Changing an induction motor voltage

Post by **acmotor** » Thu, 11 Jun 2009, 18:12

Herein presented is a king sized fiddle with internal wiring of a (small) 3 phase induction motor to change its operating voltage without doing a rewind.

This is a work in progress…..
Warning – not all terms are technically correct. I am certain someone will put me straight.

Here we go…

Take one small 4 pole (1330 ! RPM) 3 phase induction motor. Nominally 415V 50Hz connected in star. (Out of the skip bin. Working perfectly. Right price !)

Original rating 0.37kW ( 1/2hp), now make it deliver 1.5kW continuous / 4.5kW peak at 6000 RPM (less some increased losses)
How ?
Open it up. Identify the coils / wiring and re-arrange so emotor is wired for 104V. Then run it at v/f ratio of 2, up to approx 415V.
Put it another way…. Make it into a 200Hz emotor.
Ok, giving the insulation a hard time maybe !

It will make it non ‘off the shelf industrial’ for the purists but it may address our present v/f limitation on industrial AC induction motors without doing a rewind $$.

The victim...

The emotor and full coffee cup. Stator in emotor body only shown. Rotor, bearings and end caps already removed.

Nameplate 415V in star. ( It would also connect as 240V in delta.)

Existing wiring after I cut some of the tie down cords to free the wiring.

Note the interphase insulating paper, normally expected for VFD work. Tick.

Identifying the stator segments pole pitch.
Here is the coffee inspired sketch ! (actually more than one cup required). Just for me to follow you understand. You guys all know this gear.
24 segment so pitch is 6 segments per pole. 4 poles (2 pole pairs) N.S.N.S.

Terminal block.

Shorting bar goes across top 3 terminals for star and vertically over each pair for delta. Normal 3PIM connections.

Freeing up the inter coil wiring that has high temp woven fibreglass sleeving on it.

Starting the wiring cuts. Fortunately this emotor has minimal varnish splashed over the wiring so it is easy and non destructive to pull the wiring free and no damage to the enamel on the copper wiring.

There are 4 coils per phase ( in series) x 3 phases = 12 coils in total. The coils are quite separate items.
The plan is to re-connect these series coils all in parallel (observing phase). i.e. 1/4 voltage (415/4 ~ 104V in star) ( 60V in delta)… could also arrange for 1/2 voltage, but then ….go for broke ! (My Danfoss says it will run a motor down to 50V)
I played with this using clip leads and a DC power supply. Seems to make sense so far.

Screwdriver probes flux in each segment to locate coil positions when 12V DC passed through each coil. I should use a P.magnet to confirm polarity as well.

This all leads to thoughts of modifying Red Suzi's emotor.

Ok, so tell me it has all been done before. Tough.

Anyone interested in this ? Shall I post any more of this experiment ?

Post by **bga** » Thu, 11 Jun 2009, 18:39

Cool

Exactly what I was thinking about.

It may have been done before, but nobody has lived to tell the tale.
Well done.

I wonder how this would work on a 11 or 15KW 160 frame?
(my fear being completely resin glooped windings that are impossible to pick at)

Presumably. 4 pole is likely to be all series or series-parallel, allowing 1/2 or 1/4 voltage to be achieved without too much surgery.

100V sounds good, except that the current may be a bit high on a 11/15KW? - perhaps 200V is better (thinks 480V battery)

I have a set of Powerex 1200V 300Amp (average) IGBTs for my experiment.

Cheers

Post by **Johny** » Thu, 11 Jun 2009, 18:44

Yes - keep going. Good stuff. I thought the windings were all varnished and oven hardened though. Seems not.

Post by **a4x4kiwi** » Thu, 11 Jun 2009, 23:12

Isn't it amazing what gets done when the forums are down for a few days. perhaps they should die more often

Tuarn, keep up the great effort! I am tempted to make a trip to the used equipment place to pick up another motor to give it a shot. Now, to figure out what the new name plate data should be...

Post by **Johny** » Thu, 11 Jun 2009, 23:38

Rather than connect all the coils for a particular phase in parallel. How about series/parallel (2 in series then parallel them) so you have 207V Star/120V Delta. That gives you a 5050 RPM motor in Delta at 173Hz - a lot closer to 'normal' total drive-line ratio requirements.
It might also be simpler to do.

