# Not getting full power output from system



## Prop guy (Nov 17, 2014)

Hi,

I'm relatively new to electric motors and am having trouble getting my system to perform where it should be. This forum has been really helpful for me to get up to speed on some of this, so I'm hoping someone out there has an idea to help fix my problem! 

I'm using it as a testing rig for an aircraft I'm building with hopes to turn it into a vehicle when I'm done and need to get it up to >32kW output to properly run my tests but I can't manage to get it above 24kW. I bought the motor, controller, and batteries from Electric Motorsport and it should be able to put out up to 38kW of power.

*Components*
Motor/controller: PMAC 15kW Cont. / 38kW Pk. Liquid-Cooled motor with Sevcon Gen 4 controller (size 8 I think)
_-http://www.electricmotorsport.com/ev-parts/motor-drive-kits-2/brush-less-pmac-motor-kits/pmac-15kw-cont-38kw-pk-liquid-cooled-motor-drive-system-72-84v-550a.html_
Batteries: GBS 84V 100Ah LiFeMnPO4 set with BMS
_-http://www.electricmotorsport.com/ev-parts/batteries/battery-packages/gbs-84v-100ah-li-ion-battery-pack-with-emus-bms-and-charger.html_

I also have a Clearview display which lets me see the motor data in real-time, which is how I know that the batteries max out at ~80V @ 300A, while the motor is being fed ~21kW of power. I've tested it with a wide range of gear ratios and it made almost no difference.

I know the batteries are rated for 3C continuous discharge / 10C pulsed discharge for short periods and I only need higher amperage for ~5-10 seconds which the system should be more than able to handle but for some reason it's just not doing it.

Any ideas for things I could try to fix it? Could there be a controller setting that was programmed wrong? Could I have screwed up the install somehow and crossed a wire? It seems suspicious to me that I can get up to 3C of current exactly like the controller either isn't demanding more power from the batteries or the batteries are not supplying it with more. Any help or thoughts would be greatly appreciated!


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## dougingraham (Jul 26, 2011)

You have 84v of batteries. This is probably 24 cells which gives an average of 76.8 volts. At 3C they sag about 25% so at 300 amps the voltage will be approximately 58 volts. This would be 17.4kw. I have the same batteries in my car and they seem to peak out at about 1400 watts per cell. Since you have 24 cells your peak will be around 33.6kw. You can pull more amps but the output power stays the same because of the additional voltage sag.

Since you are seeing 24kw out of your batteries and 21kw into the motor you are seeing some losses between the batteries and the motor. About 88% of the power is making it or you have some sort of instrumentation error.

I would suspect some kind of limit is being applied in the controller.


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## Prop guy (Nov 17, 2014)

I tend to agree with you that it's something in the controller - the battery is a 28 cell system and drops to ~80-81V @ 3C and as soon as it reaches ~80V it stops and never goes any lower. If I could manage to get 1400W out of each cell I'd be peaking around 39kW of total power vs the 24kW I'm getting right now (~850W per cell).

What voltage per cell are you getting when you reach 1400W each? I assume the controller you're using has no problem drawing more power out of them for short bursts?


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## major (Apr 4, 2008)

Odd that the link to the motor doesn't specify a speed or give a torque curve. Just wondering what RPM you're turning, what is base speed on the motor, how you control the load, things like that which might help 

What help did the vendor offer?


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## Hollie Maea (Dec 9, 2009)

If I had to hazard a guess, I would guess that they sent you the 130V version, which does in fact have a 300A limit. Or perhaps more likely, they accidentally configured it for those settings. There is probably a sticker on the side of the controller that states the nominal voltage.

For sure there is no hard current limit being imposed by the batteries or motor. It has to be the controller.


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## Prop guy (Nov 17, 2014)

They did send me some of the torque curve and speed info (I think it's just not on the website). So far I've tried different gear ratios with the motor turning between 2600 - 4600 RPM when it maxes out and won't go any faster, all at the same power output. The motor has a max speed of 6500 and throttle is a 'pot box throttle', with no change in motor speed after throttle is 50-60% open.

Vendor has been great and I've been on the phone many times trying to figure it out but we're both at a loss right now over what could be causing it.  In fact, they're being supportive enough to bring some testing equipment over to my shop in a few days to try and get some more data on what's going on, but there's no guarantee we'll figure it out from the data they gather. 

Basically I have the motor driving a timing belt which runs to an aircraft propeller so I can test the thrust produced at different speeds, pitches, configurations, etc. I take video of the vehicle status screen which lets me go back and analyze the controller information for power output/input.  I went back and analyzed ~20 different tests and they all maxed out right at 300amps from the batteries.


