Ev motors, gears and back EMF
Discussion
Many moons ago I started a degree in electrical and electronic engineering (but dropped out to do software instead).
One thing I remember from the power engineering where we did motors was that as the load and or rpm of the motor increased, I can't remember which, the was a back emf force generated where eventually the motor stops being able to Rev faster or produce more usable power. I recall having to hold thumb and two fingers at right angles to do field, current & force but it escapes me which was which. And you used left or right hand depending on whether it was generating or running as a motor.
It makes me wonder that although all current EVs are single gear they would benefit from having a cruising gear. Or maybe they just pick a final drive ratio to get a decent top speed and the acceleration isn't affected add the torque is flat from zero. Kind wish I'd stayed on now. Think electric motors are where the money will be forel the next few years.
Might be a bit technical for PH. But in 10 years instead of talking valves and plug gaps I think a lot of people will be talking about armature windings, CMax and reduction ratios. Or something.
One thing I remember from the power engineering where we did motors was that as the load and or rpm of the motor increased, I can't remember which, the was a back emf force generated where eventually the motor stops being able to Rev faster or produce more usable power. I recall having to hold thumb and two fingers at right angles to do field, current & force but it escapes me which was which. And you used left or right hand depending on whether it was generating or running as a motor.
It makes me wonder that although all current EVs are single gear they would benefit from having a cruising gear. Or maybe they just pick a final drive ratio to get a decent top speed and the acceleration isn't affected add the torque is flat from zero. Kind wish I'd stayed on now. Think electric motors are where the money will be forel the next few years.
Might be a bit technical for PH. But in 10 years instead of talking valves and plug gaps I think a lot of people will be talking about armature windings, CMax and reduction ratios. Or something.
TransverseTight said:
Many moons ago I started a degree in electrical and electronic engineering (but dropped out to do software instead).
One thing I remember from the power engineering where we did motors was that as the load and or rpm of the motor increased, I can't remember which, the was a back emf force generated where eventually the motor stops being able to Rev faster or produce more usable power. I recall having to hold thumb and two fingers at right angles to do field, current & force but it escapes me which was which. And you used left or right hand depending on whether it was generating or running as a motor.
It makes me wonder that although all current EVs are single gear they would benefit from having a cruising gear. Or maybe they just pick a final drive ratio to get a decent top speed and the acceleration isn't affected add the torque is flat from zero. Kind wish I'd stayed on now. Think electric motors are where the money will be forel the next few years.
Might be a bit technical for PH. But in 10 years instead of talking valves and plug gaps I think a lot of people will be talking about armature windings, CMax and reduction ratios. Or something.
Indeed, the bEMF of an electric machine is linearly proportional to it's electrical frequency, and hence it's mechanical rotational speed. The power produced by the machine is proportional to the bEMF. So you are correct in saying that Emachines have a single speed at which they produce maximum power. However, due to the smart way in which we can now control the magnetic field orientation and strength within the magnetic "circuit" of the machine, (using a technique called "Field Oriented Control" or FOC) the motor can be run beyond it's natural peak power speed, and at a speed where the Bemf is actually larger than the DC supply voltage. In effect, we re-orient the magnetic field generated by the coils in the stator to "fight back" against the fundamental magnetic field in the rotor (either generated by Permanent magnets, or by induction, depending on motor architecture). As such, we can reduce the apparent magnetic field strength in the air gap, and artificially suppress the Bemf. This is not without penalty, and cannot ever make the motor more powerful than the natural peak power value (because it takes more stator current to do this so copper losses increase, and iron losses go up with commutation frequency). What it does do is to extend the power curve of the motor to higher rotational speeds. An Emachines ability to be "field weakened" like this depends primarily on it's inductance and saliency btw. Typically, the operational speed range of a typical EV PM machine can be approximately doubled.One thing I remember from the power engineering where we did motors was that as the load and or rpm of the motor increased, I can't remember which, the was a back emf force generated where eventually the motor stops being able to Rev faster or produce more usable power. I recall having to hold thumb and two fingers at right angles to do field, current & force but it escapes me which was which. And you used left or right hand depending on whether it was generating or running as a motor.
