Fractional Horsepower Motors
A fractional horsepower motor (FHP motor) is a small electric motor. Its power output is measured as a fraction of one horsepower. Typically, fractional horsepower motors are mounted on frames. Though no formal standard specifies a minimum size, most fractional horsepower motors are too large to fit into a pocket. Almost all have a power rating below that measures below one horsepower; larger motors are referred to as integral horsepower motors, and their power ratings are specified by integer multiples of one horsepower. Notably, motors on frame sizes 42, 48, and 56 are considered fractional horsepower motors even if the motor's power exceeds one horsepower.
Fractional horsepower motors are very common in residential, commercial, and industrial settings. There are two major types of fractional horsepower motors. Standard fractional horsepower motors are those larger than 1/20 horsepower. They are distributed among in kitchen appliances, grocery store conveyor belts, and vacuum cleaners. Very small motors, those of 1/20 horsepower or less, are referred to as sub-fractional horsepower motors or micro motors. They are often found powering small consumer devices such as electric toothbrushes and toys.
Some even refer to small internal combustion engines as fractional horsepower motors.
Uses of Fractional Horsepower Motors
Fractional horsepower motors are used both in industry and in consumer devices. Fractional horsepower motors are utilized in an extremely broad and diverse array of sectors, from agriculture to heating & cooling. They have thousands of applications, from powering cooling fans and electric drills to driving meat slicers and electric shavers.
The global automotive industry consumes more than a third of fractional horsepower motors sold worldwide. Most find uses in secondary applications such as starters, door openers, air conditioning, and powered seats.
More than 10% of fractional horsepower motors power domestic appliances, finding use in microwave turntables, blenders, mixers, refrigerators, washers and dryers, and garage door openers.
Large quantities of fractional horsepower motors are used in electric tools, office equipment, medical devices, and heating & cooling. Diverse industries use them for material handling and processing.
The earliest record of fractional horsepower motors of commercial importance was Nicola Tesla's 1888 U.S. patent on the induction motor. They remained uncommon until 1915, when major urban centers started receiving electrical utility services. With broadening electrification, the nascent fractional horsepower industry began to grow to meet the demand for consumer appliances such as washing machines. By 1920, over half a million fractional horsepower motors were in use in the U.S.
As fractional horsepower motors became more common, powered woodworking machines such as band saws, circular saws, and drill presses grew smaller, less expensive, and more portable. Furniture and wooden structures became much less expensive and labor-intensive to produce. Powered machine tools accelerated the industrial revolution, already well in motion since the early 1800s. Assembly lines started producing faster and required less labor, and machines both large and small, increase in supply and dropped in cost, as quality increased.
After World War II, demand for fractional horsepower motors surged as Americans used more and more electrically powered devices. Since that time, demand has continued to increase. They have become ubiquitous in automobiles, household appliances, and small machinery. In 2017 the European fractional horsepower market was estimated at around 300 million motors worth $4.5 billion USD.
Types of Fractional Horsepower Motors
There are several types of fractional horsepower motors, but all electric motors have the same basic components: a rotor, coil, field magnet, axle, and enclosure. They usually contain a stator, which may be part of the enclosure. (If you are confused, remember: the rotor rotates while the stator remains stationary.) Some motors contain internal brushes, which help conduct precisely-timed electrical currents to parts of the device when they need power. Brushless DC motors demand higher prices than regular DC motors since they require a power supply, usually located externally.
All types of electric motors use field magnets to generate power. Some motors incorporate permanent magnets; others, referred to as induction motors, contain only electromagnets. The magnets are repulse or attract when electrical current flows through the coil, generating a rotational force on the rotor that transfers to the axle.
Permanent magnet motors are more energy efficient than induction motors. Also, permanent magnet motors are more powerful than similar sized induction motors. A permanent magnet motor can have high torque over its entire speed range, even at low speeds. One major drawback of permanent magnet motors is their cost; their construction often requires expensive rare earth magnets. Permanent magnet motors run at synchronous speed, rotating in-step with the frequency of their power sources.
An induction motor operates by modulating the electrical current in the coil which wrapped around the stator. The modulation induces a rotating magnetic field, exerting a force on the rotor. The magnetic field revolves slower than synchronous speed would dictate. Three-phase induction motors are used extensively in industry, but single-phase fractional horsepower induction motors are also extremely common. They are cheap, dependable, and robust.
Choosing a Fractional Horsepower Motor
With few exceptions, fractional horsepower motors with equal specifications may be interchanged. If a motor made by one manufacturer is removed, a similar motor made by another manufacturer can typically be swapped into place with no change in operating characteristics.
