DIRECT-CURRENT. DYNAMOS AND MOTORS. ; 3 
A generator is usually required. to develop either a given 
current, C' amperes, or a certain electrical power, K.W. 
kilowatts, at a given voltage, H, and at a prescribed 
speed, n, revolutions per minute. The value of the cur- 
rent, if the problem is given in the latter form, may be 
found by dividing the output, in watts, by the #. M. F., 
in volts; ; or in symbols: 
1,000 K. W. 
PES sila is. SAI ST ee a a 1 
C= a (1) 
where C = current, in amperes; 
K. W. = electrical power, in kilowatts; 
E = KE. M. F., in volts. 
A motor is ordinarily designed to develop a given number 
of mechanical horse powers, H. P., at a certain speed, n, 
revolutions per minute, when supplied by a given volt- 
age, EH. 
5. ‘Method of Motor Calculation.—In order to enable 
a motor problem to be treated in the same manner as the 
design of a generator, the H. P. may be converted into 
K. W. by multiplying the former by .746; and the coun- 
ter EK. M. F., EZ’, of the motor may be assumed to be from 
1 to 5 per cent. less than the supply voltage, H, depending 
upon the size and type of the motor. The difference be- 
tween EF and LE’ is usually between 7 and 2 per cent. for 
large machines of 100 H. P. and more, and about 4 or 6 
per cent. for small machines of 7 and? H. P. If the 
speed of the motor is abnormally low, to enable it to be 
directly connected to printing presses, blowers, etc., the 
difference between the direct and counter E. M. F. is cor- 
respondingly greater. The following values, therefore, 
enable a motor to be treated the same as if it were a prob- 
lem in dynamo design: 
Outoute i Wo TAO PEP iit caccec sess (2) 
Counter E. M. F.: Fes 0R EE 10490 BE is oe ee (3) 
Current: O'= Aa (4) 
EX Bf. Bee 2 ee 
