HankLloydRight
No Roads
Not sure if you're joking, but there's still a difference between holding the same current through the same set of coil windings (0 mph), and alternating that current around several sets of stator coils (>0 mph).
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Not sure if you're joking, but there's still a difference between holding the same current through the same set of coil windings (0 mph), and alternating that current around several sets of stator coils (>0 mph).
The speed & torque of the motor is controlled by the amplitude and frequency of the three phase power going through the stator. The only thing that happens if the rotor is stationary is that more current is present in the rotor windings than there would be if it was turning and more closely coupled to the stator.
Induction motors do *NOT* work that way - you are thinking of a brushless permanent magnet motor, not an induction motor. For an induction motor generating torque while the rotor is stalled (which happens every time a Tesla starts moving from a stop) the field has to rotate relative to the (stalled) rotor to induce currents in the rotor. The torque is proportional to the lead or lag frequency (commonly known as the "slip"); i.e. the difference between the rotating field frequency and the 60 times the rotor RPM. When a Tesla is in "regen" (negative acceleration) the field frequency is less than the rotor frequency (negative torque) and when increasing velocity (positive acceleration) the rotor frequency is less than the field frequency. *All* of the stator windings are being driven whenever the motor is energized, not just some of them.
In hill hold mode the field drive currents and frequency will both generally be low, since it only has to provide sufficient torque to prevent rolling. The stress on the motor is far less than when performing rapid acceleration or at high speed (due to drag). For a simple example, imagine a Tesla weighing 5,000 pounds is stopped on a road with a 20 degree grade (this is the steepest paved road in the US: Canton Avenue in Pittsburgh, Pennsylvania). That requires about 1,700 pound feet of torque at the wheels, which is only about 176 pound feet at the rotor (assumes a gear reduction of 9.7:1 and no losses). This is a small fraction of the torque that the Tesla motors can produce, and it is also on the steepest paved road.
In summary, all of the poles and all of the phases are operating when an AC induction motor is producing torque. The Tesla induction drive motors have 4 poles and 3 phases, and the connections from the inverter to the motor are done with only 3 heavy conductors (3 phases). And there is no time when only one phase or pole is being driven with the others being inactive. All phases and poles are driven or else none of them are.
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HankLloydRight
No Roads
stopcrazypp
Well-Known Member
Well, I'm not going to get into the complexities of the induction motor as others have, but I will say that in your simple example of a toy DC motor connected to a battery, there is no current control or motor controller in between, so when you don't allow it to spin, the current spikes up and the motor burns out.I like driving very smoothly when there are passengers in my S, especially when stopping at a light. I obsess on bringing the car to a perfectly smooth stop without feeling the suspension jerking back. It's really hard to do it when stopping uphill thoughso I sometimes use the accelerator to keep the car at 0 speed. I know that with a small toy-sized motor if you hook it up with a battery but don't allow it to spin, it'll burn out, so that leads me to wonder if doing so with a Tesla motor also harms it. What do you guys think?
Your S has a motor controller that can limit the current and the temperature of the motor is also monitored (so if it does overheat, power can be cut appropriately). I imagine even for the permanent magnet motors used by other manufacturers the story is the same (there is no real danger of overheating the motor doing a hill hold).
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Hmm. You are correct the stator phases are energized all the time, I don't think you have considered the current going through them and the rotor. I am no way a motor expert, but here is how I see it.
1) normal rotating induction motor
The phases current of the stator is set to some value and is driven in synchrony. Currents is induced in the rotor creating the counter magnetic field causing it to rotate with the phase of the stator. Once it rotates, current will reduce in the rotor because it's rotating and magnetic field of the stator weakens relative to the rotor and unless the frequency of the stator increases, there is no increases rotational velocity. Thus the motor cools. Moreover, unless there is some hill to slow the rotor down, the magnetic field stays weakened.
2) Stalled induction motor
Same as above, but now the rotor is prevented from rotating. This current in the rotor stays the same and never decreases. Moreover, because of the "stronger" counter magnetic field from the rotor, the current of the stator also has to stay at a high level to "try" to rotate the rotor. Hence current is increased in BOTH places heating up the motor overall compared to just allowing it to freely rotate.
