Motor identification

Question: I am presently involved in motor rewinding and I come across a frequent problem with motor identification.

There are some motors that carry “dashes” (-) for horsepower ratings. At one time, I was made to understand that each dash meant ¼ horsepower. But I come across very small motors that sometimes carry four dashes and pull minimum amperage. In caparison, there are fairly large motors that carry one dash and pull 6 or more amperes.

Most technicians request motors by horsepower ratings and sometimes there is not even nameplate data present.

Can there by any simple formula that one can work out to determine horsepower rating? Also what does each dash mean?

­ -Adrian Lewis Trinidad, West Indies

Answer: Motors, which show one or more dashes in the horsepower rating section of the nameplates, are manufactured to the specifications of the customer. One, two or more dashes means the same as a blank space for horsepower rating. The customer requested that horsepower rating not be shown. There is no horsepower rating assigned to each dash.

The customer of motors furnished without the horsepower rating shown on the nameplate is usually an original equipment manufacturer. The customer applies these motors to drive loads, which vary in horsepower required at different times in the duty cycle. The motors may be overloaded for a short time and then under loaded for a time period or even run at no load for a time period. This type of duty cycle is well within the thermal capability of the motor even though it operates above its continuous horsepower rating for certain periods of time. Motors driving air compressors are examples of this type of duty cycle.

Nameplate data for a motor without a nameplate can usually be obtained from the motor manufacturer. If all nameplate data except horsepower is available, the hp rating may be calculated from this formula:

Single Phase hp = volts ´ amperes ´ efficiency ´ power factor / 746

Three Phase hp = 1.732 ´ volts ´ amperes ´ efficiency ´ power factor / 746

-Richard Beard PE, consultant