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Author Topic: Motor voltage limits  (Read 3747 times)

Online Dennis Toth

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Motor voltage limits
« on: August 29, 2014, 08:41:10 PM »
Guys,
Is there any limit on the voltage that can be applied to the normal size out runners we use? For example instead of a 5 cell pack (18.5 Volts) could we use a 6 cell ( 22.2 Volts) pack to use less amps at a reduced rpm setting? My understanding is that "current" causes heat, higher voltage lower current (less heat) for the same rpm set point. Can you go to far with the voltage?

Best,         DennisT

Offline Tim Wescott

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Re: Motor voltage limits
« Reply #1 on: August 29, 2014, 10:08:15 PM »
If you apply higher voltage to the motor itself then it would need to run faster, with less prop, for the current to be less.

This is complicated by the fact that with an ESC in there, the ESC could see lower current at the higher voltage, while the motor still saw the same voltage and current as before.
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Online Dennis Toth

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Re: Motor voltage limits
« Reply #2 on: August 30, 2014, 05:40:35 AM »
Tim,
Thanks for the response.

What I was looking at was a way to reduce battery weight by choosing a higher voltage pack with a smaller mah. If you look at what is currently used for large ships swing the 12 - 13" diameter props they run 5S1P 2700 mah packs or 4S1P 4000mah packs. The 5S packs are about 1 - 1.25 oz lighter than the 4S. They both would run the same rpm set point in the ESC
but the 5S pulls less current and thus the 2700 pack vs the 4000.

My point being could we push the voltage to 6S 22.2 V and say 2000 mah ish and save more weight (assuming you could find the a pack that did in fact weigh less then the 5S for the capacity needed to do the pattern) while running at the same rpm set point or does the motor have limits?

Best,       DennisT

Online John Rist

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Re: Motor voltage limits
« Reply #3 on: August 30, 2014, 08:10:31 AM »
Tim,
Thanks for the response.

What I was looking at was a way to reduce battery weight by choosing a higher voltage pack with a smaller mah. If you look at what is currently used for large ships swing the 12 - 13" diameter props they run 5S1P 2700 mah packs or 4S1P 4000mah packs. The 5S packs are about 1 - 1.25 oz lighter than the 4S. They both would run the same rpm set point in the ESC
but the 5S pulls less current and thus the 2700 pack vs the 4000.

My point being could we push the voltage to 6S 22.2 V and say 2000 mah ish and save more weight (assuming you could find the a pack that did in fact weigh less then the 5S for the capacity needed to do the pattern) while running at the same rpm set point or does the motor have limits?

Best,       DennisT

I am no expert but what you say may work.  However I am quite sure that the same motor will not work in all cases.  You have to match the KV ratting of the motor to the number of cells in the pack.  I suspect in the end their isn't a lot of weight savings because the total weight of the battery = the available energy.  Doesn't matter if it is high voltage or high current.  The weight advantage of HV might be in smaller wire and smaller speed controller.  But as I say what do I know.
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Offline Tim Wescott

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Re: Motor voltage limits
« Reply #4 on: August 30, 2014, 10:06:20 AM »
Hey Dennis:

The 5S, 2700mAh pack has approximately 50 watt-hours of energy.  The 4S, 4000mAh pack has about 60 watt-hours of energy.

To really compare apples to apples you'd find a 4S, 3400mAh pack and get that weight -- assuming that these hypothetical 3400mAh packs are up to snuff compared to the ones being used.

What John says about pack weight being proportional to available energy is more or less correct, although if you go do a survey with published ratings you'll find a lot of variation.

Going all the way back to your original question and spiffing it up for nit-picky engineering types, if you were to ask "what's the consequence of replacing a 4S pack in a working system with a 5S pack", the answer is that the system as a whole will be less efficient due to a phenomenon called switching losses in the ESC and to a lesser extent extra losses in the motor due to the higher voltages involved.  This probably won't have a big impact on the amount of energy drawn from the battery, but the heating in the ESC may go up quite noticeably*.  The motor probably won't get measurably warmer unless you have a well-equipped physics lab and some time on your hands.

