Is my motor big enough?

[K_IV_Push]

I built a backyard go kart using an electronic motor kit bought online. The motor is listed as 48v DC and 1800 watt output. It uses 4 12v batteries wired in series. The problem is, the motor cannot turn when the kart is stationary. However, when I push the kart and it begins to move at a low speed, the motor will then be able to propel the cart and continue to move it. My question is, is the motor large enough for this vehicle? The motor was advertised as a go kart motor so I assumed it could handle a moderate weight. Is there a way to increase the torque of the motor to give it more power to move the kart if its stationary? The motor seems to work fine once the kart is moving, but its not strong enough to start moving it on its own. This is my first attempt making anything like this and I dont know anything about electric motors.

 

[offGridDownUnder]

1800 watts / 48 volts gives 37.5 amps. Your batteries are in series, so what amps is one battery rated to provide? That is what the collection in series can provide.
That may not be the problem (or the only problem), but it's a start to consider.

Also, you don't mention - prop the driven wheels up off the ground, and start the motor with no load at all. Does the motor turn the wheels then? This will tell you if the motor needs something else to get turning and eliminates that question.

 

[fechter]

That motor should be big enough to make the cart go at least 25 mph.
My best guess would be the motor wire colors do NOT match the controller colors. This is a common problem and you may need to try some different combinations to make it run properly.

 

[K_IV_Push]

1800 watts / 48 volts gives 37.5 amps. Your batteries are in series, so what amps is one battery rated to provide? That is what the collection in series can provide.
That may not be the problem (or the only problem), but it's a start to consider.

Also, you don't mention - prop the driven wheels up off the ground, and start the motor with no load at all. Does the motor turn the wheels then? This will tell you if the motor needs something else to get turning and eliminates that question.

It looks like the batteries are only 22 amps. The batteries actually came with the motor and speed controller and everything else with the kit. Im not very familiaf with amps but 22 seems like a big difference from 37.5. Will getting 4 new 12v batteries rated for more amps help?

Also, when i pick the back end up off the ground, the motor spins just fine. the more I press the pedal the faster the motor goes, it sounds like it has 4 or 5 different speeds. With the wheels off the ground, it can spin at top speed with no problems.

 

[K_IV_Push]

That motor should be big enough to make the cart go at least 25 mph.
My best guess would be the motor wire colors do NOT match the controller colors. This is a common problem and you may need to try some different combinations to make it run properly.

I will try that, but all the wires were labeled from the company that i got the kit from and it seemed pretty straightforward to assemble and connect everything. Most connections were plastic quick-connects but the motor wires connected to the controller wires in the yellow junction box with ring terminals. I've heard of this problem with motors sold on amazon such as the Vevor motors/controllers. But this kit came from a company that seems fairly established so I feel like the wiring is correct.

 

[offGridDownUnder]

It looks like the batteries are only 22 amps

The label on the battery shown is not 22 amps - it is 22 amp-hours. This means that nominally the battery can deliver 1 amp for 22 hours, or 22 amps for one hour. This figure does not tell you what amps the battery can deliver at any given instant - it might be 1/1000 of an amp for 22,000 hours and still fit that statement.

An ampere (amp) is a measure of the quantity of electrons that flow past a certain point per second. It's like gallons per minute - it's a measure of the rate of flow. In the case of amps it is 1 coulomb per second, with coulomb being a quantity of electrons (or charge).

I do not know in this case if the amps the batteries can deliver is what determines if the motor can spin from 0 rpm or not - and we still don't know the amps the batteries can deliver. Perhaps there is some other label or some information on the seller's web site, or a web site you find by searching on the part numbers on the battery?

 

[offGridDownUnder]

I searched for this phrase: "ub12220 battery maximum discharge amperage"

Several links suggested this battery can discharge at a rate between 2 to 4 amperes. 48V at 4 amperes will provide 192 watts of power.

I'm waiting for others to chime in about whether this is the reason it can't start moving on it's own.

