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Electric Motors Winding and Repair Electric Motors Winding and Repair Discussion


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Old 03-19-2021, 04:22 AM   #1 (permalink)
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Default How to Determine current KV?

Is there a simple way to test a motor to get its KV? I recently received a Pyro 650-103 in a used Heli and on the bench my headspeed without blades was putting it closer to a 620kv rating based on a basic headspeed calculation. When I swapped in a Pyro 650-83 as a test, my headspeed increased drastically, close to the predicted headspeed calculation. This motor also looks as if maybe it was rewound as the windings look a little different compared to my other Pyro 650ís.

Is it also possible that the motor is damaged? Is there a roughly simple way of determining the health and KV without too much fancy equipment for a novice at motors?

I find fascination in rewindings and follow threads here but have not moved forward with learning how to and frankly a lot of it feels over my head.
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Old 03-19-2021, 07:02 AM   #2 (permalink)
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Motor Kv is probably the easiest thing to measure. It is very simply the RPMs the motor spins per volt.

All you need is a way to measure the motor RPM and ESC voltage (telemetry valves are sufficient)

Just disengage the motor pinion, and spin the motor at 100% throttle. Record the Main battery Voltage and RPMs. 100% throttle is important to get a good measurement. If you have batteries of different voltage/cell count; you can take the measurement at different voltages. (My ISDT chargers have an adjustable voltage supply function as well)

When measuring the RPM, you’ll need to make sure it’s the motor Rpm by setting the gear ratio to 1:1. Also make sure the motor poles are set correctly.

Once you have a measurement, just divide the RPM by the Volts:

Kv=RPM/Volts

If you have several measurements they should result in similar Kv values regardless of the battery voltage. (That’s why it’s called a ‘motor constant’)
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Old 03-19-2021, 07:03 AM   #3 (permalink)
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If you can measure RPM of the motor and voltage of the battery you can do a reasonably accurate Kv measurement. Remove the pinion so that the motor is running with no load, open the throttle wide open (no governor). Record the coincident battery voltage and motor RPM then divide RPM by voltage to get kV
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Old 03-19-2021, 07:14 AM   #4 (permalink)
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As for motor health, that’s a fair bit more difficult. (Or it’s not immediately clear how to measure it to me)

The two things that come to mind to look for is:
—a short in the windings
— overheated/Weak magnets

Measurement of the winding resistance is difficult to get a good value with a basic Voltmeter. Even on my smallest motors it barely registers, it’s basically a dead short for my meter.

The easiest way to tell if the motor is bad would then simply fall to performance. Does it put out the power that telemetry is saying? (i.e. Motor bogging during basic maneuvers but power is through the roof)

2Cents
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Old 03-19-2021, 07:20 AM   #5 (permalink)
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You can also measure Kv using a drill press:

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Old 03-19-2021, 07:27 AM   #6 (permalink)
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If you change the gear ratio in the telemetry to 1:1 will it show the 25K rpm the pyro is supposed to spin at with a fully charged 6S pack?
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Old 03-19-2021, 07:58 AM   #7 (permalink)
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You could invest in an LC meter for around $30. Or as Ace Dude mentioned, try the drill press method.


How to test a brushless motor with a $30 LC meter (6 min 21 sec)



Brushless motor testing without LC meter. (10 min 18 sec)
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Old 03-19-2021, 10:40 AM   #8 (permalink)
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Thanks everyone! This is great information and sort of inline with what I was thinking. I was hoping to hear exactly what has been said. I would like to learn more and dive a little deeper as it does fascinate me. Math had been a big weak point for me though.
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Old 03-22-2021, 04:32 PM   #9 (permalink)
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Worked like a charm, thanks guys. The Pyro 650-103 came out to be about 550kv, which makes sense because the airframe I got it with I think was set up for 12S (Voodoo 600).
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