single phase motor starting 1

[Markr]

Which is easier to start ? A single phase phase motor of a given hp fed from a single phase generator at some voltage or a three phase motor of the same hp fed from a three phase generator of the same voltage.

Does it matter if the same single phase motor is fed from a three phase generator ?

 

[ScottyUK]

Normally pushing a button takes the same effort whether the motor is 1-phase or 3-phase!

But joking aside, what do you mean by 'easier'? The 3-phase machine will have better starting torque. In almost all respects the 3-phase machine is superior to the 1-phase equivalent: they are simpler, more robust, and are inherently self-starting. The wiring will be a little more complex for the 3-phase machine, but will be of lighter gauge. The control gear - contactor and overload - will be slightly more expensive for the 3-phase machine.

A 1-phase motor on a 3-phase generator will be a desperately unbalanced load which will have a detrimental effect, possibly terminal, on your generator because the NPS currents resulting from the highly imbalanced load will cause severe heating of the generator rotor.

How big is the motor? The generator needs to be considerably larger in rating than the motor in order to ride through the motor starting without the terminal voltage collapsing.


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One day my ship will come in.
But with my luck, I'll be at the airport!

 

[rbulsara]

While what ScottyUk said above is all true specially for a sizable installation with more than one or two motors.

What is the application here? Domestic or commercial?

If you are talking about domestic or garage type application, with one or two 1HP or lesser motors, single phase units may suffice or make sense on economical basis.

For smaller application, using a 3 phase generator to drive a single phase motor may not make sense.

 

[jraef]

Assuming you are speaking of a small load as rbulsara pointed out, I totally agree with ScottyUK's point. But another thing to consider is what else you will use the generator for. If it is going to be dedicated to that motor, go 3 phase by all means. If you might want to use it for something else, 1 phase may be more practical. The difference in the motor performance would be negligible, it's just that a 1 phase motor has more parts to break down (i.e. speed switches, capacitors etc.)

ScottyUK,
Loved the joke about the push button!

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[aolalde]

Markr

Three phase motors performance is superior as compared to single-phase motors.
The initial cost is lower.
Starting torques are higher.
Efficiency is higher.
Power factor is higher.
Weight is smaller.
Keeps power phases balanced.

 

[Markr]

I guess I'm wondering about some rule of thumb numbers. I have a small diesel power plant feeding some other loads in addition to some single phase motors. We do have some trouble with starting the motors with volatge drop . I wondering about installing three phase motors. What will I save on start up ? Or is it better for me to look at soft starts for the single phase motors?

 

[Marke]

Hello Markr

Both the three phase motors and the single phase motors require a high start current when started at full voltage, so if you major concern is the voltage drop on start, you get the same problem. The start currents are not too different but it depends on the design of the motor.

You can get single phase soft starters for soingle phase motors, and you can get three phase soft starters for three phase motors. The three phase soft starter on a three phase motor is more effective than the single phase soft starter on a single phase motor. The level of performance on a single phase motor is dependent on the type of single phase motor also. The induction start (no capacitor) is the worst case that I have had to make work.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[rbulsara]

What is the size of the generator? is it 3 phase? provide some more data...Are the load balanced now?

If the gen is a 3 phase unit, a 3 motor is better, if it is more than 0.5 HP. If a single phase motor HP is more than 1/3 of a 3p phase generator size, it will create problems and 3 phase motor may make sense.

But a lot depends on your specific set up, there are no rules of thumb that fit all.