I guess getting back to my question, obviously if you had one gear you would be limited by motor rpm.
Yes. The normal approach is to choose the overall gear ratio so that the motor is at its maximum speed, or the highest speed at which it can produce the required power, when the vehicle is moving at the highest design speed. Like everything else in design this is a compromise, since a taller ratio potentially allows a higher speed, but it also reduces the power available at low speed (in the region in which the motor is limited by set current and resulting torque output).
Being that I'd have multiple gears to take advantage of, where would you want to be shifting at motor rpm wise?
Typically modern motor and controller combinations have a common set of performance characteristics:
- At low speed the motor current is limited, by controller current capacity and/or motor heating issues, so a constant current limit leads to a constant torque output. For the HyPer9 as shown in the graph, that applies up to about 3400 RPM.
- As speed increases the motor power is limited, by battery power output capability, controller power-handling capacity and/or motor heating, so a constant input power limit leads to a constant power output. For the HyPer9 as shown in the graph, that is about 80 kW and applies from about 3400 RPM to 6000 RPM. In a typical production EV, much higher battery voltage extends this range (to about 10,000 RPM in a Leaf, for instance).
- At some point battery voltage is insufficient to push enough current, and power drops off. For the HyPer9 as shown in the graph, that occurs at about 6,000 RPM.
- Eventually either zero current, torque, and power are reached due to the limited voltage, or mechanical considerations such as bearings become limiting. For the HyPer9 as shown in the graph, the test ends at 8,000 RPM but the motor is still producing lots of power, so it is a controller frequency limit, a mechanical limitation, or just the end of the test range.
I consider any performance data published by NetGain to suspect, because there are too many strange inconsistencies to give me any confidence... but the one posted above is sensible.
The best motor speed would be determined by power availability and efficiency. For power, the obvious choice is to just stay anywhere in the power-limited speed range; that's 3400 to 6000 RPM in this case. Efficiency depends on speed and load: anywhere in the power-limited range is probably reasonable, but within that range a greater load (torque, power) is probably most efficient at a somewhat higher speed, and best efficiency occurs at a moderate speed and load. There is an efficiency map in NetGain's marketing material for the HyPer9; it is a generic chart for the range of motors and is probably not correct for the HyPer9, but it shows the general behaviour.
The net result is with typical compact production car manual transmission gearing, the top ratio is pointless. Just shift up at 6000 RPM from first and by 6000 RPM from higher gears, and shift down when motor speed drops to 3400 RPM, and you'll have full power available from whatever 3400 RPM in first corresponds to... and you'll never make it to a speed that requires shifting out of 4th. You can use a short final drive ratio for better performance at the lowest speeds; if that makes fifth gear useful it also means smaller motor speed changes between gears... which you don't need. With a higher-voltage motor like a production EV, this gets to the point of never needing to shift at all, but the HyPer9 is a deliberately low-voltage motor made for industrial vehicles.
The HyPer9 was supplied to NetGain, along with suitable controller, by SME Group of Italy (who in turn built them - or had them built - in China), but SME was purchased by Dana (the truck axle people) last year, and placed in their TM4 division (TM4 was another motor manufacturer which they bought earlier). I think this product is now Dana TM4's SRI 200
line... and most of the published information has evaporated, but SME never did publish anything about the specific version that they supplied to NetGain. If anyone has the manufacturers performance data, that would be interesting to see.
Certainly you wouldn't want to run the motor up to 8000 rpms? Am I looking at this wrong?
While running anything at the very limit of any rating is probably not generally a good idea, the efficiency shown in the HyPer9 graph is essentially the same (at full load) at any speed above about 2500 RPM... probably including well past 8000 RPM if the controller and motor can go there. The SRI 200 line is rated up to 9000 RPM, which probably (but may not) apply to all models.
With an internal combustion engine you would certainly avoid high speed to reduce mechanical stress, wear, and efficiency loss due to high fluid (air and exhaust) flow. None of those factors apply to an electric motor, although there is some issue with aerodynamic drag due to air in the gap between the rotor and the stator.