Categories

# Why three phase motor not requires earthing the star point?

In a star connected motor, if the voltages are equal, and if the impedance of all the phases is equal, the star point will have a zero potential, and it won't matter if you connect it to earth. However, in case of unequal voltages or unequal impendences, there will be some current flowing continuously through the earth conductor connected to the star point, which is not a desirable condition.

There is a world of difference between a 3 Phase motor and a transformer. For a start tge centre tap on a STAR connected Tx is also used as the neutral connection, thus both are at equal electrical potential and there is a clear return path for single phase current flow and harmonics. Remember that the neutral link will always have a lower resistance than the general mass of earth so will always provide a better electrical path.

Now take a 3 phase Star connected motor. Let's assume that all currents are equal, there should then be no issue with a connection to Earth from the star point as there should always be zero current flow. But motors and systems connected to them are not created equal, and systems break down and fail. If a winding develops a higher resistance through damage or poor construction then the star connected earth will become a route for unbalanced flow to earth.

In case of loss of one phase, a much heavier current (= I x √2 ) would start flowing through the earth connection, and will continue unless the motor has been provided with protection against single phasing. This is a very unsafe condition.

In case, somehow two phases are lost, current (=I) would still keep on flowing through the remaining phase, causing overheating of the winding as the motor would not rotate and hence would not get air for cooling. This again is an unsafe condition, and I don't think any protection is provided against this.

Further, if the motor is connected through an RCD, any minor change in the phase voltages or difference in the phase impedances could trip the motor. This would cause nuisance tripping repeatedly.

Lastly, I don't find any plus point in connecting the star point to earth.

If motor loses one phase the current rise in other two phases is theoretically 1.732 times, but could go up to 2 times due to pf issue. For a lightly loaded motor, the current may not cross the overload setting, and hence it would not trip on overload. Instantaneous tripping of MCB is completely ruled out, because the current has to be at least 6 times.

As regards RCD, it contains a CBCT, and works on the principle that if the sum of the currents in all the conductors passing through it exceeds a particular limit it will trip the RCD. It is therefore not necessary to connect Neutral. Even when all the 3 phases and no Neutral is connected, the RCD would trip if there is a leakage of current from the circuit. However, RCD will not trip on any unbalance, because the sum of currents passing through it would still be zero.