Changing an induction motor voltage

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acmotor
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Changing an induction motor voltage

Post by acmotor » Thu, 18 Jun 2009, 18:46

Due credit and reference in publication. Image

A bit like the Faed Flux Finder !

At least you didn't pick me up about the exposed terminals on the 104V motor ! (I'll fix that too)

Point taken. (on this matter !) I shall lose the clamps for Mk2. Image

edit: it was a late night.
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Changing an induction motor voltage

Post by Johny » Thu, 18 Jun 2009, 19:06

acmotor wrote: Johny, it was only 12:58am in my part of the world. Don't tell me YOU were up reading it where you are Image
Ahh yes. I mistakenly assumed that - that - I don't know what I assumed. The posts are stamped that way (Eg. 2:58AM) when I read them the next day.
Anyway that makes more sense as I keep seeing posts that go up in the wee small hours.

After seeing Mal's book except I realised I have a 1944 book that my father left me that I'd sort of ignored. Full of great AC motor stuff. Including how the US switched from 25Hz to 60Hz power and somehow coped. (Didn't even know they had!)
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Changing an induction motor voltage

Post by acmotor » Thu, 18 Jun 2009, 19:21

The 2:58am would have been your local time at notice of post email.
The Forums run real time (Perth time Image ... and no daylight saving !! Image )   

Another good book ! The comment would be, it is only the development of '3 phase AC battery packs of variable voltage and frequency' Image that has allowed the best yet motor configuration to move into the vehicle traction world. (back off weber, it even shows a '2 phase' winding) (it is an old book !)Image

There have been considerable refinements over the years since that book, however the concepts are still very sound.
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Changing an induction motor voltage

Post by weber » Thu, 18 Jun 2009, 19:54

acmotor wrote: At least you didn't pick me up about the exposed terminals on the 104V motor !

But that was only for the duration of the photo, with everything powered off, so we could see how it was wired. Image
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Changing an induction motor voltage

Post by acmotor » Thu, 18 Jun 2009, 20:23

Actually, i did hang on to the terminals and run the VFD up to keep me awake. Got to ~60VAC an had to let go... too much excitement ! Image

Only as a control experiment investigating the safety issues mind you.
I guess arguing that the motor was only 104V nominal wouldn't wash ?
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Changing an induction motor voltage

Post by weber » Fri, 19 Jun 2009, 00:23

Johny wrote: Geez acmotor - you have 3 phase power too! (I'm sure I've seen a jealous emoticon somewhere).

You don't exactly need 3 phase power (or a 600 V battery) to run a VFD on the bench, although it's certainly preferable. Below are some photos showing a nasty hack, invented by some SI units who shall remain nameless, to make it run off 240 V single phase. It can probably be done to any 415 V 3-phase industrial VFD. It uses no additional components, only cable, crimp lugs, a copper link bar and heatshrink.

The trick is to turn part of the VFD's existing 3 phase rectifier (not needed in an EV), and its DC bus caps (definitely needed), into a half-wave voltage-doubler, giving 680 Vdc from 240 Vac. It relies on the fact that 415 V VFDs usually have a center-tap on the bus caps (because they stack two sets of 400 V caps to handle the max 800 V DC bus voltage).

You can only run the VFD up to maybe 1/3 of rated power with this setup, otherwise the seriously bad ripple current would overheat the bus caps. But unless it's a very small drive, you won't get that much power out of a 15 A 240 V socket anyway, without tripping the breaker.

Image

If your input rectifier does not have high-side SCRs for soft charging the caps, but has ordinary diodes, then it's a no-brainer. You connect Neutral to the bus cap center-tap (as faked in the above photo) and Active to any phase input, and of course Protective Earth to Protective Earth. But before doing this you'd better be darn sure they aren't SCRs, because if only one side of the bus caps gets charged, since the whole bus is loaded, the other caps will get reverse voltaged. And reverse voltage across electrolytics is very bad.

