How to test motor KV? Why my tested motor KV is different from the specification?

Posted by Christina Zhang on

Some notices when test motor KV with VESC?

  1. KV must be tested without load.
  2. To compatible with different kinds motors,  VESC default parameters for motors are more conservative in the design. If test by VESC, test value would be lower about 5% than actual value.
  3. Tested KV will be different under different modes and parameters. Tested value will be higher when test in following situations: with hall sensor, high angle, FOC higher than BLDC, V6 higher than V4.

 

What is KV? Does higher KV mean faster motor?

The KV rating (not to be confused with kilo-volt) is the RPM of the motor (k) per volt (V) with no load. ... A motor with a higher KV will have more top end speed, but not as much acceleration/torque. A motor with a lower KV will not be as fast, but will accelerate faster and have more torque.

 

What is RPM? 

Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or with the notation min) is the number of turns in one minute. It is a unit of rotational speed or the frequencyof rotation around a fixed axis.

The motor's Rated Operating Speed will be given in RPM, (revolutions per minute) and is the speed the motor will rotate when producing its rated horsepower and supplied its rated voltage.

 

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 (Please don't confuse Pole number and Pole pairs number, If pole number is 14, pole pairs is 7).

 

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 your test 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 affects 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 max ERPM.

 

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