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Horse Power

Modified on 2008/04/10 20:25 by Administrator Categorized as Electrical

What is Horse Power?

The term "horsepower" was coined by the engineer James Watt (1736 to 1819) in 1782 while working in the performance of steam engines. This occurred while using a mine pony to lift coal out of a coal mine. He conceived the idea of defining the power exerted by these animals to accomplish this work. He found that, on the average, a mine horse could pull (lift by means of a pulley) 22,000 foot-pounds per minute. Rather than call this "pony" power, he increased these test results by 50 percent, and called it horsepower i.e. 33,000 foot-pounds of work per minute.

Therefore, the true definition of Horse Power is: 1 hp = 33,000 ft·lbf/min{Reference:|}

So, in essence, the term "horse power" is simply a standardized measurement of how much work a power source can supply.

Motor Data

Every motor should have a data plate listing the manufacturer, motor type and electrical requirements for the motor. Each data plate will contain at least some of the following information:


The voltage listed on the data plate is the design voltage the motor was made for. The motor may be operated at any voltage within approximately 10% of the design voltage.

Full Load Amps (FLA)

This amperage is the current the motor will draw when the motor is loaded up to its rated horsepower. The motor will draw less than the listed FLA if the motor is operating at less than the rated horsepower. The motor will draw more than the rated FLA when it attempts to operate at more than the rated horsepower. Measuring the actual motor amperage and comparing it to the FLA will quickly tell if a motor is overloaded.

Locked Rotor Amps (LRA)

This amperage is the current the motor will draw when the motor is energized but is unable to turn for any reason (seized bearings, etc.) A rotor is said to be locked anytime the rotor is not rotating. The LRA rating of a motor is the most amperage the motor can draw under any condition. Maintaining this condition will quickly burn up your motor.

Efficiency %

The rated efficiency for the motor is listed on some motors but not all. Motors that fail to state their efficiency probably have poor efficiency ratings. If the efficiency is not given it can still be determined using basic math. Generally higher efficiency motors are those that use more metal in their construction and the metal is laminated and insulated between laminations to reduce eddy currents, which create heat. Heat in a motor is lost efficiency. The efficiency rating may be given as a percent such as 86%, or the same percentage may be listed as a decimal fraction like 0.86. When using the percent efficiency in a math formula, it must be used as a decimal fraction.

Power Factor

The power factor is also given as a decimal fraction and may be any number less than one. Common power factor ratings range from .70 to .98. The higher power factor is always more desirable.

The power factor is a number which tells to what extent the motor voltage and current are out of phase from one another. Unlike pure resistive circuits like electric heaters and incandescent lights, motors operate with strong magnetic fields present. The magnetic fields add a new element of magnetic resistance to the motor circuit, which throws the voltage and current out of phase from each other. When the voltage and current are not in phase Ohm's law will not work unless the power factor is used to correct for this phase difference. The power factor listed on a motor must be considered when making motor horsepower and current calculations.

Service Factor

The service factor of a motor is a number, which indicates how much more work a given motor, can do beyond the rated horsepower. This is a safety factor and is not to be considered as a part of the motor's normal useful horsepower. A motor may have no service factor whatsoever and thus has no safety factor in the event the motor becomes overloaded. A common service factor on motors is a SF of 1.15. This number multiplied times the rated horsepower gives the actual horsepower the motor could operate at for a short period of time. For example; a 10 HP motor with a SF of 1.15 could actually provide service for a short time up to 11.5 HP. A motor with a high service factor is used on applications where the load may vary and may occasionally be confronted with an unexpected overload in horsepower. Air conditioning systems often use motors with the SF rating of 1.15.

The service factor can also be multiplied times the FLA of the motor to give the absolute highest operating amperage the motor should be allowed to operate with under any conditions.{Reference:|}

Electrical Calculations

The term horsepower is used extensively when describing the size of electric motors on OWWM. So, what exactly is a horsepower? For our purposes, one horsepower also equals 745.699 watts or .746 kW (kilowatts) of electrical power. We round this off to 1 horsepower = 746 watts.


  • E = Volts
  • I = Amperes
  • EFF = Efficiency (decimal)
  • PF = Power factor (decimal)
  • HP = Horsepower

Single Phase Motor

Example: What is the actual horsepower output of the following motor if its actual measured current draw is 16 amps?

5 HP, 230 Volts, FLA 20.72

Eff(%) 0.86, PF 0.91, SF 1.15

HP = (E x I x EFF x PF) / 746

HP = (230 x 16 x 0.86 x 0.91) / 746

HP = 2880 / 746

HP = 3.86 Hp

Three Phase Motor

Three phase motors use the same calculation as was used on single-phase motors with one addition to the formula. Three phase motors have three separate voltages each 120 degrees out of phase from one another. This is what gives the three phase motor its superior starting and running power and eliminates the need for start capacitors and start relays to remove a starting winding as is often necessary on single phase motors.

The three-phase motor is 73% more powerful than an equivalent single phase motor of the same voltage and amperage. The is reflected by the factor 1.732 in the equations below. The factor 1.732 is the square root of the number 3 representing the 3 phases.

Example: Prove that the actual horsepower output of the following motor is the same as the manufacturers specifications at FLA?

1 HP, 230 Volts, FLA 2.8

Eff(%) 0.84, PF 0.82, SF 1.25

HP = (E x I x EFF x PF x 1.732) / 746

HP = (230 x 2.8 x 0.84 x 0.82 x 1.732) / 746

HP = 768.293 / 746

HP = 1.03 Hp

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