How to measure the kv of a motor

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Doctorbass   100 GW

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How to measure the kv of a motor

Post by Doctorbass » Mar 02 2015 1:17am

There is alot of new motor availlable every months and as usual people like to compare them to know witch is the best for their purpose.

Most of the time people will use the parameters specified by the manufactur and will trust them. Sometime it is not all the parameters that are availlable or sometime they will be wrong due to bad copy/past from chineese website making wrong parameters to be showed for a given motor.. or sometime the same exact image of parameters for like a dozen different motor.

In a goal of avoiding confusion and wrong data shared about motor comparaison i think it's important to have a uniform way of measuring these parameters. One of them that people are using alot is the kv of a motor.

kv represent the number of RPM per volt a motor will need or produce at no load. It also be represent as how many rpm are required to produce one volt when the motor is in generator mode ( regen)

The wikipedia description is :
The motor constant is winding independent (as long as the same conductive material used for wires); e.g., winding a motor with 6 turns with 2 parallel wires instead of 12 turns single wire will double the velocity constant, Kv, but KM remains unchanged. KM can be used for selecting the size of a motor to use in an application. Kv can be used for selecting the winding to use in the motor.

Motor velocity constant, back EMF constant
Kv is the motor velocity constant, measured in RPM per volt (not to be confused with kV, the abbreviation for kilovolt). The Kv rating of a brushless motor is the ratio of the motor's unloaded RPM to the peak (not RMS) voltage on the wires connected to the coils (the back-EMF). For example, an unloaded motor of Kv, 5,700 RPM/V, supplied with 11.1 V, will run at a nominal 63,270 RPM (5,700 RPM/V × 11.1 V).

(Please note that i think that usually we measure the RMS voltage of the motor ad not the peak in our ebike purpose.. )

We will concentrate the following description for the hub motor only.

The kv can be affected by many factors and if all these are not take in account this will false the calculated and measured kv value. Here is some inportant details that will help to get better accuracy:

- When measuring the kv, the best way is to use a hub motor without wheel and tire ( a real no load ! )
- The controller throttle must be set to the max and you need to ensure that the PWM is also at the max witch mean 100% modulation or 100% speed.
- Keep all tuning like flux weakening, OVS or any timing advance disabled

Now some more fancy details for the purist! 8)

- The motor must have spin for couple of minutes at least.. a brand new motor out ot the box will sometime have the bearing seal and other additional friction added that will. slower the motor.
- The multimeter that measure the voltage of the battery must be accurate and measured with probe the closest to the controller, not at the battery terminal.
- the motor should be at ambiant temp and not come still frozen from the box you got freshly shipped at your door!, allow few minutes to have the winding to get ambiant and stable temp.

Some fact:
-A higher turn count motor: ex 3T (meaning 3 turns) will sping faster than a 4T motor for the same controller voltage and same throttle position.
- The wider the stator is the slower the motor should be for the same turn count. Ex a 5304 with 4T will spin faster than a 5404 also with 4T
- The flux weakening or timing advance when activated will modify temporarely the kv of the motor.

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