3 phase motors

meego · Jun 17, 2011

Why do theese motors require a start up circuit?
What is the function of the phase monitor?

johnboy6083 · Jun 17, 2011

they require a starter in order to provide, overload, short ciruit and undervoltage protection.
a phase montior, i read as a phase failure relay. this will cut the supply, if one or more of the phases fail, or differ in voltage beyond a preset value. this will prolong the life of your motor.

Electro-tech · Jun 17, 2011

Once a supply is connected to a three phase induction motor a rotating magnetic field will be set up in the stator, this will link and cut the rotor bars which in turn will induce rotor currents and create a rotor field which will interact with the stator field and produce rotation. Of course this means that the three phase induction motor is entirely capable of self starting.The need for a starter therefore is not, conversely enough, to provide starting but to reduce heavy starting currents and provide overload and no-voltage protection.
There are a number of different types of starter including The Direct On-line Starter, The Star-Delta Starter Auto-Transformer and Rotor resistance.

Phase Monitor Relays protect against unbalanced voltages or single phasing regardless of any regenerative voltages.

Engineer54 · Jun 17, 2011

Electro-tech said:
Once a supply is connected to a three phase induction motor a rotating magnetic field will be set up in the stator, this will link and cut the rotor bars which in turn will induce rotor currents and create a rotor field which will interact with the stator field and produce rotation. Of course this means that the three phase induction motor is entirely capable of self starting.The need for a starter therefore is not, conversely enough, to provide starting but to reduce heavy starting currents and provide overload and no-voltage protection.
There are a number of different types of starter including The Direct On-line Starter, The Star-Delta Starter Auto-Transformer and Rotor resistance.

Phase Monitor Relays protect against unbalanced voltages or single phasing regardless of any regenerative voltages.

A short but precise response to the O/Ps questions. He should now know the reasons behind his whys...lol!!!

topquark · Jun 17, 2011

Here's the full doc: http://www.djtelectraining.co.uk/downloads/ADL 3 Ph Induction Motors.pdf

Engineer54 · Jun 17, 2011

OK, ....haha!! a cut and paste job then!! ....At least it answered the O/Ps enquires!!

pmac10 · Jun 17, 2011

phase monitor relays can also protect against incorrect direction of rotation, that can also be quite important.

Knobhead · Jun 21, 2011

Phase monitoring or phase failure? To be honest they are often one and the same, they check all three phases are present, the voltage is within a set tolerance and the phase rotation is correct.

I’ve only time I've ever seen them fitted was to passenger lifts. There the phase rotation is a waste of time as about the only thing that could change the rotation is a motor change, the relay monitors the supply to the panel not at the motor so it’s a waste of time.

Mind you they could come in handy when Brussels decides to change the rotation of the National Grid!

pmac10 · Jun 22, 2011

i,ve worked on different types of machine where phase monitoring/ failure relays are fitted as standard as incorrect rotation can be disastrous. ive seen a machine moved to another location in the same room but plugged into a different 16A socket with expensive consequence.

Knobhead · Jun 22, 2011

Shoot the electrician that wired the sockets up with reverse phase rotation. It’s not rocket science to get it right. And to my mind more fool a company that has vital equipment on plugs and sockets.

The first plants I worked on all had reverse phase rotation due to the company having it’s own power station in the 1930’s. The biggest laugh was a contracting foreman assuring me that every drive in a newly built plant would go the right direction first time. R>U Y>V B>W will give clockwise rotation looking at the motor shaft, or so he thought. I hadn’t the heart to tell him everything would go backwards! 90 drives had to be reversed at the motor terminal block!