trevortron
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Re: steep hill vehicle stall

Wed Dec 25, 2019 4:33 pm

I recently had a 'panic moment' on a steep incline. Not towing or anything, but (having previously owned a Tucson 4X4 which would go anywhere) I had reverse-parked down a fairly, but not excessively steep incline, thinking driving out would be a breeze. How wrong was I? When I tried to leave, I did the classic 'handbrake start' i.e. holding it on the handbrake to stop it rolling backwards further down the hill and releasing when ready to go. But would it go? Nope. All I could hear was the whine. I tried every option I could thinkof - 4WD lock, save & charge options to run the ICE but she wouldn't budge - and something started to smell a bit 'hot'. And this is when the panic set in. It was a remote location with no mobile phone coverage - that's why I was there, installing a TooWay satellite Internet system. So no chance of phoning for assistance, not that they'd have found me anyway!
Fortunately there was a bit more hill behind me which flattened out, round a corner & behind a shed. I nervously (because there was a near sheer drop to the other side) rolled back and took a run at it, got further up but it still stalled. It was third time lucky. After unloading maybe about 100kg of gear I took it right round the bend and floored it up the hill and just made it. (Then had to carry the gear up the hill)
Seriously, this did not feel like a 200HP car, the old '04 Tucson diesel (101HP) would have knocked spots off it!

jaapv
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Re: steep hill vehicle stall

Wed Dec 25, 2019 7:03 pm

The handbrake bit is superfluous, the car has a anti-roll-back feature, but I wonder a bit about your difficulties starting up a steepish hill. Did you engage 4 WD Lock, which distributes power equally between front and rear motor? It seems to be the same problem described above; the electric motor must be turning in order to build up power.
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AlexBorro
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Re: steep hill vehicle stall

Thu Dec 26, 2019 7:22 am

greendwarf wrote: This is beyond my A-level physics! Why can't you find an electrical engineer when you want one. :lol:
I'm mechatronics engineer and I will give my 2 cents ;-)

One possible problem I see in this situation might be the lack of torque multiplier in the transmission. Let's do some math:

A regular ICE-only car:
a) First gear reduction: around 3.5:1
b) Differential reduction: around 4:1
c) Engine torque: suppose 18Kgfm

That means a torque delivered to the wheels around 250 Kgfm. Besides that, we have more torque multiplication in the Torque Converter (automatic gearbox) or in the Clutch (manual gearbox), which can multiply this torque a few more.
- Please some more experienced person correct me if I'm wrong -

In the case of the PHEV, the front motor has 14Kgfm and the rear motor 20Kgfm, summing 34Kgfm. I have no idea about its gearing system to calculate the total torque delivered to the wheels, but I guess it is a lot lower in comparison to a regular ICE car with gearbox. The lack of a gearbox also lowers the maximum delivered torque.

That might be an explanation of why an EV car has a start torque on the wheels lower than a car with a gearbox.

About how the electric motors work and how the maximum torque is achieved, let's start with some concepts:

a) The motor torque is only proportional to the current drawn by the motor;
b) The motor rotational speed is only proportional to the voltage to the motor voltage;
c) The motor electric Power is Current * Voltage;
d) The motor mechanical Power is Torque * Rotational Speed.

When the car is stuck and you try to start the movement, the motor inverter is supplying a huge current to the motor - probably the maximum allowed by the system - but the motor voltage is low, because it is stopped. Do not confuse battery voltage with the motor voltage, they are completely different. That means the total electric power drawn by the motor is quite low, not even close to the maximum power (120kW).

In the case you are requesting the maximum torque from the motors and even though it is not enough to move the car, the motor control unit just stop feeding the motors because it is useless (the motors are not moving) and all that energy is being converted to heat. By the way, a warm electric motor has less torque than a cold one.

I hope I could explain why a stalled motor can provide full torque at low power consumption.

If there is any further question, let me know.

Cheers.

Alex.

