AC motors, multipoles, torque

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weber
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Post by weber »

Johny wrote:
woody wrote:Also: please note that ABB will not cover warranty if motor is run at 100Hz as intended by customer. <- Harsh but fair
Hmmm. 100Hz is no higher speed than a 2 pole. Too harsh! Image Image

I agree and I expect that's a mistake. I suspect the 4-pole quote was done by editing the 2-pole quote. For the 2-pole, I mentioned wanting it balanced for 6000 rpm and I assume they translated that to 100 Hz and forgot it would be 200 Hz for the 4-pole.

It's fair for the 2-pole, but get back to Dave Gayner to have it removed or changed to 200 Hz for the 4-pole. But lets face it, that aint the only thing we plan to do that will violate the warranty. Image

Many thanks Johny (via email from Woody), for this interesting page on insulation temperature rating and lifetime.
http://tristate.apogee.net/mnd/mfnrins.asp
It helped us decide not to bother about class H insulation.
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Post by acmotor »

Sorry to interupt the Hz and RPM discussion but I was wondereing what on earth makes people think that 3 thermistors would be required for a motor running off a VFD ?
Voltages and currents are well balanced and the windings (ends, where the thermistor is located) are in good wrapped, overlayed, thermal contact.
A phase loss DOL would be the situation that 3 thermistors MAY be useful (if there was electronics to drop out the supply), but this is not required with a VFD.

The VFD also calculates the thermal load on the motor reasonably well (after all it is supplying the current) and will act upon your settings as required with or without a thermistor.

One is enough ! Image
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Post by woody »

weber wrote:
I still don't understand why the most powerful 2-pole in a given frame size is typically one size more powerful than the most powerful 4-pole.
In simple theory, should be double. IE a 2 pole version of the 15 kW 4 pole ABB 132-316 would be 30kW...   

here's a few ideas to start, not necessarily mutually exclusive why that doesn't happen:

1 More losses at higher RPM:
a) increased frequency of magnetic cycles applied to the stator result in higher losses
b) fan losses
2 2 pole is a less ideal design
a) windings have a greater spread for sinusoidal distribution therefore less room in the middle slots for enough windings.
3 continuous cooling limitation of the frame - can't dissipate twice the power

please add/shoot down...

To counteract these, the manufacturer can reduce V/F which would reduce over-magnetisation and therefore losses, but also peak torque and power. This explains why the peak torque is down, as well as the nominal.

Some of these apply to 4 poles at higher frequency, some don't.
As someone who's looking to buy a 4 pole, I'm hoping that most of it is problems with 2 pole windings...
ACP, Tesla, Azure all seem to use 4 pole, even at 12000rpm/450hz

cheers,
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Post by acmotor »

I'll just sit back and wait for weber to flame that ! Image ......




edit:
BTW, prius uses 8 pole
Last edited by acmotor on Sun, 05 Jul 2009, 09:35, edited 1 time in total.
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Post by coulomb »

woody wrote: In simple theory, should be double. IE a 2 pole version of the 15 kW 4 pole ABB 132-316 would be 30kW...   

Is that assuming that all 4-pole motors use consequent poles?

Otherwise, I suppose that since current capability is proportional to the wire cross section, and the volume of the windings is proportional to the number of turns, then the number of amp-turns you can fit into a given frame (therefore a given total usable slot area) would be about the same. Amp-turns are proportional to torque, is that right?
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Post by woody »

3 thermistors - I was thinking since a4x4kiwis motor got hot I might need some.
1 is enough as you say.
I'm probably going to lie to the VFD, so the calculated protection may not be ideal...
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Post by coulomb »

acmotor wrote:BTW, prius uses 8 pole

But surely PM synchronous is a special case. Rather different to induction motors.
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Post by woody »

coulomb wrote:
woody wrote: In simple theory, should be double. IE a 2 pole version of the 15 kW 4 pole ABB 132-316 would be 30kW...   

Is that assuming that all 4-pole motors use consequent poles?