Of course, when replacing a fractional horsepower motor, the replacement motor must meet the correct specifications. The easiest way to match motor specifications is to copy the nameplate information from the broken, worn or defunct motor.
When replacing motors that failed for unknown reasons, it is a good idea to consider whether the motor specifications are appropriate for the application. If the motor was too small or not rugged enough, a different type of motor should replace it.
It is worth noting that sometimes a motor will be conspicuously marked as a special or custom motor; special motors should ordinarily be replaced by exact duplicates.
A wide range of fractional horsepower motors exist, but nearly all general-purpose motors can be seen as variations along the following dimensions: power supply, motor type, power characteristics, enclosure, bearings, the direction of rotation, mount, and thermal protection. Each of these dimensions should be examined in detail before buying a fractional horsepower motor.
Choosing a Motor: Power Supply
Fractional horsepower motors can be alternating current (AC) motors or direct current (DC) motors. AC motors can be either single-phase motors or three-phase motors. Most AC fractional horsepower motors are single phase, but a three-phase motor cannot be placed on a single-phase power supply. Some AC motors, typically those for American markets, need a 60 Hz power supply. Motors for the European and Asian markets typically demand a 50 Hz power supply. Voltage is an important consideration for both AC and DC motors; the power supply voltage must match the motor specifications.
Continuously variable speed motors depend on a controller that regulates electrical current supplied to the motor. The controller must be matched with the motor type and available power sources. The controllers may operate by applying power at intervals to selected poles of the motor, by varying electrical power levels or by modulating the AC wavelength.
Stepper and servo motors have their own power supply and timing requirements. These must be matched to meet specifications.
Choosing a Motor: Power Characteristics
Motors have three major power characteristics. Horsepower is the capability to do a certain amount of work in a certain amount of time. Torque is the degree angular force that a motor can exert. Speed is the rate at which the motor turns. These three factors conform to a simple relation. Speed multiplied by torque is directly proportional to horsepower; gear mechanisms can trade speed for torque or vice versa. There is no advantage to purchasing a motor with excessive horsepower, but the horsepower must be sufficient to meet the motor's maximum workload. In applications where extremely high torque or extremely low speed is required, gearmotors may be appropriate.
Choosing a Motor: Motor Class
The precise details of the inner construction of fractional horsepower motors vary. Some have brushes; some are brushless. Some require a large capacitor to provide starting current. Some have large amounts of speed and torque to start, and some gradually build up torque as they build up speed. Some are reversible, some are variable speed. Some have two or more windings; some have only one. It is almost always possible to choose an appropriate motor type by looking at maximum starting current and starting torque.; choose the motor type based on the power supply and startup requirements of the motor.
Choosing a Motor: Enclosure
The motor enclosure must be the right size to inter-operate with the machinery that it powers. The enclosure should also protect the motor from the environment. Motors used in marine environments or highly corrosive environments such as chemical plants may need to be sealed in a full enclosure. Motors which power devices outdoors may need drip-proof enclosures.
Choosing a Motor: Bearings
Motors should have suitable bearings for their expected use. Sleeve bearings are often appropriate, but ball bearings may be required to support a heavy axial load. Needle bearings may be appropriate for high-speed applications. Also, if a motor is mounted vertically, the bearing must be suitable for a vertical orientation.
Choosing a Motor: Rotation
It may seem obvious, but the motor must rotate in the correct direction. Some motors can be reversed by reversing the poles of the power supply; some cannot. Typically, the direction a motor rotates is specified on the end opposite the shaft. On most motors, a notation of CW indicates that they rotate clockwise and a notation of CCW indicates that they rotate counterclockwise. Note that on pump motors, this notation is reversed.
Choosing a Motor: Mount
The motor must have mounts that match the machinery or supports to which it will be attached.
Choosing a Motor: Thermal Protection
To protect the motor from overheating, many motors shut off when they reach a specified operating temperature. This capability is called thermal protection. There are two major types of thermal protection switches. Automatic Reset switches are reset automatically when the temperature drops. Manual Reset switches require the operator to push a button or throw a switch to power up the motor again. An automatic reset switch might be appropriate on a fan but would be a serious hazard on a circular saw. Wherever the automatic restart of a motor could create a safety hazard, manual reset should be considered mandatory.