ThudnBlundr
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Re: steep hill vehicle stall

Fri Dec 27, 2019 6:13 am

I think that the electronics actually limit the maximum torque when the motor is stationary to prevent overheating of the components. As soon as the car is moving, the torque can be increased, so it's a bit chicken and egg...
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AlexBorro
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Re: steep hill vehicle stall

Fri Dec 27, 2019 4:58 pm

ThudnBlundr wrote:I think that the electronics actually limit the maximum torque when the motor is stationary to prevent overheating of the components. As soon as the car is moving, the torque can be increased, so it's a bit chicken and egg...
The controller can apply full torque to a stationary motor, it just cannot keep it for too long if no rotation is detected.

That is the reason you hear that whining sound for about one second when the car is stuck - the controller is probably applying full torque, realize the motor is not spinning and then turn it off. The controller can even measure the coils temperatures.

Cheers.

Alex.

jaapv
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Re: steep hill vehicle stall

Sat Dec 28, 2019 3:26 am

In other words, the car will simply stall when an incline is too steep, just like a conventional car does, and instead of burning out the clutch, it will switch off without damage. And just like a conventional car, reversing and adding some momentum for the steep bit will overcome the difficulty.
As to torque comparisons, if we do not take the weight of the car into consideration, they are meaningless.
The drawback of the PHEV or any other electrically driven car is that it has no facility to multiply torque by applying a low gear, which will remain the province of conventional vehicles. (unless somebody comes up with an electric Landcruiser sporting a low-gear box)
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greendwarf
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Re: steep hill vehicle stall

Sat Dec 28, 2019 5:07 am

jaapv wrote:In other words, the car will simply stall when an incline is too steep, just like a conventional car does, and instead of burning out the clutch, it will switch off without damage. And just like a conventional car, reversing and adding some momentum for the steep bit will overcome the difficulty.
Yes, but rather oversimplified surely - as the "brick" test shows, which the PHEV can't climb over from stationery.

An electric motor has no physical connection to transmit the energy. Although the current induces an initial turning effect on the armature, it needs the repeated kicks of turning on and off to keep turning hence the large mass to provide the momentum necessary for this. If the initial turn is stopped (e.g. by the brick) there are no further kicks and the motor stalls but the current rapidly heats up the coils.

The clutch (which is not essential in an ICE - e.g. simple aero engine) provides a physical connection that transmits all the energy (apart from transmission losses) direct to the wheels with a larger initial kick than in an electric motor because there is nowhere else for the energy to go, and so bumps over the brick easily. Of course, clutches will "burn out" but only if they start slipping, otherwise an ICE won't receive further energy once it is stopped from turning.

At least this is how my 50 year old memory of A-level physics informs me.

jaapv
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Re: steep hill vehicle stall

Sat Dec 28, 2019 7:34 pm

The kerb is the bit that you left out in your quote... It is straight up, that requires a lot of torque. Unless you are in first gear, a conventional car won't manage it either without a bit of rollback.
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greendwarf
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Re: steep hill vehicle stall

Sun Dec 29, 2019 2:03 am

Well of course, you wouldn't be attempting it from a standing start with an ICE in other than 1st. But I still think the energy delivered from a single ICE detonation is more than that incrementally induced in our electric motors at switch on. However, you also wouldn't normally start an ICE "in gear" - so when you attempt the curb by releasing the clutch with engine running you have even more energy instantly delivered via the transmission to the wheels.

Where the EV has the advantage over the ICE is that so long as it is still moving, even very slowly it will continue to climb unlike the ICE which will stall before coming to a stop - unless you slip the clutch.

As with apples and pears, you can make cider with both but they have different tastes. :D

AlexBorro
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Re: steep hill vehicle stall

Sun Dec 29, 2019 4:33 am

greendwarf wrote:But I still think the energy delivered from a single ICE detonation is more than that incrementally induced in our electric motors at switch on.
Believe me, an electric motor is capable of full torque when standing still. The full torque takes place when the angle between the magnetic field and electric field is 90° no matter if it is running or stopped, and, of course, you have maximum current at the coils to maximize the electric field. It is quite complex for the inverter to find this sweet spot (90° angle) when the motor is standing still, but it is completely possible.

I didn't quite understand in your previous message what you mean by "it needs repeated kicks to turn on". Maybe you are confusion BLDC motors with another old kind of motors.

Cheers.

Alex.

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