Two battery packs- One for torque, one for speed

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speedyebikenoob

1 kW
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Feb 6, 2019
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I had an idea but it sounds too good to be true. A motor rated for 2000 watts means in theory I could supply 40v and 50 amps or 100v and 20 amps without it overheating, right? So what if you had a 40v battery pack for lower speeds so you could take advantage of more low end torque but less speed and also a 100v battery pack for higher rpms but less torque? You can have a switch to shift to the 100v pack when you want more rpm. Sorry if this is a stupid idea, I was just curious.
 
Maybe there's a combination that would give you both? Upping the voltage less dramatically may provide the torque boost you're looking for, and provide a speed increase. If you don't need the speed often, then field weakening can give you more speed on top of that.

https://ebikes.ca/tools/simulator.html?motor=M2707_SA&batt=cust_40_0.2_20&axis=mph&cont=cust_50_100_0.03_V&cont_b=cust_38.46_76.8_0.03_V&motor_b=M2707_SA&batt_b=B5220_GA&bopen=true
 
100v is stretching it, but I get what your getting at. The problem would be the LVC of the controller, random #'s here but 100V would cut off at say 70V so your 40V wouldnt even work. But having a 48V controller with 120V caps (if they exist or is a thing for controllers) would work with 100v battery, just hope you have lvc in the bms of the 48V battery.
 
A motor rated 2000 watts can be fed 10kw acceleration bursts, conservatively. That is 100 Amp battery current on 24s.

20 to 24s lico chemistry high C-rate quality battery is best for both, power and speed. A second battery should be exactly the same as the first.
 
speedyebikenoob said:
I had an idea but it sounds too good to be true. A motor rated for 2000 watts means in theory I could supply 40v and 50 amps or 100v and 20 amps without it overheating, right? So what if you had a 40v battery pack for lower speeds so you could take advantage of more low end torque but less speed and also a 100v battery pack for higher rpms but less torque? You can have a switch to shift to the 100v pack when you want more rpm. Sorry if this is a stupid idea, I was just curious.
Click to expand...


It's phase amps (controller output) not battery amps
(controller input) that create torque. A controller that could use both batteries is probably going to create about the same phase amps at the same speed with either battery. So the 40v battery with be lower speed without any extra torque. Play around with the simulator E-HP linked.

The idea that you had, run low voltage high amp at low speeds and high voltage low amps at high speed, is exactly how these motors work. When the full battery voltage isn't required for whatever speed your at, the controller uses pwm to output a lower voltage and higher amps to the motor.
 
Partial throttle at the high voltage works exactly the same as full throttle at the lower voltage. Neither has any torque advantage either. The difference in any losses in the controller are miniscule, and what goes down the phase wires to the motor is the same either way, so don't bother and just set it up for the top speed you want.

Torque limits of a motor are set by the magnetic circuit, ie the stator steel and magnets. Supply voltage only affects maximum rpm. I don't know where this sudden rash of people thinking winding makes a difference in torque potential or efficiency, or thinking Delta/WYE switching offers some advantage or other things like this idea (running 2 different voltages), but none of it results in any kind of torque or efficiency advantage.
 
John in CR said:
Partial throttle at the high voltage works exactly the same as full throttle at the lower voltage.
Click to expand...

Ya, looks pretty darn close
https://ebikes.ca/tools/simulator.html?motor=MGRIN2708_SA&batt=cust_40_0.2_30&cont=cust_50_100_0.03_V&hp=0&axis=mph&cont_b=cust_20_40_0.03_V&motor_b=MGRIN2708_SA&batt_b=cust_100_0.2_30&hp_b=0&bopen=true&throt_b=38.8
 
John in CR said:
... I don't know where this sudden rash of people thinking winding makes a difference in torque potential or efficiency, or thinking Delta/WYE switching offers some advantage or other things like this idea (running 2 different voltages), but none of it results in any kind of torque or efficiency advantage.
Click to expand...
Sellers continue to name low Kv ‘Torque motor’ and high Kv ‘Speed motor’. Newbs simply adhere to this nonsense and perpetuate the myth.

