Industrial AC Controllers + Rated Torque/Power

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Post by Johny »

I would really love to see a torque graph for a typical type general purpose motor pushing with an adequately EV sized VFD.
We know we get pullout torque at rated speed and higher torque at lower speed but can we calculate a realistic graph of expected torque and motor current for a general purpose 11kW motor (any would do).
Say going up from 50RPM in 50RPM steps (or bigger as we get highr) to around 4000RPM.
Some of these could be near guesses but your empirical data would help a lot.
Eg.
RPM   DC current(A) Motor current(A) Torque(NM)
50    ?             ?               ?
100   ?             ?               ?
200   ?             ?               ?
500   ?             ?               ?
...
1500 ?             22               72
3000 ?             ?               ?

I'm asking to be spoon fed I know but while I understand broadly where highest torque and lowest torque live I lack the knowledge to properly pin this down.

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Post by woody »

Hi Johny,

as you can see I'm still working it out, but here goes:
(ignoring slip).

At 50Hz, your motor should be able to get the breakdown/stall/max torque (different vendors call it different things), usually about 3 x the nominal torque.

So for the high performance ABB 11kW, this is 73 x 2.5 = 182.5 Nm.
Motor current should be roughly proportional to Torque, so 21 x 2.5 = 52.5A

At different speeds, this torque limit scales inversely proportional i.e. at 1000 rpm which is 2/3 of Nominal, you can get 3/2 = 1.5 x this torque and current i.e. 273.75Nm @ 78.5 A.

At some lower RPM, you will hit your controller motor current limit, below this RPM the torque will be limited to a flat value, for a Danfoss 5042 that's 97.6 A ~ 348Nm below about 315 rpm.

Above the nominal speed, it scales the same way i.e. 3000rpm is double the speed, you get half the torque / current, about 91Nm / 26 A.

I've heard people say 3x sync speed is about at fast as you'd want to run an industrial motor, so I'd say 4500RPM redline with 61Nm (1/3) of torque at 18A.

If your controller is too small, it will be the limit rather than your breakdown torque, so you want a controller about 3 x the rated output of your motor.

As far as battery current goes, above 315 RPM you're producing about 27.5 kW @ 90% motor efficiency, 97% controller efficiency, so you should be drawing 27.5/0.9/0.97 = 31.5 kW from your pack. @ 600V, this is 52.5 Amps. Below 315rpm, it will proportional to speed, about 1Amp per 6rpm.

I hope that makes sense and isn't too far from the truth.

cheers,
Woody
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Post by Richo »

woody wrote: Above the nominal speed, it scales the same way i.e. 3000rpm is double the speed, you get half the torque / current, about 91Nm / 26 A.

I've heard people say 3x sync speed is about at fast as you'd want to run an industrial motor, so I'd say 4500RPM redline with 61Nm (1/3) of torque at 18A.
This doesn't seem quite right.
To increase the RPM above sync the voltage can't go any higher as the DC bus limit is reached.
As a result the Torque drops off.
BUT this does not reduce the current.
The torque producing current lowers but the total current stays the same as it goes into prodcuing the power to get the higher RPM.

EDIT: added an example
ACIM_example.pdf
Last edited by Richo on Sat, 23 Aug 2008, 13:34, edited 1 time in total.
So the short answer is NO but the long answer is YES.
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Post by acmotor »

Correct.
Kw remains constant above sync speed therefore battery current and motor current are constant. Frequency (RPM) increases while available max torque drops off.

woody,
Your torques and currents agree with what I see at the bottom end.
I run 500V motor setting and see 40A from the battery at 40kmph on heavy acceleration.
I never see more than 60A from the battery (36kW).
Image

I'm now set up to log all Danfoss data via USB to laptop. It will be interesting to mull over some run time data. Just tidying some wiring at present.
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Post by antiscab »

Hi tuarn,

does that mean you have already hooked up the lithium cells to your vfd?
or are you some how getting around 700v from your henda batts to your vfd?
otherwse how are you getting 500v at the motor?


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Post by acmotor »

Matt,
"I run 500V motor setting" does not mean I supply 500V.
That setting tells the Danfoss to use a base V/F ratio of 10V/Hz.
so at say 25Hz the motor sees 250V RMS. It never gets to 500V at 50Hz as this is battery pack voltage limited.
This seems to be the max I can set without reaching magnetic saturation.
Danfoss pushes V/F further and overmodulates (changes form factor - becomes squarer - more current - more torque) as well at lower frequencies.
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Post by woody »

Richo wrote: This doesn't seem quite right.
To increase the RPM above sync the voltage can't go any higher as the DC bus limit is reached.
As a result the Torque drops off.
BUT this does not reduce the current.
The torque producing current lowers but the total current stays the same as it goes into prodcuing the power to get the higher RPM.
Yep, that's definitely right. I'm not sure where the extra current goes though :-(

At the other end, is less current needed below sync speed for the same torque? The more I learn, the less I know. I _really_ should have done Comp Eng, not Comp Sci!

