Three phase motor onto inverter

[OldWood]

I have an interesting challenge and would appreciate anything anyone can add.

I recently bought an elderly (late 1960's possibly) Cooksley P/T. The main motor is no problem as it is just a change of jumpers star to delta.

The problem i've found (once I got it home!) is that it uses a seperate motor gearbox for the thicknesser rollers, and to compound the problem this is a 1450/2900 rpm three phase motor - fortunately in some ways it is not a Dahlander configuration but a 2 pole/4 pole stator with two independent sets of coils, both set up as star - and no jumpers option!!

I successfully did a star to delta conversion for a Graduate lathe some 15 yeras ago and that is still working, but can't remember much of what I did, and am wondering if there's anyone here who can remind me of what the hazards are and how to avoid them. I'm not finding much on the internet on re-wiring star to delta.

The motor is dismantled and the body is seperated out.

Thanks
Rob

 

[guineafowl21]

The main hazards would be:

Digging out something that’s not the star point - avoided by measuring resistance of star-connected coils before you start, and ensuring the resistance between the new connections and old is half that.

Generally causing damage to the windings. Disturb as little as you can - the star point is normally quite superficial, and protected by a sleeve. Keep heat shrink tubing available. The last one I did had the upper part stitched with cotton thread, which I was able to replace with butcher’s twine, soaked in varnish.

Two sets of coils might mean two conversions, and an inverter with two default settings. On the other hand, you will have speed control so it may be worth only converting the higher speed 2 pole set and using the speed control on that.

Thread here: Wadkin RBD planer

 

[Inspector]

I may be off but I remember reading somewhere that you can't run two different motors from the same VFD at the same time. You might need two. A look into it might be worthwhile.

Pete

 

[OldWood]

The main hazards would be:

Digging out something that’s not the star point - avoided by measuring resistance of star-connected coils before you start, and ensuring the resistance between the new connections and old is half that.

Generally causing damage to the windings. Disturb as little as you can - the star point is normally quite superficial, and protected by a sleeve. Keep heat shrink tubing available. The last one I did had the upper part stitched with cotton thread, which I was able to replace with butcher’s twine, soaked in varnish.

Two sets of coils might mean two conversions, and an inverter with two default settings. On the other hand, you will have speed control so it may be worth only converting the higher speed 2 pole set and using the speed control on that.

Thread here: Wadkin RBD planer

Hi GF

1. Should I soften any existing varnish with a hot air gun when starting off .?
2. And re-varnish afterwards? - I'm going to use long thin cable ties to clamp the coils afterwards.
3. My thinking was once the motor was re-assembled to check the current consumption on both sets of coils and go with the one that had the greater power, and adjust the speed controller correspondingly. Logic says 4 pole would have more torque but that does depend on how much current is in the field coils. High 1 to High 2 (in star) is 49 ohms, equivalent in 4 pole mode is 115 ohms.
4. I've got two controllers. I won't really want to change the main motor speed as the cutter head is 3 knives and at 4000 rpm they will be doing a good enough job anyway. I'm not sure as yet how much benefit variable speed will be on the thicknesser rollers - that will be a suck it and see when we get there.

 

[guineafowl21]

Hi GF

1. Should I soften any existing varnish with a hot air gun when starting off .?
2. And re-varnish afterwards? - I'm going to use long thin cable ties to clamp the coils afterwards.
3. My thinking was once the motor was re-assembled to check the current consumption on both sets of coils and go with the one that had the greater power, and adjust the speed controller correspondingly. Logic says 4 pole would have more torque but that does depend on how much current is in the field coils. High 1 to High 2 (in star) is 49 ohms, equivalent in 4 pole mode is 115 ohms.
4. I've got two controllers. I won't really want to change the main motor speed as the cutter head is 3 knives and at 4000 rpm they will be doing a good enough job anyway. I'm not sure as yet how much benefit variable speed will be on the thicknesser rollers - that will be a suck it and see when we get there.

1. Might help - I’ve never done it so go carefully.
2. Either varnish or epoxy seems to work well. Check the temp rating of your cable ties.
3. & 4. See how it goes.

I bet @Myfordman would be interested in this...

 

[Spectric]

Hi there

Yes you can only run a single load from an invertor so you would need two but it would solve the speed issue. If you do any invasive work on the motor then it is important that you perform an insulation test with a mega.

 

[Myfordman]

I bet @Myfordman would be interested in this...

The OP and I have been emailing on this job for the last few weeks

 

[guineafowl21]

The OP and I have been emailing on this job for the last few weeks

Secret motor discussions!

How are you dealing with the dual speed motor?

 

[Myfordman]

Secret motor discussions!

How are you dealing with the dual speed motor?

