Industrial AC Controllers + Rated Torque/Power

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Richo
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Post by Richo »

No - You just get more heat that the motor can't cope with.
218Nm in a Cortina not enough?!?
So the short answer is NO but the long answer is YES.
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Post by Richo »

The locked rotor current shouldn't be the same as the current used when at a certain slip that gives you the same torque output. Image
So 138Nm is unlikely to need 160A.

The limiting factor will be the temp class and insulation class.
Most motors avail are about the same class unless you get it rewound.
They do have different "S" ratings which makes them more suitable for higher short peak loads.
I think this is in the physical design of the motor though.
Most of the ones I have seen don't really state the "S" rating.
So the short answer is NO but the long answer is YES.
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Post by acmotor »

Just checking you follow these numbers for AC induction motors…

Full Load (normal) torque Nm   - the torque the motor will supply continuously in an industrial situation. ~ 73Nm on an 11kW 4 pole motor

Locked rotor torque - usually given as the ratio to full load torque if the motor were started DOL (direct on line) to 50Hz mains with rotor locked (not rotating). Basically irrelevant when using a VFD in an EV.
BTW it is deliberately kept low in design so DOL is possible without having to ring the power station before starting motor.
Maybe 140Nm on above motor.
The motor is 100% lost sync in this mode so the current (usually some large number) is not representative of anything to do with EV operation.
As I said, applies to DOL starting.
One fact that comes from it though is that the motor is designed to carry such a large current for up to 60 seconds without damage. It will be hot in the windings (150deg C) and expect a rest after ! Such is the industrial world !

Break down or pull out torque is the max torque that the motor can supply before ‘losing the plot’ i.e. losing sync with supply frequency.
Usually given as a ratio, typically 3 times the normal torque at rated voltage and frequency.
Around 220Nm on above motor.
Note this is at normal voltage and frequency. When operating on a VFD with load dependent (not fixed) V/F (voltage to frequency ratio) this is a variable up to nearly 5 x at low frequencies provided the controller has the current capacity.
Bargain on at least 3 x at synchronous revs (1500RPM)

Image


Induction motor slip. At no load the rotor rotates at the magnetic field speed of the stator (supply frequency) 1500RPM on 4 pole motor at 50Hz. As the load is increased the rotor speed slips by a few percent so a magnetic field can be generated in the rotor to pull it along.

Most motor manufacturers have similar offerings (each has the best !).
You need to check kW for size and weight as these vary.
Some variation in pull out torque as well. (more is better)

Limiting voltage on the motor is the point of magnetic saturation.
Premium efficiency motors are less sensitive to over-voltage than standard efficiency motors.
2 and 4 pole motors are less sensitive to overvoltage than 6 or 8 pole.
Around 500V at 50Hz seems to be the limiting voltage on the above 11kW (old) motor that I have.
Remember this would be 1000V at 100Hz. (not practical from battery pack or motor winding insulation point of view)
500V is around 20% over normal and this gives torque increase of 40% from most data I have seen. This seems to correspond to the Danfoss controller’s overall 466% indication. Remember this is a low revs.

Does any of that help ?

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Richo
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Post by Richo »

At the end of the day what performance were you hoping for?
Top speed x km/hr?
0-100km/hr in y seconds?
Operating efficiency z Wh/km @ z1 km/hr?

Sometimes it's better to look at the end game and work back...
So the short answer is NO but the long answer is YES.
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Post by woody »

Hi Richo,

I'm hoping for more Lotus Cortina than 1200cc Cortina Performance, but somewhere in the middle would be fine.

Top Speed original 125kph is fine.
0-100 Lotus 10secs would be great, original 26 secs would be disappointing
Efficiency: hoping to get under 100Wh/km @ 50kph, but I really don't mind at all. Electricity is cheap compared to batteries/battery life.

You'll probably want the Cd for your calcs, I can't find one anywhere, I've been using 0.4-0.5, frontal area is 18.7 sq. ft.

My spreadsheet reckons 0-60 3.7s 0-100 10.2s, with a 95Nm 4 pole (15kW) motor with controller maxing out at 48 kW. But this is naively assuming flat torque curve at 466% of 95Nm until it hits that power limit.
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Post by Richo »

weight, diff ratio and tyre size would be handy also.
So the short answer is NO but the long answer is YES.
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Post by woody »

acmotor: your info is gold

Locked rotor current 7.8 * nominal current 20.5 =~ 160A for 60s which will almost (but not quite) cook it.

Your explanation of Breakdown torque is tops.

