Matching speed controller to Motor

[Bluebeagle]

Hi all,
We have been slowly assembling an electric go cart with some gifted parts, in particular a motor, speed controller and battery. The motor is great. It's looking like we may need to go with a new speed controller too. Here's the question. We have a burly 24 v 1875w 2.5hp motor. Does the speed controller have to match the potential power output of the motor? Easy to find a 24v speed controller, it is near impossible to find a 24v/2000 w speed controller. They mostly exist with 48v motors. In other words could we go with a 24 v 1000 w speed controller?

Thanks for the feedback!

Jeff

 

[Rat]

Hi all,
We have been slowly assembling an electric go cart with some gifted parts, in particular a motor, speed controller and battery. The motor is great. It's looking like we may need to go with a new speed controller too. Here's the question. We have a burly 24 v 1875w 2.5hp motor. Does the speed controller have to match the potential power output of the motor? Easy to find a 24v speed controller, it is near impossible to find a 24v/2000 w speed controller. They mostly exist with 48v motors. In other words could we go with a 24 v 1000 w speed controller?

Thanks for the feedback!

Jeff

You could but the penalty would be a slight overall power reduction, you'd be better off over watting than over volting though. Watts are a bit more forgiving than volts in my experience

 

[EpsilonZero]

I "overvolt" most of my motors. You mostly need to worry about running them beyond the RPM they can mechanically handle or generating too much heat (too much power/watts). The issue you run into is usually the cheaper motors don't provide you with their design limits... or really much information at all. I have a Currie motor rated for 24v 450w that I run at 36v 1000w without a problem. I run my Motenergy ME1719 "60v-72v" motor at 81-87.6v. The "72v" controller is run at the same higher voltage, but everything is within design limits... e.g. motor under max RPM, controller under max pack voltage.

If you want to step up on the controller power at that voltage, Kelly makes 24v controllers that can handle around 12000w. Have a pack that can handle 500A may be a different story, though.

 

[Bluebeagle]

Interesting comments guys, thanks for the replies. This motor was previously used in battlebot competitions and was commented on that it can handle 36 v. Just wondering if this motor is too power hungry. We initially did a bench top power supply at 24 v and the max supply output of 5 amps. The motor drew about 14 volts at 5 amps. We put the cart on the ground under own weight (first load test) and got zero torque/ would not move! We did out first battery test today and connect a 24v/7ah battery with a temporary on/off switch. flipping the switch we got a momentary BIG burst of power that caused the cart to jolt forward under my son's weight and the cart weight. Promising however we lost power immediately and discovered the battery had 3 amps left. attaching a photo but just know we went with our skill set and available material so already know it's our clunky masterpiece.

 

[EpsilonZero]

Interesting comments guys, thanks for the replies. This motor was previously used in battlebot competitions and was commented on that it can handle 36 v. Just wondering if this motor is too power hungry. We initially did a bench top power supply at 24 v and the max supply output of 5 amps. The motor drew about 14 volts at 5 amps. We put the cart on the ground under own weight (first load test) and got zero torque/ would not move! We did out first battery test today and connect a 24v/7ah battery with a temporary on/off switch. flipping the switch we got a momentary BIG burst of power that caused the cart to jolt forward under my son's weight and the cart weight. Promising however we lost power immediately and discovered the battery had 3 amps left. attaching a photo but just know we went with our skill set and available material so already know it's our clunky masterpiece.

5A current is very low. Running it unloaded on a bench is not going to draw much current. Most of the controllers for those little motors allow for more like 40A. You need a controller rather than just an on/off switch. Did you mean the battery had 3V left rather than 3A? That's very dead.

 

[Bluebeagle]

We've since learned trying to measure amps on a volt meter is not accurate. That said, you are correct, they are very dead. The motor is drawing at least 80 amps; In essence we were attempting to power a Tesla with a 9v battery. Amazing. We would need on the order of a 24 v 100 ah battery to provide enough power. Unfortunately, that takes LIfePO4 off the table due to cost. So we will either go AGM route and stick with this motor or switch to something like a 36v/500 w motor. I've guesstimated our cart weight will approx. ~100lbs. + rider weight of ~125. Based on this guide, we could get away with this motor and reduce our battery requirements significantly.

 

[EpsilonZero]

You don't have to run it at 80A. A controller can limit the current drawn from the battery at the expense of maximum torque. If you want to run that kind of amperage, eight 38120HP LiFePO4 cells in series can handle 24v 80A continuous and 200A max if you want to build your own little battery. They are $7 each from BatteryHookup, used. Just make sure you use a BMS.

 

[Bluebeagle]

Hi EpsilonZero, Thanks for the feedback and reference for used batteries. This will be VERY useful for future builds! As our knowledge base and confidence grows, building a battery pack is very high on the list but will require a small investment in a spot welder and a lot of reading. For now we are making great progress, albeit incremental. We now have very capable 30v Nicd battery packs. Our current conundrum is that our Victor 883 speed controller uses a servo tester with a PWM signal to control the motor speed and direction via a potentiometer. This is another oddity of inheriting a motor/battery/controller system but we're working through it because it's otherwise a great system and has saved us a lot of money. The issue is it is very impractical to use a pot. to control the cart. We have a hall sensor effect pedal that we want to replace this input with. It's also a three wire pos/neg/signal connection but not sure how this will work. Current plan is to wire a rev/neutral/ forward switch between motor and speed controller and connect the pedal to the PWM input on the speed controller and see what happens. Aware this probably all sounds like crazy talk but any thoughts appreciated.