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EV Cars

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Re: EV Cars
Post by aairfccha   » Mon Feb 26, 2018 1:27 am

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Imaginos1892 wrote:If they're using gears to effect reverse, they're even stupider than I thought. Reversing an electric motor is trivial.

Using one motor and a set of differential gears instead of two motors is only slightly less stupid.


So what's your opinion on a multi-speeed transmision in an EV, is a predicted 15% gain in efficiency stupid?
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Re: EV Cars
Post by Joat42   » Mon Feb 26, 2018 7:36 am

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aairfccha wrote:
Imaginos1892 wrote:If they're using gears to effect reverse, they're even stupider than I thought. Reversing an electric motor is trivial.

Using one motor and a set of differential gears instead of two motors is only slightly less stupid.


So what's your opinion on a multi-speeed transmision in an EV, is a predicted 15% gain in efficiency stupid?

I haven't looked that up, but I guess it has to do with the duty cycle efficiency of the electric drive train. Driving some loads at certain rpms is less efficient that just putting in a simpler multi-speed transmission to make sure the motor always works in it's most efficient rpm range. You can get quite high losses due to components heating up on low rpms and high load.

If that results in 15% better efficiency it's a good thing since you get better range and battery life time.

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Re: EV Cars
Post by Imaginos1892   » Mon Feb 26, 2018 11:09 am

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aairfccha wrote:So what's your opinion on a multi-speeed transmision in an EV, is a predicted 15% gain in efficiency stupid?

Who predicted it? The same folks that want to make electric cars as much like gas-engine cars as possible?

I'm sure they could get that, or better, with the right motor and PWM controller design. Electric motors do not have a narrow 'powerband' like a piston engine. They deliver near-constant torque from a dead stop until the rotor flies apart.

A car's wheels need to go from 0 to about 1,400 RPM. At 8 KPH (5 MPH), they're turning 70 RPM, which is fast enough for a 24-pole electric motor to be efficient. How much time do you expect the car to spend going less than 8 KPH under a heavy load? Enough to be worth the excess weight and friction of a transmission?

Alternately, the motors could be geared down — say 4:1 — making the top motor speed 5,600 RPM and the 8 KPH speed 280 RPM.
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Re: EV Cars
Post by aairfccha   » Mon Feb 26, 2018 11:13 am

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Imaginos1892 wrote:I'm sure they could get that, or better, with the right motor and PWM controller design. Electric motors do not have a narrow 'powerband' like a piston engine. They deliver near-constant torque from a dead stop until the rotor flies apart.

But not near-constant efficiency. Fundamental difference.

https://newatlas.com/antonov-3-speed-transmission-ev/19088/ wrote:
Electric motors on the other hand have full torque at 0 rpm and a much wider operating range. That's why most EVs have a single speed - or in some cases
dual-speed - transmission. Despite this, the efficiency of electric motors still varies at different speeds - they operate at a peak efficiency of around 90 percent but this can fall to 60-70 percent, particularly at low speed. The question is whether or not it's worth adding a multi-speed transmission to the EV drive-train to optimize efficiency at all speeds. According to Antonov, the answer is definitely yes.
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Re: EV Cars
Post by Joat42   » Mon Feb 26, 2018 6:40 pm

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Imaginos1892 wrote:
aairfccha wrote:So what's your opinion on a multi-speeed transmision in an EV, is a predicted 15% gain in efficiency stupid?

Who predicted it? The same folks that want to make electric cars as much like gas-engine cars as possible?

I'm sure they could get that, or better, with the right motor and PWM controller design. Electric motors do not have a narrow 'powerband' like a piston engine. They deliver near-constant torque from a dead stop until the rotor flies apart.

A car's wheels need to go from 0 to about 1,400 RPM. At 8 KPH (5 MPH), they're turning 70 RPM, which is fast enough for a 24-pole electric motor to be efficient. How much time do you expect the car to spend going less than 8 KPH under a heavy load? Enough to be worth the excess weight and friction of a transmission?

