Hardware Reference
In-Depth Information
Figure 4.35: EMF induced in a single conductor rotating at constant speed.
For a DC motor, similar to the case of torque generation, increasing the
coil number and distributing the coil symmetrically in the space can reduce
the harmonics of the EMF output from the brush. Figure 4.37 shows the EMF
output of ∆-connected 3-phase symmetric coils.
In normal operation of an electric motors, the EMF induced in the wind-
ing always opposes the variation in the input current. Therefore, in electric
machine analysis, the EMF induced is also called the back-EMF.
Similar to EM torque generation in DC motor, increasing the number
of winding (or phase number) makes the back-EMF approximately constant.
Equations 4.45 and 4.46 can still be used to describe the relationship between
the peak and average values of EMF. When the motor rotates, the EMF of
each coil varies linearly with the rotational speed. So the EMF from the com-
mutation system can be described by
E = K
e
Ω,
(4.57)
where, K
e
is constant for a given motor and is known as the back-EMF con-
stant. Its value depends on the motor and the winding structure. For a multi-
coil DC motor, equation 4.57 is a fairly accurate description of the relationship
between back-EMF appearing on the brush-pair and motor speed. For BLDC
motor with multi-phase coils, the relationship described by equation 4.57 is
still valid if high order harmonics are neglected. Introduction of back-EMF
constant enables us to analyze DC motors and BLDC motors from the point
of view of electric circuits, where K
e
describes motor's EM structure and Ω
describes motor speed. The equivalent circuit of motor shown in Figure 4.38
can be used to analyze electrical performance of a DC motor.
