Atahree Phase Motor

[Jbird]

Hello all. I have a three phase motor about 8hp and it still works fine driveing a saw blade for cutting marble etc. The blade runs free and there is no sign of obviose bearing ware when gently rotated by hand. The problem is when you turn the motor off it comes to an abrupt stop ie it doesnt spin down slowly. when the blade has finaly stops there is a noise, not loud, but a sort of rough hum as if the motor was jamed, but its not as the motor starts again fine. I am wondering if there is a phase down or a short of some sort? I am confused as these problems only occur post power being turned off.

Any help will be much appreciated.

JB

[Hymie]

The symptoms you describe suggest to me that your disconnecting device (switch) is not disconnecting all the phase conductors simultaneously – resulting in one or more phases remaining powered for a short while, causing the abrupt braking and humming sound.

Try stopping the motor using an alternate method (pulling the plug if there is one) – or isolating further back on the supply side, to test my theory.

[beso]

A three phase motor that is already turning will normally continue to to run on a single phase but with reduced torque.

Maybe a phase is being reversed which will cause the motor to be powered backwards bringing it to a very sudden halt.

[heathfield]

My Elu radial arm saw does exactly this. The motor is wired in such a way as to allow a 'back EMF' (Electro Motive Force) to act as a brake and quickly stop the blade rotation. This is a safety feature, in case you're impatient and try and do something while the blade's still turning. It sounds alarming to hear the hum once the blade's come to a stop, but it's nothing to worry about.

[BRIGHT SPARK]

Has the motor always done this or has it just developed.If its always been like this then I agree with heathfield. On switch off the rotor is effectively shorted out and the motor becomes a crude generator and will halt rotation.A similar system is used on electric trains and the resultant power is fed back into the supply system.

[bentaxle]

Never seen ac three phase motor that will generate emf for electrical braking ,

[heathfield]

There's more than one means of electrically braking a three phase motor. I reckon one of them is incorporated in Jbird's saw, and it's acting the way it's supposed to. (I wish my Metabo mitre saw did the same, instead of taking an age for the blade to come to a stop!),

[beso]

Any ordinary induction motor can used to generate power and can be electrically braked.

Some of you would be familiar with induction motors being driven above synchronous speed to generate power but they can also be used as stand alone generators.

The rotor of an induction motor will retain a small residual magnetic field after it is disconnected. When it is rotated that field will generate voltage in the stator windings. However, due to the inductance of the system the field is weakened as is the case in any generator subjected to an inductive load.

On the other hand, as in any generator, a leading power factor will increase the magnetic field, building up the field strength and terminal voltage. Consequently all that is required to run an induction motor as a generator is to connect capacitors across the phases.

The capacitance required depends on the operating speed. The greater the rotational speed the higher the frequency and hence the capacitive reactance and generated voltage.

Without a load the voltage will increase until the magnetic field of the motor saturates. In practice it blows the capacitors first.

Braking can be achieved by simply increasing the capacitance while maintaining a suitable load.

[bentaxle]

Thank you Beso for what you wrote it was very interesting and being in Electrical Maintenance for 60yrs and never coming into contact with it only goes to prove you never stop learning I've since found this which explains it as well ,All the best
Induction Motor Braking



In capacitor braking, the induction machine will be disconnected from the supply and connected to a capacitor bank connected in either Star or Delta. So, now the excitation would be provided by the capacitor and the machine will start acting like a generator (Why?). The generated electric power will be dissipated in the inherent resistance of the windings and thus braking is accomplished.