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4 comments
Anonymous Poster

3 Phase thermal over load relay is use for single phase ac induction motor.

09/21/2008 7:42 AM

I am using a 3phase thermal overload Relay for single phase motor protection this is right selection aither this is not proper selection which type proction use for single phase motor.

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Power-User
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Join Date: Oct 2007
Location: Christchurch, New Zealand
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#1

Re: 3 Phase thermal over load relay is use for single phase ac induction motor.

09/21/2008 6:53 PM

I am most familiar with the GE products, but my answer will probably apply to other brands as well.

3-phase thermal overloads can be used to protect single-phase motors, and will give the same level of protection if they are wired correctly. It is not sufficient to just connect through one phase of the overload relay, as these are usually designed to protect against the 3-phase motor single-phasing, and if only one phase of the overload is carrying current, this protection will operate. To make it work, you need to connect all three phases of the overload relay in series.

Not wishing to teach grandma how to suck eggs, but be aware that thermal overloads are intended to operate in conjunction with a line contactor. The overload relay does not break the load path when it trips, it switches a set of contacts from closed to open, and these are are connected so as to break the coil current in the line contactor.

The GE thermal overloads are designed to mount up to a 3-pole contactor, which is fine for your single-phase setup, but if you want to save money by using a single-pole contactor you will need the optional chassis so you can din-rail mount the two side-by-side so that you can wire them up.

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paulusgnome
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#2

Re: 3 Phase thermal over load relay is use for single phase ac induction motor.

09/22/2008 3:19 AM

I will recomend you a MCB for single phase motor.

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#3

Re: 3 Phase thermal over load relay is use for single phase ac induction motor.

09/22/2008 7:06 AM

Your solution is not ideal, but workable under constraints of availability of other means.

Please find out from the data sheet of relay about the current setting on relay - & set the relay at proper current value (full load current).

Connecting only in one phase of relay may result in nuisance tripping if relay has built in single phasing protection, but connection in series as suggested by other participant would prevent the nuisance tripping.

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Guru

Join Date: Jul 2006
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#4

Re: 3 Phase thermal over load relay is use for single phase ac induction motor.

09/22/2008 5:52 PM

Guest,

NEMA-style overload relays generally do not have phase-loss protection (so-called single-phasing of a 3-phase motor). IEC-style overload relays do. If you use a 3-phase (therefore a 3-pole) overload relay, you will do best to wire two of the poles in series (looping the load terminal of the first pole to the line terminal of the second pole). Then connect the contactor's load terminals to poles 1 & 3, and the motor to the load side of poles 2 & 3. That is usually the way manufacturers will say to do it, and compliance with their instructions is always a code requirement.

--JMM

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