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Coninsan
17 February 2011, 0513
Following the good threads example on DC motors made my Dave over at: http://www.elmoto.net/showthread.php?944-Something-I-ve-never-really-understood-battery-vs.-motor-current

I began wondering, what about AC motors?
I for example have a "allegedly" 32Kw AC-15 motor with a Curtis 550 Amp controller and 60 Ah Thundersky batteries at 72 Volts.

What methods can be used on AC motors to calculate battery side draw?

lugnut
17 February 2011, 0559
I began wondering, what about AC motors?
I for example have a "allegedly" 32Kw AC-15 motor with a Curtis 550 Amp controller and 60 Ah Thundersky batteries at 72 Volts.

What methods can be used on AC motors to calculate battery side draw?

The old stand-by (Power Out = Power In * eff) will work. For kicks, take 550A * 72V * 81% = 32076W ~= 32kW which is the alleged rating. However we know you're battery will be a lot less than 72V when putting out 550A, if that is even possible. But such is the nature of ratings.

As for your question, methods to calculate battery current, what are you starting with? What knowns, or measurements, or assumptions? The actual calculations can be complex involving RMS and 3 phase to DC for non-sinusoidal wave forms. Let's hope the above power method will suffice :-)

chef
17 February 2011, 0713
I might be misunderstanding the question but it sounds like you're going to be battery limited. 550A for TS 60Ah is almost 10C. The batteries could only deliver that for milliseconds before sagging way down. You'll want to set the controller to limit it to 2C continuous (120A), with 3C peaks of something like 10s. Discharge above that and the cells may overheat -> electrolyte boils off and loss of capacity.

mpipes
17 February 2011, 0734
32kW at 72v is 321 Amps 3 phase on the motor side. There are calculators on the internet for converting HP, kW, kVA with AC and 3 phase. Of course this doesn't change power in/power out so in theory it's still 72v/550A at stall on the battery side..

Coninsan
17 February 2011, 0753
I might be misunderstanding the question but it sounds like you're going to be battery limited. 550A for TS 60Ah is almost 10C. The batteries could only deliver that for milliseconds before sagging way down. You'll want to set the controller to limit it to 2C continuous (120A), with 3C peaks of something like 10s. Discharge above that and the cells may overheat -> electrolyte boils off and loss of capacity.

You might be. At full power the equation should look like: 32000W / 72V = 444A which equates to 7.4C. TS cells can handle 3C continuous and 10C pulse.
Being that the 32Kw rating of the motor is also peak/pulse, the batteries should be fine.
Also, 550 is the Controllers max amp rating, not the substained current going to the motor. 550 amps would be 550A * 72V = 39.6Kw Max allowed motor output from the Controller.

I just dug this out; the maximum substained outout of the motor is 13.5Kw so.. 13500 W / 72 V = 187.5 A Which is just above the safe 3C of the TS cells, but only by .125C
This is based on the motors power output, so I can't know for sure if this will equate to the same current being drawn on the battery side = the reason for me asking this question.

Let me refraise my question, since it looks like it was badly formulated to begin with. How do I calculate the current load on the battery side with a AC system?

Tony Coiro
17 February 2011, 0950
32kW at 72v is 321 Amps 3 phase on the motor side. There are calculators on the internet for converting HP, kW, kVA with AC and 3 phase. Of course this doesn't change power in/power out so in theory it's still 72v/550A at stall on the battery side..

I concur. My friend Sean, who I am having the AC vs. DC showdown with, is running a Curtis 1238R (650A output) however, from his research, other people using that controller have seen almost 800A battery side, due to the Curtis outputting 3-phase AC. Fortunately, since his pack is 11.5kwh, 800A only ends up being a little under 8C.

lugnut
17 February 2011, 1056
32kW at 72v is 321 Amps 3 phase on the motor side.

Not so sure. With a 72V battery you cannot get 72V AC output to the motor. And you also need to include Power Factor (PF) to calculate AC current from real power.


Of course this doesn't change power in/power out so in theory it's still 72v/550A at stall on the battery side This certainly would not be the case at stall. Maybe at peak power at base speed. But at stall (zero speed) the controller will reduce the voltage to the motor and therefore the battery current will be a lot lower than the phase current in the motor.