Ye olde danfoss 5042 can put out a max of 97.6 Amps in high overload torque mode, so that looks like the limit here: 97.6/20.5 = 476% (346Nm!). Working backwards I guess your old ABB is 97.6/466% = 21A.

As for how this is limited with rpm, I'm not sure I'm keeping up with you, but I'll give it a bash...

At some point below the nominal speed, the voltage at which magnetic saturation occurs gets higher than the controller can put out? (2^0.5 * Battery pack voltage less IGBT voltage?)

Higher than that speed the torque is calculated by (caution I'm making this up as I go now) working out the proportion of the saturation voltage that the controller can put out, this is the proportion of the max overload torque that the motor can give?

Also at some point the controller will run out of kW either by internal limits or input current limit and battery voltage sag.

Does the AC motor efficiency really go south in this area of operation?

Where did that graph come from? I should probably read that whole book :-)

How did I do?
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Post by Richo »

Sorry even playing around with the numbers I can't get down to 10sec.
Seems to be around the 13-15 sec mark depending on other variables.
Should get under 100Wh/km@50kph.
So the short answer is NO but the long answer is YES.
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Post by woody »

Richo: working with 1000kg, 4.4:1, 165/75R13 = 280.6mm radius. Using 0.015 for combined tyre/steering/brake drag I get about 147N of Rolling Resistance.
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Post by acmotor »

woody,
You've thought of the saturation voltage in a whole new way !
I'll have to mull that one over.
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.

AC motor efficiency is not 'going south' at higher revs. It is still as efficient (maybe a few percent down in eddy current and iron losses at high Hz). It is just that you run out of voltage as above.

The graph came from a publication by Danfoss called 'Facts worth knowing about variable frequency drives' I'll see if I have a link to it.
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Post by Johny »

I just hung up from CMG. It appears the 11kW high output motor that I've been raving on about didn't pass the MEPS (Government efficiency) rating so they don't bring it in any more. Funny I wasn't told that 2 months ago when I enquired. I just have to hope that ABB's 132 frame 11kW high output is still allowed or else I just got 20kg heavier.
OH. I did say it wasn't going be running from standard 3 phase power and that it was an independent power source - nope, hit a brick wall there!
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Post by Richo »

Yeah they have rules and regs to abide by.
Still no harm being more efficient in your eV as well.
So the short answer is NO but the long answer is YES.
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Post by AMPrentice »

bump

heres a 30hp ac drive for a good price, can it be used?

Image

30hp ac drive
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Post by Johny »

It looks useful. Allen Bradleys tend to have transformers in them which have to be replaced with DC-DC converter. I think the transformer is for 24V but not sure. I have just bought an under powered (for me currently) Lenze EVF9327 - 15kW. It is in the same frame as the 22 and 30kW and has 180% overload for 60 seconds - so I'm hopeful. It was cheap! I am now pestering anyone who services Lenze worldwide for info on a possible power hardware upgrade. (I hadn't heard of Lenze up until 2 days ago)
One thing is that anything from the US appears to cost about $16 per kg to get here so it jacks the price up.
Had I not just bought the Lenze I'd be asking about shipping on that AB now!

I sent off an email regarding shipping shortly after writing the above...
Last edited by Johny on Tue, 19 Aug 2008, 05:59, edited 1 time in total.
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Post by AMPrentice »

glad I could help john
I think if you ask for economy mail i think
it will be okay if just under 30kg.
Shouldnt cost more than 200 au then landed.
Measuring my bricks at home and the ones on
image I guess its about 30cm wide by 60cm tall.
I dont know about its output in amps probably
not enough?
heres an image of the inside of a 40hp one
look at the IGBTs same as the toyota prius Image
I think if you ask for economy mail i think
it will be okay if just under 30kg.
Shouldnt cost more than 200 au then landed.
Measuring my bricks at home and the ones on
image I guess its about 30cm wide by 60cm tall.
I dont know about its output in amps probably
not enough?
heres an image of the inside of a 30hp one
Image
ebay 30hp
and the 40hp one
Image

I guess 90hp output is possible.
Last edited by AMPrentice on Tue, 19 Aug 2008, 18:46, edited 1 time in total.
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Post by woody »

Richo wrote: Sorry even playing around with the numbers I can't get down to 10sec.
Seems to be around the 13-15 sec mark depending on other variables.
Should get under 100Wh/km@50kph.
Cheers Richo,
for my calcs I'm assuming:

48kW max motor power (Danfoss 5042).
torque of 476% of nominal available until I hit that 48 kW. (maybe not very realistic according to above acmotor post!)

