The Ramp Test is performed with a slowly rising voltage. The final voltage is the Hipot test voltage. The voltage and measured current is recorded every second or few seconds during the ramp test.
The initial increase in the current is mainly due to capacitive inrush current, IC. This current quickly stabilizes and remains constant throughout the test as long as the voltage increase, dV/dt, is constant.
IC = C *dV/dt C = winding capacitance
There may also be some absorption current throughout the ramp test.
The increase in the current as the test proceeds is due to surface leakage current. If the insulation is weak or breaking down, there will also be an accelerating volume conduction current through the insulation.
For a good motor, the slope of the curve is fixed (i.e. the curve is close to a straight line) since a higher voltage typically produces a proportionally higher measured current. The amount of surface leakage current indicates how contaminated and/or moist the windings are.
An acceleration in the total measured current after the initial current inrush has settled indicates a breakdown of the insulation.
At the end of the voltage ramp, the voltage is held steady for a short while. The capacitive inrush current rapidly drops to zero, and the remaining measured current is mainly due to surface currents and conduction leakage currents if the winding insulation is breaking down.