Phase Failure Relays

Objectives

After studying this unit, the student will be able to:

• Explain the purpose of phase failure relays

• List the hazards of phase failure and phase reversal

If two phases of the supply to a three-phase induction motor are interchanged, the motor will reverse its direction of rotation. This action is called phase reversal. In the operation of elevators and in many industrial applications, phase reversal may result in serious damage to the equipment and injury to people using the equipment. In other situations, if a fuse blows or a wire to a mo tor breaks while the motor is running, the motor will continue to operate on single phase but will experience serious overheating. To protect motors against these conditions, phase failure and reversal relays are used.

One type of phase failure relay, figure 22-1,

uses coils connected to two lines of the three-phase supply. The currents in these coils set up a rotat ing magnetic field that tends to turn a copper disc clockwise. This clockwise torque actually is there sult of two torques. One polyphase torque tends to turn the disc clockwise, and one single-phase torque tends to turn the disc counterclockwise.

The disc is kept from turning in the clock wise direction by a projection resting against a stop. However, if the disc begins to rotate in the counterclockwise direction, the projecting arm will move a toggle mechanism to open the line contactors and remove the motor from the line. In other words, if one line is opened, the polyphase torque disappears and the remaining single-phase torque rotates the disc counterclockwise . As a resuit, the motor is removed from the line. In case of phase reversal, the polyphase torque helps the single-phase torque turn the disc counterclock wise, and again, the motor is disconnected from the line.

Other designs of phase failure and phase reversal relays are available to protect motors, ma chines, and personnel from the hazards of open phase or reverse phase conditions. For example, one type of relay consists of a static, current-sen sitive network connected in series with the line and a switching relay connected in the coil circuit of the starter. The sensing network continuously monitors the motor line currents. If one phase opens, the sensing network immediately detects it and causes the relay to open the starter coil circuit to disconnect the motor from the line. A built-in delay of five cycles prevents nuisance dropouts caused by fluctuating line voltages.

A solid-state phase monitoring relay is shown in figure 22-2. This relay provides protection in the event of a voltage unbalance or a phase rever sal. The unit automatically resets after the correct voltage conditions return. Indicating lights show when the relay is activated.