I suspect in general, regardless of what type of motor, stalling a motor is not a good idea. But I do agree if only a small torque is required to counter the torque caused by gravity to pull the car down the hill and the time duration of the stall is minimal, I don't think it will hurt the motor/inverter much if they are not stalling it for super long time on a hill. And what i mean by super long time is probably hours. I also agree that induction motor tends to be bigger and is probably much more resilient to stalls compared to other types of motors.
I mean if you think about it, in energy wise terms, you are not moving so all the power your drive train is using has to go somewhere.
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I sometimes amuse myself by maintaining position on a hill at a stop sign without using the brake. It's not difficult. Doesn't harm the drivetrain.
In principle, it can, but in practice I don't think it is likely to be a problem unless you're holding exactly stopped for an extended period on a rather steep hill.
If you manage to hold perfectly stationary, you'll be hitting the same set of poles in the rotor with current over and over again - while not hitting the rest of the rotor.
Since a fraction of the energy gets dissipated in the rotor as heat, this has the theoretical potential to heat one portion of the rotor differently than the rest of it - leading to differential thermal expansion caused issues and/or localized overheating.
But as I said, I think the power levels involved are low enough it's not a practical problem - and since the motor is geared at over 9:1 to the wheels, even a very slight rotation of the wheels will mean hitting different sets of poles anyway.
If you manage to hold perfectly stationary, you'll be hitting the same set of poles in the rotor with current over and over again - while not hitting the rest of the rotor.
Since a fraction of the energy gets dissipated in the rotor as heat, this has the theoretical potential to heat one portion of the rotor differently than the rest of it - leading to differential thermal expansion caused issues and/or localized overheating.
But as I said, I think the power levels involved are low enough it's not a practical problem - and since the motor is geared at over 9:1 to the wheels, even a very slight rotation of the wheels will mean hitting different sets of poles anyway.
NO! All the rotor is engaged in a electromagnetic tug of war with all the stator. This is a three phase induction motor. @No2DinosaurFuel has a valid point. The rotor has an induced current present at the surface due to the induced field present in the stator. That current traveleing through the rotor is producing heat. Prof. Knowitall challenges all the Real Nerds to calculate the power necessary to maintain the position on the hill @Zetopan describes. Remember that power and energy are different and pounds are really a measure of force not mass.
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Hmm. GM specifically recommended not holding on hills with the drive motor in the Volt for this exact reason, but that's a permanent magnet A/C motor where the fields are kept synchronous with a phase lag for torque rather than the induction motor's slip ratio.
I had assumed that it would still apply, but I'm not enough of an EE to know for sure.
I'm pretty confident it's perfectly fine to hold the car with the accelerator. The inverter creates a rotating magnetic field in the stator and the rotor follows it. In an induction motor (used in the Tesla) there is a certain slippage. The rotor kind of lags behind the rotating field in the stator. It is by design and important for the motor to work. The slight slippage is what creates and induction thus a magnetic field in the rotor that will then follow the rotating field in the stator. It always lags behind a little. It means that when you slightly press the accelerator on a hill just enough to hold the car still, the magnetic field in the stator is actually rotating slightly while the rotor is still.
The situation for the motor itself is no different than driving. There is still a rotating field in the stator and with some slip the rotor follows. In this particular case the rotor speed happens to be zero but this is in no way a special or harmful state. Just to us it seem odd as the car is not in motion, but to the motor it makes no difference.
(when watching the video, keep in mind that in an EV we do not have a fixed three phase current as it comes from the grid. The inverter modulates the amplitude and phase speed to achieve various levels of power and speed)
The situation for the motor itself is no different than driving. There is still a rotating field in the stator and with some slip the rotor follows. In this particular case the rotor speed happens to be zero but this is in no way a special or harmful state. Just to us it seem odd as the car is not in motion, but to the motor it makes no difference.
Right. That's the difference. A permanent magnet A/C motor is totally different. In those motors you have to "position" the magnetic field of the stator using specific coils. This increases the duty cycle of those coils beyond what they would normally have if the motor is spinning. In 3 phase motors the magnetic field is always going around the rotor, and the amount that the rotor is coupled to the field depends on the amount of induced field in the rotor. That's kind of a sloppy explanation, but @David99 has a good explanation posted.
One point here, in hill hold mold there is no current going to the motor as it is using the friction brakes to hold the car from rolling. As soon as you apply pressure to the accelerator it releases the friction brakes.
Right, of course there would be no generated current or field in the rotor if everything was stationary. The stator has to be cycling to hold the car stationary. DC fields do not induce currents in stationary coils.
