# Electronic – 24V 250W motors

batteriesmotormotor controllervoltagewatts

I am trying to make an electric go-kart. I am new to the forum, so I may be missing some information but any help is greatly appreciated. After doing some math and discussing with friends I found the following with the given information. I have 4, 24V 14A motors, all connected to a read drive axle. I have 2 pairs of motors on each side of the drive axle. The motors have an 11 tooth sprocket that is chained to a 68 tooth sprocket that is connected to the drive axle. I currently have 2, 12V 7Amp 20 hour lead-acid batteries, which provide my 24V. Each pair of motors is connected in parallel to ensure they both get the 24V. However, the motors do not spin, even when there is no load (The wheels aren't contacting the ground). I believe that this may be because the motors do not have enough amps, only receiving 7 Amps of the 14 Amps from the batteries, because there are 2 motors in each pair. My main goal is to get the kart moving. I weigh about 130lbs, and the kart is likely around 30lbs. Would getting a battery/batteries with move amps (28A) allow the motors to run properly? Any advice is really appreciated.

All the information I know

Each motor is a DC motor that needs 14A and 24V
The motor has a sprocket of 11 teeth
The Watts of each motor is 250
The motors are rated to have an RPM of 2750 or around 45.83Hz

The drive axle has a sprocket of 68 teeth.
The 68 toothe sprocket is 5.5"
The wheel diameter is 10"

If my math is correct, given the ratio of the motors to drive shaft. Each motor produces 5.45Nm of torque, and after the sprockets, the torque is 33.69Nm. I don't know what impact the 4 motors would have on the torque, nor do I know what the torque is for each side. I really just want this kart to move.

Would removing motors allow my vehicle to move? I think that my power supply does not provide enough amperage to my motors. Would having 1 or 2 motors be better in this situation?

Properly geared and powered a 250 Watt motor will move a gokart at modest speed. Probably 5-10 kph and maybe more. More Watts does better. At slow speed power increases about proportionately with speed. As speed picks up and wind resistance matters power needed increases with cube of speed !

Wheel diameter influences driving torque.
The power source must be able to supply 10A at 24V for about 250 Watts of power. More for more.
You need to specify wheel diameter and power source.

Watts ~= kg.m torque x RPM
For 250 Watts torque in kg.m = Watts/RPM
Wheel RPM is Motor RPM x gear ratio.
= 2750 x 11/68 ~= 440 RPM.
So torque is ~= Watts/RPM = 250/2750 =~ 0.1 kg.m
If wheel diameter is say 12 inches = 0.3m
= 0.1/0.15 =~ 0.66 kg force.

That's "rather small".
It may be that your reduction drive requires more force than that to overcome static stiction. If you prop it up and power it and hand turn the wheels, does it spin?

You need substantially more down gearing.

Derivation of the above "approximate power" formula:

The formula Power = Torque in kg.metres x RPM is an approximation that is about 2.5% low.

It works because various things just happen to cancel.
Power = Force x distance
= (kg x g) x 2.Pi.R x RPS = kg x R x (g x 2.Pi x RPS) versus
. kg x R x (rpm)
So we need g x 2 x Pi x RPs to equal RPM
= 9.8 x 2 x 3.14 = 61.544 RPS = 1.0257 RPM

so Kgm x RPM is ~= 2.5% low.

Despite the slight' inaccuracy this is an immensely useful formula for in-the-field use.