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## major (Apr 4, 2008)

Prop guy said:


> They did send me some of the torque curve and speed info (I think it's just not on the website). So far I've tried different gear ratios with the motor turning between 2600 - 4600 RPM when it maxes out and won't go any faster, all at the same power output. The motor has a max speed of 6500 and throttle is a 'pot box throttle', with no change in motor speed after throttle is 50-60% open.
> 
> Vendor has been great and I've been on the phone many times trying to figure it out but we're both at a loss right now over what could be causing it.  In fact, they're being supportive enough to bring some testing equipment over to my shop in a few days to try and get some more data on what's going on, but there's no guarantee we'll figure it out from the data they gather.
> 
> Basically I have the motor driving a timing belt which runs to an aircraft propeller so I can test the thrust produced at different speeds, pitches, configurations, etc. I take video of the vehicle status screen which lets me go back and analyze the controller information for power output/input.  I went back and analyzed ~20 different tests and they all maxed out right at 300amps from the batteries.


Can you post that torque curve/speed info? Or determine what the base speed is for your voltage? Typically controllers set current limit on phase current, not battery current. Strange that you hit 300A max battery over such a wide speed range. Do you know the range of phase current for those test points?

Nice to hear you are getting vendor support. I think this is the same motor winding we discussed some time ago concerning a discrepancy in the ad. It would be nice to hear what the vendor says on the subject, and it may relate to your issue. Ref: http://www.diyelectriccar.com/forums/showthread.php?t=92045&highlight=liveforphysics


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## rwaudio (May 22, 2008)

Prop guy said:


> I tend to agree with you that it's something in the controller - the battery is a 28 cell system and drops to ~80-81V @ 3C and as soon as it reaches ~80V it stops and never goes any lower. If I could manage to get 1400W out of each cell I'd be peaking around 39kW of total power vs the 24kW I'm getting right now (~850W per cell).
> 
> What voltage per cell are you getting when you reach 1400W each? I assume the controller you're using has no problem drawing more power out of them for short bursts?


Well there's a good chance what you said right here holds the answer.
As soon as it drops to 80v it doesn't go any lower....

What's the low voltage limit on the controller? 80v? Is it programmable?
Change it to 75 or 70v and you will likely find more power..


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## dougingraham (Jul 26, 2011)

rwaudio said:


> Well there's a good chance what you said right here holds the answer.
> As soon as it drops to 80v it doesn't go any lower....
> 
> What's the low voltage limit on the controller? 80v? Is it programmable?
> Change it to 75 or 70v and you will likely find more power..


On 28 cells 80 volts is 2.85 volts per cell. For the winter when the batteries get down to 0C I used to set the low voltage cutoff to 1.6 volts per cell because they sag so terribly. Now I just set it to 1.73 volts per cell (90 volts) so my DC-DC doesn't have any issues. People wig out when they see voltages below 2 or even 2.5 but voltages under load don't mean anything relative to state of charge.

If you want to get full power out of the cells set the low voltage limit on the controller to 1.6 volts per cell or in the case of 28 cells 44.8 volts. This is half nominal voltage and you will have reached the max power point on the cells at this level. You will never see this level of sag unless it is horribly cold or you are pulling 600 amps for longer than a few seconds.

My guess is that the controller is set to limit to 300 battery amps and 80 battery volts in a misguided attempt to protect the batteries.


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## dougingraham (Jul 26, 2011)

Prop guy said:


> I tend to agree with you that it's something in the controller - the battery is a 28 cell system and drops to ~80-81V @ 3C and as soon as it reaches ~80V it stops and never goes any lower. If I could manage to get 1400W out of each cell I'd be peaking around 39kW of total power vs the 24kW I'm getting right now (~850W per cell).
> 
> What voltage per cell are you getting when you reach 1400W each? I assume the controller you're using has no problem drawing more power out of them for short bursts?


I don't know the voltage. I have not logged the data. What I do know is whp on the dyno and I see it come up to a certain level and hold there. The controller is perfectly happy passing along up to 1000 amps to the motor. At 1000 amps the voltage would be sagged to 1.4 volts per cell but I have a higher voltage limit than this and I have a battery current limit of 600 amps. But these things all interact in a somewhat complex fashion. The only thing the motor controller can actually do directly is lower the battery voltage that the motor sees. Everything else is a result of this. You give a motor a voltage and it wants to turn at a certain RPM. If it cant reach that RPM it pulls more and more amps in order to try to get there. The controller will enforce limits on voltage and current by changing the voltage sent to the motor. Reducing the motor voltage will lower what the motor wants to pull for current. If the current is too high on the batteries they sag so the controller lowers the voltage the motor sees and it draws less current so the batteries see less load.

I said I have the same batteries, I have 100AH cells and you have 60AH cells so not exactly the same. Since I have a 600 amp (6C) limit on my batteries I am probably seeing about 2.33 volts per cell in order to reach 1400 watts. And the 1400 watts is from Wheel Horse Power so motor HP would be as much as 15 percent more than this meaning 1600 watts per cell output of the motor and if the motor is 90% efficient then another 10% more than this or around 1770 watts per cell. I guess I need to log some data into the motor to see what is really happening.


