The purpose of measuring the insulation resistance of the motor winding is to check the moisture and contamination of the winding insulation material, as well as to determine whether there is a short circuit between the winding and the casing or between the three-phase windings. This is to ensure the safe operation of the motor.
Generally, the larger the insulation resistance value of the winding, the better. If the insulation resistance is zero, it indicates a short circuit between the winding and the casing or between the phases. If the insulation resistance is not zero but the value is very small, it is also considered不合格. Usually， 1000 ohms per volt of working voltage is required. For example， for a motor with a rated voltage of 380 volts， the insulation resistance should not be lower than 0.38 megohms. The insulation resistance we usually measure is the cold-state insulation resistance， which is the insulation resistance when the motor is not operating.
When the motor is running， the current passing through the winding always generates heat， causing the motor temperature to rise. The temperature change will affect the resistance of each part of the motor. The insulation resistance value will decrease as the temperature rises. Therefore， the insulation resistance value of general small and medium-sized motors should not be lower than 0.5 megohms. When measuring the insulation resistance of large motors， in addition to measuring the insulation resistance， in order to determine the moisture condition of the high-voltage winding insulation， the absorption ratio (also called absorption coefficient k) should also be measured. The absorption ratio refers to the ratio of the insulation resistance R60 at 60 seconds to the insulation resistance R15 at 15 seconds (R60/R15). Usually， K ≥ 1.3； it can be considered that the winding insulation is dry (moisture-damaged insulation or defective ones usually have an absorption ratio close to 1).
When measuring the insulation resistance of the motor， the following matters should also be noted：
1. Before measuring the insulation resistance， the power supply of the motor to be measured should be cut off and short-circuited for discharge to ensure the safety of personnel and instruments. There are two situations that need attention：
(1) For single-phase capacitor motors， the capacitor connection lines should be removed；
(2) For variable frequency， soft starters， thyristors， and other non-mechanical contact (soft switches) controlled motors， the winding connections should be completely disconnected from the above electrical control equipment to avoid damaging the above control equipment.
2. After power-off， when measuring the insulation resistance with a load， wait until the motor with load has completely stopped before proceeding.
3. For motors with a rated voltage of 3000 volts or above， after each measurement of insulation resistance， the winding should be connected to the casing for a period of time. For motors below 1000 kW， it should be no less than 15 seconds， and for motors above 1000 kW， it should be no less than 1 minute.
Post time: Aug-21-2026