Types of Motors
There are many types of electric motors, some smaller than a human hair others
large enough to power a locomotive. For the purpose of this page we will be
discussing induction motors which are typically found on most workshop
machinery such as air compressors, drill presses, table saws, band saws, jointers,
shapers and lathes. These types of motors have no brushes and work only on
alternating current. They may range in size from 1/4 horsepower up to 20
horsepower or more.
Types of Motors
The split phase motor is mostly used for "medium starting" applications. It has start and run windings, both are
energized when the motor is started. When the motor reaches about 75% of its rated full load speed, the starting
winding is disconnected by an automatic switch.
This motor is used where stops and starts are somewhat frequent. Common applications of split phase motors
include: fans, blowers, office machines and tools such as small saws or drill presses where the load is applied
after the motor has obtained its operating speed.
This motor has a capacitor in series with a starting winding and provides more than double the starting torque
with one third less starting current than the split phase motor. Because of this improved starting ability, the
capacitor start motor is used for loads which are hard to start. It has good efficiency and requires starting
currents of approximately five times full load current. The capacitor and starting windings are disconnected from
the circuit by an automatic switch when the motor reaches about 75% of its rated full load speed.
Common uses include: compressors, pumps, machine tools, air conditioners, conveyors, blowers, fans and other
hard to start applications.
Horsepower & RPM
Electric motors are rated by horsepower, the home shop will probably utilize motors from 1/4 HP for small tools
and up to 5 HP on air compressors. Not all motors are rated the same, some are rated under load, others as peak
horsepower, hence we have 5 HP compressors with huge motors and 5 Hp shopvacs with tiny little motors.
Unfortunately all 5 HP compressor motors are not equal in actual power either, to judge the true horsepower the
easiest way is to look at the amperage of the motor. Electric motors are not efficient, most have a rating of about
50% due to factors such as heat and friction, some may be as high as 70%.
This chart will give you a basic idea of the true horse power rating compared to the ampere rating. Motors with a
higher efficiency rating will draw fewer amps, for example a 5 HP motor with a 50% efficiency rating will draw
about 32 amps at 230 VAC compared to about 23 amps for a motor with a 70% rating.
TRUE HP AMPS at 115VAC AMPS at 230 VAC
1/4 3.2 - 2.3 1.6 -1.2
1/3 4.3 - 3.1 2.2 - 1.5
1/2 6.5 - 4.6 3.2 - 2.3
3/4 9.7- 7.0 4.9 - 3.5
1 13.0 - 9.3 6.5 - 4.6
1 1/2 19.5 - 13.9 9.7 - 7.0
2 25.9 - 18.5 13.0 - 9.3
5 64.9 - 46.3 32.4 - 23.2
A quick general calculation when looking at a motor is 1 HP = 10 amps on 110 volts and 1 HP = 5 amps on 220
The shaft on a typical shop motor will rotate at either 1725 or 3450 RPM (revolutions per minute).
The speed of the driven machine will be determined by the size of pulleys used, for example a 3450 RPM motor
can be replaced by a 1750 RPM motor if the diameter of the pulley on the motor is doubled. The opposite is true as
well but if the pulley on the 1750 RPM motor is small it is not always possible to replace it with one half the size. It
may be possible to double the pulley size on the driven machine if it uses a standard type of pulley, (not easily
done on air compressors for example).
Electronic speed reducers such as the ones sold for routers will not work on induction type motors.
Phase, Voltage & Rotation
Whether or not you can use a motor will likely depend on these factors.
Ordinary household wiring is single phase, alternating current. Each cycle peaks and dips as shown. To run a
three phase motor a phase converter must be used, usually this is not practical, it is often less expensive to
change the motor on a machine to a single phase style.
This is used in industrial shops, rather than peaks and valleys the current supply is more even because of the
other two cycles each offset by 120 degrees.
Many motors are dual voltage, by simply changing the wiring configuration they can be run on 110 volts or 220
volts. Motors usually run better on 220 volts, especially if there is any line loss because of having to use a long
wire to reach the power supply.
Motors are available for both AC and DC current, your typical home wiring will be AC, there are DC converters
available which are used in applications where the speed of the motor is controlled.
The direction the shaft rotates can be changed on most motors by switching the right wires, there is usually a
diagram on the motor.
The direction of rotation is usually determined by viewing the motor from the shaft end and is designated as CW
(clockwise) or CCW (counter-clockwise). Note: Some manufactures may have a different method of determining
shaft rotation but will usually make a note of it.
Motors are built to standard specifications, such as shaft height, shaft diameter, and style of mounting. The
different styles are defined by a number and lettering system developed by Nema (See Reference Chart).nema
Types of Mounts
The three most common types of mounts you will find are:
Is bolted, welded or cast on main frame and allows motor to be rigidly mounted.
Has isolation or resilient rings between motor mounting hubs and base to absorb vibration and noise.
NEMA C face mount
Has a machined face which allows direct mounting, bolts pass through mounted part to threaded holes in the
The two most commonly used styles are:
An ODP enclosure on a motor means "Open, Drip Proof". They are relatively inexpensive motors used in normal
applications. The construction of an ODP motor consists of a sheet metal enclosure with vent stamped to allow
good air flow. The vents are designed in such a way that water dripping on the motor will not normally flow into the
motor. A fan is mounted on the motor's rear shaft to pull air through the motor to keep the motor cool.
A TEFC enclosure on a motor means "Totally Enclosed, Fan Cooled". This is probably the most commonly used
motor in ordinary industrial environments. It costs only a few dollars more than the open motor, yet offers good
protection against common hazards. It is constructed with a small fan on the rear shaft of the motor, usually
covered by a housing. This fan draws air over the motor fins, removing excess heat and cooling the motor. The
enclosure is "Totally Enclosed". This ordinarily means that the motor is dust tight, and has a moderate water seal
as well. Note that TEFC motors are not secure against high pressure water.