Then, many are misunderstanding the laws of electricity. Here we have the typical mistake of thinking that power is Amp instead of watt, thus believing that 100 Amp X 50v does result in more power than 50 Amp X 100v.
 
E-HP said:
John in CR said:
Partial throttle at the high voltage works exactly the same as full throttle at the lower voltage.
Click to expand...

Ya, looks pretty darn close
https://ebikes.ca/tools/simulator.html?motor=MGRIN2708_SA&batt=cust_40_0.2_30&cont=cust_50_100_0.03_V&hp=0&axis=mph&cont_b=cust_20_40_0.03_V&motor_b=MGRIN2708_SA&batt_b=cust_100_0.2_30&hp_b=0&bopen=true&throt_b=38.8
Click to expand...

Notice that the torque, phase amps and rpm are identical, so the pulses of electricity sent to the motor is identical like I said. The difference in losses the simulator is coming up with are elsewhere, and maybe even an inaccuracy in the simulator. You can change the sim to the same controller, since you wouldn't switch to a different controller for doing the voltage switch and still get the same thing.
 
What John said.

Increase both amps and voltage to get more torque. Essentially, increase watts to get more torque. Increase both amps and volts will increase torque very well, but you may get plenty at less than 100v. Try a 48v-72v controller for a good flexible setup. Run at less than full throttle when you want to go slower.

Motor rated to 2000w can handle about all you can handle, or at least the bikes tires and frame anyway.

500w rated motor can be pushed to 1500w easily, 3000w briefly. So a 2000w rated motor can take 10,000w briefly. Enough torque to make keeping the front wheel down your main problem.

The battery of course, becomes your limiting factor when you go for big wattage. It has to be able to supply the amps the controller allows without the battery sagging to death.
 
I see, I looked at the simulator and seems like I've been misunderstanding how this stuff works. So a voltage increase not only increases speed and rpm, but torque as well?
 
Watts are watts, no matter what. :wink:

Not only higher voltage does make more watts at equal Amperage, it also relieves the stress on the circuit as compared to making more watts from higher Amps.

Let’s say you want 20kw of power. You can make it 100v 200 Amp as well as 50v 400 Amp. Same watts, same power. Yet, running 400 Amps does require bigger wires, bigger connectors, bigger everything in the electrical circuit.
 
speedyebikenoob said:
I see, I looked at the simulator and seems like I've been misunderstanding how this stuff works. So a voltage increase not only increases speed and rpm, but torque as well?
Click to expand...

Increasing voltage with everything else the same ends up increasing torque a bit through the mid-range speeds, because the torque curve is broadened, and since the BEMF from the motor is the same at any given rpm the higher pack voltage enables the same controller with the same current limit settings to push more current to the motor against the BEMF coming from the motor.

If you really want greater performance with any hubmotor, don't just increase pack voltage. Decrease the wheel diameter by less than the % increase in voltage. Then you'll not only get a higher top speed, but you'll get more thrust at all speeds as well as greater efficiency. With hubmotors nothing trumps going with a smaller wheel as long as you don't go too small and impact top speed or push the motor to rpm so high that the iron losses become too high.
 
speedyebikenoob said:
I had an idea but it sounds too good to be true. A motor rated for 2000 watts means in theory I could supply 40v and 50 amps or 100v and 20 amps without it overheating, right? So what if you had a 40v battery pack for lower speeds so you could take advantage of more low end torque but less speed and also a 100v battery pack for higher rpms but less torque? You can have a switch to shift to the 100v pack when you want more rpm. Sorry if this is a stupid idea, I was just curious.
Click to expand...
Like other people have said, if you have a good controller there's not too much difference between those two cases. With the higher voltage you'll see a bit more loss in the controller. Ideally you want the battery voltage compared to motor kV to be such that you are at base speed near your top speed. Doing phase advance to get to your normal cruising speed is pretty inefficient.
 
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