I'm not doing proper integration, just calculating torque available at each kmph, working out how long it will take to accel to the next km/h using the torque left over after drag, and how far I will have travelled in that time, and then summing.

Mucking around with the numbers, I find that controller power greatly affects 0-100 times (72 kW -> 7.35 secs, 48kW -> 10.5 secs, 36 kW -> 15 secs, 25 kW -> 24 secs), whereas nominal motor torque does not (50Nm -> 11.5 secs, 72.7Nm -> 10.5 secs, 95 Nm -> 10.2 secs, 146 Nm -> 9.9 secs)

Where does your modelling differ?

cheers,
Woody
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Post by Johny »

Woody. The torque available below about 500RPM will be effected by the Drive power - up to a limit where extra drive power doesn't help (this is acmotor's 466%). From 500 to 1500 is moving from drive effect to mostly pullout torque of the motor. At 1500RPM and assuming normal voltage then it's the pullout torque of the motor - roughly 3 times rated torque. Above 1500 RPM the drive has no effect - torque drops off as speed increases.
The torque available MUST effect your accel. times.
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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.
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)
I should be working, but instead I've done some sums on this V/F ratio:
(makes up for some unpaid overtime)

I think it only comes into play (on 4 pole motors) if your controller e.g. Danfoss 5042 is much bigger than your motor (e.g. 7.5kW / 50 Nm). Otherwise the controller's power limit kicks in before the V/F ratio becomes a limiting factor.

If you have a 6 pole motor, it becomes an issue too.

I noticed CMG will sell you a motor with dual wiring (e.g. 4 + 6 poles), is this of any benefit?

How would the calcs differ for a delta wired motor, if at all?

cheers,
Woody
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Post by Johny »

The 4/6 pole motors have less overall torque and are generally lower power for the weight - I wouldn't bother, just use a 4 pole and overload it as Tuarn says. Generally decide your torque based on motors pullout or stall torque - usually 2.7 to 3 times rated torque. The drive provides higher v/f ratio at low speeds so < 500RPM count on about 4.5 times rated torque so drive must supply rated current x 4.5 - 90amps for 20 amp 11Kw motor. As said in an ealier post I am only going to start with a 48Amp max drive so I can measure the change when I up it to 100Amp.

Star/Delta has little effect on power.
Motors <=3kw are 240 delta/415 star.
Motors >3kw are 415 delta/720 star.
You can get a special motor intended for the 220V 3 phase market and that is an option if we bring them in - see below. Down side is they are actually 220 start/460 delta so this opens up lots of worm-cans.

By the way. I have tried to order an 11kw 132 frame high output from CMG and they no longer bring them in due to Govt regs (MEPS). I have been chasing alumunium frame motors all over Australia and UK (like you - when I should be working). I can get it from UK but am chasing ABB now for a comparison again. Also have found a Brook Crompton in 160 frame but it weighs 84Kg for an 11kW (CMG was 61Kg). They all come out around AU$1000.
Last edited by Johny on Mon, 15 Sep 2008, 06:32, edited 1 time in total.
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Post by woody »

Johny wrote: Woody. The torque available below about 500RPM will be effected by the Drive power - up to a limit where extra drive power doesn't help (this is acmotor's 466%). From 500 to 1500 is moving from drive effect to mostly pullout torque of the motor. At 1500RPM and assuming normal voltage then it's the pullout torque of the motor - roughly 3 times rated torque. Above 1500 RPM the drive has no effect - torque drops off as speed increases.
The torque available MUST effect your accel. times.
I realised my mistake, I was calculating the VF Ratio Torque based on the drive limited torque, which is wrong. The VF ratio torque limit is the breakdown/pullout torque ratio. I.E. at 8.3V/Hz you can get the pullout torque.


If I make that correction, I get (with our N/A CMG 11kW motor + Danfoss 5042):
Controller Current Limited Torque of 465Nm up to ~700 rpm
VF Ratio limited torque thereafter (600V pack).
The Controller Power limit doesn't come into it at all.
0-100 in 17 secs
400m in 19 secs

If I upgrade to 15kW CMG, I get:
Current Limited torque of 337Nm (lower? - is that right)
VF ratio limited torque from 1200rpm & up
Controller Power limit is near (<10Nm) the VF Ratio limit.
0-100 in 12.3 secs :-)
400m in 18 secs

Looks like I'm getting closer to reality all the time, thanks all for your help.

cheers,
Woody
PS: If anyone wants my spreadsheet, drop me a line
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Post by woody »

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Post by acmotor »

Yep, that's the book.
I read it about 15 years ago when it came out.
A bit of a sales pitch for Danfoss, but then they have earned the right ! Image
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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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