 

[neptronix]

If you had a voltage meter, i'd bet you'd see those batteries' voltage sagging like crazy. That might be part of your issue.

Regular SLA is only really good for ~0.25C-1C. Pretty sure you're torturing those things. This is a job for lithium!

 

[K_IV_Push]

If you had a voltage meter, i'd bet you'd see those batteries' voltage sagging like crazy. That might be part of your issue.

Regular SLA is only really good for ~0.25C-1C. Pretty sure you're torturing those things. This is a job for lithium!

Any sources where I could get one? As long as I can figure out how to wire it to the controller and motor i'll try lithium

 

[fechter]

OK, if the wiring was previously tested on that controller, it shouldn't change.

If your batteries are good, they should be able to supply 100A for a short burst no problem. But they might not be good.
So the test is to measure the battery voltage while you try to get it going on the ground and see what happens. If the voltage drops below a certain point (which should be listed on the controller label), it cuts power to the motor. This might happen pretty fast, so apply throttle slowly.

 

[K_IV_Push]

OK, if the wiring was previously tested on that controller, it shouldn't change.

If your batteries are good, they should be able to supply 100A for a short burst no problem. But they might not be good.
So the test is to measure the battery voltage while you try to get it going on the ground and see what happens. If the voltage drops below a certain point (which should be listed on the controller label), it cuts power to the motor. This might happen pretty fast, so apply throttle slowly.

Ive been at work the past few days but today I wanted to give an update on this project. First of all, the cart runs very well.

I was able to ride on it with my son. Combined we weigh roughly around 250-280 pounds. So the motor is definitely strong enough for this cart. The frame (which came with the chain) was very used and old. When I assembled the chain and sprocket with the motor, the chain was very tight. After messing with the cart and taking it on test runs for about a week, the chain seemed to have loosened up a bit and now there is some slack. This makes me think that perhaps the motor would not turn at first because the chain may have been too tight. I dont know for sure.

I also used a multimeter to check the voltage of the batteries. Output was right around 50 volts. I checked the voltage coming off the batteries with the back wheels of the cart off the ground and the motor spinning at full speed still had around 50 volts output from the batteries. I was attempting to check the voltage with the wheels on the ground (this is where the cart would usually get stuck and the motor wouldnt turn) but whenever I would step on the acceleration pedal, the cart would start moving. After a few attempted I just sat in the cart and tried to drive, and sure enough it ran just fine.

Some other things to point out: Today when the cart started running, I had all wires from the batteries tightened. Initially when it wasnt working, the battery cables were only hand-tightened. There was connection but the nuts/bolts were not fully tightened. Maybe this is why the motor had trouble turning. I did not check with the multimeter before when the wires were loose, so I dont know how many volts the batteries were actually putting out to the motor.
Another thing that I did not mention in my initial post that I think is important. When the cart was not running, the motor would not turn but it would make a continuous noise as if it were trying to turn but was stuck. I dont know if this is because mechanically it was not strong enough to turn the axle (perhaps because the chain was too tight) or because electrically the motor wasn't getting enough volts (since before the connections were not fully tightened).

Unfortunately I cannot determine exactly what the issue was with the cart. Maybe the old chain was too tight and now that its loosened a bit, the motor can turn easier. Or maybe the wires from the batteries just werent getting a good connection and the motor wasnt getting the full 48 volts. Either way, the cart runs great and it can carry a lot of weight. Very fun. Maybe someday I can upgrade to a more efficient battery. Pretty good for a first project I think.




I was not able to check the voltage output of the batteries while I was riding it but I'll see if I can do that later on

 

[fechter]

Nice! How fast does it go? Looks like a lot of fun.

If you can apply the brake while the motor is running you can test for voltage sag with the wheels off the ground. Just don't overdo the brakes. But if it's running well, I wouldn't worry about that too much.

Once your batteries die (which doesn't take long with lead-acid batteries), you can think about a lithium upgrade. Much longer run time and battery lifespan. Usually lighter too.