The single phasing preventer cum high low and unbalanced-supply voltage cutoff relay described here ensure complete safety for 3-phase machinery.
Generally, induction motor burn out due to excessive voltage during off hour, very low voltage during peak hours, unequal phase voltage and single phasing (missing of one phase completely from 3-phase supply). It is therefore essential to install single phasing preventer in addition to the starter. Starter provide only overload protection and therefore cannot prevent the motor from burning out under above- stated conditions of supply voltage. But this preventer protects the motor by stopping it automatically under such conditions.
Circuit and Working of Single Phasing Preventer
For simplicity, this circuit can be divided into three parts, viz, power supply, unbalanced sensor and cut-off relay.
As shown in figure 1, primary of transformer X1 is connected to 400V supply line. The induced 12V AC in transformer’s secondary is rectified by the bridge rectifier comprising diode D1 to D4 and the smoothing capacitor C1. Capacitor C1 is deliberately chosen low in value for obtaining poorly regulated rectified voltage to ‘sense’ the variation in the line voltage from the specified voltage level.
As shown in figure 2, the unbalanced supply sensing circuit comprises RC network, bridge rectifier and transistor T3. The three phase of the supply line R, Y and B are connected to the corresponding terminals of the RC network. By virtue of this design, the terminal ‘P’ and ‘Q’ stay at same potential when a balanced supply (balanced 3-phase supply consists of three equal voltage with phase difference of 1200) is fed to the RC network. By any means, if any one of the phase is absent, or the voltage are unequal, or even if the phase sequence is reversed, there exists a certain potential difference between terminals ‘P’ and ‘Q’.
The potential difference is fed to the bridge rectifier through a preset. As a result, the bridge rectifier conduct to forward bias transistor T3 . The transistor, in turn actuates the cut-off relay circuit to disconnect the supply to motor. Preset VR3 can be adjusted against the unbalanced supply. In other words, if 5% or 10% unbalanced is admissible, VR3 can be adjusted so as not to actuate the circuit when unbalanced is less than 5% or 10%.
Cut-off Relay Circuit
Cut-off relay circuit is a unique bistable multivibrator. As shown in figure 3, it forms an unconventional multivibrator. In this circuit, only one transistor conducts at a time.
When the supply is within limits, transistor T2 alone conducts and T1 stays in its non-conducting state, Hence, relay RL1 stays energized. The relay contacts are connected in series with the no-volt coil of the starter.
If the line voltage falls very low, zener diode Z2 will not break down (under good supply condition Z2 is adjusted by preset VR2 to its breakdown state). This in turn, removes all forward bias from transistor T2. Hence, T2 does not conduct and the relay is de-energized, and stops the motor by breaking the no-volt circuit of the starter,
If the voltage rises excessively, the zener diode (which is adjusted by preset so as not to conduct when the line voltage is within the specified limits) will break down and drive the transistor T1. Consequently, the motor stops.
Testing of Single Phasing Preventer
Testing can be carried out by the techniques used in repairing transistorized circuits. The following steps are recommended:
- Measure the HT supply. If absent, search for open circuit in primary or secondary winding of X1, or perhaps C1 is short or diode D1 to D4 are faulty.
- If proper HT is present, but the relay is inoperative, try up and down adjustment of VR1 and VR2. If there is no change, search for emitter-to-collector short circuit of T1 and T3, or faulty zener diodes and transistor T2, or open circuit of VR2, R1 or RL1.
- If the relay is in operative state, adjust VR1 and VR2 such that the relay gets de-energized at certain points. If the relay stays at the same energized position, search for short in T2 or ZD2.
- Disconnect one phase from RC network and observe the relay’s state; it should be de-energized. If not, suspect faulty components of RC network, bridge rectifier and T3.
Setting of Single Phasing Preventer
As shown in figure 4, setting is carried out by four variable auto-transformer and four AC voltmeters of 500V range. The following are the setting operations:
- Vary all the variable auto-transformer to read 415V in voltmeter V1 and 220V in V2, V3 and V4. Now, set VR1, VR2 and VR3 such that the relay stays in its energized state.
- Decrease the voltage across X1 by variable auto-transformer 1 to 360V and VR2 to de-energized the relay.
- Increase the voltage to 470V and set VR1 to pull down the relay.
- Set the voltage t 415V across X1 and turn auto transformer 2, 3 and 4 to read the voltage of 250, 220 and 190 volts in V2, V3 and V4 Now, set VR3 to pulldown the relay.
Installation of Single Phasing Preventer
This preventer installation is shown in figure 5. The figure indicates:
- R, Y, and B are three phase of supply
- 1, 2 and 3 are starter’s R, Y and B terminals.
- RL2, RL3, RL4, RL5 are no-volt relay coil contacts
- M1, M2 and M3 are motor’s R, Y and B terminals
- NV1 and NV2 are preventer’s relay contacts
- N1 is Neon Lamp
- ESW is emergency switch, provides to operate the motor even under serve conditions of supply, at our own risk.
- TS is a six-terminal, terminal stay connector.
PARTS LIST OF SINGLE PHASING PREVENTER
|Resistor (all ¼-watt, ± 5% Carbon)|
|R1 = 470 Ω, 1W
R2 – R7 = 120 KΩ, 1/2W
VR1, VR2 = 5 K KΩ
VR3 = 220 KΩ
|C1 = 470 µF, 16V
C2 = 200 µF, 16V
C3 = 330 µF, 50V
C4, C5 = 0.01 µF, 500V
|D1 – D8 = 1N4007
T1, T2 = CIL333
ZD1, ZD2 = BZ148 (10%)
|X1 = 440V ac to 12V, 500mA step down transformer
RL1 = 12V, 450Ω Relay