After a few messages back and forth we have established that it is not a Dahlander wired motor so it is a case of doing a star delta conversion on the most powerful winding set and using an inverter to perform the speed selection. I'm not happy with Rob switching the windings on an inverter.

 

[OldWood]

Secret motor discussions!

How are you dealing with the dual speed motor?

At the moment finding other jobs to do having finally got the motor apart - that is putting off the moment of commitment of delving into the motor windings to find two star points and convert them both to delta so that as Myfordman indicates I can work out which set is the more powerful when running off an inverter. That will be done by changing the wiring from the inverter for as MM says inverters are not at all happy at changing loads mid-stream.

i don't know enough about 3 phase motor theory to know how to make an intelligent guess as to whether I'm going to be better running with the two poles at 50Hz and turning the inverter down for low speed, or use the 4 pole windings and raise the inverter frequency upwards for a high feed rate.
Rob

 

[guineafowl21]

i don't know enough about 3 phase motor theory to know how to make an intelligent guess as to whether I'm going to be better running with the two poles at 50Hz and turning the inverter down for low speed, or use the 4 pole windings and raise the inverter frequency upwards for a high feed rate.
Rob

The better inverters have parameters to adjust when altering speed to ensure the motor remains correctly fluxed, for example, see here: How to get Maximum Torque at Low Speed from a Simple VxF Inverter

In this example, a fast motor is being slowed efficiently using the drive’s built-in capabilities. It may well be possible to go the other way, but my money would go on using the 2-pole and slowing it down.

 

[Jackbequick]

Why are you doing this (what's the point? what do you hope to essentially and practically achieve?) What's your actual theory in converting 'two' star points to delta'? What sort of control presently exists? ...respond if you want. Inverters range come in various forms and some are programmable. When switching star/delta when motor running quite an arc can be drawn and amateur design could be pricey. I'm wondering whether this is going to turn out with tears shed.

 

[Jackbequick]

I should have added typically in 3 phase motors under 5Hp star is use...when required the motors start in star (reduced voltage/torque...a little like a soft start on inverter) then using a timer, switch to Delta. What Hp is your motor and is it star/delta? How does it start? What is the point in starting in delta?...torque?...whatever the device it drives, it should not be used until up to speed anyway. Leave it there.

 

[OldWood]

I should have added typically in 3 phase motors under 5Hp star is use...when required the motors start in star (reduced voltage/torque...a little like a soft start on inverter) then using a timer, switch to Delta. What Hp is your motor and is it star/delta? How does it start? What is the point in starting in delta?...torque?...whatever the device it drives, it should not be used until up to speed anyway. Leave it there.

Yes - there are bits of information missing in the original post specifically the motor powers and the intention to run the P/T on a suitably rated electronic inverter - now two inverters! On the otherhand many of the people here know the type of motors used in the typical equipment mentioned.

The motor causing me concern is a 1 hp 440vac, 3 phase, 2 pole/4 pole externally switched in star configuration. In order to run on 240vac 3 phase from an inverter, it needs to be reconfigured to delta, as does the main 2 hp motor but that has jumpers. My intention is to re-wire both star points to delta as my knowledge of ac motor theory is too thin to know whether it will operate better in 2 pole or 4 pole configuration.
I hope this answers your questions
Rob

 

[Jackbequick]

Hi Rob. I do know a fair bit about motors and also that at 1hP which is a small motor it would be cheaper to buy a single phase motor (maybe for both motors is sap;ace permits)and same base, same shaft and adapt it. Ok I am not 100% clued-in yes...is this a planer? You probably know much of what I am writing but anyway...

Transmission is generally in star connection...provides the neutral for the 3 phases which are organised to try to balance phases for the alternator supply system.

Motors are commonly star connected to start (reduced current) then switched to delta. At the instant of starting current is 'locked rotor' that is no current generated by the motor is fed back into the 'grid', diminishing motor current. IN choosing an inverter look for at least 4 times the full load running current (see motor spec plated). For 3Hp running look at 25 amperes for short period. It could well be less... I don't know the arrangement. That's all worked on starting at full load....starting at virtually no load then hooking-in the load might see 15 amps being adequate provision by the inverter. One could go less if considering that the 1 hP motor (746 watts) would draw about 3.5 amps on full load single phase but that on 3 phase that is shared amongst 3 phases..., call it say 1.5 amps per phase full load.

Obviously buying 3 phase gear to use on single phase comes with problems. If you have 2 motors, however, essentially once you provide 3 phases of adequate current both can work off one inverter...no differently form were the inverter a mains supply...T

The pole switching I gather is extant..(?) all you need is 3 phases of adequate current availability. That includes start up current...which some inverters handle through quadratic design and 'soft start' capability...never start motors on load unless there is no option. In formular below your pf is unlikely to be better than 0.8 and your efficiency also unlikely to be better than 0.8

When a motor is pole-switched arcing occurs. One can use a timer to reduce that by not switching "in" instantly. Your equipment if made 'standard' with pole switching will have included all that design consideration.

