iciency varies with their size, ranging from 40 to 60 per
cent in very small motors, and up to 95 per cent in very large ones.
The following table may be taken as a guide in calculating the power
required by motors, on 110-volt circuits:

1/4 Horsepower 2-1/2 amperes, or 275 watts
1/2 hp 4-1/2 amperes, or 500 watts
1 hp 9 amperes, or 990 watts
2 hp 17 amperes, or 1.97 kilowatts
3 hp 26 amperes, or 2.86 kilowatts
5 hp 40 amperes, or 4.40 kilowatts
7-1/2 hp 60 amperes, or 6.60 kilowatts
10 hp 76 amperes, or 8.36 kilowatts
15 hp 112 amperes, or 12.32 kilowatts

An electric motor, in operation, actually generates electricity, which
it pushes back into the line as a counter-electromotive-force. The
strength of this counter force, in volts, depends on the motor's
speed, the same as if it were running as a dynamo. For this reason,
when a motor is started, and before it comes up to speed, there would
be a rush of current from the line, with nothing to hold it back, and
the motor would be burned out unless some means were provided to
protect it for the moment. This is done by means of a starting
rheostat, similar to the regulating rheostat on the dynamo
switchboard. This resistance box is connected in "series" with the
armature, in the case of shunt and compound motors; and with the
entire motor circuit in the case of a series machine.

A _series_ motor has a powerful starting torque, and adjusts its speed
to the load. It is used almost altogether in street cars. It can be
used in stump pulling, or derrick work, such as using a hay fork. It
must always be operated under load, otherwise, it would increase in
speed until it tore itself to pieces through mechanical strain. The
ingenious farmer who puts together an electric plow, with the mains
following behind on a reel, will use a series motor.

A _shunt_ motor should be used in all situations where a fairly
uniform speed under load is required, such as separating, in milking
machines, running a lathe, an ensilage cutter, vacuum cleaners,
grinders, etc.

The _compound_ motor has the characteristics of the series and shunt
motors, giving an increased starting torque, and a more nearly
constant speed under varying loads than the shunt motor, since the
latter drops off slightly in speed with increasing loa