If the motor is not put into operation immediately upon arrival, it is important to protect it against external factors like moisture, high temperature and impurities in order to avoid damage to the insulation. Before the motor is put into operation after a long period of storage, you have to measure the winding insulation resistance.
If the motor is kept in a place with high humidity, a periodical inspection is necessary. It is practically impossible to determine rules for the actual minimum insulation resistance value of a motor because resistance varies according to method of construction, condition of insulation material used, rated voltage, size and type.
In fact, it takes many years of experience to determine whether a motor is ready for operation or not. A general rule-of-thumb is 10 Megohm or more.
The measurement of insulation resistance is carried out by means of a megohmmeter - high resistance range ohmmeter. This is how the test works: DC voltage of 500 or 1000 V is applied between the windings and the ground of the motor. During the measurement and
immediately afterwards, some of the terminals carry dangerous voltages and MUST NOT BE TOUCHED.
Now, three points are worth mentioning in this connection:
Insulation resistance, measurement and checking.
Insulation resistance
• The minimum insulation resistance of new, cleaned or repaired windings with respect to ground is 10 Megohm or more.
• The minimum insulation resistance, R, is calculated by multiplying the rated voltage, Un, with the constant factor 0.5 Megohm/kV. For example: If the rated voltage is 690 V = 0.69 kV, the minimum insulation resistance is: 0.69 kV x 0.5 Megohm/kV=0.35 Megohm Measurement
• Minimum insulation resistance of the winding to ground is measured with 500 V DC. The winding temperature should be 25°C +/- 15°C.
• Maximum insulation resistance should be measured with 500 V DC with the windings at a operating temperature of 80-120°C depending on the motor type and efficiency.
Ground insulation test
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