apparatus for testing the performance of electrical machines by producing a direct digital readout and display of per unit rotor slip in electrical motors from no load condition through loads in excess of maximum rated load . the apparatus includes speed sensing means for detecting the rotor speed of the motor under test and means for producing a voltage output that is proportional to the rotor speed . a reference voltage is developed by sensing the frequency of the input voltage to the motor under test . the difference between these two signals , representing the actual rotor speed and the synchronous rotor speed , is divided by the reference voltage to produce the per unit slip of the motor under test .