Does the Danfoss have some upper frequency limitation? I thought it was 132Hz???

Post by **Tritium_James** » Fri, 12 Jun 2009, 00:14

But then you have to run in Delta. You're more likely to have circulating currents around the motor if your phases are not EXACTLY matched to each other, which cause losses, heating, etc. Running in Star is better if you have a choice.

Post by **acmotor** » Fri, 12 Jun 2009, 00:15

There are two selectable ranges on my Danfoss. 0-132Hz and 0-1000Hz
Some models have 0-400Hz single range. I think the 132Hz is to stop people over-revving their motors too easily ! (there probably is a real reason for the number 132, I haven't given it any thought)

Johny, push 'till it breakes mate ! If it can survive the 104V in star then I can back off ! (or try the 60V in delta

)

Post by **acmotor** » Fri, 12 Jun 2009, 00:29

TJ, talk me through these circulating currents and why star is better than delta for this ? Is it because it finds its own neutral point ?

When running off a VFD (as opposed to DOL) wouldn't phase voltages be perfectly matched anyway since they come from the same source ?

Post by **Tritium_James** » Fri, 12 Jun 2009, 01:16

The output voltages from the VFD will be perfectly matched, but the back-EMF on each motor phase may not be. So say A-B is developing 100V across it, so is B-C, but C-A is at 102V. You'll have 2V across whatever the A-B-C winding resistance is (probably not much!) which will give you potentially quite a high current flowing.

You'll get a mismatch on the phases if they're not all exactly the same length, or they're in a slightly different position in the slot, that sort of thing. Depending on the degree of mismatch, and the resistance of the windings, it may not be an issue in the slightest.

I just know in the solarcar motors we usually see, they are very low resistance, so it doesn't take much voltage offset to get quite high currents flowing. They always run in star.

Post by **coulomb** » Fri, 12 Jun 2009, 01:56

acmotor wrote:interested in this ? Shall I post any more of this experiment ?

Yes, yes, yes, YES!

This is exactly the next thing for White Suzi's 7.5 kW motor. I think I'll attempt to overvoltage it first, then get the bearings and balancing done.

I'll want to 1/3 voltage it. I have 36 slots, where you have 24.

The issue I have is finding the places where the wire "jumps to a new slot". In your case, there seems to be a big flashing pointer in the form of the tubing; is it likely that all motors will have it?

Also, I didn't follow what the screwdriver was doing. Surely you can see what slot the wire goes down? Is it to find the other slots that are in series?

I suspect I won't have the tubing, and hence it will be hard to find out which one of about 62 wires in the slot changes to another slot. I'm looking for some shortcut, like a square wave generator at one end of the winding, and looking for the wire with the least output (furthest from the square wave generator). Is there a commercial gizmo to do this? Or do I just get real and do one myself?

Thanks for starting this thread!

Post by **acmotor** » Fri, 12 Jun 2009, 02:25

Thanks TJ, star it is then.

Coulomb, slots have nothing to do with the number of poles. It is just a suitable number for fabrication and dealing with air gaps and eddy currents and also fitting in the coil wires AFAIK.
This little motor has one coil per slot and one slot per coil i.e. 3 phase 4 pole must have 24 slots minimum ??? (or share slots ?)
More slots means the coils are distributed over additional slots (possibly making them harder to identify ?) Slot numbers would increase in multiples of 12. Hey, but then what do I know ?

The screwdriver was just my way of learning what was going on. It made quite an effective flux sensor !

edit: spelling

Post by **a4x4kiwi** » Fri, 12 Jun 2009, 02:36

You could use a small compass to identify the N/S poles.

Post by **weber** » Fri, 12 Jun 2009, 05:10

ACmotor, you the man! This is insanely great. I don't know if it has been done before, but publication is everything. And you've done it so beautifully with clear photos and explanations.

This (1/4 voltage rewire) is what I suggested Coulomb should do to a 7.5 kW 4-pole 132-frame 415 V star motor, for his Suzuki Swift. Put it in delta to get down to 240 V, then do the 4 times pole rewiring, taking it down to 60 V, but then feed it only 200 V max from a 200 V 37/45 kW VFD with a 312 V battery. So overall it's being overvolted by a factor of 200/60 = 3.33, and max power will be at 5000 rpm minus slip.

