# Bench Power Supply for ESC and KV on motor

I have a bench power supply it can provide ~10 amps and ~30 volts. Working on an autonomous car project I have an ESC to manage a brush-less motor.

Two questions I cannot seem to wrap my head around:

1. The ESC has a 35A requirements. With the bench power supply I have I can only go to 10A. If I am powering the ESC with this what are the risks? Over heating the supply? I set the volts to 13v based on the 35A specs.

2. The motor stated 3600kv or 36v (if I understand correctly). Do I need to be concerned with this? The ESC manages that so as long as I manage the 35A the motor is fine. Why do I need to know this metric?

• When discussing motor ratings, you would say that the motor Kv = 3600. The units for that are RPM/volt. This establishes a relationship between motor speed and battery voltage. A 3.6V supply will drive that motor at 3.6 * 3600 = 13,000 rpm, roughly (take all these numbers with a grain of salt). Try to figure out how many batteries most people are using with that particular motor. Is it 1S or 2S or??? You want to drive it at the same voltage that others are using successfully. I am guessing it is 1S. 1S means one lithium ion battery in series, nominally 3.6V. Jan 17, 2021 at 1:41
• kV is totally different. That is the abbeviation for kiloVolt (1000 Volts). It has nothing to do with your motor. I recommend you always use either kV for kiloVolts or Kv for motor voltage constant. Don't capitalize it any other way (no kv or KV). Volts are always capitalized. Never v, always V. This is not just about grammar, It helps avoid confusion. Jan 17, 2021 at 1:43
• @mkeith This was super helpful. I guess I need to get a better grasp on volts and amps. Jan 17, 2021 at 2:20

The ESC has a 35A requirements. With the bench power supply I have I can only go to 10A. If I am powering the ESC with this what are the risks? Over heating the supply?

The 35A specification means that the ESC can safely draw 35A from the supply. The amount it actually draws will depend on the motor you drive with it, and the load you put on that motor.

I set the volts to 13v based on the 35A specs.

That's somewhat nonsensical. Current depends on the load and the voltage, and you haven't tied those together yet. You can have 35A at any voltage if you get to choose the load.

The motor stated 3600kv or 36v (if I understand correctly). Do I need to be concerned with this? The ESC manages that so as long as I manage the 35A the motor is fine. Why do I need to know this metric?

If you're deriving the 36V from the Kv = 3600 -- no. Model airplane motors are rated in with a voltage to speed constant called Kv, which is in units of RPM/volt. For a motor that's driven with $$\v\$$ volts p-p at the coils, it will turn $$\v \cdot K_v\$$ RPM. That's pretty close to the speed it'll turn with $$\v\$$ applied to the battery terminals of the ESC and the ESC at full throttle.

Given that Kv rating, the motor is probably designed to run off of a single Li-Ion cell. That's consistent with a propeller speed of a bit more than 10,000 RPM, which is typical. 12V with that Kv would be over 40,000 RPM -- I wouldn't attempt that without a scatter shield, or at least without making sure that there wasn't anything in the path of the debris when the motor explodes from centrifugal effects.

• Thank you this is helping me see some of my flaws. I am not powering this motor yet I want to make sure it is a safe endeavor. The 13v was bases on the datasheet unless I am reading that wrong. It say at 35 amps I should be using 5 to 13 volts. Jan 17, 2021 at 0:29

Do more research before connecting anything up. Too many gaps in understanding that I can't pin down. But here are some:

1. The motor stating $$\3600K_v\$$ means that for every 1V you apply it will spin 3600RPM. If you tried to apply 36V to it, it would spin at 100,000+RPM and blow itself apart. The motor's bearing are not rated for that, the motor's structural integrity is not designed for that, and the motor is not well balanced enough to run at that speed. This happens even if the motor is unloaded and drawing minimal amps.
2. Some loads, like propellers, for example have a load torque that is dependent on speed. For a propeller, more voltage = more RPM = more torque = more current. This isn't true of other loads like wheels where the torque is relatively constant regardless of how fast the wheels are spinning.
3. A motor of a particular size can only output so much power. The power is split between RPM and torque. kV is important because it means that some of that power is going into more speed than you need thereby taking away from the torque that you need, or is going into too much torque and taking away from the speed that you need (and gearboxes can be used to convert undesired speed into more torque, or undesired torque into more speed).
4. For a car, too much torque and not enough speed means your car is unstoppable but moves really slow.
5. For a propeller, too much torque and not enough speed means don't get off the ground.
6. For both a car and propeller, too much speed but not enough torque means your motor stalls, overheats, and burns out as well as possibly your ESC and battery.
7. The ESC does not have a 35A requirement. It has a 35A capability.
8. Whether you can run using the 10A supply depends on how much you plan on loading up your motor.
• @mkeith As you wish. Jan 17, 2021 at 1:57