# Need help killing an electronic gremlin [electric go-kart]



## asoap (Aug 7, 2011)

Hey Guys,

I've built an electric go-kart and I'm having some issues getting it up to full speed. As current increases we're getting a voltage drop as is to be expected, but as current gets over 50+ amps we're seeing a massive voltage drop after the speed controller only. 

Here are the details:

I'm using this speed controller, the pm 362901: 
http://kellycontroller.com/pm36201200a36vwith-regen-p-543.html

And I'm using three of these batteries hooked up in series 12v each:
http://www.power-sonic.com/images/powersonic/sla_batteries/psh_series/PSH-1280FR_11_Jan_12.pdf

And we are using two of these motors in parallel:
http://tncscooters.com/product.php?sku=106150

We are measuring the voltage in two places, on the batteries before the speed controller, and at the motors after the speed controller. But we are only seeing a massive drop at the motors, the batteries will drop to 31.6 volts, but the motors will drop to 20 volts!

I've done a bunch of tests and these are the results I've gotten which you can see for yourself:
http://www.caffeinedreams.ca/0to60/battery_controller_voltages.pdf

Just a note. The amp readings are wrong. We're using kelly controller's amp meter which connects to the controller. It does show current draw, but the numbers are wrong. 200 amps on the meter is closer to 53 amps.

What's going on? Why would the speed controller cause a voltage drop like that? Is there something we are not providing? Do we need to add more volts, more amps? Replace the controller?

Thanks for help,
-Derek


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## Tesseract (Sep 27, 2008)

asoap said:


> Why would the speed controller cause a voltage drop like that? Is there something we are not providing? Do we need to add more volts, more amps? Replace the controller?


You need... more cowbell.

Sorry, couldn't resist.

First off, it may be that the controller - despite being a Kelly - is working fine. Do you understand how PWM controllers work? It's been explained dozens (if not hundreds) of times here, but, well... no one ever uses the search function so might as well explain it again. 

PWM controllers are essentially buck converters. They chop up the battery voltage, so the ratio of on time to off time determines the average output (or in this case, motor) voltage. But power in this type of converter is conserved, so if output voltage is lower, then input current must be lower by the same proportion.

In other words, if the battery voltage is, say, 100V and the motor voltage is 50V, then the ratio of on time to off time - the duty cycle - is 50%. That means that if the motor current is, say, 200A then the battery current will be 100A.

Try putting a DC current clamp meter on the motor positive cable and the battery positive cable at the same time to see this in action. If the current on both sides is the same then the duty cycle is 100% and the voltage should be the same, too, barring a volt or two in loss.


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## asoap (Aug 7, 2011)

*slaps forehead* of course, MORE COWBELL!

I understand the concept of a PWM, and mosfets. How mosfets work exactly I don't know. I do understand duty cycle and that it's turning on/off power at thousands of times per second. 

Initially when we started this we had a problem with smaller controllers. The car would run but at a VERY slow speed. The previous controllers couldn't get passed the startup current. We hooked up an oscilloscope to it, and found that it was indeed at fully duty cycle. It was rather cool to see the duty cycles displayed like that. We then switched to the 200 amp Kelly controller and fixed that problem. But I digress.

Also if I currently run the car at 50% duty cycle I will see that the output of the motors is 50% the volts of the batteries.

When you say DC current clamp. Do you mean something like this?










I did indeed use a tool like that to measure amps. This was mild load, the amp meter on the car which was reading 100amps (and is clearly the wrong number). Anyway I measured on the motor side to get 37 amps, and the battery side to get 35amps. I don't know if that 2amp difference was because I only had one meter and had to run the test twice or not. Also I am getting the amp readings while holding the breaks on the car. So let's just say that they are pretty much the same.

But if you look at the PDF I supplied in my original post. I used two multimeters at the same time for measuring voltage. For most conditions there is a slight voltage drop from the batteries to the motors. Which is what is expected as the controller eats up a 2-3 volts. It's when we get to 50+amps that there is a 10 volt drop between the two.


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## major (Apr 4, 2008)

asoap said:


> The amp readings are wrong. We're using kelly controller's amp meter which connects to the controller. It does show current draw, but the numbers are wrong. 200 amps on the meter is closer to 53 amps.


Hi asoap,

If the controller displays 200 amps, it probably thinks it is supplying 200 amps to the motor and therefore is in current limit which reduces the output voltage. Did you confirm the duty cycle on this test? Sounds like something in the controller is in need of calibration, or your instruments 

How about some photos of the machine in question 

major


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## asoap (Aug 7, 2011)

Thanks for the reply major. I'm not thinking that it's an internal reading of the controller, as I've seen it at 200 amps+ (gauge reading not actual) and under certain circumstance it's still behaving fine. It's not having a big voltage drop. Or in certain circumstances be bellow 200 amps and still have a big voltage drop.

I emailed kelly controllers also with the same information. And this is the reply they gave me:

The current is opposite to the speed.
The speed is proportional to the battery voltage.
The controller is designed with constant power.
If the current will increase more,the speed or voltage will be reduced.
If it is reduced too much,you may consider checking one of the pack.
One of cell is damaged.

Woops. I forgot photos:

http://www.caffeinedreams.ca/0to60/motors_3.jpg
http://www.caffeinedreams.ca/0to60/motors_1.jpg
http://www.caffeinedreams.ca/0to60/final_work_1.jpg
http://www.caffeinedreams.ca/0to60/final_work_13.jpg
http://www.caffeinedreams.ca/0to60/final_work_3.jpg
http://www.caffeinedreams.ca/0to60/final_work_4.jpg
http://www.caffeinedreams.ca/0to60/final_work_5.jpg


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## asoap (Aug 7, 2011)

I've had a few email bouncing back and forth from Kelly Controllers. Apparently the amp meter should be reading correctly, and because it's not it might mean that it is thinking that it's over the 200 amp limit and going into a safety mode. So I'm going to be replacing it with a 48v, 120 amp continuous, 300 amp spiked version.

But before I send the controller back, I got some new batteries today which I'm going to replace and give it a few tests first.


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