66 DIRECT-CURRENT DYNAMOS AND MOTORS. 
tors, from the large sizes of 200 K. W., or more, down to. 
about 10 K. W., below which the bipolar form is gen- 
erally used for the reasons given. Between 200 and 400° ° 
revs. per min. 6 poles are commonly adopted, the cor- 
responding frequencies being from 10 to 20 cycles. ‘This 
range of speed comprises practically all generators directly 
connected to high-speed engines, from the largest to the 
smallest, excepting combinations with very high-speed 
engines of 600 revs. per min., or more, for which 4 poles. 
would be preferable. When a dynamo is directly driven 
by a steam turbine, at an extremely high speed without. 
reducing gear, the field should be of the bipolar type. 
For speeds below 200 revs. per min. the number of 
poles is generally increased to 8 or more. This applies to 
most generators directly connected to low-speed engines. 
In some cases motors of various sizes, even down to Jor 2 
H. P., are required to run at low speeds in order to be 
connected directly to the machines which they drive. 
For this purpose 6 poles are suitable for speeds from 200 
to 400 revs. per min., and 8 or more poles if the speed is be- 
low 200 revs. permin. Electric railway motors are nearly 
always constructed with 4 poles, the speed being very 
variable, but having a maximum value in most cases of 
about 800 revs. per min. This gives a rather high fre- 
quency of 26% cycles, but,as the maximum speed is rarely 
maintained for more than a few minutes at a time, the 
heating due to hysteresis and eddy currents in the arma- | 
ture core does not rise above the limit allowed. The 
average speed corresponds to a moderate frequency of 15 
to 20 eycles. 
54. Sectional Area of Magnet Frame.—The proper 
number of poles having been decided upon, the next step 
is to find the approximate size of the field magnet required 
in a given case. 
The cross section of the field magnet is determined by the 
total flux which it has to carry, sufficient area being 
allowed to avoid excessive flux density in the iron. The 
