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Single phase preventer functional principle

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harii07

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Hello to all

I need a clarification on functionality of single phase preventer.

Will single phase preventer work when supply is connected with 3 phase capacitor (KVR) for power factor correction?
 

As I understand, you are talking about a loss of one incoming phase. So you test each phase voltage, so any capacitor connected across the phases will not change the voltage. The reason for a loss of phase detector is because with electric motors, if a phase goes missing, if the motor is heavily loaded then it would burn out due to excessive current in the connected phases or would trip its local current overload. If you had a factory with 100s of machines it would take for ever to reset them all! Best to turn the lot off until all three phases return.
Frank
 
Thanks Mr.Chuckey

I have a single phase preventer (SPP) for my 5hp submersible motor (used for agricultural purpose). I have connected PF correction capacitor (KVR). When any one of the phase is cut, the SPP unable to switch OFF the stater but, without KVR the system is functioning well. May i know exact reason?.

Thanks
 

What is happening is that when one phase disappears on a three phase running motor, the motor generates the missing voltage. This principle is used for rotary phase converters, see :- https://en.wikipedia.org/wiki/Rotary_phase_converter .
You need a large load connected between the motor and the point where the phase goes missing, so the motors tries to power this load with the generated phase, which it can't do, the voltage falls and the phase detector kills the supply. So I have only seen them right on the incoming mains supply of large installations.
Frank
 

Dear chuckey
Thanks for ur valuable reply....
The preventer works well when there is no KVR capacitor connected. It switch OFF the starter when a phase is failed..
Please clarify this.
 

Dear Mr.Chuckey

Yes, The KVR is connected parallel to the motor and there is no neutral connected
 

The simple ones work somewhat like this:

All the three cables corresponding to the thee phases go through a toroid on which the secondary is wound. The cable acts like a one turn of a transformer.

When one of the cable loses power, there will be less current and the voltage induced will fall significantly. The circuit breaker trips under such condition.

If you have no power factor capacitors connected across the supply, the motor skips one phase but still runs with reduced torque.

If you have PFC capacitors installed correctly, they will try to balance the currents in the three phases and when one phase fails, this will not be possible.

It is important to install the PFC because your meter will run slower (if it is KVAH type).

It is not advisable to run a 3 phase motor on two phases only.
 

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