replacing a 3 phase with a single phase

[unvago]

I have a 3 phase 1750 rpm motor on my compressor.
I would like to replace it with a single phase motor.
Can I just replace and all will be ok?

[tfi racing]

Sort of.Your new single phase motor will draw almost double the current for the same hp,the control and wiring will likely need to be upsized as well.Post some more info,I'm sure this has been done in the past by some of the other members.

[mmg440]

Originally Posted by unvago

I have a 3 phase 1750 rpm motor on my compressor.
I would like to replace it with a single phase motor.
Can I just replace and all will be ok?

Yes and no. It doesn't always go without a hitch. A three phase motor has great start up torque. normally greater then that of even a single phase with the start and run caps. So a three phase compressor while having a cyl head relief and so on does not always require systems to function for the compressor to be operational. So if the pressure and or flywheel weights (or any other start up aid) reliefs are not working correctly, the single phase motor would not be able to start the compressor even though it seemed to be functioning fine with a three phase motor. How may horsepower is the three phase motor. I would recommend going with a slightly high horsepower if going single phase. I am only basing this on my own experiences and others may disagree but I feel in general 3 phase motors are more commonly rated with stronger service factors. So a HP rating of a single phase motor may be actually less then the comparable HP motor of the three phase.

[mrb]

what about getting a VFD and keeping the 3ph motor? They have really come down in price over the last couple years.

[rsanter]

go to ebay and get a bargain priced VFD to run the thing

bob

[MXtras]

Another vote for a VFD.

All responses so far are great.

Scott

[rinny_tin_tin]

Originally Posted by unvago

I have a 3 phase 1750 rpm motor on my compressor.
I would like to replace it with a single phase motor.
Can I just replace and all will be ok?

Yes you can replace it horsepower for horsepower w/o a problem cept your single phase running current single will be 73% higher than it will be at 3 phase. So - for a 5 hp motor, it would be 20 A instead of 12 A.

However, your inrush current (5 X cont. running current) would also be about 73% higher - or 100 A vice 60A - so be prepared for 8 % or greater voltage dip - dimming lights etc.

[bimmer1980]

the variable frequency drives can be hard on the 3-ph motors. It's not a true 3-ph. I've got a 3-ph vertical mill and I'm currently looking for a rotary phase converter. They are much better for creating 3-ph from single phase.

Another option-- buy a panel from ebay that allows you to "create" a rotary phase converter. you could source your own 3-ph motor and have real three phase.

[walrus]

How old is the 3 phase motor?, If very old I'd up the HP on a new single phase

[mrb]

Originally Posted by bimmer1980

the variable frequency drives can be hard on the 3-ph motors. It's not a true 3-ph. I've got a 3-ph vertical mill and I'm currently looking for a rotary phase converter. They are much better for creating 3-ph from single phase.

Another option-- buy a panel from ebay that allows you to "create" a rotary phase converter. you could source your own 3-ph motor and have real three phase.

how is that? they provide a softstart function limiting startup current and stress on the motor. How are they not true 3ph? They use a 3ph inverter to create the output to the motor. (note: I have not looked at the waveform of the output of one on a scope)

[79firebird]

a lot of it depends on what size the motor is my works new location they moved to 5 years ago can only get single phaze in there 400amp service and we are maxing it out. 10hp single phase will run 60amps and we have a 5hp runs 30 and fan anouther 30 and every time the compressor starts up you see the lights flicker due to the big draws on it. loking back on it we are wishing we with with a phase converter. Curently have 4 3phase motors sitting there been thinking of getting one big one so i can make a converter to run them

[tdkkart]

Originally Posted by mrb

how is that? they provide a softstart function limiting startup current and stress on the motor. How are they not true 3ph? They use a 3ph inverter to create the output to the motor. (note: I have not looked at the waveform of the output of one on a scope)

I've looked at the 3 phase coming off my Boston Fincor VFD(driven with single phase), and it looks a hell of alot better than most "real" 3 phase.
Unless you spend big bucks on a rotary convertor most of them are a far cry from "real" 3 phase too.

As far as being "hard" on 3 phase motors, if that's the case there's alot of 3 phase motors out there that are about to smoke. Look around the typical factory these days, except for fan motors and other basic operations, 3 phase motors in most automated machines are running very happily off of VFD's. Open the control cabinets and you'll find a host of VFDs right along side the PLCs, or integrated into the PLCs.

My vote is for the VFD.....