Thread: rewire 3 phase motor to single phase

Originally Posted by jhovel

I agree with Bob. For a 1Hp motor you don't need anything bigger than a 0.75kW VFD.
However: that 1HP motor will produce very little torque at low frequncy! And believe me, you WILL use the VFD at very low frequency - just because you can and it was never an option before....
At that point you'll kick yourself for not getting a bigger VFD becasue you will want to put a bigger motor in you lathe so that you have more torque at low motor RPM......
. . . . . . . There is some thing eerie about turning big diameters at low revs with the VFD making just 15Hz or so.... and the motor still having plenty of torque to spare!

As long as you are aware that if something catches, a bigger motor will not stall as easily and all that torque may seriously damage or, if the motor is big enough, even tear a small lathe apart.

When choosing a VFD to fit a machine tool motor, it is not mandatory but highly advisable to select the VFD one ore two sizes larger than the motor. Depending on operating conditions. The reasons for this are as follows:

- VFD's are usually rated at 20C room temperature. For every 10 degrees C above room temperature you need to de-rate your VFD output power by approximately 10-15%. If you install your VFD in a minimal size dust proof enclosure, and your workshop is not aiconditioned, the temperature inside the enclosure can on a hot summer day easily reach 60C and more. That is 40C more than the VFD is rated, and it is likely to throttle back automatically its output power by some 40 - 60%. Which is substantial, say a 1HP motor may only deliver 1/2HP on a hot day.

- VFD's are usually NOT rated to deliver full power when choosing the highest chopper frequency. If you do not like the squealing motor noise and want to run the VFD at its higher chopper frequency, you may have to derate the maximum rated output by 10 to 15%. The VFD manual will give the exact figures.

- A VFD larger than the motor rating will not get as hot in operation. The life span of a VFD is inversely proportional to its operating temperature. A rise of 20C in operating temperature may translate to a halving of VFD life expectancy.

- If driving say a lathe, you may be interested in dynamic braking to bring the spindle to a stop faster than when letting it coast. A larger VFD will have a more powerful built-in brake chopper transistor and resistor, giving better braking. Should you go for a smaller VFD and then find out the breaking is less than you expected, the purchase of an external braking option may add up to be more expensive than if you went for a larger VFD to begin with.

- should one day circumstances change and you remove the VFD, it will be easier to find a new use for it if it is not too small.

Originally Posted by BobL

As long as you are aware that if something catches, a bigger motor will not stall as easily and all that torque may seriously damage or, if the motor is big enough, even tear a small lathe apart.

True but for example a Hercus lathe, 3/4hp motor doing 1440rpm, reduced to 50rpm through back gears and V-belts is has a hell of a lot of torque as well... I would imagine a lot more then a 750rpm 3hp motor running at 15Hz..

When choosing a VFD motor for a lathe, you need to consider how you intend to use it. It will affect how big a motor you need. And it will greatly affect how much the VFD conversion is going to cost.

Lets assume you currently have a lathe with 6 spindle speeds and a 1HP motor.

A)
If you are happy to keep changing gears (or belt position), you can use a 1HP motor for your VFD. You will change gear much less often than before, particularly so if you use a modern "sensorless flux vector" type of VFD. But in the situations when you need high torque at low rpm, you will have to change gear. VFD conversions of lathes used for home/hobby are mostly done this way. It is the cheapest way to to do it.

B)
If your intention is to do away with some or all gears (or belt positions), you will need a much greater motor for your VFD. As a rule of thumb, you need to multiply the motor rating by as many gear or belt position you wish to remove.
Leaving off one gear means you need a 2HP motor, 2 gears 3HP, 3 gears 4HP, 4 gears 4HP, 5 gears 5HP, or if you want to leave away all 6 gears you will need a 6HP motor and VFD. A 6HP motor/VFD would ensure that you always have at least the same torque available in a direct drive setup, as you would have with a 1HP motor and a 6-speed gearbox. Industrial CNC lathe spindle drives are mostly done this way.

VFD upgrades of manual lathes used to earn a living, are most often a compromise between A and B.

Originally Posted by .RC.

True but for example a Hercus lathe, 3/4hp motor doing 1440rpm, reduced to 50rpm through back gears and V-belts is has a hell of a lot of torque as well... I would imagine a lot more then a 750rpm 3hp motor running at 15Hz..

True but its not the low speeds I'm worried about. I have visions of a lathe with bracings and bearings designed to run with a 1/2 HP motor but now running a 4HP motor at 50Hz catching something. Hopefully belts will slip but if not I can see something getting bent. I think Joe said he did run his belts on the loose side?

Originally Posted by BobL

True but its not the low speeds I'm worried about. I have visions of a lathe with bracings and bearings designed to run with a 1/2 HP motor but now running a 4HP motor at 50Hz catching something. Hopefully belts will slip but if not I can see something getting bent. I think Joe said he did run his belts on the loose side?