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## Prop guy (Nov 17, 2014)

Hollie Maea said:


> If I had to hazard a guess, I would guess that they sent you the 130V version, which does in fact have a 300A limit. Or perhaps more likely, they accidentally configured it for those settings. There is probably a sticker on the side of the controller that states the nominal voltage.
> 
> For sure there is no hard current limit being imposed by the batteries or motor. It has to be the controller.


I checked and it's the beefy 650A controller, but that's a good idea to verify the settings weren't accidentally set for the 130V version! 



major said:


> Can you post that torque curve/speed info? Or determine what the base speed is for your voltage? Typically controllers set current limit on phase current, not battery current. Strange that you hit 300A max battery over such a wide speed range. Do you know the range of phase current for those test points?
> 
> Nice to hear you are getting vendor support. I think this is the same motor winding we discussed some time ago concerning a discrepancy in the ad. It would be nice to hear what the vendor says on the subject, and it may relate to your issue.


After looking for the files I realized I don't actually have the torque curve like I thought I did. I think I have an efficiency curve for different speeds but it's not that helpful here  



rwaudio said:


> Well there's a good chance what you said right here holds the answer.
> As soon as it drops to 80v it doesn't go any lower....
> 
> What's the low voltage limit on the controller? 80v? Is it programmable?
> Change it to 75 or 70v and you will likely find more power..


Unfortunately I don't have a computer interface for the controller, just a Clearview Display. The only setting I was able to find is an 'Undervolt limit' which is set at 52V. I think the vendor is bringing an interface in a couple days and I can check then if there's an internal setting that's limiting it to 80V




dougingraham said:


> My guess is that the controller is set to limit to 300 battery amps and 80 battery volts in a misguided attempt to protect the batteries.


I'm starting to agree with you that there must be an internal setting that's artificially limiting it to stop drawing more current once it hits 80V. I'll play around with the few settings I have access to tomorrow and see if it makes any difference.


I do want to say I appreciate all of your help so far! Any advice on where to get a computer interface for a Sevcon Gen 4 controller? The ones I saw have very long delivery times 

Here's a picture of my set-up in case you're curious. I gets pretty loud with a 44" propeller turning at 3400 RPM


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## piotrsko (Dec 9, 2007)

Bet your tips are going supersonic.


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## dedlast (Aug 17, 2013)

piotrsko said:


> Bet your tips are going supersonic.


You got me curious.
Circumference is 2*pi*R, yes?
So, 2*3.14*22 = 138.16 inches
138.16*3400 rpm = 469,744 inches per minute
469,744/12 = 39,145 feet per minute
39,145/5280 = 7.414 miles per minute
7.414*60 = 444.8 MPH

YMMV (your math may vary, but I think it's right)

Not quite supersonic but still really fast. Don't stick your finger in there.

B


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## Prop guy (Nov 17, 2014)

Yup - you're math's pretty good! I've gotten these up to ~670mph before using my gas engine with no problem, but above that you run into supersonic and compressibility issues.  In short, it's terrifying when this is running.

Right now they're running mach ~0.58 which is well within their operational range. I need data up to mach ~0.7 to be able to optimize my speed / pitch properly which is why this issue is vexing me so much. 

But yes, I keep my fingers far away


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## aeroscott (Jan 5, 2008)

After you get the power problem solved. I was thinking of how much thrust you are after.Of coarse the most possible . If you can increase swept area and slowing tip speeds (limited by top speed and airframe constants), you can increase thrust hugely .What brought this home was a story about a copy of the Wright brothers plane. They used a modern engine and prop of the same hp but could not get airborne . The Wright's 2 props , 6ft. dia. and very low rpm, which had huge low speed thrust on very low hp.
edit; props 8'6" , 12 hp ,airplane speed 30mph


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## Duncan (Dec 8, 2008)

Hi Aeroscott

The best way to think about it is as a reaction drive

Thrust = Mass flow rate times "exhaust velocity" (momentum equation)
Energy required = 1/2 Mass flow rate times "exhaust velocity" SQUARED (energy equation)

So a larger mass flow rate with a smaller "exhaust velocity" will give the same thrust but require much less energy

With a larger prop and the same speed you have a larger mass flow rate

Which is why helicopters are practicable but something like a Harrier requires a LOT more power


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## aeroscott (Jan 5, 2008)

If we take 1/2 the prop speed(inverse squared) , add a second prop (2X). 
I can't think where we end up with thrust and power? help


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## Tesseract (Sep 27, 2008)

aeroscott said:


> If we take 1/2 the prop speed(inverse squared) , add a second prop (2X).
> I can't think where we end up with thrust and power? help


Look up "affinity laws".

Airflow is directly proportional to fan speed.
Air pressure is proportional to the square of speed.

So power required by a fan is proportional to the cube of speed, all other things (diameter, air density, temperature, etc.) kept equal.

If you cut prop speed in half then you will reduce airflow velocity by half and pressure to 1/4th. Ie - 1/8th the power. Add a second fan and you get back to 1/4th the power of a single prop running at twice the speed.


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