The Motor Specialist

Knowledge Center

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Quick References and Formulas

Insulation Class Temperature Limits (°C)
A1) 105
B1) 130
F2) 155
H2) 180
  1. Hydroscopic insulation - avoided for open and outdoor locations - shortened lifetime if condensation in the insulation occurs
  2. Non-hydroscopic insulation - preferred for open and outdoor locations, higher ambient temperatures.

Heat gain from electrical motors to the surroundings
Size of Motor (kW) Heat Loss (Watts/kW)
0 - 2 250
3 - 15 150
15 - 150 100
150 - 80
  • 1 kW = 1.34 hp
  • 1 hp = 0.746 k W

Alternative - with Imperial Units
*Heat Loss to Room Air (Btu per Hr per Rated Hp)
Nameplate Rating (Hp) Motor Efficiency Average Motor in Room Driven Device Inside Room* Motor in Room Driven Devise Outside Room* Motor Outside Room Driven Device inside Room*
1/8 to 1/2 0.60 4250 1700 2550
1/2 to 3 0.69 3650 1100 2550
3 to 20 0.85 2950 400 2550
  • 1 Btu/h = 0.293 W

  • Full Load Amps (Aka nameplate amps) – The amount of current a motor draws under full load.
  • Locked Rotor Amps (Aka locked rotor current) – The required energy to start the motor when full voltage is applied.
  • Service Factor Amps – The amount of current the motor will draw when it is subjected to overload above its nameplate horsepower. It is a multiplier that indicates the amount of additional load the motor can handle.
  • Code Letter – The code letter is an indication of the amount of locked rotor current that's required by a motor when it is started.
  • Design Letter – A letter that indicates the shape of the torque speed curve when graphed. For more information, check out or
  • Full Load Speed – The approximate speed that a motor will run when putting out maximum torque or horsepower.
  • High Inertia Load – Loads that have a relatively high flywheel effect.
  • Efficiency – Refers to how effective a motor is at converting input power to output work. Check the motor's nameplate to find this figure.
  • Frame Size – Standard sizes designated by NEMA that dictate which sized frame will be used with which sized motor.
  • Frequency – Refers to the cycle frequency.
  • Insulation Class – Refers to the thermal capabilities and resistance of the insulating components to heat in a motor.
  • Constant Horsepower – Used in describing loads in which the torque requirement is reduced as the speed is increased, and vice-versa.
  • Constant Torque – Used to describe load types where the amount of torque required to drive the machine is fixed, regardless of speed.
  • Variable Torque – Describes loads having characteristics requiring low torque at low speeds and increasing values of torque as the speed is increased. E.g. – a fan.
  • Phase – describes the type of power supply to a motor. The two major types are single phase and three phase. Most industrial motors are three phase.
  • Poles – The number of magnetic poles that appear within the motor when power is applied.
  • Power Factor – The measure of a particular motor's requirements for magnetizing amperage.
  • Service Factor – A multiplier that indicates the amount of overload a motor can be expected to handle.
  • Slip – The difference between the synchronous speed of the magnetic field and the full load speed (or shaft rotating speed) of a motor.
  • Time Rating – The time that a motor can operate at full load torque continuously without overheating.
  • Torque – The tendency of a force to rotate an object about an axis. For motors, it's the twisting force exerted by the shaft of a motor.
  • Synchronous Speed – The speed at which the magnetic field within the motor is rotating.
  • Voltage – The voltage rating for which the motor is designed.

Synchronous Speed
  • 120 x frequency / No. of Poles
Horsepower (HP)
  • Torque x Speed / Constant
  • Speed in RPM
  • Value of Constant depends on units used for torque
  • Torque units:
  • Pound Feet = 5252
  • Pound Inches = 63,025
  • Ounce Inches = 1,000,000
Horsepower Required by Pumps
Centrifugal Pumps
  • HP = Gallons per minute x Head in Feet / (3960 x pump efficiency)
Hydraulic Pumps
  • HP = Gallons per Minute x Pounds per sq. inch / (1714 x pump efficiency)
Fans and Blowers
  • HP = C F M x Pressure (Inches of Water) / (6356 x efficiency)
Air Compressor Rule of Thumb
  • 1 HP produces 4 CFM @ 100 PSI
Approximate Full Load Amps (3 Phase Motors)
  • Amps = HP x 1.2 x (460/ Motor Voltage)
  • Motor Watts (at full load) = HP x 746 / Efficiency
Operating Cost on Motors
  • Kilowatt Hours = Average Load HP x .746 x Hours of Operation / Motor Efficiency (Decimal)
  • Annual Operating Cost = Annual Kilowatt Hours x Cost per KW Hour
Horsepower Calculations for Speed Changes on Variable Torque Loads
  • HPnew = HPoriginal x (speed new / speed original)3
Electric Power Formulas
  • Power in Watts = Voltage x Current in Amperes
  • Power in Watts = Resistance in Ohms x Current in Amperes 2
  • Power = Voltage2 / Resistance in Ohms
Electric Current Formulas
  • Current = Voltage / Resistance in Ohms
  • Current = Power in watts / Voltage
  • Current = (Power / Resistance) ½
Electric Resistance Formulas
  • Resistance = Voltage / Current
  • Resistance = Voltage2 / Power
  • Resistance = Power / Current 2
Ohms Law
  • Voltage = Resistance * Current
  • Voltage = Power / Resistance
  • Voltage = (Power*Resistance)1/2
Electrical Motor Efficiency
  • Efficiency = 746*output horsepower / input electrical power
Electrical Motor Power
  • Power (3-Phase) = (Volate*Current*Power Factor* 1.732) / 1000
Three Phase Real Power
  • Real Power in Watts = 31/2 * Voltage / Power Factor(.0-.95)
Total Horsepower
  • Real Power in Watts = 31/2 * Voltage * Current


The number of poles within a motor is always an even number. In an AC motor, the number of poles work in conjunction with the frequency of the motor (cycles) to determine the synchronous speed of the motor. Below is a table outlining common arrangements.

Synchronous Speeds
Poles 60 Cycles 50 Cycles
2 3600 3000
4 1800 1500
6 1200 1000
8 900 750
10 720 600



  • Variable Frequency Drive Setup Instructions
  • Demonstration and Benefits of Using a Ball Detent Torque Limiter
  • Matching Mechanical Features to Meet Application Needs for AC Motors
  • AC Motor Service Technology
  • Closed Loop Control and Vector Control

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