Electric Motors and Drives, 2nd Edition

Conventional

d.c.

Motors

101

Usually, field

and

armature circuits

are

interlocked

so

that

if

the field

is

lost,

the

armature circuit

is

switched

off

automati-

cally.

TRANSIENT

BEHA

VIOUR

-

CURRENT

SURGES

It

has

already been pointed

out

that

the

steady-state armature

current depends

on the

small difference between

the

back

e.m.f.

Ε

and the

applied voltage

V. In a

converter-fed drive

it is

vital

that

the

current

is

kept within safe bounds, otherwise

the

thyristors

or

transistors (which have very limited overcurrent

capacity) will

be

destroyed,

and it

follows from equation

3.8

that

in

order

to

prevent

the

current from exceeding

its

rated

value