No Load Test (AC Motors)

The motor is run at rated voltage and frequency without any shaft load. Input power, current, frequency and voltage are noted. The no load P.F. is quite low and hence low PF watt meters are required. From the input power, stator I2R losses under no load are subtracted to give the sum of Friction and Windage (F&W) and core losses. To separate core and  F & W losses, test is repeated at variable voltages. It is useful to plot no-load input kW versus Voltage; the intercept is Friction & Windage kW loss component. 

F&W and core losses = No load power (watts) – (No load current) 2 × Stator resistance

Stator and Rotor I2R Losses

 The stator winding resistance is directly measured by a bridge or volt amp method. The resistance must be corrected to the operating temperature. For modern motors, the operating temperature is likely to be in the range of 1000C to 1200C and necessary correction should be made. Correction to 750C may be inaccurate. The correction factor is given as follows :

R1/R2 = (235+t2)/(235+t1)

Where, t1 = ambient temperature, oC & t2 =- operating temperature, oC. 

The rotor resistance can be determined from locked rotor test at reduced frequency, but rotor I2R losses are measured from measurement of rotor slip. Rotor I2R losses = Slip × (Stator Input – Stator I2R Losses – Core Loss) Accurate measurement of slip is possible by stroboscope or non-contact type tachometer. Slip also must be corrected to operating temperature.