Electric Motors and Drives, 2nd Edition

92 Electric Motors

and

Drives

Nm/A,

and for k

e

the

units

are

volts/rad/sec. (Note, however,

that

k

e

is

more often given

in

volts/1000 rev/min.)

In SI

units,

the torque

and e.m.f.

constants

are

equal,

i.e. k

t

= k

e

= k. The

torque

and e.m.f.

equations

can

thus

be

written

T=kl

(3.5)

E

=

ka>

(3.6)

Equivalent circuit

The equivalent circuit

can now be

drawn

on the

same basis

as

we used

for the

primitive machine

in

Chapter 1,

and is

shown

in

Figure

3.6.

ι

Figure

3.6

Equivalent circuit

of a d.c.

motor

The voltage

V is the

voltage applied

to the

armature

terminals

(i.e.

across

the

brushes),

and Ε is the

internally

developed motional

e.m.f. The

resistanc ...