ADC/Curtis-reduced torque at start up?

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Peter C in Canberra
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ADC/Curtis-reduced torque at start up?

Post by Peter C in Canberra » Tue, 22 Feb 2011, 03:40

Hi,
My car has been going just fine for nearly two years with ADC 8" motor and Curtis 144V/500A controller. The ramping rate and current limits are and have always been fully clockwise-maximum. Lately I have been feeling that the car just doesn't quite have the torque it used to have at low speed/low revs. Once I get going at a few krpms it is as good as ever. None of the connections are loose or getting excessively warm. Battery current and voltage look normal. I gave the Curtis adjustment pots a twiddle in case they were not contacting well or dirty but it made no difference. Since the car performs well once going I don't think it can be a major fault, more likely something subtle. I also have the feeling the delay at lower power before the power kicks in after starting is a bit longer. I used to take off at a respectable rate at lights but now it is a bit embarrassing, especially up a hill. Any suggestions?
Thanks,
Peter C.
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ADC/Curtis-reduced torque at start up?

Post by coulomb » Tue, 22 Feb 2011, 04:58

Peter C in Canberra wrote: The ramping rate and current limits are and have always been fully clockwise-maximum. ... Any suggestions?

It sounds to me as though the motor current limit is reduced. The current might be measured by a current shunt (internal to the controller), and perhaps the shunt has gone a little high resistance (so the controller sees the current limit before the real current is at the real limit, if that makes any sense).

Perhaps you could use a clamp meter (perhaps borrow one if necessary) to see whether you are getting 500 A or close to that at start-up. At higher speeds, you may well be battery current limited, so the lower current limit might not be noticed, or indeed have any effect at all.

You might want to use the peak measuring ability of a suitable meter, if you can't find a volunteer to watch the reading under the hood with their legs sticking out vertically while you accelerate the car Image A fancier meter might be able to graph the current for you. But just knowing the peak motor current might be all you need.

The other possibility might be to convert your ammeter from battery current to motor current temporarily, but that could be a lot of work.

Unfortunately, if it is the current shunt in the controller, there may not be a whole lot to do about it other than replacing the whole controller. Curtis controllers are usually fully potted, which makes them tricky to repair at home. Of course, some have done it, and I'm sure you can find instructions on the web on how to do it successfully and replace the potting when you're done with the repair.

Perhaps there is a repair facility, but it possibly involves a lot of downtime. Maybe you could make use of David Sharp's hirable controller if yours had to go overseas for some weeks to get repaired.

But hopefully others will suggest something much cheaper and simpler that you can try first. For example, perhaps the main contactor is pitted or worn and dropping several volts at higher current.
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ADC/Curtis-reduced torque at start up?

Post by Richo » Tue, 22 Feb 2011, 20:31

coulomb wrote: But hopefully others will suggest something much cheaper and simpler that you can try first.


Clean Motor Brushes
So the short answer is NO but the long answer is YES.
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ADC/Curtis-reduced torque at start up?

Post by Peter C in Canberra » Tue, 22 Feb 2011, 21:55

Richo wrote:
coulomb wrote: But hopefully others will suggest something much cheaper and simpler that you can try first.


Clean Motor Brushes

Ah! Now that's a plausible and appealing option I had not thought of. I guess I had just assumed they were sort of self-cleaning as they rub on the spinning bit (technical term, sorry), but perhaps not. Anything obvious to do or look out for? I have never taken out the brushes having been told they last for ages.
Thanks,
Peter
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ADC/Curtis-reduced torque at start up?

Post by Johny » Wed, 23 Feb 2011, 15:07

Hi Peter.
You don't have to remove the brushes. Visual inspection is adequate to check brushes. See if you can remove a brush cover or get to have a good look at the brushes and brush springs. The commutator should be a consistent darkish copper colour where the brushes run, with no obvious scrapes or deep lines.
Example:
Image
Check if if the brush springs have good tension by lightly pulling on the springs with a piece of "hooked" wire or such. Check all brushes.
If there is a lot of dust from brush wear in the motor. Vacuum this out, don't blow it. (Maybe make up a q&d vacuum cleaner hose reducer with plastic tubing and duct tape.)
The normal motor cooling should be keeping it fairly free of dust.

