Electric Motor Insulation Resistance Measurements

Abstract:
Insulation resistance measurements provide a means for testing the integrity of wiring systems, i.e. motors. By frequent testing and comparison against baseline data, fluctuation from normal resistance values is determined and equipment is scheduled for repair prior to failure. This type of preventive maintenance testing program assists in safeguarding against motor short circuits, overheating and ultimate motor failure.

Basic concept:
To provide a reasonable factor of safety against short-circuits, grounds and insulation degradation, completed wiring systems and motors undergo an insulation resistance test. The proper instrument for making this test is a megohmmeter ("megger"). A megohmmeter is an instrument frequently used to measure high resistance. A typical megger consists of a magneto that is turned by a crank on the side of the case. The scale is calibrated directly in ohms. The resistance to be measured is connected across two terminals. The crank is turned at a moderate speed until the pointer or meter reaches a steady deflection.

Cable and conductor installations provide a wide variation of conditions with respect to the resistance of insulation. These variations are due to the many types of insulating materials used, insulation thickness, voltage rating and length of the circuit or windings. Long circuits and motor windings may be subject to wide variations in temperature that influence the insulation resistance values in a given test.

An alternative method to using a megger entails utilizing a 500-volt dc source and taking readings with a voltmeter. Typically, the ungrounded side of the system is connected to all the motor terminals through the voltmeter. The opposite or grounded side is connected directly to the motor frame through a fuse of 10 amps or less. The resistance is then found by calculation.

While insulation resistance readings are quantitative, they are also relative and comparable. It is this comparability which forms the heart of any insulation resistance testing program. Readings are utilized to indicate the presence of moisture, dirt and deterioration and impending motor failure.

Uses:
The primary objectives of an Insulation Resistance Measurement Program are:

General Procedures:
Motors to be tested are removed from the ride or device and relocated to a dry and temperature stable environment. While in this environment, the motors are cleaned, inspected and serviced as necessary. The motors are kept in this environment for at least one week prior to testing to "normalize", dry out and standardize temperature between the various motors. All insulation resistance testing is performed at a time when temperature and humidity levels are as close as possible to the previous tests to ease data interpretation and comparisons.

Although the operation of the megohmmeter is fairly simple, it is important to properly interpret the results. The instructions provided with the test equipment must be read and understood before performing any testing. DUE TO THE DANGEROUS ENERGY LEVELS INVOLVED DURING THIS TESTING PROCESS, ALL SAFEGUARDS AND PERSONAL PROTECTION PROCEDURES MUST BE FOLLOWED.

After inspection, servicing and testing, the motors are stored in a warm, dry location. Prior to re-installation on the ride or attraction, a final inspection should be performed to insure proper re-assembly, proper wiring connection and a check for any damage incurred during storage.

The priority of inspection, testing and measurement of electric motors is governed by many factors. Listed below are some of the more common factors:

Terminology:

Megohm - One million ohms.

Ohm - The resistance through which a difference of potential of one volt will produce a current of one ampere. The ohm is symbolized by the Greek letter omega (W ). An ohm can also be described as the resistance at 0⁰ C., of a uniform column of mercury 106.300 cm long and weighing 14.4521 grams.

Ohm’s law - The voltage across an element of a dc circuit is equal to the current in amperes through the element, multiplied by the resistance of the element in ohms. Ohm’s law is expressed mathematically as E = I x R in all forms.

Resistance - A property of conductors which determines the current produced by a given difference of potential; that property of a substance which impedes current and results in the dissipation of power in the form of heat. The practical unit of resistance is the ohm.

Volt - The unit of measure of the difference in electrical potential.

Documentation:
Below is an example Recreation Engineering, Inc. motor log sheet which can be utilized as a guide for documentation purposes in a resistance measurement / inspection program.