Image

The hack in the photo above is for a system with high-side SCRs. In addition to the Neutral going to the bus-cap center (not shown), it isolates the low-side diode in an unused phase and connects it in parallel with the high-side SCR of the Active phase. Schematic below.

You also need to provide an external soft charge resistor, not shown. The people in question used a 500 W 240 V floodlamp in series.

Image

Of course we would never do this to our brand-new drive, because it would void the warranty. And don't try this at home kiddies, unless you're a licenced electrician or electrical engineer, and you really know what you're doing, particularly since it involves voltages that can make you dead. And if you try it, and blow up your drive, don't say Uncle Weber didn't warn you.

Since it gives 680 Vdc (higher than the 590 Vdc you'd get from 415 V 3-phase), it may also occur to you that it could be used as a bad-boy charger for your bank of 200+ LiFePO4 cells, if you were desperate ... and certifiably insane. Although ... with a timer ...
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Changing an induction motor voltage

Post by coulomb » Fri, 19 Jun 2009, 00:42

I would also add that such a hack would surely have some sort of inrush protection, possibly wired in series with the mains lead. I'd imagine that they'd use large incandescent loads in series with the active lead, useful because of their nonlinear resistance verses current. After charging the capacitor bank, the lamps would cool down and offer reasonably low resistance.

In fact, it'd probably look something like this:

Image

Is that what those nameless units (or was it unitless names?) in long white coats did, Weber?

Edit: oops, I didn't see that part of the post. I was right: incandescent lamps!
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Changing an induction motor voltage

Post by Squiggles » Fri, 19 Jun 2009, 01:37

You guys related to Evel Knievel by any chance?

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Changing an induction motor voltage

Post by woody » Fri, 19 Jun 2009, 02:51

No wonder the yanks stay away from those dodgy SI units.
Would a safer alternative if you're friendly with the neighbours is to run extension leads from your nearest ones? Don't usually the hand out the same phase to every 3rd house... That way you'd get 3 phases, 3 neutrals, 3 earths...
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Changing an induction motor voltage

Post by Johny » Fri, 19 Jun 2009, 02:51

I really don't think that half wave rectified DC would be very nice for my caps - massive ripple current considering they are sized for 3 phase ripple. I plan on running my new motor (when it arrives) and the no-load current is likely to be around 14 Amps (yes I know it's on output side - still it'd be good to get it up to speed as well).

Check my blog weber and you'll see I run from single phase already, using spare transformers as auto-transformers. I've just scored 3 x 240 to 40 VAC 15 Amp transformers (on secondary) from work (free) so I can up the current to 15 Amps.

Real 3 phase would be very nice though. I have it at work but the extension lead came out at > $1,500,000.

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Changing an induction motor voltage

Post by acmotor » Fri, 19 Jun 2009, 03:31

... Or you could just use a 240V to 240V isolating transformer and stack the outputs to give 480V AC (VFD accepts 500VAC).
Connect to two 'line in' terminals and then the internal inrush system of the VFD (High side phase angle triggered SCR or PTC/Contactor) would all work as normal. No voided warranty.
Image
Ok, you need an isolating transformer. I just happened to have a number of 2kVA and 5KVA units (out of the skip) that were popular in the past.

My first charger for red suzi used this isolating transformer system for charging so I could charge from 240V.
It had an AC choke (inductor) (actually an old sodium vapour lamp ballast) in series with the VFD inputs to limit charging current to 2A.
Very crude but functional. Don't go out for the night !

A mod to the idea would be to use the brake resistor output to operate an end of charge dropout relay and have the braking voltage match the battery end of charge voltage. (no... don't go there)

BTW, wouldn't this system give you far less ripple as it is full wave rectified ?

Another system I used on the very first VFD (SCR type) I used in red suzi years ago, was a cap start, smaller cap run single to 3 phase convertor using a 3 phase motor. This is very common on farms (where red suzi started). Go google. It worked fine, but almost as primative as DC ! Image http://www.phasechanger.com/

http://www.metalwebnews.com/howto/phase ... erter.html
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Changing an induction motor voltage

Post by acmotor » Fri, 19 Jun 2009, 03:41

Also, on a brighter note.
I bought this today for Mk2 dyno.
It is new, old stock and $50 as it is not Oz voltage (at least on the label !)