But he's got this crazy idea of rewiring it for 1/3 voltage. He wants to change slot-pairs (not poles) from series to parallel, which I think requires him to find 30 correct places to cut, not merely the 9 correct places of your experiment.

However, what I think we have found, in the second-hand 4-pole motor (7.5 kW 132-frame 415 V delta) that Coulomb bought off eBay, is that there are only two actual pole windings per phase (diametrically opposed and of the same polarity), with the other two poles per phase being consequent poles (automatically halfway between wound poles and of opposite polarity). It seems to match this 4-pole winding diagram.

Here's a 2-pole winding diagram for comparison.

Is there any difference?

By the way, this looks like a good tutorial on AC motors:
http://www.ibiblio.org/kuphaldt/electri ... AC_13.html

Post by **Richo** » Fri, 12 Jun 2009, 05:39

Yeah I suppose I should have looked at my motor before I had it re-wound to 220V with class H.
On the plus side I could change the connections for 200kW

Here's a picture of a 1.1kW 4-pole motor.
The windings are all gooped in so would be much harder work with.
There is something going on in the circled area.
Probably just some wire holding the windings together.

Post by **acmotor** » Fri, 12 Jun 2009, 06:32

weber,

Both those look like 2 pole diagrams. Clearly only 2 poles per phase are to be seen.
I was thinking that a 36 slot motor needs to be 2 pole if it has a 3 slot coil spread (12 slot coil span called 'phase belt' so I see).

A 4 pole 36 slot would need to have a 9 slot phase belt with adjacent phase coil overlaps.

Back to my emotor.

Post by **weber** » Fri, 12 Jun 2009, 07:25

acmotor wrote: weber,
Both those look like 2 pole diagrams. Clearly only 2 poles per phase are to be seen.

Only 2 windings per phase are to be seen. You can have 4-pole with only two windings. I thought someone explained about consequent poles on this forum before. Maybe Richo? The original context of that first image (as well as its filename) make it clear it is intended as a 4-pole. But unfortunately it doesn't show how the two windings for the same phase are connected up. To be a 4-pole, one winding of each pair has to have its connections swapped relative to how they are in the 2-pole diagram in the second image.

Some Google Book sections on Consquent Poles.
Practical electric motor handbook, by Irving M. Gottlieb
Transformers and Motors, by George Patrick Shultz

Your motor may well have 4 windings per phase, but it seems that other 4-pole motors do not, and so are not amenable to a "simple" 4 times rewire.
[Edit: "4 poles" -> "4 pole motors" for clarification]

Post by **coulomb** » Fri, 12 Jun 2009, 13:55

Ok, so I need to brush up on consequent poles; I'll do that soon. Thanks for the links.

But to quarter voltage the motor (if wound per Weber's recent coloured diagram)... wouldn't that require finding a centre tap? Just paralleling the two windings per pole would only achieve a voltage halving, right?

Post by **weber** » Fri, 12 Jun 2009, 17:16

coulomb wrote:But to quarter voltage the motor (if wound per Weber's recent coloured diagram)...

The diagram comes from "zubbly", here
http://www.greenbits.com/User+Pages/zub ... st+edition

wouldn't that require finding a centre tap?

Yes, and I don't consider that feasible. If you end up with different numbers of turns in parallelled windings you're screwed, by circulating currents. And if the turn-count is odd, I note that there's no such thing as a half a turn (the circuit has to be completed somewhere). And in any case one half of the pole winding will have a different diameter to the other half because they are concentric which will cause harmonic circulating currents. I believe this problem applies to your coil (slot-pair) parallelling proposal for achieving a 1/3 rewire.

Just paralleling the two windings per pole would only achieve a voltage halving, right?

Right. My recent suggestion of a 4 times rewire for your project (which I believe acmotor suggested in general a long time ago) was based on the assumption that it would have a physical winding for each pole. It may take some careful wire-following to be certain that your eBay 4-pole motor uses consequent poles. I'm just going on the apparent coil spans I saw. I wish they would use different coloured enamelled wire for the 3 phases to make life easier for us hackers.

