362 THE INDUCTION MOTOR. 



To bring 



Dyne-centimetres to gramme-centimetres divide by 981 

 ,, ,, ,, kilogramme-metres divide by 981 x 10 5 



,, ,, ,, pound- feet divide by 13-56 x 10 6 



Power expressed as the product of the absolute units 

 of force and velocity, i.e., dynes x cm. per second is 

 in ergs per second. This also holds good if the power 

 is expressed in torque of dyne-centimetres multiplied 

 by angular velocity in radians per second. 



To bring 

 ergs per sec. to watts divide by 10 : 



,, ,, kg. cm. per sec. ,, ,, 9'81 x 10 7 

 ,, ,, ft.-lbs. per sec. ,, ,, 1-356 x 10 7 



,, ,, horse-power ,, ,, 746 x 10 7 



The discussion given above is intended to give an idea 

 of the relation between the various factors in the induction 

 motor in order to explain its behaviour. The formulae 

 are therefore not put in the form in which they would 

 be used by a designer, and the torque or power of a 

 motor would not in the ordinary course be obtained 

 from the expressions given. The complete mathematical 

 determination of the leakage coefficient and other 

 factors involves the consideration of too many matters 

 to be suitable for discussion here. 



Measurement of Slip. Since the slip of a motor is only a 

 small percentage of its speed, it cannot be accurately 

 measured by a tachometer of the usual type, as the 

 readings of such an instrument cannot usually be read with 

 a closer approximation than about 1 per cent, on account of 

 the large range of the scale. Even this degree of accuracy 

 can hardly be relied upon when two belt-driven tacho- 

 meters are employed one on the generator and one on 

 the motor. As the slip of an induction motor usually varies 

 between 2 and 5 per cent, of its speed, it is evident that 

 other methods of measurement must be employed. 



The simplest method is to apply simultaneously a 

 speed counter to the shaft of both motor and generator 

 for one minute. If both machines are in the same room, 

 two observers can easily signal to each other, so as to 

 obtain exact coincidence in- the times of reading. If the 

 generator is at a distance, it is often possible to run a 

 temporary pair of wires for a signal bell or lamp from 