ImageImage

I was good and put a piece of cardboard down before putting it on the loungeroom floor. Image
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Post by weber » Fri, 19 Jun 2009, 04:26

Johny wrote: I really don't think that half wave rectified DC would be very nice for my caps - massive ripple current considering they are sized for 3 phase ripple.
Yes. I did mention that above. And, as you imply, the component of ripple current due to the input rectifiers is roughly proportional to the input current (reactive load current not relevant). But it only causes heating of the caps, and they have a reasonable thermal mass, and in the Control Techniques case a temperature sensor and fan, so short bursts shouldn't be a problem.

Yes, they were aware of the auto-transformer-like options, and would have used them if they'd had a free multi-kVA transformer lying around, like you lucky chaps.
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Changing an induction motor voltage

Post by coulomb » Fri, 19 Jun 2009, 05:05

My next holidays out to the wild West I think I'll go dumpster diving with my pal acmotor Image

Edit: I'd better bring a forklift as well, if I'm likely to find 5 kVA transformers in those skips. (Must be solid skips!!)
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Changing an induction motor voltage

Post by acmotor » Fri, 19 Jun 2009, 05:11

We have an unofficial agreement with departments to email around the workshops when it is seagul time. Call it re-cycling. Mostly junk mind you. Problem is storing the stuff until the right project comes along.
Worst is when you know what things are, how much they cost (in 1851?) and what they could be used for (one day). Image
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Changing an induction motor voltage

Post by coulomb » Fri, 19 Jun 2009, 05:13

acmotor wrote: ... Or you could just use a 240V to 240V isolating transformer ... Connect to two 'line in' terminals and then the internal inrush system of the VFD (High side phase angle triggered SCR or PTC/Contactor) would all work as normal. No voided warranty.

I'm not so sure actually. I have a suspicion that they wait for the difference between phases to be a certain percentage of peak amplitude to start firing the SCRs. So you'd need at minimum different waveforms on all three line inputs, possibly also needing them to be 120° (edit: was 60°) apart.

Have you tried this? If it worked, what model of controller was it?
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Changing an induction motor voltage

Post by coulomb » Fri, 19 Jun 2009, 05:28

acmotor wrote: We have an unofficial agreement with departments to email around the workshops...

Workshops! Ah, they were the days. I used to work for Telecom Workshops at Bulimba, Brisbane, as an electrical engineer class 2. Now the whole workshops are gone - we had numerically controlled punch presses, milling machines (one a nice Swiss auto), there was cords and coils, repair and test (with ATE), dial reconditioning, printed circuit board manufacture and loading, metalwork, paint. Now if you use Google maps you'll find it's all apartments. "Buy, don't build" was the catch cry of the day. You could get some decent foreigners through that place.

Ok, back to the thread. Nice motor!
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Changing an induction motor voltage

Post by acmotor » Fri, 19 Jun 2009, 06:12

120° on this side of the country.

There is no neutral connected to a VFD, only 3 actives.
A Danfoss VLT5042,5001,5004,2815,2840 et.al. work with the above transformer input. (tried and tested) They are not SCR highsiders though.

You can simply lift one phase input wire for exactly the same input affect as the transformer input described earlier.
VFD just thinks there is one phase connection lost, but only knows it (or worries) by DC bus ripple if there is enough motor load.

On <3kW systems, it is common to run 240V 3 phase VFDs off 240V single phase and run the motor in delta. (remember most <3kW motors are ~240V delta)

But haven't tried any SCR highsiders. I will when I get a chance. It is likely your suspicion is right.

The way around that would be to use your own single phase bridge (<15A after all) and your own inrush limit that you have ? to build anyway, to feed to the DC bus terminals. Still a better ripple option. Image
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Changing an induction motor voltage

Post by Squiggles » Fri, 19 Jun 2009, 20:15

Guys,
Can you list the factors that limit power output of a motor.
Assume an unlimited power source.