It makes me wonder that although all current EVs are single gear they would benefit from having a cruising gear. Or maybe they just pick a final drive ratio to get a decent top speed and the acceleration isn't affected add the torque is flat from zero. Kind wish I'd stayed on now. Think electric motors are where the money will be forel the next few years.
Might be a bit technical for PH. But in 10 years instead of talking valves and plug gaps I think a lot of people will be talking about armature windings, CMax and reduction ratios. Or something.
This allows EVs to get away without a multi speed gearbox, although in all cases the vehicle would be both faster, and often more efficient with a gearbox. (generally, cost and complexity prevents there use)
Edited by anonymous-user on Tuesday 28th January 17:26
No practical EV can avoid having some sort of speed reducing gearbox. As the power of a motor is broadly speaking proportional to it's rotational velocity, a small fast spinning motor, geared to the wheels will always be more efficient (in terms of cost and power losses) than a direct drive "hub motor".
NVH does become an issue to the lack of combustion noise, but tbh, on most modern cars, it is the road / wind noise that now dominates rather than engine noises. The lack of low speed torsional vibrations also means you can make the powertrain torsional stiffness lower without significant penalty, and the lack of uncertain inertial loadings (no clutch to "dump" or gears to mishandle etc) means the gear tooth (and casing) design can be more optimised for noise and efficiency.
In future, as the costs of Emachines, inverter and battery systems fall with increasing manufacturing volumes, i suspect we will see a return to multispeed (maybe 2-3) transmissions to maximise powertrain performance across a wider road speed range.
NVH does become an issue to the lack of combustion noise, but tbh, on most modern cars, it is the road / wind noise that now dominates rather than engine noises. The lack of low speed torsional vibrations also means you can make the powertrain torsional stiffness lower without significant penalty, and the lack of uncertain inertial loadings (no clutch to "dump" or gears to mishandle etc) means the gear tooth (and casing) design can be more optimised for noise and efficiency.
In future, as the costs of Emachines, inverter and battery systems fall with increasing manufacturing volumes, i suspect we will see a return to multispeed (maybe 2-3) transmissions to maximise powertrain performance across a wider road speed range.
Max_Torque said:
No practical EV can avoid having some sort of speed reducing gearbox. As the power of a motor is broadly speaking proportional to it's rotational velocity, a small fast spinning motor, geared to the wheels will always be more efficient (in terms of cost and power losses) than a direct drive "hub motor".
NVH does become an issue to the lack of combustion noise, but tbh, on most modern cars, it is the road / wind noise that now dominates rather than engine noises. The lack of low speed torsional vibrations also means you can make the powertrain torsional stiffness lower without significant penalty, and the lack of uncertain inertial loadings (no clutch to "dump" or gears to mishandle etc) means the gear tooth (and casing) design can be more optimised for noise and efficiency.
In future, as the costs of Emachines, inverter and battery systems fall with increasing manufacturing volumes, i suspect we will see a return to multispeed (maybe 2-3) transmissions to maximise powertrain performance across a wider road speed range.
I think the forthcoming Colibri single-seat commuter has a two speed automatic box aimed at increasing efficiency at higher cruising speeds.....NVH does become an issue to the lack of combustion noise, but tbh, on most modern cars, it is the road / wind noise that now dominates rather than engine noises. The lack of low speed torsional vibrations also means you can make the powertrain torsional stiffness lower without significant penalty, and the lack of uncertain inertial loadings (no clutch to "dump" or gears to mishandle etc) means the gear tooth (and casing) design can be more optimised for noise and efficiency.
In future, as the costs of Emachines, inverter and battery systems fall with increasing manufacturing volumes, i suspect we will see a return to multispeed (maybe 2-3) transmissions to maximise powertrain performance across a wider road speed range.