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ADC/Curtis-reduced torque at start up?

Post by Richo » Wed, 23 Feb 2011, 20:42

The brushes themselves may last a while but
If grease/grime build up it could create a non conductive coating.

Sorry as I dont have a DC motor I don't know the prefered method of cleaning them.
If the brush assembly can be removed then the rotor contacts could be wiped with Isopropyl alcohol.
The brush assembly could then be given an air blow clean.
So the short answer is NO but the long answer is YES.
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ADC/Curtis-reduced torque at start up?

Post by T2 » Sun, 24 Apr 2011, 20:26


I would be interested to see what your DC voltage bus looks like at low speed. It would seem that the main caps may be drying up. Sometimes one capacitor will "let go" simply by going open circuit.

Let us say your PWM output is on a 10khz carrier i.e. 100 musec period.

Then at 10% output voltage the output pulses are 10 musecs.

If you are trying to pull 500 Amp this current can only come from the caps. It is too short an elapsed time to get current from the battery.

I = C dV/dT is the formula at work here where the delta V should be a few volts only and represents a slight dip in voltage on the DC bus - that's if the caps are working. You can see what happens in this equation if C is somehow reduced, dV will increase to compensate. If the capacitance is low enough the voltage may collapse enough that the 500 Amps is not acheived before the pulse terminates.

Of course as the vehicle speeds up the pulse width becomes longer so the dependency on the capacitors being up to snuff decreases. And that's what we are seeing here.

T2

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ADC/Curtis-reduced torque at start up?

Post by Peter C in Canberra » Sun, 24 Apr 2011, 21:07

Interesting. I guess a test of your hypotheis might be to install a suitable capacitor close to the controller. If you are correct and the in-built capacitor(s) are not functioning but open circuit I should restore their function. Is your suggestion based on anyone confirming this as a common failure mode by measuring actual Curtis caps or is it just a plausible hypothesis based on the symptoms?
Thanks,
Peter.
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ADC/Curtis-reduced torque at start up?

Post by Tritium_James » Sun, 24 Apr 2011, 23:53

It's plausible. The caps in the Curtis aren't rated for anywhere near 500A ripple, so they heat up and wear out.

I think they're something like 220u/200V, so probably about 2.5A ripple per part. Something like 30 of them in there? so they're rated for 75A ripple in total. If they're good caps this would get you 8000 hours operation. But they're being pushed about 5x harder then they should be (some ripple will come from the batteries) so they run much hotter, and don't last very long.

BTW, worst case ripple occurs at 50% duty cycle, ie half speed. I don't know what sort of currents you normally do when driving (motor current, not battery current) but if it's more than 75A the caps will be wearing out pretty quick. The rule of thumb is every 10°C rise above normal halves the lifetime. I think for electrolytic caps it is worse than this because the electrolyte starts decomposing and boiling out.

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ADC/Curtis-reduced torque at start up?

Post by Peter C in Canberra » Mon, 25 Apr 2011, 00:26

Tritium_James wrote: It's plausible. The caps in the Curtis aren't rated for anywhere near 500A ripple, so they heat up and wear out.

You would think there might be a cottage industry somewhere refurbishing Curtis controllers with new caps if it was so predictable that they would wear out, even upgrading with higher spec components if they could fit. Does anyone know if Curtis does that?
I have no idea what motor side current I have might have in normal driving. Much of it is on the arterial road around Canberra at 80kph or so with hills so working pretty hard. I have had the controller for 2 years and 20,000km but it had an uncertain history, apparently second hand but little use.
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Post by Tritium_James » Mon, 25 Apr 2011, 00:41

No electrolytics that will fit in that size box will be rated any higher, so the best you'll be able to do is get it back to new again, but it will then wear out just as fast.