Nuts & Volts
17 February 2011, 1103
Battery current will = motor current*0.7 at lower speeds. This motor current is the RMS value that controller sends through each phase, but this can change as the speed of the motor increases. As motor speed increases, motor voltage increase and as motor voltage becomes closer to battery supply voltage it becomes harder to push current through the motor (BEMF). I believe that is what causes the 0.7 ratio to change slightly along with motor efficiency changes.

lugnut
17 February 2011, 1104
...is running a Curtis 1238R (650A output) however, from his research, other people using that controller have seen almost 800A battery side, due to the Curtis outputting 3-phase AC.

So this would confirm that the controller is rated by battery voltage (DC) and motor current (AC). Seems a bit strange.

lugnut
17 February 2011, 1113
Battery current will = motor current*0.7 at lower speeds.

At lower motor speeds, the controller reduces the motor voltage using PWM very similar to a DC motor controller. So there is current multiplication, input to output. Battery current will be lower than AC output current at lower output voltage at low speed and significantly lower at stall.

chef
17 February 2011, 1131
You might be. At full power the equation should look like: 32000W / 72V = 444A which equates to 7.4C. TS cells can handle 3C continuous and 10C pulse.
The TS spec sheet says otherwise. It's 2C continuous, unless they have a new cell with different specs. At 3C the capacity is reduced quite a bit. There's a graph posted showing 2C & 3C discharge capacities of TS vs CALB. Are you by chance thinking CALB?
In any case, the motor & controller are way oversized compared to the TS 60Ah cells.

Coninsan
17 February 2011, 1150
The TS spec sheet says otherwise. It's 2C continuous, unless they have a new cell with different specs. At 3C the capacity is reduced quite a bit. There's a graph posted showing 2C & 3C discharge capacities of TS vs CALB. Are you by chance thinking CALB?
Nope. The little booklet I got in the crate with my TS cells says 3C continuous and 10C pulse discharge. So I'm trusting those numbers.

The motor may be a 32Kw peak unit, but according to mapping of the motors output, it will maximum continuously output 13.5Kw, which should equate to 3.125C draw on the batteries, which would be in the upper limits of the battery packs capabilities but not by much.
So why excactly is the motor and crontroller way oversized?

frodus
17 February 2011, 1203
DO NOT trust thost numbers. Manufacturers state specs to sell batteries. Trust what people have tested in the real world.

It's not about "can it do 3C", its about how much the capacity falls and how much the voltage drops.


So why excactly is the motor and crontroller way oversized?
The controller and motor will draw more power than the batteries can put out. Potentially 550A+, so thats like 10C+ and those batteries are the limiting factor, not the motor and controller.

Look at it either way, you undersized the batteries or oversized the motor, and the batteries are the limiting factor in the system.

chef
17 February 2011, 1204
Dug up the test results:
http://evworks.com.au/tech/ThunderSkyVsSkyEnergy/

At 3C the capacity drops to ~50%. The rest of the energy is lost to heat. It's very likely the cells will suffer capacity loss. Not sure why your sheet says 3C, maybe you have a new improved generation? I'd be interested to hear how your cells perform and how long they last. Might be a year before any conclusions can be made.

PS: do a web search for TS 60Ah specs and virtually all of them state 2C continuous.

Nuts & Volts
17 February 2011, 1217
TS used to state 3C on everything. But now i think they are being more real by stating 2C continuous

Coninsan
17 February 2011, 1220
TS cells is what I bought, TS cells is what I have and that is what I'm gonna use.
The booklet says 3C, seeing and remembering the test, I would believe that they are only capable of 2C, but I remember something over from Elmoto 1.0 where info surfaced that TS had launched a new generation of batts with improved performance. I'll have a look around and see what I can find.

I plan on having this bike for years, so when this pack runs dry in 2 years time there should be a stronger battery alternatve available.
Man.. I feel is bit stupid just about now.. And I'm the one people come asking questions on the Danish Forums.. I've picked the wrong line of work heh..

chef
17 February 2011, 1235
If you push them too hard, the risk is that they may not even last 2 years. But they're your batteries, do with them as you please. Just be honest when you report back with the results ok? ;)

Coninsan
17 February 2011, 1336
Sure thing ;)
The wise thing would be to limit the nominal draw to 2C, to get the pack to last. Limited to 120A it should hopefully last at least untill I get my bachelors degree.

But I'm wondering. What influence would the limit in current have on the torgue of the motor?
Most likely, it would be limited too, but by how much?

larryrose11
17 February 2011, 1714
With a 72V battery you cannot get 72V AC output to the motor.

