Motor Constants

Have you ever wondered if it's possible to measure the Kv of an electric motor yourself? The answer is "yes", and it is actually quite easy to do at home with relatively inexpensive equipment that you may already have. All that is required is a way to measure volts accurately (multimeter), a tacho to measure RPM and a drill press or another method to spin the motor at a constant speed.

Once you know how to measure Kv, you'd want to do it whenever you buy or rewind a motor, to confirm the Kv value.

On commercial motors, the stated Kv is usually a nominal value, because no two electric motors are completely identical.

The three motor constants which are very handy to know about any electric motor are:

Using these 3 constants, the performance of any electric motor can be predicted fairly accurately with simulation software.

Obviously, the more accurately the constants are measured, the

It is surprisingly easy to measure the Kv of an electric motor.

The unit for Kv is RPM/volt, and in effect Kv is exactly that - the RPM that a motor will turn when one volt is applied. For brushed motors, this is straightforward: Kv = RPM / Volts.

With brushless motors however, the formula becomes slightly

Io is the no-load current of a motor and in my opinion is best measured at intended operating speed. Usually, a reading is taken at 10 Volts, however I prefer to measure Io at the RPM at which the motor will most likely be going to work. There are other methods, i.e. the 'four constant model' which is outside the scope of this article.

With outrunners, it's quite easy to measure RPM when two marks or lines are placed on opposite sides of the outside of the rotor. Then a tachometer set to 2-blade setting can be used to measure RPM.

It's best to use a variable power supply and varying the RPM by adjusting the voltage until the desired operating RPM is reached with the ESC at max. speed, and not by varying the speed with the ESC.

Rm (or Ri) is the resistance of one phase of a motor (after termination). In other words, if one measures the resistance between any two motor leads of a brushless motor, that value would be the Rm. Unfortunately, this measurement is a little more tricky to perform than meets the eye, because a multimeter is just not sensitive enough for this purpose.

But fear not. There is a method to measure Rm which is within the capabilities of the average DIY person, yet it's very accurate and only requires a few pieces of inexpensive equipment.

It's a variation of the Kelvin (4-wire) method and has served me well for many years.

Equipment needed: Ammeter, voltmeter, 10-20 Ohm (10 Watt) resistor and a high capacity battery of about 12volt. The exact values are not important, because the accuracy gets taken care of by the calculation. A 2000mAh 3S or 4S LiPo battery is ideal.

The ammeter and DVM (digital volt meter) can be cheap multimeters, as long as the one used for amps can measure current to at least 5A.

Being an electric model flyer, I already had an accurate ammeter in the form of a Hyperion e-Meter. Although the standard 100A shunt will work, I chose to use the 20A shunt because then Amps is displayed with an extra decimal.