I think the reason there's no people offering upgrades is that the controllers are potted in place - isn't it basically impossible to get one apart without breaking things?

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Post by Electrocycle » Mon, 25 Apr 2011, 01:29

The end covers are cast in place, but if you get them loose (people use heat guns / hot water / hammer, etc) you can pull the board out reasonably easily.
There are a lot of screws on the bottom, under a cover plate / sticker from memory, securing the heatsink to the case.
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ADC/Curtis-reduced torque at start up?

Post by T2 » Mon, 25 Apr 2011, 17:14

Thanks for corroborating my suggestion T_J, and yes I agree 50% duty cycle is absolute worst case but mentioning that might spoil the good story I had going !!

If my experience is in question, I have seen only the one beer can size go OC and it was in a group of four. The caps were not potted and were in the path of fan cooling. The cap terminals were connected with M4 screws so checking was easy in this case. BTW the head of the screw is quite important.

I know of one manufacturing operation where the pan head screws were inadvertantly replaced with screws of much smaller head size. The reduced contact size to the bus bars increased the contact resistance. This added to the effective ESR of the capacitor. The effect of this increased resistance to the main input filter on production 24 volt 100 Amp regulated supplies caused them to go unstable. It was a call to the capacitor manufacturer - luckily an american company - that resolved the problem.

For individual components soldered to PCB I've replaced quite a few. That the switcher will start squealing and squawking is usually the first clue along with loss of ability to maintain output voltage on load.

However if we are talking arrays of radial legged components all in close proximity and in parallel which is typically done to get the lowest ESR in the smallest footprint. Not seen that yet. I know that office UPS's which use them are often thrown out just becaused the sealed lead acid accumulators have dried out. The owners never realised that these large replaceable components existed inside !!

Have you tried contacting Curtis ?? Since the controller is still functional they might be motivated to replace it for a few hundred dollars. 20k kms is quite respectable for an EV They might be interested in some forensic investigations. That would be 10 years of use for a golfcart or forklifter - their main business.

I was thinking of going for a Curtis myself but encasing the caps in potting compound may be perfectly OK for the latter applications but for a highway capable EV I am not so sure.

If Curtis doesn't come through, and you have to decide whether to sell your controller with appropriate disclaimers or try the capacitor replacement route please let us know. I could possibly spare a few dozen Nichicons (200v 470 mu date code 8838) but you need to specify the form factor in case they may not fit.   
T2

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Post by Electrocycle » Mon, 25 Apr 2011, 17:41

quick search on google found this: http://youtu.be/afuMg2Jmilo


As you can see, a bunch of caps in the middle of the heatsink.
From memory they are about the size TJ mentioned.
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ADC/Curtis-reduced torque at start up?

Post by EV2Go » Mon, 25 Apr 2011, 18:49

The next video illustrates it coming out the end Youtube video as per the video Andrew linked, you can better see how those bolts prevent you taking off the heatsink.

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Post by Salty9 » Wed, 27 Apr 2011, 16:29

lou-ace at:

http://www.diyelectriccar.com/forums/sh ... 57893.html

has been having problems similar to yours. Don't know if they are related but I found this article about low speed Curtis 1231:

http://cafeelectric.com/curtis/FirstFeet.html
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Post by Johny » Wed, 27 Apr 2011, 16:43

Links fixed.
Salty9 wrote: lou-ace at:

http://forums.aeva.asn.auhttp://www.diy ... 57893.html

has been having problems similar to yours. Don't know if they are related but I found this article about low speed Curtis 1231:

http://forums.aeva.asn.auhttp://cafeele ... tFeet.html

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ADC/Curtis-reduced torque at start up?