Otherwise, I suppose that since current capability is proportional to the wire cross section, and the volume of the windings is proportional to the number of turns, then the number of amp-turns you can fit into a given frame (therefore a given total usable slot area) would be about the same. Amp-turns are proportional to torque, is that right?
I was going super-simple - same torque - double speed - ignoring the reality.
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Post by acmotor »

Mal's motor only gets hot IMHO when he ventures past pullout. I have not seen heat problems (this using computer fans now) once I understood what was going on.

BTW, on that topic, thermistors as used in emotors are very non linear and give no warning of temperature rise, just that a temp has been reached. This is bad news for EV emotor that may be at 150° in the winding and you are about to floor it (likely that the next burst will cause temp rise that thermistor can only tell you has happened... too late). i.e. when overrated 3x or 10x (me preference) you need trend information about temperature so you don't floor it since you have no I^2t capacity left !

The prius example was for the topic of pole numbers only.
PM sync. motors are not all that special. In fact many VFDs like my Danfoss FC302 will drive both 3PIM and PM motors. The emotor market place has many offerings of PM sync as they can offer some cruise efficiency (no magnetising current). Similar torque curve but with lower peak and flatter torque below pullout. But considerably more expensive from a quick look around. But that is off topic !

Woody, half torque double speed, same power ?
The inconsequentialist pole idea may be the difference. 2 pole motor can't have one ? Image
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Post by woody »

woody wrote: ~$800 Special winding insulation for VFD supply (405)
www.kilowattclassroom.com wrote:*Inverter Duty Motors - Initially standard AC motors were employed on inverter drives. Most motor manufacturers
now offer Inverter Duty Motors which provide improved performance and reliability when used in Variable Frequency
Applications. These special motors have insulation designed to withstand the steep-wave-front voltage impressed by
the VFD waveform, and are redesigned to run smoother and cooler on inverter power supplies.
Is this ABB option 405 it's talking about?
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Post by woody »

woody wrote: Is this ABB option 405 it's talking about?


ABB-NEMA-Motors PDF says:
ABB wrote: 405 Special winding insulation for frequency converter supply.
Required for operating voltages 500 V and higher.
So maybe we don't need it for 400V AC EV...
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Post by Johny »

[quote="acmotor"]BTW, on that topic, thermistors as used in emotors are very non linear and give no warning of temperature rise, just that a temp Lenze 9300 series (and I assume many other VFDs) actually have 2 temperature settings that you enter the PTC thermistor values for calibration.

It then uses the winding temperature actively in controlling the motor and also ramps back torque as it approaches a maximum temperature parameter. They (Lenze) are quite specific about the thermistor type if it is to be used in the control loop.

Apparently temperature is way more important if you are using sensorless control as the winding resistance varies considerably with heat and is detrimental to the VFDs internal motor model.

Regardless of all that, you can have the motor temperature up on a display and I would imagine it won't take much driving experience to know when you should glance at it.

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Post by weber »

acmotor wrote: Sorry to interupt the Hz and RPM discussion but I was wondereing what on earth makes people think that 3 thermistors would be required for a motor running off a VFD ?
Voltages and currents are well balanced and the windings (ends, where the thermistor is located) are in good wrapped, overlayed, thermal contact.
A phase loss DOL would be the situation that 3 thermistors MAY be useful (if there was electronics to drop out the supply), but this is not required with a VFD.

The VFD also calculates the thermal load on the motor reasonably well (after all it is supplying the current) and will act upon your settings as required with or without a thermistor.

One is enough ! Image

I totally agree. It's just the standard offering from ABB. But I wouldn't expect there to be any significant difference in price between one and three. The thermistors themselves only cost cents and they are simply wired in series.

However if you want max performance I think you need a thermistor or three, as the VFD's thermal calculation would need to be conservative to allow for the information it _doesn't_ have such as degree of fan cooling and ambient temperature.
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Post by weber »

woody wrote: In simple theory, should be double. IE a 2 pole version of the 15 kW 4 pole ABB 132-316 would be 30kW...

My default assumption is: A given amount of steel, a given amount of copper and a given frequency should put an upper limit on the rate of conversion of energy (the power). A 2-pole seems to approach that upper limit more closely (at least in a 3PIM from ABB), despite the extra wasted copper in the end-turns.
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Post by Tritium_James »

Re thermistors, you need to check what sort they are! Quite often, and probably the sort ACmotor mentioned, are the PTC type that change resistance abruptly at a certain trigger point. These are almost useless for a temperature measurement, but are good for a go/nogo indication for a PLC or some relay type controller.

