14S VESC ?

General topics and discussions about the VESC and its development.
rew
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Re: 14S VESC ?

Postby rew » 29 May 2016, 08:04

Those simple equations still work.

Say the example of my friends.... They had a motor doing 50V, 10k RPM. at 200A. 10kW. When they run it at 2500RPM, the motor effectively sees only 12.5V, and assuming the 200A max still holds, the maximum power is only 2.5kW. If the battery is still 50V, the motor fries after a while at 50A battery current (200A through the motor). And the motor will tolerate 300A for a while, but you blow up the 300A ESC after a few seconds of 75A battery current.

So, with 12S, the 80KV motor will be running at 3500RPM. You should check that it can do more than 3500RPM, and then gear the motor so that it runs near maximum RPM at the maximum speed.

Normal "rule of thumb" is 80% of no load RPM. But when you're running at 80% of the no load RPM you're losing at least 20% of your power into losses in the motor. For modern, highly compact motors that is usually "too much". So I recommend trying to stay above 90% no-load-RPM. For short bursts, the 80% max RPM is acceptable.

notger
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Re: 14S VESC ?

Postby notger » 29 May 2016, 09:32


jaroslavnovotny
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Re: 14S VESC ?

Postby jaroslavnovotny » 29 May 2016, 09:53

I think everyone is over-complicating things.

What matters is Efficiency of motor, which is specific to each motor.

Running at peak efficiency means you get the best ratio between mechanical power produced and heat losses. Efficiency is only dependent on motor RPM, which is a function of motor Voltage. Running at peak efficiency will also get you the best battery life.

You get the maximum (peak) power, at half the maximum RPM. You never get the max possible power at max efficiency. So if you need 2500W and you want to run at max efficiency at the same time, you need a more than 2500W motor (cca. 3000W - depends on type of motor and it's efficiency curve).

If you have a motor you need to drive well, you can change it's gearing or the voltage to operate it at max. efficiency RPM. If you cannot change the gearing (mechanical or size limitations), voltage is the only option and you might need to go higher than 12S.

Of course it's best to design the whole thing and buy a motor with appropriate kv, so you can run at the voltage you want. If you have four 5S batteries, you can connect them parallel into 10S2P pack, which is equivalent to 20S pack in power stored and power it can deliver.

One last thing that is relevant - a high kv geared inrunner will be more energy efficient than less-geared low-kv outrunner. The first package will also have better power-weight ratio.

rew
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Re: 14S VESC ?

Postby rew » 29 May 2016, 13:45

Oldfashioned motors could be stalled almost indefinitively. So you'd have a 12V, 12W motor, and it would have a 12 Ohm copper resistance resulting in max 1A through the motor.

The torque curve is a straight line from the maximum torque at 12V 0RPM, dropping down to zero torque at max RPM. This results in the "max efficiency" at the halfway point.

Nowadays they make the motors a little better. This has improved power density by a whole lot. However this also means that you can no longer apply the full voltage across the copper at zero RPM. So nowadays the spec is not "your motor keep on working as long as you don't apply more than 12V", but "the maximum for this motor is 15A". This "about the same size" motor as my first example will now have a resistance of about 0.1 ohm. Applying the full 12V will cause a 120A current to flow. This can only hold up for a short while. Applying the 15A current limit, will mean that you can only apply 1.5V when it is stalled.

On my bike, I've set a 15A current limit. This means that at 10km/h my motor will only put out only about 150W. But at 30km/h (30V) I get 450W.

Measuring the effective voltage at your motor is difficult unless you connect a computer and have BLDC_Tool running with realtime data active.... So when you measure the battery current, find that my motor can easily do 450W, level at 30km/h you'd think it would be able to push you uphill at 450W at 10km/h as well. Set the limit to 45A to make that possible and.... the motor will overheat.

scienceworks
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Re: 14S VESC ?

Postby scienceworks » 29 May 2016, 14:27


rew
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Re: 14S VESC ?

Postby rew » 29 May 2016, 18:10

Yes. That's the fallacy. Those who can easily measure the battery current will think that current is also the motor current, but in fact the motor current can easily be many times more.

jaroslavnovotny
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Re: 14S VESC ?

Postby jaroslavnovotny » 29 May 2016, 18:44

Stalled motor is a heater, it is not doing any mechanical work. You basically require infinite power to turn an infinite load. Every stalled motor will overheat and burn, it is only a matter of cooling if it won't or when it will.

You design the system (cooling, motor size and power,..) for max load you aim to throw on the motor. You can usually overload the motor for a moment without any harm. If you design the system for going on straights then you have not enough power to go up-hill or you overload. That is imho a badly designed system. The system should be designed for uphill (if you need) and on straights it will be under-performing.

rew
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Re: 14S VESC ?

Postby rew » 29 May 2016, 19:51

nope

Hummie
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Re: 14S VESC ?

Postby Hummie » 30 May 2016, 20:03

Is the current limit on the vesc the true current to the motor?

What I'm reading is that the benefits of higher voltage are greater top speed and the ability to put more amps to the motor. The downside of higher voltage being that if you don't ride at a higher speed you lose efficiency.

In practice though, on higher voltage I can get to the top of the hill faster, with the amps coming quicker, and ultimately I stay cooler

rew
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Joined: 25 Mar 2016, 12:29
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Re: 14S VESC ?

Postby rew » 02 Jun 2016, 06:34

In the VESC you can configure motor-max-current and battery-max-current separately. Configure them to whatever your motor and battery allow.

With modern motors you are not limited in amps by the battery voltage. My testing-motor does 50A, 50V or 2500W. Nice! It has 14mOhm resistance. So of 24, 36, 48 or whatever volts, about 0.7V is taken for "causing the amps", the rest is used for "speed".

Efficiency of the VESC is IMHO very good, I have not noticed a difference in running my motor from a "much too high" battery voltage. (on the other hand, I don't have a real VESC).


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