That depends on the drive control.
Vector style drive control allow the motor voltage to be a factor of 1.10 greater than the input.
Reference:
page 107.
http://books.google.com/books?id=vDQHzeEmSfUC&pg=PA106&lpg=PA106&dq=Vector+AC+drive+voltage+limits&source=bl&ots=SKnU0fYd96&sig=JOWRlTtWjGHPhvDQInTveAQR2B8&hl=en&ei=A7ddTdbADNDegQf524mYDQ&sa=X&oi=book_result&ct=result&resnum=2&sqi=2&ved=0CBsQ6AEwAQ#v=onepage&q=Vector%20AC%20drive%20voltage%20limits&f=false

lugnut
17 February 2011, 1744
That depends on the drive control.
Vector style drive control allow the motor voltage to be a factor of 1.10 greater than the input.
Reference:
page 107.
http://books.google.com/books?id=vDQHzeEmSfUC&pg=PA106&lpg=PA106&dq=Vector+AC+drive+voltage+limits&source=bl&ots=SKnU0fYd96&sig=JOWRlTtWjGHPhvDQInTveAQR2B8&hl=en&ei=A7ddTdbADNDegQf524mYDQ&sa=X&oi=book_result&ct=result&resnum=2&sqi=2&ved=0CBsQ6AEwAQ#v=onepage&q=Vector%20AC%20drive%20voltage%20limits&f=false

Thanks for the reference. I didn't read it word for word, but notice that the Vin they refer to is the AC voltage into the rectifier bridge on the typical industrial inverter VFD connected to the 3 phase mains. Here the inverter is powered by a battery connected to the DC link. You are not going to see an AC RMS voltage output from the inverter greater than the DC link voltage. Even if you have a perfect square wave, zero dead time, its RMS value would just equal the DC link voltage in a perfect world.

chef
18 February 2011, 2234
Sure thing ;)
The wise thing would be to limit the nominal draw to 2C, to get the pack to last. Limited to 120A it should hopefully last at least untill I get my bachelors degree.

But I'm wondering. What influence would the limit in current have on the torgue of the motor?
Most likely, it would be limited too, but by how much?
A friend tested my 24-cell TS pack that was shot from both over-charging and drawing too much current (thank you EMS and Modalis). He's got some fancy industrial gear that precisely measures a large range of currents. They do high power LiPo so their gear has to handle alot of juice. Anyway with the pack done, I asked him to test the best cell at 2C & 3C discharge rates. At 2C the cell got warm but was able to deliver 35 out of 37 Ah, very respectable. However at 3C it fell apart, delivering 19Ah while getting almost too hot to touch.

You may not be happy with the performance at 2C and might up it a bit for peak draws. You'll want to pay attention not to open it up too long while also keeping an eye on traffic and not getting run over. I wish that controllers supported limited-duration peak draws in addition to plain old continuous max. For instance, the controller should allow 2C for an indefinite amount of time (until the LVC hits) while also allowing 3C for up to 10s. After 10s it should limit the draw to 2C until a certain rest interval has passed (say, 50s). Anyone know of a controller with that type of feature?

Richard230
19 February 2011, 1250
I am convinced (or at least I have convinced myself) that battery heat is the real problem keeping LiFePO4 batteries alive. I found that my controller was set for 250 amps and we reset it for 150 amps maximum. (Not that I ever saw more than 180 amps draw on the CA screen.) This still might be too much, but we have yet to try it going up a hill and I have to be able to ride up a 10 percent slope at 35 mph for at least 5 minutes in order to get anywhere in my neighborhood.

I keep wondering if maybe there is a limit to the number of battery cells that you can connect in series. Ever since I found that one of my batteries was shorted out, apparently from the day I bought the bike, and replaced it, giving me 81 volts and all 24 batteries on-line, the last two batteries in my pack have been overheating, swelling and dieing. Could it be that the batteries just can not deal with more than a certain amount of current and by the time the batteries are all tossing their 150 amps into the circuit, the last two batteries in the pack, before the main power cable goes to the motor, just can't conduct that many watts for more than a few seconds before overheating? That would make sense and dialing down the maximum power might start saving me the cost of buying replacement batteries. I agree that 2C sounds better than 3C, no matter what the manufacturer says the batteries can handle on their website.