Back to my danfoss book :-)
acmotor wrote: I'm now set up to log all Danfoss data via USB to laptop. It will be interesting to mull over some run time data. Just tidying some wiring at present.
Looking forward to that! have you got a GPS too for double happiness?

cheers,
Woody
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Post by Johny »

Thanks woody, acmotor. That all sounds good. I assume that where you hit saturation at low RPM will also limit the torque. That may vary for a given motor and acmotors empirical data will help clear that up.
I'm really trying to figure what current limit will do to drivability with my low power Lenze.
Vector space control also effects the lower speed range and again empirical data will be valuable.
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Post by Richo »

woody wrote:I'm not sure where the extra current goes though :-(


Basically the extra current goes into pushing the magnetic field frequency higher.
ie RPM
If you are limited to constant power over sync (stopping thermal runaway) then as the frequency increases the current to produce the higher frequency increases so in turn the torque producing current must drop to keep the constant power.
So the short answer is NO but the long answer is YES.
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Post by acmotor »

I've heard of a GPS logging program that calcs the kW from position, speed and vehicle weight. Anyone got a copy ?
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Post by a4x4kiwi »

Here is a spreadsheet from Cameron Software.

Note it dosn't account for the Peukert effect if you are using lead acid, but it gives and idea.

Silicon is just sand with attitude.

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Post by woody »

acmotor wrote: woody,
You've thought of the saturation voltage in a whole new way !
I'll have to mull that one over.
Hmm, I think I was thinking about it in a wrong way. Over the weekend a few thoughts have solidified in my mind:

* Torque Declines by the square of the stator voltage (from the danfoss book, in a section about soft starting)

Applying this to V/F ratio as a limit to motor torque - this limit kicks in harder than I previously thought.

At double sync speed (~3000RPM), the controller can only give about half the voltage required for full magnetisation, i.e. you only get 1/4 of the breakdown torque, (probably at full slip though).

At triple sync speed (~4500RPM), it's much worse, 1/9 of breakdown torque.

If I'm right, then the motor current is limited by this from sync speed and up. Which means even with an ABB 18.5 kW motor, my poor little cortina will take 30s to get to 100kph, despite 0-60 in 5secs and 0-80 in 10 secs.

This is probably why Red Suzi only does 80kph.

* Reactive Current is proportional to drive frequency and drive voltage, whereas the Active Current is proportional to torque. The apparent (controller - motor current) is the RMS of these 2 currents.

I haven't finished working out what this means, probably that my current-limited torque numbers will change a bit (upwards by about 10% at the bottom end, and downwards at higher revs).
acmotor wrote: The issue above normal synchronous speed is that the controller / battery no longer keeps up with the V/F ratio that the motor requires.
(around 8V per Hz in my case)
This is where the AC150 and Siemens motors step in. They are effectively around 100V motors but the controller goes to 3 times that voltage so revs keep going up at full torque. At 3 times the revs you get 3 times the power at the same motor current. Motor is happy with that as it really only knows about the current (provided insulation handles the voltage)
This is why I would like to try a 200V motor on a 500V controller.


Yep, looks like a definite advantage.

Shifting from delta to star would be interesting too.

Looking forward to some real world numbers :-)

cheers,
Woody
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Post by Richo »

woody wrote:
If I'm right, then the motor current is limited by this from sync speed and up. Which means even with an ABB 18.5 kW motor, my poor little cortina will take 30s to get to 100kph, despite 0-60 in 5secs and 0-80 in 10 secs.


True the input current should be limited above sync speed to get constant power.
However the data I sent still shows around 13-15sec on 15kW motor.
Maybe send me your data and I'll look at where the difference is.

So the short answer is NO but the long answer is YES.
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Post by Johny »

This gets interesting. Since I can't order the motor I want from Australia I have already got some work colleagues (I only met them once but they owe me) chasing shipping costs from UK to Aust. The motor is available but I have to handle shipping - still finding out about that (at least they are). The thing is, should I be pursuing a US style motor which is 230 star, 460 delta. The 460 makes things worse (then a 415 motor) if shifting isn't practical. Star/delta starters were (pre solid state softstarters) common industry practise for starting big motors.
The issue is can we shut the VSD down to coast mode for a few hundred ms then do a 'flying start' to catch the motor once the windings have been switched? Most VFD offer this 'flying start' operation. This is getting away a bit from a simple AC drive solution but it's still inexpensive if we can get the right motors - just a 3 phase contactor or two.
Now, where to get 11-18kW 230 VAC, 4 pole aluminum frame motors...
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Post by woody »

I think rewinding a 2nd hand motor locally for 230 star / 460 delta with higher insulation is one way to go here. I don't think this is expensive. Also I think Richo and/or CMG quoted custom winding prices for new motors at around $500 extra.
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Post by acmotor »

Software flying start is slow (several seconds at least) and quite weird. Seems to start at a high freq and run down looking for motor revs.
Best is to use an RPM pickup, even 1 pulse per rotation with an inductive pickup wired straight to the controller.
On red suzi the controller knows motor RPM and pickup is in ms from flying start.

