22 
DIRECT-CURRENT DYNAMOS AND MOTORS. 
210, 
211. 
212. 
213. 
214. 
215. 
216. 
21%. 
What are the usual limits of the frequency in direct- 
current machines? 
Give the usual average values of the frequencies for low- . 
speed, medium-speed, and high-speed direct-current 
machines, respectively. 
Give formula for obtainiag the proper speed of a machine 
corresponding to a given frequency. 
What must be the speed of a 6-polar dynamo, if a 
frequency of 15 cycles is desired? 
Compute the maximum permissible speed of an 8-pole 
direct-current machine. 
By transposition of the terms in the frequency formula 
deduce an expression for the proper number of pole- 
pairs for a dynamo. 
From the formula given in answer to the preceding ques- 
tion, find the number of poles for a medium-speed 
machine running at 400 revolutions per minute. 
State the proper number of poles for a low-speed 125- 
revolution machine, by means of Table 23, instraction 
Book. 
Between what limits may the pole number be selected for 
a 175-revolution high-speed dynamo? 
For what kind of machines is the bipolar ra almost 
always adopted? 
In what machines is the 4 pole construction ‘generally 
used? 
Name the kinds of .machines in which 6 poe or more are 
employed. 
What determines the cross-section of a aynamo magnet 
frame? | | nt: 
