SPEED ELECTRIC MOTOR

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