# 3 Phase Induction Motor On Single Phase

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I have a 5.5hp induction motor running a compressor, but it is 3 phase.

Can I run it on single phase? I assume I need to connect it in delta to work at 230V and put a capacitor between one supply lead and remaining winding. What size cap is needed? I managed to get it to move slightly in star connection with 170uF cap. Am I on the right track?

Rick

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Hello ricard

The best way to do this, is to convert the motor to 230 volts if possible (not always possible to do) and use a single phase input, three phase output variable speed drive to run the motor.

Adding capacitors on the third phase can work, but the motor must be seriously derated and thus is not very effective.

Best regards,

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Dear

Generally, 1kw motor requires 65uf capacitor for starting it on single phase but keep it in your mind that the torque would be reduced 1/3rd percent. For your compressor application, where a high starting torque requires because of air which presses the piston, this experience would be failed. For experiment, start the motor on delta configuration.

"Don't assume any thing, always check/ask and clear yourself".

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Dear

Generally, 1kw motor requires 65uf capacitor for starting it on single phase but keep it in your mind that the torque would be reduced 1/3rd percent. For your compressor application, where a high starting torque requires because of air which presses the piston, this experience would be failed. For experiment, start the motor on delta configuration.

Correction, the torque is not reduced by 1/3, it is reduced to 1/Sq.Rt 3 (1.732) which comes out to be 58% of full torque, not 66.7%. In addition, since the rated speed is the same, the torque loss would reduce the shaft power with it, so your mechanical kW output (HP) would drop by an equal amount as well. For example let's say you had a 100kW motor which was sized for a particular load, in this case the compressor at full compression stroke and speed. With 1 phase input, even if you could get it started, it would only be capable of putting out 58kW, so it will likely overload as soon as the compressor loading valve closes. If you could reduce the loading by reducing the compression stroke or change the pulley ratio so that you needed less shaft power per stroke, it might possibly work, but it will take twice as long to deliver the same volume of air (assuming the motor is rated for longer duty cycles).

"He's not dead, he's just pinin' for the fjords!"
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Thanks for correction.

"Don't assume any thing, always check/ask and clear yourself".

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• 2 years later...
Correction, the torque is not reduced by 1/3, it is reduced to 1/Sq.Rt 3 (1.732) which comes out to be 58% of full torque, not 66.7%. In addition, since the rated speed is the same, the torque loss would reduce the shaft power with it, so your mechanical kW output (HP) would drop by an equal amount as well. For example let's say you had a 100kW motor which was sized for a particular load, in this case the compressor at full compression stroke and speed. With 1 phase input, even if you could get it started, it would only be capable of putting out 58kW, so it will likely overload as soon as the compressor loading valve closes. If you could reduce the loading by reducing the compression stroke or change the pulley ratio so that you needed less shaft power per stroke, it might possibly work, but it will take twice as long to deliver the same volume of air (assuming the motor is rated for longer duty cycles).

Question, I have an application where the required torque is only about 40% of three phase rating without a difficult start up. Where which line do I connect the feed of the capacitor to. first or second 115V input, and then connect the secondary side of the capicator to the third phase of the motor?

I apprenticed in a machine shop which had a Bridgeport connected similarily, with a momentary switch for the start only and then it ran with somewhat less power but ok with slowed down feed rates. Thanks for any help...cliffwieser

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Hello cliffwieser

I expect that you can connect to either but the direction of rotation will be determined by were it is connected.

Best regards,

Mark.

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Hello cliffwieser

I expect that you can connect to either but the direction of rotation will be determined by were it is connected.

Best regards,

Mark.

Thanks Mark, This makes perfect sense amd confirms another resource. Thanks again, Cliff Wieser

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• 1 month later...
Question, I have an application where the required torque is only about 40% of three phase rating without a difficult start up. Where which line do I connect the feed of the capacitor to. first or second 115V input, and then connect the secondary side of the capicator to the third phase of the motor?

I apprenticed in a machine shop which had a Bridgeport connected similarily, with a momentary switch for the start only and then it ran with somewhat less power but ok with slowed down feed rates. Thanks for any help...cliffwieser

You need to add a start capacitor between the generated leg and one of the two supply single phase legs. Using a start capacitor of 65 mfd per HP and controled by a momentary switch (NC). Manual Mode

Or a normally closed relay with a 110 volt coil can be used, with one leg of the coil wired to the generated 3 phase leg and the other leg to ground. This leaves the start capacitor of (65 mfd per hp) in the circuit in the off/or static position, this enables the start capacitor to kick the third leg and start the motor, and when the third leg comes up to voltage will open the NC relay removing the start cacacitor from the circuit. Automatic Mode

To balance the generated leg to increase % add 25 mfd (Run Capacitors) per HP. Without these capacitors the motors duty cycle of a 100% motor is 60%.

These vaules are a starting point, but I have had one like this for 25 years.

You need to add a start capacitor between the generated leg and one of the two supply single phase legs. Using a start capacitor of 65 mfd per HP and controled by a momentary switch (NO). Manual Mode

Or a normally closed relay with a 110 volt coil can be used, with one leg of the coil wired to the generated 3 phase leg and the other leg to ground. This leaves the start capacitor of (65 mfd per hp) in the circuit in the off/or static position, this enables the start capacitor to kick the third leg and start the motor, and when the third leg comes up to voltage will open the NC relay removing the start cacacitor from the circuit. Automatic Mode

To balance the generated leg to increase % add 25 mfd (Run Capacitors) per HP. Without these capacitors the motors duty cycle of a 100% motor is 60%.

These vaules are a starting point, but I have had one like this for 25 years.

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