The other sort quite often found in motors are the NTC type, which while still non-linear, usually have a much more gradual change in resistance and can be used to measure temperature quite well.

If you actually want to get a good measurement, I've also seen a platinum (Pt100 or Pt1000) element embedded in the motor. Usually with a 3-wire connection so you can null out wiring resistances and get an accurate measurement.

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Post by acmotor »

ABB offer a thermistor, PTC type selected to motor max operating temp 130 or 150 ° +-5° at 3kohm nominal switching point. Not useful for actual temperature measurement ? Would not suit Lenze 9300 function ?

Do they or others offer any different types of sensor ?

I like the sound of a more linear sensor.
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Post by coulomb »

Farnell have heaps of them.

Surface mount, already wired for a motor, all kinds.
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Post by Squiggles »

Odd, but when I think of temperature sensing I think PT100 or LM35. Due of course to familiarity. I am sure there are others, these are pretty simple to use though, and adequately reliable.

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Post by Tritium_James »

There's a 3-wire Pt100 embedded in my 11kW 2-pole IM, but that motor has been rewound so I'm not sure if the sensor was in there from the factory as well, or added as an extra during the rewind process.

Squiggles, an LM35 is not so good for motor temp sensing, if embedded in the guts of the motor it can fairly easily go past the max temperature rating of the sensor (150°C) if you're pushing things a bit with the motor.

ACMotor, this one here http://au.farnell.com/vishay-bc-compone ... dp/1187036 is what's in the stator of the CSIRO solarcar motors, but they only run at lower temperatures - the epoxy the stator windings are potted in (no iron, remember) goes soft and warps at about 120°, so nobody runs them over about 100.

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Post by acmotor »

Nice. Thanks TJ.
Where are the thermistors placed ? In the loop ends of the winding ?
i.e. where is the hottest part of the winding ?

How good is the electrical insulation around the thermistor ?
Danfoss warn that the VFD control module is no longer ELV galvanically isolated if there is a thermistor connected since they are not in control of a thermistor insulation failure.

BTW, have you fired your re-wound 2 pole in anger yet ? Image

I guess that people would be disappointed in the CSIRO solarcar motor if they HAD to run it at more than 100°C !! Image
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Post by acmotor »

Just thinking though....

These thermistors are rated to 150°C the same as LM35A.
LM35A will give 10mV/°C linear and acurate to +-0.5°C.
The thermistor is rather vague and non linear unless individually calibrated.
ABB normally fit 120 or 130°C thermistors ?

I wonder if the LM35 would be happy with the electromagnetic field in the winding ? Image
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Post by Johny »

Lenze specify a thermistor called KTY83-110 - a silicon temperature sensor. You can google datasheets on it. It is 5% accurate from -55 to +130 degrees C with increased accuracy when individually calibrated (which is encouraged by Lenze).

"Temperature detection
For motors with temperature sensing (KTY83-110), the controller can consider temperature changes in its motor model. The accuracy and stability of the vector control is thus considerably improved."

I'm not suggesting that anyone use these, just that there appears to be an advantage in the VFD using temperature data - and not just for protection.






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Post by Johny »

acmotor wrote:Danfoss warn that the VFD control module is no longer ELV galvanically isolated if there is a thermistor connected since they are not in control of a thermistor insulation failure.
A real concern - that though has occurred to me. That could be a nasty, expensive leak/flash-over.
The VFD manufacturers tend to have dedicated inputs for thermal sensors so you would hope that they provide protection. Trouble is if their protection device kicks in, it probably snuffs it as well.
For DIY thermal sensors though - lots of care required.

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Post by Tritium_James »

Which is exactly why the motor temperature (and hall effect position sensors, for BLDC motors) are isolated inputs on our controllers... Image

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Post by Richo »

Pt100(0) would be my preference.
Too much maths in the micro to calc thermistors.
But nowadays the published specs are quite good and don't really need individual calibration for thermistors.

An external temp switch would be ok as a backup "idiot light" on the dash.
So the short answer is NO but the long answer is YES.
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