So, bottom line: the weight savings is coming from reduced energy in the pack, and the change may make your ESC get warmer.

* Engineers use "quite noticeably" to hedge their bets -- it covers territory ranging from just perceivable warming to smoke coming out.  I don't think you'll see smoke.
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Offline Tim Wescott

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Re: Motor voltage limits
« Reply #5 on: August 30, 2014, 10:08:16 AM »
All my answers are made assuming that your ESC is rated for 5S, of course -- if it's only rated for 4S then you risk breaking the ESC.  Just how much depends on the ESC -- some manufacturers will be cutting corners everywhere they can, others will build in lots of margin.
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Online Dennis Toth

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Re: Motor voltage limits
« Reply #6 on: August 30, 2014, 03:48:25 PM »
Tim and John,
Agreed, most of the weight savings seems to come from having a smaller 5S pack that has enough capacity to get through the pattern compared to the available 4S packs for the same total energy use. I had not thought of the switching loss in the ESC at the higher 6S voltage being throttled down so much as to hold the same rpm. Need to look at available packs.

Best,        DennisT

Offline Dan Bregar

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Re: Motor voltage limits
« Reply #7 on: August 30, 2014, 07:37:09 PM »
Dennis

A closer comparison would be a 5S2700 with a 4S3300.   :)

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Offline Igor Burger

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Re: Motor voltage limits
« Reply #8 on: August 31, 2014, 05:41:06 AM »
If you apply higher voltage to the motor itself then it would need to run faster, with less prop, for the current to be less.

There is a difference between higher voltage throttled down, higher voltage with slower motor and higher voltage running at higher rpm.

If you use higher voltage with less prop as Tim wrote (or geared down), then also best efficiency point apears at higher RPM and it could be possibly at rpm which is not allowed for particular motor, so in that case there is a real voltage limit. Maker usualy specify highes practical number of cells.

If you use higher voltage and slower motor, then there is no "automatic" difference. Only practical reasons make some number of cell better then other.

If you use higher voltage throttled down (this is typical case with our combination with ESCs with governor), it acts like a overloaded motor, result is more heating in both motor and also ESC so the result is LOWER efficiency than properly matched battery, motor and prop. That is why we cannot use 30A ESC even out average current is at that value. Proper combination is, that motor at lowest battery voltage (discharged and loaded) reaches highest necessary RMP. It also depends if it is constant RPM setup of combo with active timer. It is usually visible on some ESC data after flight, like max throttle coming from CC escs on their chards or "max current" on Jeti Spins etc. The rule is, throttle should never reach 100%, but it should be as close as possible.

Offline Igor Burger

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Re: Motor voltage limits
« Reply #9 on: August 31, 2014, 05:51:32 AM »
My understanding is that "current" causes heat, higher voltage lower current (less heat) for the same rpm set point. Can you go to far with the voltage?

Yes it does, but you must remember that current on battery leads is not equivalent to current in motor winding (and thus current going via transistors) the rule is, that if you turn the same prop at the same rpm, you need the same torque and since torque = current * 1/KV and since KV is the same also average current is the same (ESC and motor coil is DC converter) ... and since we regulate current by duty cycle of PWM, then the only difference is, that chopped current replaces the DC current which can make for example 10A ^ 2 * 10mOhm loses what makes 1W but you feed coil by 10% duty cycle of PWM means 100 ^2 * 10 mohm /10 what makes 10000 * 10 mohm /10 and that is 10W ... so you see you can have easily 10 times more loses :- ))) ... fortunately coil in motor has some inductance, so it is not so bad, but it will not be good idea to expect the same loses if you feed coil with shorther duty cycle at higher voltage .. and yes it make more loses in both motor and ESC