On what you are doing...which I wouldn't....To wire to delta..you pull out the ends of the three start windings. For delta the end of one winding connects to start of the next. (balanced design is critical as is correct winding connections...as reversing any winding in 3 phase reverses direction.)

The option perhaps not considered is the one I'd go for if feasible. My opinion is a better long term solution as well should be taken: ....Do you actually have 3 phase at the main board or at the mains (street)supply?...which you can have hooked into your system and forever more enable you to use any 3 phase gear....N.B. always checking direction of motor before hooking to a load...your phasing might give say clockwise to some motors and anti-clock to others. That's an easy fix.

Using an inverter is ok but the expense might be better spent as providing a permanent supply as above. That way the product stays as the designer intended and you have the option for other three phase gear....and a more valuable property actually...as in 'more widely desirable/attractive'.

 

[guineafowl21]

@Jackbequick as I understand it, the OP is looking to run two three-phase, star-connected (400V) motors from a single phase (230V) supply. Commonly available inverter drives can only put out three phase 230V, hence the need to delta convert the motors to accept the lower voltage.

Star-delta starting, at least over here, is used on much larger motors which might be configured for 400V delta, allowing 400V star configuration for starting to manage inrush current. It’s not used in this machine.

One motor is switchable from 2->4 pole operation for speed control, again wired in star and intended for 400V three-phase. If both configs were delta converted, in theory it could be run from a standard inverter drive as above, but switching between the speeds could damage the unit.

So the intention is to stick with either 2 or 4 pole setup, and use the inverter drive itself for speed control.

 

[Jackbequick]

Hi yes thanks....all a bit hard in a way so I recommended getting 3 phases to the place....it ads value and makes buying gear one shouldn't have previously bought, ok to buy!!

If you have a purpose e.g. 'workshop facilites and welder' the electrical authority might connect for no charge to the point of attachment. Of course it's all a piece of string as I don't know the layout, distances or any such. I'm a 'long in the tooth' electrical contractor and engineer so have formed views about tweaking situations when changes to actually supply are more useful long term and at property resale. That said inverters and converters have come a long way and have plummeted in price since I was selling them to mining companies (the old 'remeninscence")

Star is 3 x 220-240 V usually offering mains voltage and neutral to property as single two or three phases. Delta requires the three phases.

I'm going to send some converter information later which may assist. One thing controllable/programmable inverters can do is extend motor life especially on soft start ...be that as it may a unit to suit the purpose (Australian) will cost probably around $750.00.

 

[OldWood]

@Jackbequick - in general few properties in the UK have 3 phase available and the cost and nuisance of installing it outweighs the convenience. Any investment needs to show a return and that is highly unlikely to add anything to the value of my property. Interestingly the engineering business up the road from me had 3 ph added recently having survived on a static inverter - I'll find out how much it cost him, but the massive inconvenience it will create in terms of new wiring is just not worth the candle, partucularly at my age!!
Another factor you haven't taken into account is that by doing the conversions and adding in inverters, the equipment, which is not now H&S approved for commercial use, increases in value 2 or 3 times in terms of re-sale to those with single phase.
The idea of changing to single phase motors is just not worth considering - cost, effort - particularly linking to the thicknesser gearbox, lack of speed control flexibility and so on.

@guineafowl21 - many thanks - you summarised my intentions completely. My unknown is the torque response in 2 or 4 pole connection. Once selected the speed will be via the inverter only.
Rob

 

[Jackbequick]

OK Rob, got it. Here 3 Phase is commonplace excepting outlying country areas served by single phase earth return for example. Three phase connection does increase property value or property 'desirability' if you like for people who need it, want it or could do with it...For others it wouldn't make a dent.

I'm surprised that three phase (or other polyphase) is uncommon in UK, insofar as it is the most useful way of transmission whether at 415V or at 66kV and in-between. I didn't find that polyphase deficit in France where (during 'la grande vente') I was connecting 'groupes' for the military (and Mairie) in rural areas for emergency situations.

If the designer of your machine used only two speeds for the purpose I wonder whether incrementally varying the frequency (presumably cutter speed) achieves much in better processing. It's a choice you've made nevertheless and you'll come to a conclusion on that in use. As you have someone who understands electrical matters more than I, it seems, perhaps (?) you may not be interested in the inverter and speed control information I was to send across this morning. My Regards

 

[Inspector]

Residential areas here don't have 3 phase available as a rule either. If it is it would be a second service to the property with its own meter.
Single phase is the norm and my place (2016/2017) has a 200 amp service. For a higher monthly fee I could have had 400amp.

Pete