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Post by coulomb » Fri, 19 Jun 2009, 20:30

1. Heat dissipation. This includes melting winding wires and insulation breakdown through over-temperature.
2. Breakdown torque.
3. Winding insulation breakdown due to absolute voltage difference.
4. Winding insulation breakdown due to voltage rate of change (dV/dt) value.
5. Speed limit (for bearing life, rotor disintegration).

Have I missed any?
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Changing an induction motor voltage

Post by woody » Fri, 19 Jun 2009, 20:56

coulomb wrote: 1. Heat dissipation. This includes melting winding wires and insulation breakdown through over-temperature.
2. Breakdown torque.
3. Winding insulation breakdown due to absolute voltage difference.
4. Winding insulation breakdown due to voltage rate of change (dV/dt) value.
5. Speed limit (for bearing life, rotor disintegration).

Have I missed any?
Assuming squiggles "Power source" means "battery", there are a few complexities added in by the controller:

The maximum power point of an ACIM is the field weakening point, which is the point at which the controller voltage can't meet the V/Freq. ratio that you are running the motor at. i.e. a 400V ACIM on a 50Hz 200V motor will have its peak power at 100Hz when in needs 400V, therefore:

6. The Controller maximum voltage
7. Motor nominal voltage

The motor may have a low V/Hz (e.g. 1) and the controller may have a low maximum frequency (e.g. 132). Therefore the controller would only be able to get to 132 Hz and need 132 volts which is well before the maximum power point of the motor.

8. Controller maximum frequency

The maximum torque of an ACIM requires a motor current a bit more than breakdown ratio (T[max]/T[n]) * nominal current, i.e. if the ratio is 3 then current required is about 4 times nominal. If the current required at peak motor current is not available from the controller, the maximum torque will be limited. The nominal current goes up if the motor is rewound for a lower nominal voltage, i.e. half voltage @ double current = same power. Therefore:

9. Controller current limit

The controller may have a software power limit due to cooling concerns or pricing structures. Many controllers also have high overload only available for a short time (e.g. 1 minute in every 10). Controllers also don't like to get hot and limit the power if they do.

10. Controller overload duty cycle
11. Controller power (don't know of any controllers which enforce this)
12. Controller temperature / cooling.

And for completeness (ignoring your suggestion that the power source is infinite):

13. Battery voltage
14. Battery equivalent series resistance => voltage drop under load
15. Battery maximum current output ("C" rating)

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Changing an induction motor voltage

Post by Richo » Fri, 19 Jun 2009, 21:19

The terminal velocity of ali in a rotor is about 500m/s.
So a rotor with radius of 0.1m would be over 40kRPM.
So is more likely to disintergrate from a bad balance and hit the field laminations.

Don't forget the controller to a lesser extent or bad choice.
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Post by weber » Fri, 19 Jun 2009, 22:02

Richo wrote: The terminal velocity of ali in a rotor is about 500m/s.
So a rotor with radius of 0.1m would be over 40kRPM.
So is more likely to disintergrate from a bad balance and hit the field laminations.

My understanding is that even if it is perfectly balanced and only ever goes up to 6000 rpm, if it is cycled to that speed and back to zero enough times, the aluminium end rings will fail due to fatigue.
http://en.wikipedia.org/wiki/Fatigue_(material)
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Post by weber » Fri, 19 Jun 2009, 22:19

But I should have looked closer at that S-N curve for brittle aluminium in that Wikipedia article. It suggests that even at 1/2 the terminal rpm (1/4 the terminal stress) it should last around 10 million stress cycles. That's 1 stress cycle per minute for about 20 years. So you're right. Balance is way more important.
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Changing an induction motor voltage

Post by Squiggles » Fri, 19 Jun 2009, 23:23

You guys make it sound scary Image
I was thinking in simple terms like saturation of magnetic circuit, current limit of windings...then you just blow me away!


Edit: To avoid another topic hijack I have started this viewtopic.php?p=14190&t=1260#p14190
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