Offline Igor Burger

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Re: Motor voltage limits
« Reply #10 on: August 31, 2014, 06:14:25 AM »
Going all the way back to your original question and spiffing it up for nit-picky engineering types, if you were to ask "what's the consequence of replacing a 4S pack in a working system with a 5S pack", the answer is that the system as a whole will be less efficient due to a phenomenon called switching losses in the ESC and to a lesser extent extra losses in the motor due to the higher voltages involved.  This probably won't have a big impact on the amount of energy drawn from the battery, but the heating in the ESC may go up quite noticeably*.  The motor probably won't get measurably warmer unless you have a well-equipped physics lab and some time on your hands.

Yes that is true, but I do not think "from switching loses". If I remember well 30 years my courses, switching loses are linear current * voltage (beside frequency, switching time, resistance etc ) ... means not only current. Means if the same device (ESC) manages the same power, does not matter if it is 1A and 10V or 10A and 1V, loses will be the same, also duty cycle does not change it. So going from 1V feeding to 10V feeding will not change switching loses. However what IS changed, is current in duty cycle. It is exactly the same problem as I described on motor coil resistance. Simply chopped current will make much higher loses then the same average current because it comes from sqare of current. It is necessary to understand that the current goes via transistor not only in duty cycle when transistor is open, abut also when it is closed via its recuperation diod. ESCs are rated at full throttle, but we use them at partial throttle so current is higher then what we can see on battery leads.

It is tricky, but easy to check, if you fly prop 11x5 on well matched battery, motor and prop close to its best efficiency, and ESC reaches some temperature, then changing to 11x6 prop which will need little closed throttle and also input power is noticable lower (I mean power to prop, means power on shaft), ESC can heat up MORE.

Not to mention that motor in such regime of work will easily fall to worse efficiency. The current in duty cycle is higher then optimal and motor will simply heat more then the same running at its best point in 100% duty cycle. However that will heat motor, not ESC.

Offline Tim Wescott

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Re: Motor voltage limits
« Reply #11 on: August 31, 2014, 10:16:29 AM »
Yes that is true, but I do not think "from switching loses". If I remember well 30 years my courses, switching loses are linear current * voltage (beside frequency, switching time, resistance etc ) ... means not only current. Means if the same device (ESC) manages the same power, does not matter if it is 1A and 10V or 10A and 1V, loses will be the same, also duty cycle does not change it.

Switching losses are more complicated than that, and they definitely depend on the voltages involved as well as the currents.  I'm always at my most dangerous when I'm trying to be authoritative before I've had any caffeine, but here goes:

Switching loss in a transistor happens because the transistor does not turn off immediately.  The loss happens when the transistor is trying to pinch off the flow of current.  So it is the voltage and current at the transistor, times the amount of time it takes for the transistor to turn off, times the switching frequency.  Roughly, the current is the current in the motor, at the moment.  When the supply voltage is higher than the voltage the motor needs, the current at the "more driving" transistor is higher than the average current from the supply (how much higher depends on various voltages, the switching speed, and the motor inductance).  The current depends on supply voltage, the motor's back-emf (i.e., how fast it's turning), the motor inductance, the duty cycle, and the switching frequency.  How quickly the transistor turns off depends on the motor inductance, the voltage on the transistor at the instant that turn-off is commanded, and the gate driver circuit.

In general, the switching losses are at their worst when the duty cycle is 50%, and with all things being equal they get worse as the supply voltage goes up.  But there are so many details about circuit design, switching frequency, etc., that you can only make the most general of statements about what will affect things.

My knowledge is a bit more recent than 30 years: the second to last circuit board that I designed for a customer, over the past two years or so, is a switched motor driver.  It's much like the ESCs that we use except that it's chock full of bells and whistles and there was no driving need to make it super compact, so a board that'll deliver 200W to two motors is about 130mm x 180mm, and even then my layout guy whined about it being tight.
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