Hardware Reference

In-Depth Information

Figure 4.20: The single coil and commutator (θ=90
◦
)

ThetorquerippleintheDCmotorcanbereducedsigni
fi
cantly by increas-

ing the number of coils. For the coil shown in Figure 4.19, if three coils AX,

BY and CZ are used and if they are symmetrically distributed around the sta-

tor with intervals of 120
◦
, then the torque produced by each coil is the same

as that shown in Figure 4.23 but shifted from one another by 120
◦
.There-

sultant torque is shown in Figure 4.24. Comparing the
fi
gures of Figure 4.19

and Figure 4.24, it is easily understood that use of multiple coils with proper

arrangement can reduce the torque ripple.

Let us de
fi
ne a ratio r
tam
as

Average Torque

Maximum Torque
.

r
tam
=

(4.44)

For a multi-coil system equipped with commutation device, if the number of

coil M is odd, then it can be proved that,

r
tam
=
2M

π
sin(
π

2M
).

(4.45)

And if M is even, then

r
tam
=
M

π
sin(
π

M
).

(4.46)

Therefore, when the number of coil, M, is increased to a certain level, r
tam

tends to a value close to 1, i.e., the generated torque can be considered constant

if the input current is constant.