Electrical – Formulas for AC Synchronous motors

acmotorsynchronous-motor

I have this questions:

At what frequency must a 4-pole motor be supplied so that its synchronous speed is
1200 rev/min?
The nameplate of a standard 50 Hz induction motor quotes full load speed as 2950
rev/min. Find the pole number and the rated slip.
A 4-pole, 60 Hz induction motor runs with a slip of 4%. Find:
(a) the speed;
(b) the rotor frequency;
A 440 V, 60 Hz induction motor is to be used on a 50 Hz supply. What voltage should be
used?
Why can an induction motor never run at its synchronous speed?
Choose suitable pole numbers of cage induction motors for the following applications:
a. a grindstone to run at about 3500 rev/min when the supply is 60 Hz;
b. a pump drive to run at approximately 700 rev/min from a 50 Hz supply;

I now the number of poles and slip formulas. Is there any other formula that could help me for this task?

Best Answer

S = f x 120 / N

Where:

S = Synchronous speed in RPM
f = frequency in Hz
N = number of motor poles
120 = a mathematical constant

% Slip = (S - Full load RPM / S) x 100

Use those two formulae to solve all of the above.

By the way, your thread title says "synchronous motors", which would not be inclusive of "slip".

Related Solutions

Electronic – Calculate rotor slip without knowing synchronous speed

The rated speed is usually about 90-95% of synchronous speed. You can guess the synchronous speed and number of poles by looking for a number of poles that gives a rated speed slightly less than synchronous speed.

In this case, I would guess you are dealing with a 60Hz 12-pole motor, which would have a synchronous speed of 600 rpm.

I've done a quick lookup table below for reference.

Poles    50hz    60hz
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
30       200     240
40       150     180

Speed of Synchronous motor determined by frequency, but how about current

Clearly you have to apply power to the motor and you have to apply enough supply voltage to overcome motor losses. Once you do that the motor works synchronously off-load but when mechanical load is applied to the shaft there has to be enough current in the source to be able to supply mechanical power and still overcome the losses in the motor.

As a footnote, synchronous speed means exactly that but as a mechanical load increases, the rotating shaft "slips" a small angle to accommodate that change in load and draw more current.

Best Answer

Related Solutions

Electronic – Calculate rotor slip without knowing synchronous speed

Speed of Synchronous motor determined by frequency, but how about current

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