Regulations and Standards
There exist a number of standards that apply to fractional horsepower motors. In the United States, fractional horsepower motors use frame sizes outlined by NEMA, the National Electrical Manufacturers Association. Fractional horsepower motors are ordinarily exempt from energy requirements of larger motors, including the US Energy Policy Act of 2005 and the EN 60043-30:2009 ruling of the European Commission. In many environments, they may be subject to radio interference standards, meaning that they must tolerate radio interference and cannot generate excessive interference.
Choosing a Manufacturer
Several factors should be considered when choosing a manufacturer to meet your fractional horsepower motor needs. A good manufacturer will provide a quality product that will last for a minimum of five years of use. The right manufacturer will provide support and replacements not only a few weeks down the line but decades into the future.
Fractional Horsepower Motors Types
- 1/2 HP motor are fractional horsepower motors with a power output of ½ HP, which is roughly equivalent to 373 Watts.
- 1/4 HP motor are fractional horsepower motors because they output power that is less
than one HP, or 746 watts when referring to electric motors.
are either induction or synchronous. They have a
current that flows in either direction, usually with electric power.
- have variable speed operation capabilities with current,
which flows in one direction. Often use electric power.
are motors that convert electric energy into motion using magnetism.
- FHP motors is the abbreviation for fractional horsepower motors, which are motors
that are built in a frame, traditionally with a power output of smaller
than one horsepower.
a combination of a motor and gearhead that reduce motor speed to desired
- are used when minimum speed variation requirements
are necessary at constant potential with full to no load and/or constant
- Miniature motors also referred to as micro motors or small motors,
are fractional horsepower motors that must not only fit the frame size
and/or less than one horsepower requirements, but also size dimensions
of a couple inches or less.
come in two, three or four speeds. They have varying connections
that can alter the speed at the starter, because of the way they are
- NEMA motors are electric motors that fit the standards set by NEMA, which stands for the National Electrical Manufacturers Association.
have magnets in their rotor assemblies. These
allow the rotor assembly to arrange itself in a line with the rotating
field of the stator, which results in no slip and higher torque efficiency.
- Reversible motors can be AC or DC. A typical DC motor is almost always reversible
by changing the polarity of the field; an AC motor is reversed in a
three-phase power motor by reversing connections of one leg and in a
single phase by reversing the leads.
- have less than one horsepower of power.
- Small motors use a fraction of a horsepower, or 745.7 watts, of power to run.
operate up to full load at a constant speed and are often used to maintain
an exact speed. The rotor speed and the rotating magnetic field speed
- can run on either AC or DC power.
- are capable of operating on both direct current and alternating current.
- direct the rotational speed of an AC motor and can be found in many heating and cooling systems.
are essentially multi-speed motors whose loads
have varying torque requirements along with varying speed requirements.
Found in pumps and blowers.
Fractional Horsepower Motors Terms
A device that creates mechanical motion by converting various forms of
energy to rotating or linear mechanical energy.
Electrical conductors in the core slot, insulated from the iron core.
They produce and transmit the magnetic field as current passes through.
– A mounted cylindrical
device, to which the motor brushes are attached, on the armature shaft
made of copper segments set around the shaft.
– Name used for
motors with variables speeds whose loads need the same amount of horsepower
despite their speed.
– The segment of the stator
and rotor, usually iron; made up of cylindrical laminated electric steel.
A complete reverse flow of alternating current during a rate of time.
– The comparison
of operating and rest times due to normal operating temperature.
The ratio of electrical input to mechanical output. It measures the motor's
effectiveness when converting electrical energy into mechanical energy.
– Either open or
closed, the frame or housing of the motor.
– The part of the
motor housing that supports the bearing and acts as a protective guard
to the electrical and rotating parts inside the motor. An endshield is
often referred to as an "end bracket" or "end bell."
– Work done
per unit of time: 1 horsepower equals 33,000 foot-pounds of work per minute
or approximately 746 watts. Motors for pumps are an example of a motor
that is rated in horsepower.
– A device used for
the positioning of a motor. These are adjustable screws fitted on the
base or motor frame.
– The burden of the motor
by its application. The required power to overcome the resistance of the
machine it powers.
– A device that turns
a shaft by taking electrical energy and converting it into mechanical
– Made of stacked laminations,
the rotating component of an induction motor.
– The rotating part of
the motor that protrudes past the bearings for attachment to the driven
– The laminations arranged
on a rotor or armature. These help abolish low-speed cogging effects in
an armature and reduce induced vibration in a rotor.
– In an AC induction
motor, it is made of laminations with a large hole in the center for the
rotor to turn and slots in the stator for the windings to be inserted.
– A device to detect
temperature that contains two dissimilar metals which generate voltage
as a function of temperature.