@Rajasekhar Ramireddy • 18 Dec, 2012

What is an electrical angle and how is it different from mechanical angle... guys plz help me in getting clarity over this....

@lal • 18 Dec, 2012 • 3 likes
Consider a two pole alternator where poles are on the stator and armature on the rotor.

Assume the coil is exactly midway between the North and South poles. Now the flux linkage is zero and hence the output too. Clearly the rotor has to turn 90 degrees mechanically to bring the coil under the North pole which is positive maximum in the output. Positive maximum is at 90 degrees electrical in the output.

Another 90 degrees turn will bring the coil again exactly midway between the poles. The output turns zero again. That is 180 degrees mechanically and electrically from the initial position. When rotor turns 90 degrees more (270 degrees from initial position), it comes under the south pole. Output is negative maximum in this instant. 90 more degrees mechanical turn will brings the rotor back to initial position completing a cycle. Rotor turned 360 degrees mechanically in total.

In the above case the electrical and mechanical degrees were same.

Consider a 4 pole machine now. The poles will be arranged 360/4 = 90 degrees apart along the stator.

Initial condition be when the coil is exactly midway between a north and south pole where output is zero. The rotor now has to turn only 45 degrees mechanical to come under a north pole. But when the coil is under northpole, output is positive maximum. Positive maximum in output corresponds to 90 degrees electrical, Sin(90). Another 45 degrees brings the coil exactly midway between north and south pole making the output zero again; corresponding to 180 degrees electrical in output though it has mechanically turned only 45 + 45 = 90 degrees. At 135 degrees mechanical, the coil comes under south pole and output becomes negative maximum which corresponds to 270 degrees electrical. 45 more degrees and a cycle is completed in the output.

That is, though the rotor turned only 180 degrees mechanically, the output completed a complete cycle, 360 degrees electrical.

Electrical degrees = Mechanical degrees x P/2

P : Number of poles

Mechanical angle is the angle by which the rotor turns. Electrical angle is related with the output sine waveform. A pair of poles helps cover 360 electrical degrees, but not essentially the same mechanical degrees.

I think I made it a bit complicated 😐

Assume the coil is exactly midway between the North and South poles. Now the flux linkage is zero and hence the output too. Clearly the rotor has to turn 90 degrees mechanically to bring the coil under the North pole which is positive maximum in the output. Positive maximum is at 90 degrees electrical in the output.

Another 90 degrees turn will bring the coil again exactly midway between the poles. The output turns zero again. That is 180 degrees mechanically and electrically from the initial position. When rotor turns 90 degrees more (270 degrees from initial position), it comes under the south pole. Output is negative maximum in this instant. 90 more degrees mechanical turn will brings the rotor back to initial position completing a cycle. Rotor turned 360 degrees mechanically in total.

In the above case the electrical and mechanical degrees were same.

Consider a 4 pole machine now. The poles will be arranged 360/4 = 90 degrees apart along the stator.

Initial condition be when the coil is exactly midway between a north and south pole where output is zero. The rotor now has to turn only 45 degrees mechanical to come under a north pole. But when the coil is under northpole, output is positive maximum. Positive maximum in output corresponds to 90 degrees electrical, Sin(90). Another 45 degrees brings the coil exactly midway between north and south pole making the output zero again; corresponding to 180 degrees electrical in output though it has mechanically turned only 45 + 45 = 90 degrees. At 135 degrees mechanical, the coil comes under south pole and output becomes negative maximum which corresponds to 270 degrees electrical. 45 more degrees and a cycle is completed in the output.

That is, though the rotor turned only 180 degrees mechanically, the output completed a complete cycle, 360 degrees electrical.

Electrical degrees = Mechanical degrees x P/2

P : Number of poles

Mechanical angle is the angle by which the rotor turns. Electrical angle is related with the output sine waveform. A pair of poles helps cover 360 electrical degrees, but not essentially the same mechanical degrees.

I think I made it a bit complicated 😐

@Rajasekhar Ramireddy • 26 Dec, 2012 • 1 like
thank u friend.....its not complicated....the concept u gave cleared my doubt....its clear for me now....thank u...

@rajeev213 • 14 Jun, 2015
Mr.lal, i need some help regarding Ac machine stator winding diagram. i know some data that i have mentioned below:

poles =4, pitch angle=165 electrical degree, breadth between slots =15 electrical degrees. Include enough coils to fill the slots with a double layer winding. please help me to draw the winding diagram.

poles =4, pitch angle=165 electrical degree, breadth between slots =15 electrical degrees. Include enough coils to fill the slots with a double layer winding. please help me to draw the winding diagram.

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