The speed of
an a.c motor depends on the mains frequency and number of poles of the stators winding Redesign Speed in
Bahasa
n =
|
2 .
f . 60
|
. r/min
|
p
|
Where n = speed
F = frequency
P = number of poles
The rule of
thumb for 50 Hz mains frequency is that the speed in revolutions per minute (
r/min ) is 6000 divided by number of poles. This is synchronous speed ; it can never be reached by an
inductions motor, squirrelcage or slip – ring motor.
At o load, however, the speed is practically equal to the synchronous speed ;
at rated output it is slightly lower.
The
following equation s used to calculated the slip :
s =
|
n₁ - n
|
. 100 %
|
n₁
|
||
Where s =
slip in %
n₁
= synchronous speed, r/min
n =
asynchronous speed, r/min
the slip is proportional to the power
taken from the motor.
Example
4 – pole motor, 4 kW, 380 V, 50 Hz,
1425 r/min
At 4 kW ; s =
|
1500 -
1425
|
. 100 %
|
1500
|
||
s = 5 %
corresponding
to 1500 – 1425 = 75 %
At 3 kW : s =
|
1500 -
1425
|
. 100 %
|
||||||||
1500
|
||||||||||
s =
|
3
|
.
|
1500
- 1425
|
r/min
|
.
|
100
|
3.80%
|
|||
4
|
1500
|
|||||||||
Corresponding
to :
3
|
.
|
1500
- 1425
|
r/min
|
=
|
56
|
r/min
|
4
|
Therefore n
at 3 kW will be 1500 – 56 = 1444 r/min
This slip is
inversely proportional to the square of the voltage
Example
4 – pole motor, 4 kW, 380 V, 50 Hz,
1425 r/min. Supply Voltage 346 V, 50 Hz
At
346 V
|
s =
|
(
|
380
|
)
|
²
|
.
|
1500
- 1425
|
.
|
100
|
=
|
0.6 %
|
346
|
1500
|
||||||||||
Corresponding
to :
At
346 V
|
s =
|
(
|
380
|
)
|
²
|
.
|
(
1500 - 1425 )
|
.
|
=
|
90
r/min
|
346
|
n will therefore be 1500 – 90 = 1410 r/min
The rules
above apply to moderate changes in output voltage. The speeds of the motors
when warm and at rated output are subject to a tolerance of ± 20 % of he slip. The normal testing speed for overspeed is
120 % of the rated speed for two minutes.
poles
|
synchronous speed at
|
|
50 Hz
|
60 Hz
|
|
2
|
3000
|
3600
|
4
|
1500
|
1800
|
6
|
1000
|
1200
|
8
|
750
|
900
|
10
|
600
|
720
|
12
|
500
|
600
|
16
|
375
|
450
|
20
|
300
|
360
|
24
|
250
|
300
|
32
|
18,5
|
225
|
48
|
125
|
150
|
