Motor constants

The constants KM (motor size constant) and Kv (motor velocity constant, or the back EMF constant) are values used to describe characteristics of electrical motors.

Motor constant

KM is the motor constant[1] (sometimes, motor size constant). In SI units, the motor constant is expressed in (Nm/sqrt(W)):

K_\mathrm{M} =\frac{\tau}{\sqrt{P}}

where

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).[2] 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).

The terms Ke,[3] Kb are also used,[4] as are the terms back EMF constant.[5][6] or the generic electrical constant.[3] In contrast to KV the value Ke is often expressed in SI units \frac{V\cdot s}{rad}, thus it is an inverse measure of KV.[7] Sometimes it is expressed in non SI units krpm/V.[8]

The field flux may also be integrated into the formula:[9]

E_b = K_\omega\phi\omega
where E_b is back EMF, K_\omega is the constant, \phi is the flux, and \omega is the angular speed

An inverse measure is also sometimes used, which may be referred to as the speed constant.[3]

By Lenz's law, a running motor generates a back-EMF proportional to the RPM. Once the motor's rotational velocity is such that the back-EMF is equal to the battery voltage (also called DC line voltage), the motor reaches its limit speed.

Motor Torque constant

KT is the torque produced per unit armature current.[10] It can be calculated from the motor velocity constant Kv.


K_\mathrm{T} =\frac{\tau}{I_A} =\frac{60}{2\pi\cdot K_V}

where I_A is the armature current of the machine (SI units A ). KT is primarily used to calculate the armature current for a given torque demand:


{I_A}  = \frac{\tau}{K_\mathrm{T}}

References

External links

This article is issued from Wikipedia - version of the Sunday, April 03, 2016. The text is available under the Creative Commons Attribution/Share Alike but additional terms may apply for the media files.