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3 phase motor

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Lynne17/11/2019 05:45:17
80 forum posts
27 photos

Current mill motor single phase, probably 1 hp, but not plated.

Have an Assea 3phase .55Kw motor which I am thinking of using

in conjunction with a vfd. Will it cope? Mill only gets light use.

Would appreciate members input on the feasability of this.

Thanks, Lynne

Brian H17/11/2019 08:12:43
1676 forum posts
109 photos

I make .55kw to be just under 3/4 hp so might be OK for light use. (but I'm no expert)


John Haine17/11/2019 08:53:50
3181 forum posts
172 photos

It should be just fine, why do you think your current motor is 1 hp? Rarely will you be taking such a heavy cut that you actually need all the power.

Paul Lousick17/11/2019 09:05:33
1457 forum posts
555 photos

You are replacing a 1 hp motor with one that is 3/4 hp, which as stated should be OK for anything except heavy cutting. But as you slow the motor speed down with the VFD, the power/torque of the motor will also reduce.


John Haine17/11/2019 09:30:17
3181 forum posts
172 photos

Which is why you will still be using the pulleys for big cuts.

old mart17/11/2019 13:39:01
1829 forum posts
148 photos

Another factor is the rated rpm of the motors. I have changed a 1425 rpm single phase to a 935 rpm three phase one, as the minimum lower speeds available would have been too high for larger drills and cutters. With the vfd set as I want it, the three phase motor will reach nearly 1400 rpm and go down to 467 rpm, which in conjunction with the four belt positions gives a better overall spread of power and speed. Remember, that a three phase motor on a VFD running at 25Hz is not only half the speed, but also half the power, and the power drops rapidly if the frequency is lowered further.

Martin Connelly17/11/2019 14:09:43
1401 forum posts
164 photos

Old Mart has touched on an important point. The rpm of a three phase motor depends on the number of poles it has. To swap a single phase out and three phase in it is best to know the rpm of the current motor before assuming a straight swap will give the same resulting speed range.

The nominal speed of a 2 pole three phase motor at 50hz will be 3000rpm. Double the poles and it drops to 1500rpm, triple the poles and you get 1000rpm. It should be possible to get a suitable motor to swap in but it may not be the one you have.

Martin C

old mart17/11/2019 16:04:08
1829 forum posts
148 photos

Three phase motors for small applications have up to 8 poles, the more poles, the slower the running, so with equal horsepower, the torque increases the slower the motor runs natively. The motor I bought has 6 poles, the 8 pole one was 50% more expensive.

This is the one I bought:**LINK**

Lynne18/11/2019 10:44:25
80 forum posts
27 photos

Thanks to you all for getting back to me on this .

John, All I can say is that the motor is quite a lump and heavy, though that is probably down to the fact

that it all cast iron body. The mill is a Naerock RDM 350, and what bits of info. I have gathered over

time, does'nt give any info about the motor, so it could well be 1/2 or 3/4hp. I am going to have to take the motor

apart to fit a sleeve to the shaft and machine so can I establish the number of poles fairly easily from

peering inside,but it would be useful to know exactly what to look for. Thanks again, Lynne

old mart18/11/2019 11:36:37
1829 forum posts
148 photos

The motor should have a plate on it which has the voltage, frequency, amps and rpm. You will need this information to program a VFD. A visual inspection of the windings will only be of use to an electrical engineer. You will also have to make sure it is in delta configuration, not star.

John Haine18/11/2019 11:49:31
3181 forum posts
172 photos

Lynne, photos of the spec plate on the motor and the terminals would be of enormous value in helping you.

john fletcher 118/11/2019 12:20:19
603 forum posts

Lynne, on my RDM 350 it was a four pole 1425 rpm and the motor was 3/4hp.

I made a completely new motor mounting plate and discarded the centre pulley arrangement, ( its in a cupboard) So, now have a longish link belt front to rear, it works well. The replacement motor is a 1 HP 4 pole which I already had, they are available second hand. If you are considering buying a second hand motor do check that the mounting are OK for your machine and get some one with an ohm meter and a Megger to test it for you. Some motors are very easy to change from Star to Delta, other take much Lynne,longer. There has been some excellent picture and explanations published on here in the past on the electrical change over. I made a control box which is mounted above the quill lock, with a speed control knob, stop/start and a jog button below. No reverse here.

Zan18/11/2019 23:56:34
166 forum posts
12 photos

Check that it’s a dual voltage motor if you intend to use a 240 v vdf. The plate will tell you

Lynne19/11/2019 14:50:37
80 forum posts
27 photos

Thanks to all for the comment. Certainly had a senior moment, the RPM is on the plate. Have put a few pictures in my albums, plate and motor. Change from star to delta is by links, so that is easy. The next bigge is to sort out the VFD, so I may well return on this question. Regards Lynne

John Haine19/11/2019 16:36:38
3181 forum posts
172 photos

Based on JF1's post, your ASEA motor is perfect - 0.55kW which is about .75 hp, 1410 rpm = 4 pole, star or delta connection.

not done it yet19/11/2019 17:27:02
4746 forum posts
16 photos
Posted by John Haine on 19/11/2019 16:36:38:

Based on JF1's post, your ASEA motor is perfect - 0.55kW which is about .75 hp, 1410 rpm = 4 pole, star or delta connection.

If the mill is the same, that may be so. But we don’t actually know what the ‘current’ mill is. The current mill motor might be more than one horse power. Guessing doesn’t particularly help if the real details are not known. Is the mill directly driven by belt or does it have a gearbox to consider?

John Haine19/11/2019 17:38:04
3181 forum posts
172 photos

NDIY, we do, Lynne says it's an RDM350 4 or 5 posts up.  And uploaded a photo showing pulleys on the motor.

Edited By John Haine on 19/11/2019 17:44:42

John Haine19/11/2019 17:42:56
3181 forum posts
172 photos

Lynne, rather than take the motor apart to sleeve the shaft, you could sleeve the pulley. When I fitted a 3 ph motor to my VMB I removed the intermediate pulley, which was almost exactly the same set of sizes as the spindle pulley, turned up a sleeve to fit closely in its bore, with a keyway for the motor key, and pressed / loctited it into the pulley. An excellent modification.

ega20/11/2019 14:03:51
1750 forum posts
152 photos
Posted by old mart on 17/11/2019 16:04:08:

Three phase motors for small applications have up to 8 poles, the more poles, the slower the running, so with equal horsepower, the torque increases the slower the motor runs natively. The motor I bought has 6 poles, the 8 pole one was 50% more expensive.

This is the one I bought:**LINK**

An interesting quote from the linked page:

"From Jan 1st 2017, three phase electric induction motors with a rated output of 0.75kW to 375kW and efficiency less than IE3 must be equipped with a variable speed drive (Inverter Drive). For exceptions, see EC Commission Regulation 640/2009."

Would anyone care to decode this for me?

Mike Poole20/11/2019 14:32:33
2620 forum posts
63 photos

I think that the crux of the document is about saving energy and a VFD is a useful device to manage a motor to its load. As most of our home workshops do not have three phase and we either use a VFD to convert a three phase machine to run on single phase or we choose to convert for the improvement in control and smoothness of a machine then we comply with the regulation even if we may not be required to.


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