You can tell a 4HP VFD not to exceed say 1HP output. But a 4HP motor still has a lot more inertia to do damage in a collision. Most lathes come from factory with some sort of safety mechanism(s), be it a slipping flat belt, or a safety clutch, or shear pins, or plastic gears designed to strip in a collision...... At the end of the day there is no 100% effective protection. It is better to stay alert and avoid collisions.


A bigger worry with VFD conversion of lathes are the spindle bearings and chucks. Depending on what number of poles are chosen for the new motor, it is easy to double or triple the maximum spindle RPM.

- Say a lathe was in top gear capable of 2000rpm using a 4 pole single phase motor. The lathe is now upgraded to VFD with a 4-pole 3 phase motor. The lathe spindle will now still do 2500rpm with the VFD set to 50Hz. But the VFD can now be set to 100Hz for a spindle speed of 4000rpm. And that may be too much for some spindle bearings like sleeve bearings or large diameter roller bearings. And it may exceed the safe speed for larger cast iron chucks.

- Say the same lathe was upgraded to VFD with a two pole motor. Now in top gear the spindle is doing 4000rpm at 50Hz, and close to 8000rpm with the speed pot at maximum. Now it is not only bearings at risk, most chucks regardless if cast or steel are now at risk, and even the motor rotor is at risk. With a 2 pole motor the max frequency of the VFD needs be limited to something like 70Hz, and the top gear speed may need to be disabled altogether.

What's being compared here is,

On the one hand a 1hp 3 phase motor with gearbox and just direct 415V 3 phase mains connection. With only a MCCB for overload trip, and whatever mechanical crash protection designed into the lathe, like shear pins, slipping belts etc..

On the other hand we have a 3hp 3 phase motor powered from a 240V VFD with all the overload and additional protection you could ever want to protect against a machine crash.

No contest. Go with the better solution. As Joe says the extra torque at low revs, ( even with just a V/F drive, not a vector drive) is very nice to have.. plus better crash protection that the original 1hp. Think of it as an electronic programmable shear pin if that helps understand the difference in crash protection.

Ray

Originally Posted by RayG

1. I guess you could open up the motor and bring out the star point, then re-wire to delta, hook up a VFD and re-wire the controls.

2. Buy or build a phase converter that will generate 415 3 phase, commercial units are available for a few thousand..

3. Run on reduced power by running a 240V VFD as per option 1, but with the motor still in star.

Ray

Hi i thought 3phase has to be Delta running a from 240V via VFD to 3phase Motor

Originally Posted by closeshave

Hi i thought 3phase has to be Delta running a from 240V via VFD to 3phase Motor

Many (probably most) 415V 3 phase motors under about 5HP are wired in star. If wired in delta, the input voltage required will be 240V, 3 phase, which is what is synthesised by a suitable VFD. There are a small number of models of VFD comparitively speaking which can output 415V 3phase with a 240V single phase input, but they have been relatively hard to come by and somewhat more expensive than the usual types, so most people would reconfigure their star connected 415V motor to delta, and use the 3phase but 240V output from an easily sourced VFD.
On the other hand most motors over 5HP are already configured in delta, so if they were to be run in star, a 415V in delta motor would need a voltage of 718 if the motor was rejigged to star configuration.
Rob.

Originally Posted by closeshave

Hi i thought 3phase has to be Delta running a from 240V via VFD to 3phase Motor

A motor designed to run on 415V 3P Y will run on 240V 3P but at approx half power. If all you have is access to are Y connected motors, then a solution to this is to use a motor that is double the HP of the original motor. This is what I have done on my WW lathe.

Originally Posted by DavidG

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yep, you might put an ears dropping electricians nose out of joint. Give him the shiiites. But then again, he might have a NEED to bitch about something. At the very least something to do rather than just stare at his beer with nothing to say. (this of course is just a generalisation: He may not be a beer drinker…..or bitch much at all. He might just like Orange juice and only bitches when his feet hurt…who knows)

Disclaimer: this is just an opinion. I'm not an expert on off electricians.

ps. I can't believe just how many blokes here are giving electrical advice ! honestly.

Originally Posted by apricotripper

yep, you might put an ears dropping electricians nose out of joint. Give him the shiiites. But then again, he might have a NEED to bitch about something. At the very least something to do rather than just stare at his beer with nothing to say. (this of course is just a generalisation: He may not be a beer drinker…..or bitch much at all. He might just like Orange juice and only bitches when his feet hurt…who knows)

Disclaimer: this is just an opinion. I'm not an expert on off electricians.

ps. I can't believe just how many blokes here are giving electrical advice ! honestly.

What the #### are you trying to say bud.

Originally Posted by apricotripper

ps. I can't believe just how many blokes here are giving electrical advice ! honestly.

I'd like to think that at least some of the advice is use to help a novice discuss their issue with an electrician.

Originally Posted by Techo1

What the #### are you trying to say bud.

don't get your panties in a knot. bud.

I'm taking the on over caution….on the disclaimers... The whole boring lot.

I've discussed electrical ideas in the past myself. only Once. And wasn't really giving advice, and off electricians as if I was the only one who does his own electrical work when he can. As if they don't do things to try and save money.

And yet, here, post after post of advice.

<- forcing a smile despite feeling green under the gills.