Mixing Motors?

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acmotor
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Mixing Motors?

Post by acmotor » Sat, 02 Aug 2008, 18:23

I have had a dabble with small format LiPo (4.4Ah) and the real issue is the series and parallel arrangements.
The RC pack as above is a 6 series x 2 parallel arrangement with 2.6Ah cells of 3.7V
There is no BMS on discharge and RC helicopter people seem to be getting as few as 10 cycles before killing a cell as the ESC cut-off only looks at pack voltage not cell voltage.
The better balanced the charge capacity of the cells the better the life.
All this becomes progressively worse as the pack voltage is higher.
BTW there are cell level monitor systems comming on the market that will improve the situation.
rc hobbies cell monitor

The BMS on charge is supplied by external charger via the charge voltage balance connection to each cell. You can safely double the price of the cells alone to get a full BMS'd pack.
I think it has been mentioned elsewhere that LiPo as with NiMH has issues with load sharing (and ability to detect a faulty cell) in parallel arrangements.
Simple answer.... use large format cells like Kokam.
kokam
You can put them in one of these...
pro ev kokam racer

If you must use small format cells, then arrange the LiPos as strings at the working voltage of the full pack and then parallel at that point, not at each cell. Each string current must be monitored. This means a lot of BMS but the pack may last and failing cells can be tracked and replaced before they 'take the wall down'.

Woody, I agree with your peak power calcs. It just takes a lot of care to get a second cycle !
I have a lot of time for the LiPo cell. It may well replace TS style Lithium.
The first fixed wing electric aircraft to fly was LiPo powered.
LiPo flight

....all a bit off topic, but it was said earlier, think batteries first.

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acmotor
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Mixing Motors?

Post by acmotor » Sat, 02 Aug 2008, 20:11

Matt,
You should consider voltage switching on your pack.
Start with 144V x 2 in parallel to deliver high current at take off and switch to 288V as the revs come up.
This will more than halve the losses in the battery pack and match the impedance (ESR) more correctly to the motor at low voltage / high current conditions. Sort of an electronic gear change.
Use a diode switch (requires high current diodes, regen no go)
or use contactors in place of diodes (probably cheaper and less losses)
     
Image    

You would then get 900A @ 5C from 90Ah TS at 100V.
I would guess that this may take you up to maybe 40kmph twice as solidly as the 288V at 450A. Assuming controller can handle current.

Do you have motor data on this ? You know, motor current vs terminal voltage at different revs.

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Last edited by acmotor on Sun, 03 Aug 2008, 18:47, edited 1 time in total.
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antiscab
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Mixing Motors?

Post by antiscab » Sun, 03 Aug 2008, 10:39

Tuarn,

900A at 100v is the same as 450A at 200v = 90kw.
Id be surprised if the added losses of the diode is less than the efficiency gains in the controller.
The battery losses should be the same either way since the current drawn is the same.

The motor i intend to use, if i can talk rob into it, is a kostov 11. my knowledge on this motor is limited, however i believe it is approximately similar to a warp 11.
the warp 11 goes into saturation at around 300A.
torque at 2000A (the limit of my controller) is around 900Nm.
the max power plateu (90kw 45v at 2000A) starts at around 800rpm (22kmh with a 4.1 diif).
the max power plateu ends at (90kw, 200v 450A) at around 3200rpm (88 kmh with 4.1 diff).

Matt

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acmotor
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Mixing Motors?

Post by acmotor » Sun, 03 Aug 2008, 18:58

Clang !

Matt, I think you missed my point. Image

Motor torque is directly proportional to motor current. Put your kW thinking aside for a moment, it is clouding your logic. Your motor can't develop kW if there is a voltage or current limit. At low revs there is only a current limit.
I'll use your 2000A number here (It keeps me warm)

As your motor numbers show, 45V @ 2000A @ 800RPM and quite linear up to 200V @3200.

So this means that the motor will run at 2000A up to 100V = 44kmph or thereabouts. So you don't need 288(200)V battery up to that speed. Think of this as first gear.
Place the cells in parallel at 144(100)V and they will only need to supply 1000A and not 2000A each.
(ignoring the current gain (20%) that a good controller - and this is not most - can make, most are just basic PWMs and not buck converters with 2 x current at half the voltage. This is because most controllers have no internal inductance to use as a charge pump, would be a big inductor at 10-20kHz anyway. They rely on motor inductance and an 11" has a lot less than a 9"... and BTW again, using the motor inductance results in increased heating in the windings, the motor will work better with smooth DC. If we persisit to use DC then controllers need to join the 21st century and move to Mhz PWM and DC-DC converter design.
Oh and BTW yet again, the MAXIMUM STRESS ON A CONTROLLER is when you try to put 2000A into a motor at low RPM(V). It is all about the instantaneous heat across semiconductor juntions. The higher the battery voltage the worse the problem. I am happy to explain this further but this is all another topic)

Now in parallel, the cells are at 1000A each not 2000A (BTW, way too much xC for TS) and will have a lot less internal loss because the source impedance is a better match for the load impedance = maximum power transfer. It's not rocket science ! ... and is more the issue than simple P=VI. Pack voltage will also be up to 0.5Vpc higher due to the lower current per cell.

Now when you consider this, you could now draw 4000A up to 40kmph with the battery voltage halved (parallel) with the same stress you place on your battery now. Something is certain to break ! The motor will handle it for the 0.5 seconds, the controller will need to be Ian's new 4k model !Image

Rob's series parallel motors PROOVE the theory of impedance matching.
I think if he had battery voltage switching rather than motor switching he would get the same result.(leave motors in parallel) (currents would be higher in controller though)

As I said, use contactors rather than diodes to reduce losses.
Some neat switching arrangement is required. Maybe Zeva could build this into a controller for us ?
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EV2Go
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Mixing Motors?

Post by EV2Go » Mon, 04 Aug 2008, 17:24

Thanks for that info guys, I realise there is still so much more to learn (but that comes as absolutely no surprise to me), it is great that people like yourselves with so much knowledge are willing to share with a total newbie like me Image

Matt I might have a crack at putting that formula for current draw into a small application (.exe) if anyone might find it of use.

As Tuarn said batteries first... I definitely think I need to start trying to work out the physical dimensions and capabilities of the batteries before worrying about what motor I might use to propel the car.

BTW I found a handy calculator on the net which takes into consideration weight, Cd, frontal area etc to calculate battery and HP requirements.

I think if I could legally register a formula 1 car and remove the down force, I would have the ultimate EV vehicle.
Last edited by EV2Go on Mon, 04 Aug 2008, 07:31, edited 1 time in total.

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acmotor
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Mixing Motors?

Post by acmotor » Mon, 04 Aug 2008, 18:44

Ian has quite correctly shifted the thread to a new topic series/parrallel switching of batteries and motors
Have fun Image
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zeva
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Mixing Motors?

Post by zeva » Mon, 04 Aug 2008, 18:48

Oh it wasn't me who moved it.. I just replied to it Image

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