It will be interesting to see how the "chippers" move over to EV's. There are certainly some easy bodges that might increase performance (like shunting the phase current sensor output downwards...) but due to the interconnectedness of the multiple system controllers, it could be hard to avoid tripping diagnostic and safety limits in all the various modules when only simple changes are made (kinda the equivalent of the old coolant temp or MAF 'resisitor' trick ;-)
Max_Torque said:
It will be interesting to see how the "chippers" move over to EV's. There are certainly some easy bodges that might increase performance (like shunting the phase current sensor output downwards...) but due to the interconnectedness of the multiple system controllers, it could be hard to avoid tripping diagnostic and safety limits in all the various modules when only simple changes are made (kinda the equivalent of the old coolant temp or MAF 'resisitor' trick ;-)
Already there 
Interesting indeed and very achievable with the G1 Insight and HCH1 Civic hybrid. (My specialty)
The five current sensors can be fooled to allow almost +50% the standard current in the Insight and +20% in the Civic.
Upgrade the stock 100A fuse to 175A. Check.
Fit decent IMA battery using new 160A capable 8ah cells. Check
Fit 1k bypass resistors to the current sensors. Check
Power increase = +30% more oomph which makes a big difference in one of those.
Want more then you have to disarm the IGBT overcurrent sensing, lower the the bypass resistors values a bit more and you can have +44% assist
!!The IMA motor seems to be able to take ~20kw that's double the stock 10kw system.
peterperkins said:
Max_Torque said:
It will be interesting to see how the "chippers" move over to EV's. There are certainly some easy bodges that might increase performance (like shunting the phase current sensor output downwards...) but due to the interconnectedness of the multiple system controllers, it could be hard to avoid tripping diagnostic and safety limits in all the various modules when only simple changes are made (kinda the equivalent of the old coolant temp or MAF 'resisitor' trick ;-)
Already there Interesting indeed and very achievable with the G1 Insight and HCH1 Civic hybrid. (My specialty)
The five current sensors can be fooled to allow almost +50% the standard current in the Insight and +20% in the Civic.
Upgrade the stock 100A fuse to 175A. Check.
Fit decent IMA battery using new 160A capable 8ah cells. Check
Fit 1k bypass resistors to the current sensors. Check
Power increase = +30% more oomph which makes a big difference in one of those.
Want more then you have to disarm the IGBT overcurrent sensing, lower the the bypass resistors values a bit more and you can have +44% assist
!!The IMA motor seems to be able to take ~20kw that's double the stock 10kw system.
Accepting a failure at some point was all part of the project to see how far you could go.
I dial it back a bit for routine use and +30% works well day to day with no issues or other mods reqd on the G1
I've got plenty of spare IGBT blocks and IMA motors.
Haven't blown one yet at +44% and the igbt just shuts itself down momentarily if it needs too. By disabling the over current feedback circuit it prevents the igbt drive system throwing an obdII P code fault and needing a ign/on/off reset. You just get a momentary stutter now as the igbt shuts down for half a second or so at the extremes.
Only happens with regen at +44% which is harder on the system.
So I use a relay switch able system which provides +44% for assist but only +30% for regen.
These % do not trigger the igbt over current limiter.
I dial it back a bit for routine use and +30% works well day to day with no issues or other mods reqd on the G1
I've got plenty of spare IGBT blocks and IMA motors.
Haven't blown one yet at +44% and the igbt just shuts itself down momentarily if it needs too. By disabling the over current feedback circuit it prevents the igbt drive system throwing an obdII P code fault and needing a ign/on/off reset. You just get a momentary stutter now as the igbt shuts down for half a second or so at the extremes.
Only happens with regen at +44% which is harder on the system.
So I use a relay switch able system which provides +44% for assist but only +30% for regen.
These % do not trigger the igbt over current limiter.
Edited by peterperkins on Friday 31st January 22:32
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