The Horsepower Output or Polyphase Induction Motors

The continuous horsepower rating that is assigned to a given induction motor is determined mainly by the full load temperature rise of the machine. The temperature rise is dependent on both the losses in the motor and the effectiveness of its ventilation. While an accurate theoretical prediction of temperature rise is very difficult, a simple empirical method is presented here which gives good results over a wide range of motor sizes. The familiar D²l equation for horsepower output is discussed from the point of view of temperature rise, and a new form of equation is suggested which takes into account both ventilation and motor losses. By relating the horsepower output directly to the major physical dimensions of the induction motor a consistent basis of design is established for machines of widely different sizes and speeds.