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Universal motor concepts

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Universal motor concepts

Motor voltage can be expressed in an equation consisting of three components, Resistive,

Electro-motive, and Inductive.

Equation 1

expect. When the motor is not spinning the only thing limiting the current is the winding

resistance.

The inductance allows for a constant or smooth phase current when the PWM frequency is

set such that the inductance supports the current in the windings during the PWM off-time.

The BEMF is the voltage that is generated by the motor as the permanent magnet armature

spins inside (or around) the stator. As the rotor spins the windings experience a changing

magnetic field. This changing magnetic field generates the BEMF voltage that is directly

related to the speed of the changing field.

There are two universal concepts in motors that are always true The first is that BEMF is

directly proportional to armature speed and really, nothing else. The back EMF equations

spell this out quite clearly:

Equation 2

Where:

•

N is the number of coil turns

•

B represents the magnetic field

•

A the area encompassed by the motor magnetic field

•ω is the angular speed

Notice that N, B, and A are all constants specific to the motor construction. They do not

change. The end result is that Back EMF (emf) is directly proportional to motor speed (

ωt).

The next universal concept is the relationship between motor torque and motor current.

Again, the equations describe this clearly:

Equation 3

V

Motor

i

Motor

R

Windings

BEMF

L

Windings

di

Motor

dt

-----------------

++

⋅

=

bemf

N

–

BA

ωωt

()

sin

⋅⋅ ⋅ ⋅

=

T

pN

2

π

--------

θI

=