Post by Peter C in Canberra » Fri, 29 Apr 2011, 04:04

Thanks for the comments. The surging issue I had in 2009 did turn out to be fixed by a new potentiometer in the pot box. The replacement has been good for 18 months or so. The low torque at low speed is a different issue. I am considering a new controller, either the ZEVA 600A controller or an open source DIY design that a Canberra branch member has built. If these fix the issue I will know that the controller has a problem as suggested above, if it does not fix the issue I will know to look elsewhere and have a spare controller!
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ADC/Curtis-reduced torque at start up?

Post by Peter C in Canberra » Tue, 17 May 2011, 01:08

An update:
I borrowed a clamp meter which let me record the maximum DC through one of the motor leads. So, I kept resetting and then driving a short distance, then stopping to check the maximum seen. The experiment results were not quite as conclusive as I would have liked but I have ordered a Kelly 1000A controller. I would have been happier if I had seen either the Curtis' rated hard limit of 500A (no controller problem) or much less current (say 300A, definitely a controller problem).
The Curtis is rated to deliver 500A for 2 minutes. At low speed from stationary, even with my foot to the floor, I consistently saw 400-420A max when I should have had 500A. This was with sluggish take off. I could imagine that 20% more current might have been enough to have the adequate-but-not-especially-impressive initial torque I was used to having in 3rd gear, but I am not as confident as I would have been if I had seen less than 400A.
When I continued up to higher revs and speed and got good performance once moving I could get up to 460A max. I should really have been getting no more current than at the start. As revs rise the motor voltage should climb and the motor current should fall or at least not rise. I checked the clamp meter gave the same peak reading on the battery side as my built in metering that measures the voltage across a small series resistance. After lots of shorter or longer runs and always seeing several hundred amps I checked the temperature of the brushes and commutator by touching them. They were only slightly warm and no more so than the body of the motor.
I was surprised that I never saw less than 200A even for most modest carpark manoeuvring, not asking for any significant performance. On the battery side of the controller I had only a few 10s of amps at such times.
The only non-controller problem I can think off now would be shorted turns in the motor. That would be consistent with high current and poor torque but I don't think it is consistent with perfectly good normal performance at higher revs with plenty of torque and the motor not overheating.
Cheers,
Peter.
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ADC/Curtis-reduced torque at start up?

Post by Peter C in Canberra » Sun, 29 May 2011, 04:44

A further update:
I bit the bullet and bought a Kelly KDH14100D from EV Works. I programmed it to limit the motor current to 800A and the battery side to 270A (3C for my 90AH Thunderskys). I have an 8" ADC motor which is supposed to only be rated for 120V so I limited the motor voltage to 120V also. Those are nice features to have. Anyway, I accidentally spun the wheels taking off in 3rd gear on my test drive so I think the motor is OK! I wonder if the problem with the Curtis is something to do with the freewheel diode. I saw a comment somewhere that this part is a bit fragile. Could it be the controller would pur a burst of current into the motor but it could not keep flowing when the transistors turned off?
Anyway, so long as the Kelly does not blow up tomorrow, I seem to have solved the problem. The odd thing is that whatever is wrong with the curtis is quite subtle. Performance is OK if you use the gears for more torque from stationary and it is the same as ever once you are underway. It's just that it is not the same as new.
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Post by Electrocycle » Sun, 29 May 2011, 17:36

the freewheel diodes usually seem to fail to short circuit, and then spectacularly destroy all the FETs as well as themselves...

The Curtis is pretty old school analogue circuitry, so it's hard to work out exactly what's going wrong because a small problem can cause everything to look slightly out of whack, making it very hard to diagnose.
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ADC/Curtis-reduced torque at start up?

Post by Peter C in Canberra » Sun, 29 May 2011, 18:10

I assume there are some things that are relatively simple to fix that can go wrong because I found several places on line in the US who will sell you a refurbished Curtis then you send them your dead/damaged one. I think I will try to sell mine now (acknowledging the history above) since someone could get a car going with it-it did still perform adequately-and it could then be traded against one working fully up to spec. Might be attractive to someone on a tight budget.
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