synchronously, there is no ac component of rotor field current and, therefore, no relay current. If the machine is out-of-step with the system, a current of slip frequency exists; if it is of sufficient magnitude, the relay picks up. During the starting period, this relay must be blocked. This scheme is not adaptable to motors with a brushless excitation scheme.