Introduction Methods of resistance measurement

• For measurement of armature and field winding resistances i.e. low and medium resistance ranges, voltmeter-ammeter method is used.
• For measurement of insulation resistance ( i.e. high resistance), megger method and voltmeter method are used.

]]> Run1 - Measurement of Armature Resistance

• Two voltmeters used to measure voltage drop across armature winding, one including the brushes and another by excluding brushes, with range 0 to 30V.
• Ammeter used to measure the injected armature current has range 0 to 20A.
• Rating of load box to be used 110V, 20A.
• Use combination of switches to inject armature current in steps of 2A upto 110% of rated voltage.
• Note down the readings of the ammeter and two voltmeters at each step, and then calculate the resistances with and without brush for each step.

]]> Run2 - Measurement of Field Resistance

• A DC Voltmeter (0-150A) and a DC Ammeter (0-20A) are used to measure excitation voltage applied to field winding and the current flowing through it.
• A Rheostat (100 ohm,3A) is used as a potential divider across the power supply for varying the voltage to the field winding.
• Vary the voltage across the field winding in steps of 10A up to rated voltage of field winding.
• Note down the excitation voltage and field current at each step using ammeter and voltmeter.
• Calculate the value of field resistance at each step using Ohm’s law and calculate the mean value from it.

]]> Run3 - Measurement of Insulation Resistance(Voltmeter Method) Let R_X be the value of unknown resistance between armature to yoke or field winding to yoke.

• Use a voltmeter with known internal resistance R_V and note down this value
• Note down V₁ as the Voltmeter reading with only the voltmeter in the circuit and V₂ as the Voltmeter reading when the unknown resistance is connected in series with the voltmeter
• Calculate R_X using the formula, R_X = R_V (V₁ / V₂ - 1).

]]> Run4 - Measurement of Insulation Resistance(Megger Method) Let R_X be the value of unknown resistance between armature to yoke or field winding to yoke.

• Connect the line terminal of the Megger to the either the armature terminals shorted together or the field terminals shorted together, and the earth terminal to the surface of the yoke.
• Rotate Megger handle steadily, at a moderate speed.
• Note down the Megger reading when it’s pointer becomes steady, which is the insulation resistance being measured.
• Insulation resistance for field winding and armature winding have to be measured independently.

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