Post by **woody** » Fri, 12 Jun 2009, 19:30

weber wrote:
The diagram comes from "zubbly", here
http://www.greenbits.com/User+Pages/zub ... st+edition

Hey, that's hot, he has the mythical 12 wire motor wiring on the first page.

Post by **acmotor** » Fri, 12 Jun 2009, 19:50

Are yes, consequent poles, seems so far back now. AC motors 101. Thanks weber.

But weren't there negatives to consequent pole design e.g. only one coil per pole pair (less flux / torque) and a larger dependence on the magnetic path through the stator iron (also bad news for HF motor design) ?

This is what the two speed motors suffer from. Bad power to weight ?
i.e. large stator iron and limited torque for size.

TJ, where are you ? You'd be up to speed on this.

edit: consequent poles are less issue with PMs ? although I though CSIRO Halback array was to address that efficiency. Digression - sorry.

Post by **bga** » Fri, 12 Jun 2009, 20:04

weber wrote: ACmotor, you the man! This is insanely great. I don't know if it has been done before, but publication is everything. And you've done it so beautifully with clear photos and explanations.
...
However, what I think we have found, in the second-hand 4-pole motor (7.5 kW 132-frame 415 V delta) that Coulomb bought off eBay, is that there are only two actual pole windings per phase (diametrically opposed and of the same polarity), with the other two poles per phase being consequent poles (automatically halfway between wound poles and of opposite polarity). It seems to match this 4-pole winding diagram.

Here's a 2-pole winding diagram for comparison.

Is there any difference?

By the way, this looks like a good tutorial on AC motors:
http://www.ibiblio.org/kuphaldt/electri ... AC_13.html

Only a reversal of polarity of one of the coils on each phase, not shown. - My guess.

A giant (or purpose built ~ Tritium?) drive would be needed to handle the much larger motor currents, so 1/4 voltage may be impractical from that point of view.

This would make a dual speed 1500/3000 motor easy to do, so it must have been done before. The magnetic paths would be longer in the 2 pole configuration ao the performance would be altered.

At the expense of making the controller more tricky, it may be possible to double the output H-bridges and switch electronically between 2 and 4 pole as an alternative to a big contactor set.

There's always a rewind with multifilar windings to keep the wire manageable and the eddy current losses down and would be good for at least a weekend's entertainment.

[edit: make more sense]

Post by **Tritium_James** » Fri, 12 Jun 2009, 20:19

acmotor wrote:TJ, where are you ? You'd be up to speed on this.

Well I thought I was, but then I read this http://www.amazon.com/Induction-Machine ... 0849300045 and realised that I know nothing.

Post by **coulomb** » Fri, 12 Jun 2009, 21:18

This is how I was going to "third voltage" my motor:

in other words (correct me if I get the nomenclature wrong), parallel the three windings (winding pairs?) per group.

But I have no idea how well this would work with consequent poles, voltages not being identical, etc.

I suppose I could run it in star mode, since 1/3 of 690 is 230, and with fresh batteries I can generate over 440 VAC from a 624V nominal pack. That's about enough to overvoltage it 2x; with 3x overcurrent, I could get close to the original power of 50 kW (7.5 x 2 * 3 = 45 kW).

Weber has pointed out that this requires finding 24 new "junction points" in the motor, which either have to be brought out separately (to what?!

) or joined to other wires (which all takes up space).

A lower voltage pack (~ 320 VDC) would be nice, from the point of view of less battery connections, BMS, hazard (arguably), contactors, etc etc. So other configurations are worth looking into as well.

This motor is cheap enough to attempt something drastic, yet big enough to actually use in the unlikely event that all this works

Post by **acmotor** » Fri, 12 Jun 2009, 21:48

I know what you were thinking but the diagram has passed its use by date for clarity ! You meant the inner COIL, middle coil and outer coil all in parallel, yes ? Provided the coils are not arranged as a tapered phase belt then there may be different numbers of turns from each coil in each slot causing issues with the arrangement ?

Looks OK to me, but then I just looked at JT's book and in the words of Schultz, I know nutting !

Maybe the blue electrons will not like the green wire patch near terminal 12 ?

Post by **acmotor** » Sat, 13 Jun 2009, 08:03