348
MODERN GAS WORKS PRACTICE
and the mechanical power obtained is derived from the energy of the current flowing through the armature in a magnetic field, There are three possible types
of motors which may be used, the characteristics being as folio ws:—
Fig. 251.—Standard Enclosbd Vbntilated Motor.
Shunt motors: These machines run at approximately constant speed with constant voltage, and may be employed where the load is approximately constant. They haVe not, however, the heavy over-load capacity of series or com-pound-wound motors, and are most suitable for such duty as driving fans or pumps.
Series motors: These
motors are only suitable when direct geared to the load they liave to drive and should not
be used for continuously running plant. They are mostly used for hoists, cranes, lifts, etc., and must never be used where there is a possibility of the motor running unloaded, as their characteristic
is such that the speed will rapidly increase as the load decreases until a dangerous point is reached at which the machine may fly to pieces.
Compound-woimd motors: These motors combine to some extent the characteristics of both the shunt and series motors and are generally suitable for driving gasworks plant. Their cliaracteristic is that the speed decreases to a certain extent with increasing load, and rises with. a decreasing load. Owing, however, to the shunt winding, this rise in speed is limited, and a machine reasonably com-pounded would decrease in speed
by about 15 per cent, from no load FlG’ 252-~Siandakd Tota^ly-Enclosed Gaswobks u. .c ii i i mi 7 . Type Motor.
to tul! load. They have the ad-vantage that they will give a heavier overload than the shunt machine, and are capable of developing a particularly heavy starting torque wliich is of