How to Test Motor KV? Is it Normal the Motor KV I Test is Different?

How to test motor KV with VESC?

1. KV must test without load.
2. To compatible with different kinds VESC, The default parameters are more conservative in the design of the motor running speed, if test by VESC, test value would be lower about 5% than actual value.
3. Test KV would be different under different mode and parameters, KV would be higher when test in following situations: with hall sensor, high angle, FOC higher than in BLDC, V6 higher than V6.

What is KV?

KV is unload rotation speed (RPM) per Voltage, which is the increasing speed value of brushless motor unload RPM when increase 1V input voltage. The input voltage of brushless motor and the motor unload RPM follow a strict linear proportional relationship.

Different Modes in VESC has different influence on Motor KV: 

VESC_TOOL Real-time interface can display many information, you can calculate KV with Electrical RPM (ERPM), voltage, pole pairs and throttle ratio .

KV=ERPM / V / Pole Pairs / Throttle Ratio

Under different mode ERPM is different, which makes KV different, so when you test the motor KV that you received the result might be different from the KV showed on the product.

1. BLDC without hall sensor: BLDC ERPM has large fluctuations, ERPM changes in 2% even with full throttle, and even the max value would be lower than other modes.
2. BLDC with hall sensor: ERPM is higher 2% than without hall sensor.
3. FOC mode: ERPM is more stable than BLDC, fluctuation range is about 0.5%, no matter with or without hall sensor, ERPM is equivalent to average ERPM under BLDC with hall sensor mode.
4. Timing is different, timing is traditional controller concept, it’s almost like the “Cycle Integrator Limit”value in VESC , this value is to speed up and strengthen power, also effect KV.

5. Software and hardware in V6 has been upgraded compared to V4, full throttle running speed is higher 1%-2% than V4.
6. Motor actual rotation speed and ERPM depends on load weight, over-weight would highly decrease ERPM, so can’t test KV with load.

What is ERPM?

Every time the motor coil rotates a magnetic pole, the current direction would be changed, when rotate to next pole, current direction change again.

Every 2 times change is a period, current direction change period in 1 minute, is ERPM.

This parameter can only be tested and displayed when use VESC, other ESC will not record.

Motor RPM=ERPM / Poles Pairs (Noted, not Pole number, pole pairs number, If pole number is 14, pole pairs is 7)

Maytech VESC Speed Controller based on V4 and V6: click below picture to check more specs, all in stock, ready to ship: