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Electrical Motors - Heat Loss

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Heat Loss from Electric Motors during Operation

Heat loss from electric motors due to inefficiency:  

Electrical Motors - Heat Loss vs. Efficiency
Size of Motor
Heat Loss
0 - 2 75 250
3 - 15 85 150
15 - 150 90 100
150 - 92 80
  • 1 kW = 1.34 hp
  • 1 hp = 0.746 k W

Example - Heat Loss from an Electric Motor

The heat loss from an electric motor supplied with 10 kW can with the heat loss 150 watts/kW from the table above be calculated as

(10 kW) (150 watts/kW)

= 1500 watts

= 1.5 kW

The rest of the energy

(10 kW) - (1.5kW)

= 8.5 kW

is output mechanical energy. 

If the motor and the mechanical equipment connected to the motor is in the same room - or enclosure - all electric energy supplied to the motor will convert to heat and temperature increase in the room.

Heat Loss from an Electric Driven Device to a Room

The heat transferred to a room or enclosure depends on the location of the motor and the driven device related to the room. 

  1. motor and driven device in the room - all energy to the motor is at the end transferred to the room (heat loss from the motor and frictional energy from the driven device)
  2. motor in the room and device outside the room - only heat loss from the motor is transferred to the room - frictional energy from the driven device is lost outside the room
  3. motor outside the room and device inside the room - frictional loss from the driven device is transferred to the room - energy loss from the electric motor is lost outside the room
Electrical Motors - Heat Loss to Room Air
Nameplate Rating
Motor Efficiency
Heat Loss to Room Air
(Btu/hr per rated hp)

Motor in Room
Driven Device inside Room
Motor in Room
Driven Devise Outside Room
Motor Outside Room
Driven Device inside Room
0 - 1.5 0.80 2542 508 2034
1.5 - 12.5 0.85 2542 381 2161
12.5 - 0.90 2542 254 2289
  • 1 Btu/h = 0.293 W 
  • 1 horsepower = 2542 Btu/hr
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