EVs and traffic (paging Max torque)
Discussion
After greg said this
An electric motor can be driven slowly in either direction at low speeds
Can then be locked at 0 rpm?
So could you have it act as a parking brake when in traffic
Greg_D said:
when you stick it in park, put the parking brake on automatically
put a bit of creep into the transmission so it doesn't roll back when you come to a standstill
a thought cropped into my headput a bit of creep into the transmission so it doesn't roll back when you come to a standstill
An electric motor can be driven slowly in either direction at low speeds
Can then be locked at 0 rpm?
So could you have it act as a parking brake when in traffic
The i3 has a 'park' function. It must engage some sort of pin lock or something because you can hear it operate.
You can hold it on a hill with the motor but that's using power so not the best idea.
The electronic handbrake (separate to the park function on the transmission) automatically disengages when you move off too.
You can hold it on a hill with the motor but that's using power so not the best idea.
The electronic handbrake (separate to the park function on the transmission) automatically disengages when you move off too.
With Field Oriented Control (FOC) that is used by all of the motor control systems (inverters) in modern EVs, torque can be delivered down to zero speed. i.e. you can "lock" the rotor in a fixed position and it would only move when forced by a load greater than the torque capability of the Emachine. Unfortunately there is a catch:
1) Uneven Thermal Loading: Inverters and motors can't actually deliver "full" torque at zero speed, because at zero speed you are using only 1 pair of phases in the motor and 1 set of (power silicon) switches in the inverter. This means that unlike during normal running, where the thermal load is shared across the whole motor/inverter, under stall conditions the system must be derated to avoid failure. (typically around 1/3 of peak torque, still plenty to "hold" and EV on a decent slope)
2) Efficiency: Holding a motor at stall is an "Active" drive scenario, and takes battery power, whilst this is fairly minimal for short durations and on low gradients, it makes sense to have a mechanical lock for steeper/longer events. As your car must have a completely seperate "parking brake" to be road legal, you have one of these anyway, and so it makes sense to use it.
1) Uneven Thermal Loading: Inverters and motors can't actually deliver "full" torque at zero speed, because at zero speed you are using only 1 pair of phases in the motor and 1 set of (power silicon) switches in the inverter. This means that unlike during normal running, where the thermal load is shared across the whole motor/inverter, under stall conditions the system must be derated to avoid failure. (typically around 1/3 of peak torque, still plenty to "hold" and EV on a decent slope)
2) Efficiency: Holding a motor at stall is an "Active" drive scenario, and takes battery power, whilst this is fairly minimal for short durations and on low gradients, it makes sense to have a mechanical lock for steeper/longer events. As your car must have a completely seperate "parking brake" to be road legal, you have one of these anyway, and so it makes sense to use it.
It's also worth noting that a lot of drivers are comfortable with a bit of "creep" calibrated in. IE when the brake is released the car slowly accelerates up to around 3mph without any accelerator pedal input. This automatically gives a degree of "hill hold" just like in a conventional ICE+automatic transmission etc
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