How to determine the lowest safe ESC current rating for a drone build?

Question

I was planning to build a quadcopter and was wondering how I should decide what ESC to get. I'm asking about ESC current ratings. I am building a 7 inch drone using 4s with 2306 1900Kv motors. I have seen ESCs ranging anywhere from 25 amp to 50 amp frequently used on 4s quadcopters. I am looking to get the lowest amp ESC that will work for my drone because of cost. Are there ways to figure out the minimum ESC amp-rating that will work safely for my drone?

Answer 1

I think most people just copy other builds, and step up a size if they burn out any ESCs.

If you want to be more scientific you could test one motor with a watt-meter to measure the current draw. Throttle up slowly so you can watch the current build, to protect the ESC you're using for the test. Also check the motor temperature after 30 seconds or so.

This will give you the maximum current that each ESC needs to handle.

There are lots of different designs of watt-meters available for hobby use. They typically measure up to 100amps so should all be adequate for this test, and cost less than a set of ESCs.

A typical multi-meter will only measure up to 10amps and would not be adequate. Clamp meters typically only measure AC current, and we need to measure the DC current from the battery to the ESC.

Fix the motor to something solid, like a plank of wood that is clamped to a bench. Be careful that the wires and other loose material will not be sucked into the prop, and don't stand in-line with the prop, just in case breaks and throws the blades out at high speed. Even a couple of hundred watts is quite intimidating in a bench test.

Alternately, you could search the forums to see if anyone else has tested a similar motor with a similar voltage battery and prop.

Answer 2

You can calculate the rating using one or both of the following equations; just rearrange and/or substitute values as needed:

1. P = I x V
   Power (Watts) = Current (Amps) x Voltage (Volts)

2. V = I x R
   Voltage (Volts) = Current (Amps) x Resistance (Ohms)

What you want to solve for is current. You can get some or all of the above numbers from the manufacturer’s datasheet or specifications, depending on what that specific manufacturer lists. Voltage will depend on the battery you want to use. Some iteration might be necessary (e.g. run the numbers for a 3S and 4S setup.)

When you have your motors current consumption it is a good idea to add a 20% safety margin, to account for inefficiencies in the motor and ESC cooling. Then use at least the next value up (e.g. if you calculate 26A, use a 30A ESC - not 25A, even though 25A is 'closer'.)

You can prove the setup before flight by bench-testing; measure the current consumption and ESC temperature to ensure they are within specification. (You want to test at full load, but slowly ramp-up so you can stop if the values are going out of range.)

Answer 3

There's a couple of things to consider when choosing the right ESC rating for your build in addition to the other answers here. The primary thing is that it is not in-air current draw that typically kills the ESCs in multi-rotors. Static load measurements gained through testing at full throttle on the bench are only reflective of worst-case (or almost) scenarios in flight. When a multi-rotor is flying forward the current draw dissipates very quickly thanks the to lower angle of attack, which dramatically reduces current draw. The only time you will see close to or greater than static load is for very short times when the motors are at high throttle but the movement is either zero or negative to the direction of thrust, those situations don't typically last very long, and your battery can't sustain that level of current delivery for long before sagging out reducing the current output. Essentially, as soon as the movement begins to follow the direction of thrust the, the effective angle of attack decreases and the current draw starts dropping immediately. Due to this static thrust numbers can be considered as 2-3 seconds of burst at the maximum. Combine that with the fact that most batteries we use in mini-quads won't be able to deliver over about 80A for more than 10 seconds or so, and you realize quickly that sustained draw is not an issue, and even a 20A ESC would be sufficient if we were only looking at in-flight sustained current requirements.

The real killer of ESCs comes in two varieties. The first is damage from repetitive large spikes that happen when the ESC rapidly changes the throttle output to the motor. On significant and rapid throttle changes, the moment when the duty cycle of the FETs has changed but physical acceleration in the motor has yet to begin massive amounts of current are dumped into the coils. This dissipates very quickly as soon as the motor begins accelerating, but spikes close to 180A for upwards of 2-3 ms are typical in most mini-quad motors. Spikes will be even larger with physically larger motors and higher torque load. Some of this depends on the Kv and resulting Kt of the motor and how much if a torque load the given prop mass and pitch creates, but the spikes are huge compared to what we see in sustained draw. While these spikes don't necessarily fry a motor instantly in the air, they do cause wear on the FETs, and if an ESC is pushed like this over long periods of time, this can end up leading to catastrophic failure in the FETs. If pushed hard enough, it can blow a FET on rapid throttle changes outright, but you would have to have a pretty under-specced ESC or dramatically overloaded motor for it to happen instantly.

The second issue has to do with the stall torque of the motor. When a motor is blocked while trying to spin it will pretty much instantly hit the stall torque current which can be huge. It is typically high enough that it will blow an ESC or burn the resin costing off the windings in a matter of seconds, sometimes less. This is probably one of the primary causes for failure in mini-quad and racing drones, right after physical damage to the components. (We crash a lot at very high speeds 😂)

There's a bit more detail in a similar answer here ( Effect of motor load on ESC? ) along with some charts from data I've collected.

So what's the takeaway here?

Basically, the higher rates ESC also happen to have FETs and supporting circuitry that can handle higher pulsed current (those millisecond peaks) and higher short bursts ratings that make it more likely they will survive abuse over time. You need to look at your required use case for the ESC. If it is being used for an application where accleration will be minimal and there is not much risk of crashing or blocked props, then you can probably get away with very minimally rated ESCs. If you plan on crashing or otherwise abusing your motors, the higher the rating the more likely they will be to survive situations that would blow lower rated ESCs. This is the real reason you see folks pushing 55A and 60A rates ESCs (though to be honest those ratings are probably dramatically inflated). If you have real questions about the specific application, you can always find the datasheet for the FETs being used in the ESCs in question and look at the pulsed current rating, thermal dissipation limits, and suggested maximum sustained current draw and make a decision from that, but generally speaking the states rating is sufficient in most cases, assuming sufficient quality control on the product.