In three phase supply system, if any one of the three lines fails or open circuited, the load current flows between the remaining two lines only and this fault is known as single phasing. To prevent this fault, use single phasing preventer relay.
What is the Effect of single phasing?
The effect of single phasing is depend upon the types of loads such as heating load or three phase induction motor.
Effect on 3-phase heating loads: The heat produced by heating element is decreases around 50% but it does not harm the equipment.
Effect on three-phase motors: If the single phasing occurs during starting, the motor fails to start due to lack of proper rotating magnetic field. But at same time motor draws a very large current and motor windings gets heated up. This cause the motor winding gets damaged.
If the single phasing occurs during running, the motor may or may not be run depending upon the load condition. At that time the motor will draw a large current and the winding is likely to burn out due to overheating.
Necessity of single phasing preventer.
In three-phase induction motor, if two phases of the supply are interchanged, the motor will rotate in reverse direction. This is called as phase reversal. In the operation of many industrial applications, phase reversal may result in serious damage to the equipment and it may injure to operator also.
In other situations, if a fuse blows or a wire breaks while the motor is running, the motor will continue to operate on two phase but will experience serious overheating.
So, to protect motors against these faulty conditions of phase failure, a single phase preventer is used.
Type of Single Phasing Preventer.
Single phasing preventers are available in three types.
2) Current sensing
3) Voltage sensing
1) Mechanical type:
One type of single phasing preventer is included with bimetal relays which opens the NVC (no voltage coil) circuit similar to that of normal OLR (over load relay). But this type of single phasing preventer is slow in operation and also not fully reliable. Hence it is not preferred nowadays.
In second type of mechanical phase failure relay, coils connected between two lines of the three-phase supply.
The currents in these coils set up a rotating magnetic field that tends to turn a rotating copper disc clockwise.
This clockwise torque actually is the resultant of two torques acting in opposite direction. One torque rotate disc clockwise and one rotate disc in anti-clockwise. With this disc some toggle mechanism is connected to open the starter and disconnect the motor from the line.
In other words, if any one phase disconnect, the torque of this phase is disappear as a result the motor is disconnected from the line.
In this way mechanical type single phasing preventer protect the motor.
2) Current sensing type:
The current sensing type single phasing preventer operates on the principle of equal currents with balanced loads developing secondary voltage on CT (current transformers).
These secondary of current transformer are connected to ADD and the added signal is rectified, filtered and sensed and applied to operate a relay which use to close the NVC (no voltage coil) circuit of the starter.
In normal operating condition motor starter contact connection is always in closed condition.
Figure shows the block diagram of a current sensing single phasing preventer.
3) Voltage sensing Type:
In an AC three-phase supply the order of three phase voltages reach the maximum value in particular sequence is known as phase sequence.
The phase voltage reaches their maximum positive value one after another at 120o in clockwise direction is known as positive phase sequence. And in anti-clock wise direction known as negative phase sequence.
In the case of single phasing or phase reversal or unbalanced voltages or no voltage in a line it results in a super-imposition of negative phase sequence over the normal positive phase sequence of supply voltages.
This negative sequence is filtered by a resistance capacitance or resistance, capacitance and inductor network and de-energies the relay in single phasing preventer.
Figure shows the block diagram of a typical voltage sensing single phasing preventer. In this a resistance, capacitance network is utilized to sense the negative phase sequence.
When phase sequences and voltages are correct, no voltage difference will be generated across the filtered output, so no relay operation takes place.
When the negative sequence occurs due to unbalanced supply voltage or phase reversal, a voltage is developed across relay.
Some of the single phasing preventers are provided with the facility to adjust unbalanced settings.
MCQ Question PDF of Module 03 : AC Three Phase Motor.
- Principle of induction motor
- Construction of a 3-phase squirrel cage induction motor – relation between slip, speed, rotor frequency, copper loss and torque
- Classification of squirrel cage motors
- Insulation test on 3 phase induction motors
- Starter for 3-phase induction motor – power control circuits – D.O.L starter
- B.I.S. symbols pertaining to contactor and machines
- D.O.L. starter
- Numerical problems in ac 3-phase induction motors
- Jogging (inching) control circuits for motors
- Rotary type switches
- Manual star-delta starter
- Semi-automatic star-delta starter
- Automatic star-delta starter
- Three-phase, slip-ring induction motor
- Resistance starter for 3-phase, slip-ring induction motor
- Method of measurement of slip in induction motor
- Efficiency – characteristics of induction motor- no load test – blocked rotor test
- Characteristics of squirrel cage induction motor
- No-load test of induction motor
- Blocked rotor test
- Efficiency from no-load and blocked rotor test
- Effect of external resistance in slip ring motor rotor circuit
- Auto-transformer starter
- Single phasing preventer / phase failure relay
- Braking system of motors
- Method of speed control of 3 phase induction motor
MCQ Question PDF of AC 3 Ph Motor