A 220V delta motor is usually 380V in star (the ratio is 1.73 (root 3))
Voltages are specified for different frequencies as well. A 380V 50Hz motor may also be spec'd for 460V 60Hz.

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Post by Johny »

It's not as bad as you think woody.
Here is some graphs from:
"Control Technologies Manual
PWM AC Drives
Revision 1.0"

This first one is typical v/f control:

Image

The document then goes on:

"Field Oriented Control
What distinguishes a product using Field Oriented Control from a traditional vector product is its ability to separate and independently control (or regulate) the motor flux and torque."

Can't remember where I got this document (yes I do, Allen Bradley) but I don't think it matters who's drive they are talking about as I think it is a few years old anyway.

Image

Notice in both cases that Torque is down at near 100Hz but is still around 1 times rated torque. It is interesting that the drop off is quite steep in the v/f version though - around 73Hz there is a knee.

Google: ac drive "force technology"
The first hit should be it. PWMDrives01.pdf
Last edited by Johny on Mon, 25 Aug 2008, 11:42, edited 1 time in total.
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Post by Johny »

I hadn't realised that it was 60Hz 460V. I kept seeing it on US intended motors and hadn't checked. Thanks acmotor. So there is no disadvantage to getting a 'special' in that case. Other than more leg work.
acmotor, what is your motor RPM to 'distance covered on the ground' relationship in Red Suzi? i.e. Gear ratios, wheel diameter.
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Post by woody »

Johny wrote: It's not as bad as you think woody.
Here is some graphs from:
"Control Technologies Manual
PWM AC Drives
Revision 1.0"

"Field Oriented Control
What distinguishes a product using Field Oriented Control from a traditional vector product is its ability to separate and independently control (or regulate) the motor flux and torque."

Can't remember where I got this document (yes I do, Allen Bradley) but I don't think it matters who's drive they are talking about as I think it is a few years old anyway.

Notice in both cases that Torque is down at near 100Hz but is still around 1 times rated torque. It is interesting that the drop off is quite steep in the v/f version though - around 73Hz there is a knee.

Google: ac drive "force technology"
The first hit should be it. PWMDrives01.pdf
Thanks Johny,

I haven't done a graph, I'll see what shape it is.

I still think at double sync speed you'll get approx 3/4 nominal torque (1/4 times pullout torque). This drops off quite quickly to 1/3 nominal torque at triple sync speed.

I hope you're right, I guess acmotor's hard numbers will tell us soon :-)

I'll read that PDF, and see if I can't find something else I'm missing!

Hopefully the re-wind will be the answer if I'm wrong :-)

cheers,
Woody
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Post by acmotor »

Nice graphs,
Torque scale is around 3.4 at nominal V/F on my ABB 11kW. This varies with make / model.

132Hz is 82 kmph. Drive ratios on evalbum/1149

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Post by woody »

Hmm, mucking around with numbers from a 230V rewind, the current limit of the controller danfoss limits the top speed to 93 kph, so you'd shift from 400V to 240V between 50 & 90 kph, and then back to 400V for the final 15s slow climb to 100kph.

Hopefully I'm wrong somewhere :-)

The "Field Oriented Control" looks more promising, and danfoss VVC-plus hopefully does the same thing ...

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Post by woody »

acmotor wrote: 132Hz is 82 kmph. Drive ratios on evalbum/1149

Image
I though Mr Danfoss could do 0-1000 Hz ?

Does the suzuki accellerate briskly to 82 and then feel like you've hit a wall ?

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Post by woody »

Hmmm, I've put Red Suzi's numbers in, and I get 0-60 of 12 secs and top speed of 75, limited by V/F ratio, so my spreadsheet is a bit too pessimistic at the moment :-)
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Post by acmotor »

Yes but ABB suggest max 4500RPM on the motor,
otherwise you spin the rotor apart and you've already run out of voltage well before then. Thus my thoughts of star delta switching.

BTW,I can assure you, 82kmph is more than fast enough on a 1982 sierra !

Best acceleration is between 20 and 45kmph.
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Post by Johny »

I been collecting torque curves again...

The motor is a 4 pole running at 50Hz. Rated RRM 1420 and rated torque 71.3 NM.
It gives a good idea of what is possible depending on the output current capability of the controller.

Image
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