INDUCTION-MOTOR REGULATION

131

2. FREQUENCY PULSATION

81. If the frequency of the voltage supply pulsates with
sufficient rapidity that the motor speed can not appreciably
follow the pulsations of frequency, the motor current and torque
also pulsate; that is, if the frequency pulsates by the fraction,
p, above and below the normal, at the average slip, s, the actual
slip pulsates between s + p and s — p, and motor current and

MP.
150 140 130 120 110 100 90 80 70 60 50 40 30 20
10 0
— — -
---
^— !-
,. --- -

.»— «: ---*

&
tORC N.W
(,UTET8
8000 7000 GOOD 5000 4000 3000 2000 1000 0 -1000 -2000 -3000 -4000

— — * _-~— •
*~^
^
•^
-su--
N
--
S-s,v
N >
^
^
^
4,

,— - -
.—-1— '

\

\

\

txV
V.
k
& \\

^

\
\
'A
(TT

\
v 1^
i
\

^— .
•— ^
"^SS.

^ '

~*-^
•**-* ^
>^.
£5*
^

\
*

^

\
X
A-x
•^
3-^^ :"»X^
^

%

\\

\

^
^
^ "
Xj

^
:

Stj
\

^
*»
>s,
\

k

s
^

'<?:
^
>
\k

\

X
\

V<]
^
\
\
\

\

vfe.

\
Si
\

\

\

\
\

\

\

\

\

\

\

Y- 0.01- 0.1 j 2 o- 0.1+ 0.3 / Zi- 0.05+0.15 j FREQUENCY FLUCTUATING BY s-±0.02 OR 5 PER CENT
\

N

/

^ \

\

\

)•

JLIP
FRACTION
OF PYNCH
FtONI
m
'
\,
/
\

16 0.
150.
L4 0
13 0,
12 0
no]ioo.
39 0.
08 0,07 0,
OGO
05 0
04 0
030,
02 0
1 0

FIG. 50.—Effect of Frequency Pulsation on Induction Motor.
torque pulsate between the values corresponding to the slips,
s + p and s — p. If then the average slip s < p, at minimum
frequency, the actual slip, s — p, becomes negative; that is, the
motor momentarily generates and returns energy.
As instance are shown, in Fig. 50, the values of current and
of torque for maximum and minimum frequency, and for the
average frequency, for p = 0.025, that is, 2.5 per cent, pulsa-
tion of frequency from the average. As seen, the pulsation of
current is moderate until synchronism is approached, but be-