Alternately, the motors could be geared down — say 4:1 — making the top motor speed 5,600 RPM and the 8 KPH speed 280 RPM.

All electrical motors have a designed sweet spot where they have the best efficiency for a given RPM and load. When you are operating outside that RPM and load the engine starts to loose efficiency fast. That means you need to up the voltage to the motor to compensate for the torque loss which means overall energy efficiency drops considerably. A simple gearbox can quite easily compensate for that loss by extending the sweet spot over a wider range.

Looking at the prototype gearbox they have 2 axles with helical gears. A helical gear assembly has about 96% efficiency per consecutive gear pair + additional losses usually around 3-5% (for smaller hydraulically controlled gearboxes) which means that I guesstimate that the proposed gearbox has an efficiency about 90-93%.

So if they can have the electrical engine work at +90% efficiency at a sweet spot that has 3 times the rpm-range than otherwise it's no brainer to use a gearbox.

And no amount of trickery with PWM can best that. Although, when we get the technology where recharging a battery pack takes the same amount of time as filling up a tank with gas the gearbox will probably be superfluous.

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Re: EV Cars
Post by Imaginos1892   » Mon Feb 26, 2018 11:52 pm

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Joat42 wrote:All electrical motors have a designed sweet spot where they have the best efficiency for a given RPM and load. When you are operating outside that RPM and load the engine starts to loose efficiency fast.

Brush-and-commutator motors have such a 'sweet spot' but electronically-controlled brushless permanent-magnet motors can apply just the right current to the right poles at just the right time for any motor speed. As I noted before, microcontrollers to manage that timing can be had for less than $3.00 in quantity and the programming only has to be done once.

As I also noted before, brushes and commutators are for motors you want to fail.
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Re: EV Cars
Post by zyffyr   » Tue Feb 27, 2018 12:10 am

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Imaginos1892 wrote:As I also noted before, brushes and commutators are for motors you want to fail.

We are talking about the automotive industry. Birthplace of "Planned Obsolescence". They do in fact want the motors to fail. Just not until some time after the warranty ends.
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Re: EV Cars
Post by aairfccha   » Tue Feb 27, 2018 1:19 am

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Imaginos1892 wrote:As I noted before, microcontrollers to manage that timing can be had for less than $3.00 in quantity and the programming only has to be done once.

You also need the power electronics to actually control the current.
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Re: EV Cars
Post by Joat42   » Tue Feb 27, 2018 3:10 am

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Imaginos1892 wrote:
Joat42 wrote:All electrical motors have a designed sweet spot where they have the best efficiency for a given RPM and load. When you are operating outside that RPM and load the engine starts to loose efficiency fast.

Brush-and-commutator motors have such a 'sweet spot' but electronically-controlled brushless permanent-magnet motors can apply just the right current to the right poles at just the right time for any motor speed. As I noted before, microcontrollers to manage that timing can be had for less than $3.00 in quantity and the programming only has to be done once.

As I also noted before, brushes and commutators are for motors you want to fail.
———————————
Sanity is like anything else — best practiced in moderation.

A permanent-magnet motor also has an efficiency that is directly correlated to rpm and load. The are a couple of differences between permanent-magnet motors and induction motors, the permanent magnet motor needs a driver plus the efficiency is higher - but it's power draw is actually higher when you start it.

In the end, what you choose to use as a power train in a car has more to do with economics than the best engineering solution because most people buying a car are more interested in something that takes them from point A to point B in relative comfort without it costing them an arm and a leg.

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Anyone who have simple solutions for complex problems is a fool.
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Re: EV Cars
Post by Lord Skimper   » Wed Feb 28, 2018 7:23 am

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No gears, 1 speed motor forward or reverse. I say gear out of convention. Has neutral but it isn't really a neutral 'gear'. Has a drive selector. Park by button, N D L (L is regen max mode)
R is off to the side similar to a manual 'gear box'. Has a further Regen hand brake. Foot brake and auto brake within certain speeds. Also has a hand brake button that can be used when driving.

Well worth a look and can change a few minds on Electric.
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