Motor voltage questiuon

I think what you are missing is the relationship of RPM and Torque to Horse Power. HP = RPM x T / 5252. You are looking at a 6.2 Kwh motor or roughly 6 to 7 HP at some RPM and Torque value. You have to sacrifice Torque to get higher RPM's.

Example you have two 48 volt motors:

1. Generates 6 HP @ 3200 RPM with 9.8 ft-lbs of torque.
2. Generates 6 Hp @ 10,000 RPM with 3.2 ft-pounds of torque.

So which motor do you use. Well you use the one that generates the most power at the maximum engine RPM required for top speed. So if your target speed is 30 mph. you would need to know the motor RPM, differential ratios, and tire diameters to work that out.

You are correct in thinking higher voltage = higher RPM's as a general rule. HP is just like electrical power watts. Watts and Horse Power are products of Voltage and Current for watts, and RPM and Torque for HP.

I can get say 6000 watts with:

6000 Watts = 1 Volts x 6000 Amps turning a motor at 1 RPM with 31,512 ft-pounds of torque. You could pull a train a few inches per minute

or

6000 watts = 6000 volts x 1 Amp. Turning a motor at 6000 RPM with 5.3 ft-lbs of torque. making your bike go say 30 mph with the right transmission system. That is assuming that is enough power to overcome weight, rolling resistance, and COD.


So you could pull a train with one motor, o rmake your bike go 30 mph with a speed motor, but forget about pulling a train with it.

brian_ said:

Also, if 6.2 kW is just enough to keep it moving at 90 km/h, it will never get there because at the bike gets close to that speed there is almost no extra power to accelerate it. You need enough to keep the bike moving, plus some extra to account for headwinds and uphill grades, plus more power to accelerate it.

Click to expand...

This is an excellent point, and one often missed by DIY. If you look at any commercial passenger vehicle like a car and how the motor or engine is sized if it takes say 26 hp to make your Honda Civic cruise at 70 mph, you want a motor with 3 to 4 times more peak Horse Power 75 to 100 HP for passing and acceleration.

So if you calculate it will take the full 6 HP of your motor to make the bike go say 60 mph top speed means it is a 30 mph cursing vehicle using 1.5 to 2 HP. Enough left to get up to 40 mph to pass, or accelerate 0 to 30 mph just fine but getting to 50 will take a while, and 60 will only happen down hill.


Another thing you have to look at is you have a fixed Differential Ratio transmission just like a golf cart with Direct Drive. You can get a golf cart to go 60 mph and faster, but not with a brushed DC Motor. You need a high speed motor with flat torque up to 5000 RPM, and flat HP up to 10,000 RPM because even with 22 inch tires with a fixed ratio of 12:1 up to 17:1 requires a very high PPM using Direct Drive. You would have to use an AC Induction Motor or BLDC in a golf cart. A DC motor operates at much lower range of RPM and the peak power bandwidth is in the lower half of RPM range. To get both Torque and Speed is with a Transmission to change differential ratios to get higher wheel rpms. HPEV makes one heck of a 48 to 96 volt golf cart motor called the AC9. 6 HP continuous at 8000 RPM, and 40 HP peak @ 3700 RPM.

Well, that's a lot of stuff to take in. Thank you guys for the quick responses.


I don't think the torque will be a problem. The motor will do ~300 Nm on the wheel I think. The motor has a torque of 24 Nm x 4.13 (gear ratio) / 0.31m (r of the wheel) =~ 300 N/m. If I take @brian_'s formula, I think I would only need 80 Nm to maintain the cruising speed?



@Sunking Agreed. The 6.2 kW would be the nominal output power for the given maximum cruise speed. The motor has a maximum power output of 16kW, so there is extra short-term power available to take it up to speed, climb small hills etc.



I've looked into AC motors, but the higher rpm almost always means you need an extra reduction to get the correct wheel speed. I would like to keep the design as simple as possible. This is also the reason I want to go for a 48 Volt setup. Building a 13S (18650 battery pack) is not really exotic in terms of BMS and charging. I'm looking at 13S20P of 2.6 AH cells to give me 52 Ah (~2400 Wh) for the range I'm going for, about 30 KM.


If it turns out this is not feasible, I will have to adjust my expectations for a lower voltage system. This is supposed to be a "cruise around the block" bike. I do not need highway speeds, but it's nice to have enough power/speed if need be.

So let's try again. If I'd only saved my text...


Thank you guys for the quick response, lots of info to take in.

I don't think the motor torque would be a problem. The motor does 24 Nm on the shaft. So this would be be 24*4.13 (gear ratio) / 0.31 (r of the wheel) =~ 319 Nm on the wheel. If I fill in brian's formula with the known parameters, I would only need ~80 Nm at cruising speed?


@Sunking. Agreed. The 6.2 kW is not the maximum motor power. Maximum short term power output is 16 kW, so there should be enough power available for getting up to speed, climbing small hills, etc.


I also looked into AC motors, but the main issue I have is that they need an extra reduction because of the higher RPM. I would like to keep the build as simple as possible. This is also one of the reasons I'm going for a lower voltage, if possible. Making a 13S pack is a lot less exotic then a 20S pack in terms of BMS and charging. This is going to be a "taking it for a spin around the block" build. I'm not looking for highway speeds, but a little extra power when needed would be nice. If this is not possible with the lower voltages, I will have to lower my expectations a little bit, which is not really a problem. That's why it's always nice to have some people with more experience to look and see if you have most of the basics right.


As a last note, this is the calculator I'm currently basing my choices on: http://www.electricbikesimulator.co...ge=en&speed=90&range=30&weight=250&voltage=48

And we try again. I hope these will not become double posts...maybe it's because of the link?


Thank you guys for the quick response, lots of info to take in.

I don't think the motor torque would be a problem. The motor does 24 Nm on the shaft. So this would be be 24*4.13 (gear ratio) / 0.31 (r of the wheel) =~ 319 Nm on the wheel. If I fill in brian's formula with the known parameters, I would only need ~80 Nm at cruising speed?


@Sunking. Agreed. The 6.2 kW is not the maximum motor power. Maximum short term power output is 16 kW, so there should be enough power available for getting up to speed, climbing small hills, etc.


I also looked into AC motors, but the main issue I have is that they need an extra reduction because of the higher RPM. I would like to keep the build as simple as possible. This is also one of the reasons I'm going for a lower voltage, if possible. Making a 13S pack is a lot less exotic then a 20S pack in terms of BMS and charging. This is going to be a "taking it for a spin around the block" build. I'm not looking for highway speeds, but a little extra power when needed would be nice. If this is not possible with the lower voltages, I will have to lower my expectations a little bit, which is not really a problem. That's why it's always nice to have some people with more experience to look and see if you have most of the basics right.