Difference between lower and higher current rated ESCs and electric skateboard questions

Question

I am starting a project and would like to build an electric skateboard. Here are the specs for the parts that I have gathered so far:

a. Two 90mm wheels that each contain a built-in BLDC motor. The power rating for each motor is 550W and the voltage rating is 24V.

b. Approximately 24V battery in 6 series 2 parallel (6S2P) configuration.

Using the formula P = IV, the max total current usage per motor would be 22.917A.

My questions are:-

1. Does the weight of the person riding the skateboard affect any of the parameters for the project?

2. Does an ESC that is rated for 6S(24v) and 30A suffice?

3. Say my project requires a maximum of 30A (an example), what's the difference between using a 40A ESC and a 100A ESC?

Thank you in advance. :)

Answer 1

Does the weight of the person riding the skateboard affect any of the parameters for the project?

Not the ones you have stated. The cargo weight effects the acceleration at any fixed power setting. It also effects the maximum speed you can go uphill at any fixed power setting.

Does an esc that is rated for 6S(24v) and 30A suffice?

You say the maximum operating point is 24 V and 23 A, so a 24 V and 30 A rating should suffice, if the specs you quoted really are the worst case.

Say my project requires a maximum of 30A (an example), whats the difference between if i use a 40A ESC and a 100A ESC?

Each should be able to handle the problem. The higher power unit will likely be larger, heavier, more expensive, and possibly less efficient at low power levels.

Answer 2

I am starting a project and would like to build an electric skateboard. Here are the specs for the parts that i have gathered so far:

You've had a few cautionary warnings, but let's add another: Large lithium ion battery packs are effectively firebombs if mistreated and you must as a result treat this project with great respect. You need to make sure your battery charge and protection circuits are up to snuff and you should probably use thermal protection and possibly even heat sink your batteries. Regardless of what you build, treat it with great respect and caution.

a. 2 90mm wheels that contains a built in BLDC motor in each. The power rating for each motor is 550W and the voltage rating is 24V.

Your motors are wheel motors so they're hopefully high torque, but it wouldn't hurt to mention the kV rating.

b. Approximately 24V battery in 6 series 2 parallel (6S2P) configuration.

That is a serious, high powered battery pack. You need to make every effort to ensure that it is charged properly, not overcharged or over discharged or discharged at too great a rate, that it does not overheat, is not punctured and is able to dissipate power. Making sure it is shock isolated, mechanically secure and isolated from moisture would be wise. When building it's enclosure/shroud, put thought into how the battery case will vent if it does explode.

Using the formula P = IV, the max total current usage per motor would be 22.917A.

Yeaaarp. Note that you don't necessarily have to run them at absolute full blast.

My questions are:-

Does the weight of the person riding the skateboard affect any of the parameters for the project?

Yes, this will affect your max velocity, your velocity of maximum efficiency, and how hard the motor has to work to maintain any given velocity.

Does an esc that is rated for 6S(24v) and 30A suffice?

It should, but you should be certain your motor is current controlled and current limited. If you can find information to investigate the nature of the ESC, some things you might want to concern yourself with are:

-that if it has a BEC and you intend to use it, ensure that it is switching

-ESCs with built-on waterblocks or heatsink and fan arrangements are readily available, and you may want to consider closed loop water cooling. Whatever your cooling arrangement, you not only want to keep ESCs cool to improve efficiency, but you should also ensure that they are not transferring their thermals to the battery pack.

-Efficiency will be very important

Say my project requires a maximum of 30A (an example), whats the difference between if i use a 40A ESC and a 100A ESC?

Switching regulators have efficiency curves and some types have lower efficiency at low load, so depending on the regulator, using an overpowered regulator may cause your efficiency to suffer. Find efficiency curves on datasheets or manually test it. Ideally your motor's maximum efficiency range should be close to intended operating speed and the ESC should have either a flat efficiency curve, or be close to max efficiency at the drive current required to pull the skateboard under intended circumstances.

A final note about the battery packs, if you can find flat ones with just a single layer of batteries you can probably cool them more easily.

Play safe.

Answer 3

Does the weight of the person riding the skateboard affect any of the parameters for the project?

Acceleration current is I=(Vavg)/Rdc (motor coils) is up to 10x max rated (e.g.10x22.9A= 229A on full start.

From Physics, F=ma, so mass certainly affects acceleration and heat loss when it starts at 5500 watts and reduces to 550w rating at some speed and load due to generated back EMF. Obviously, it cannot dissipate 5kW very long without overheating. The ESC job for a propeller is much lower mass.

Does an ESC that is rated for 6S(24v) and 30A suffice?

Sounds big, but you must work out the weight, size and heat loss of battery capacity, ESC and motor for heat rise.

Say my project requires a maximum of 30A (an example), whats the difference between if i use a 40A ESC and a 100A ESC?

About 250% You need to control 230A worst case unless you get better specs.

This is why you should not try to learn Physics more before trying to design something.

Others with far more design skill than you in China and have had millions of skateboards recalled and lawsuits due to fires.