A fraction of the Horsepower!  A look at fractional horsepower motors and some old standards

Electrical power is generally rated in Horsepower or Watts. A horsepower is a unit of power equal to 746 watts or 33,000 pound-foot per minute.  The horsepower used for electrical machines is defined as exactly 746 Watts.

A fractional horsepower motor (FHP) is an electric motor which produces less than 746 watts when operated.  FHP gets the name because it produces so little power that it cannot be rated by the conventional horsepower system of units.

FHP motors are exempt from laws which govern more powerful motors, such as European directives related to eco-design and United States Energy Power Act of 2005.

FHPs’ are used to power white goods, such as washing machines and fridges, and other household appliances such as vacuum cleaners.  You can also find them on auxiliary devices in cars – power seats and windscreen wipers for example – and this covers the largest part of the FHP market.

There are a number of FHP specialist manufacturers still about, but generally they do not tend to be a core business of bigger manufacturers.

History of FHP motors and use today

Commercially successful electric motors date back to the late 1800’s.  FHPs’ appeared with the use of alternative current (AC) in cities in about 1915.  Major manufacturers recognised the potential market for electric household appliances, and in the United States hundreds of thousands of FHPs were powering household appliances by the 1920s’.

During the post-war consumer boom, demand for the motors grew, especially in Europe throughout the 1960s and as new housing in Britain spread, with built-in ovens, refrigerators and washing machines becoming required goods.

Today you can find them everywhere in vending machines, exhaust fans, coffee-machines, dishwashers, microwaves, laptops, printers, powering the electric windows and wing mirrors in your car and in medical devices.

Roaring Twenties – along with the new household goods, the first FHP standard appears

In 1926, BSI released the specification for Electrical Performance of Fractional Horse-Power Electric Motors – BS 170.  It applied to FHP motors (including Fan Motors) of any continuous rating “less than one horse-power per 1000 revs per minute” and covered variable speed and inverse speed motors.

In 1939 the scope was expanded to cover “universal” type FHP motors, and AC DC generators.

In 1962, the standard was again revised, with major changes bringing it into line with current practice, with particular provision made for new classes of insulating materials, and guidance was given on performance characteristics.

Standards covering dimensions in the 1950s

BS 2048 appeared in 1953, establishment dimensions for two frame sizes of FHP motors, necessary so that motors of all different makes would mechanically interchangeable when mounted on the driven machines.  The dimensions specified were identical to the American NEMA FHP Motor Standard MG2:1951, so that those complying with the standard would be interchangeable between the two countries.

BS 2048 was revised in 1961 to cover more frame sizes, still identical to the US standard.

1970s until today – part of the BS 5000 series

BS 170 and BS 2048 were withdrawn in 1973 and replaced by the new BS 5000-11:1973 “Small Power Electric Motors and Generators.”  The new standard was primarily intended to cover light type single-phase electric motors, but could also “be used for 3-phase or d.c. machines, particularly where mechanical interchangeability of machines suitable for different supplies” was of importance.  It formed part of BS 5000 which was intended to eventually contain a comprehensive series of British Standards for rotating electrical machines. 

Most parts of the BS 5000 rotating electrical machines series came and went, but BS 5000-11 is still in force and was confirmed in October 2012.

What’s available in the Knowledge Centre?

BSI’s Knowledge Centre can provide access to all versions of the withdrawn BS 170, BS 2048 and old versions of BS 5000-11.  Members can view withdrawn standards without charge in Chiswick, and they are available for purchase in hard copy and PDF.  BSI members receive a 50% discount, free postage and can buy on account.
To find out more please contact the Knowledge Centre at: knowledgecentre@bsigroup.com or on +44 (0)20 8996 7004


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