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When you buy an asynchronous motor and store it in the warehouse, the motor is an investment waiting for you; however, every week spent in storage carries a silent risk for the motor's winding insulation. A motor waiting in a humid or temperature-fluctuating environment can absorb moisture without any visible damage; this moisture causes the winding to be damaged during commissioning. The measurement that reveals this risk before energization is the insulation resistance test, known in the field as the megger test. In this article, as HEM Motor, with our identity as both manufacturer and supplier, we address from a commercial perspective why and how an asynchronous motor buyer should check the insulation resistance of a stock and warehouse motor, and why this test is indispensable before commissioning.
What Are Insulation Resistance and the Megger Test?
Insulation resistance is a measure indicating how sound the insulation between the motor winding and the body (ground) is, and it is expressed in megaohms (MΩ). A high MΩ value indicates that the winding is well insulated from ground; a low value indicates that the insulation has weakened, often having absorbed moisture or dirt. The megger (megohmmeter) is the device that measures this resistance by applying a controlled high DC voltage to the winding. The measurement is performed with the motor de-energized and its terminals disconnected; thus, the motor's real insulation health is safely evaluated before it is connected to the grid.
This test is a critical post-purchase check, especially for stock motors that have waited a long time. Because no matter how high quality a motor is in the warehouse, the ambient conditions affect the insulation over time. The megger measurement shows whether the motor is healthy before it goes to the field, which protects both the motor and the system it is connected to. You can also find the general checklist for commissioning a stock motor in our article on drive and installation compatibility when migrating to an IE5 motor: commissioning checklist; the insulation measurement is also an important step on that list.
What Does the MΩ Value Mean? Acceptable Limits
A motor's insulation resistance is evaluated depending on the voltage class and temperature. As a general practical acceptance, in low-voltage motors the insulation resistance of a healthy winding is expected to be at the level of tens, often hundreds, of megaohms; the value dropping to a few megaohms is a sign of moisture or degradation. However, rather than fixating on a single absolute value, monitoring the trend is a more accurate approach: a noticeable drop in measurements of the same motor over time is a more meaningful warning than the absolute value. Temperature is also critical; insulation resistance drops with temperature, so the winding temperature should be noted when comparing measurements.
If the value is below the acceptable limit, the motor must be dried before being energized; energizing a damp winding directly can cause the insulation to break down and result in permanent damage. Therefore, a stock motor reading low insulation resistance may not be defective but often merely have absorbed moisture; it returns to health with proper drying. Making this distinction prevents unnecessary returns or panic decisions. We addressed the motor's general lifespan and the causes of early failure in our article on the electric motor lifespan and 7 causes of early failure; moisture and low insulation resistance are among the foremost of those causes.
PI and DAR: Reading Insulation Health in Depth
A single instantaneous MΩ value says a lot about the insulation; however, to understand the insulation's real condition more deeply, two time-based tests are used: the polarization index (PI) and the dielectric absorption ratio (DAR). PI is the ratio of the insulation resistance value at the 10th minute to its value at the 1st minute; DAR is the ratio of the value at the 60th second to the value at the 30th second. In a healthy, dry insulation, resistance rises over time, because clean insulation polarizes under the applied voltage and the resistance increases. In a damp or dirty insulation, the resistance does not rise much over time; this manifests itself with low PI and DAR values.
In practice, a high PI value indicates that the insulation is dry and clean, while a low PI indicates the presence of moisture or dirt. These two tests provide a far more reliable picture than an instantaneous measurement alone, especially in stock motors that have waited a long time in storage or come from a humid environment. Because the DAR measurement takes less time, it is a practical quick check under field conditions; PI provides a more comprehensive evaluation. As HEM Motor, we recommend performing these tests before commissioning, especially for motors that have waited a long time.
Moisture Control in Stock and Warehouse Motors
A significant portion of project motors wait in the warehouse for weeks, sometimes months, until installation time comes. During this process, it is possible for warehouse humidity, temperature changes and condensation to accumulate on the winding. Therefore, measuring the insulation resistance before commissioning a motor that has waited a long time is the simplest and most effective way to protect the investment purchased. If a low value is read, it is usually possible to restore the insulation to a healthy level by drying the motor (for example, with controlled heating methods). This simple check prevents the winding failure that could occur at first energization and the related warranty disputes.
Storage conditions also directly affect the moisture risk. Storing the motor in a dry, ventilated environment with low temperature fluctuation slows the deterioration of the insulation. In dusty or humid warehouses, an IP55 protection class provides some advantage; however, no protection class fully compensates for long-term incorrect storage. You can examine protection class selection in our article on IP protection class selection in electric motors; and the effect of winding and insulation class on durability in our article on winding and insulation class (F/H) in IE3 motors.
Pre-Commissioning Tests and Other Checks
The insulation resistance measurement is at the center of the pre-commissioning checklist; however, it is not sufficient on its own. For a healthy commissioning, besides the insulation test, the following checks should also be performed: confirmation of free rotation by turning the shaft by hand; verification of phase sequence and rotation direction; measurement of body grounding; tightness of terminal connections; and clearance of cooling channels. We addressed rotation direction and phase sequence checking in detail in our article on motor rotation direction and phase sequence. For plants that want to continuously monitor winding temperature, it is also possible to evaluate the PT100 or PTC thermistor options; we addressed this topic in our article on motor winding temperature monitoring: PT100 and PTC thermistor.
All of these checks let you safely run the motor at first energization and capture the full performance you purchased on site. Periodic insulation measurement as part of regular maintenance is the most reliable way to monitor the insulation health of a running motor over the years. We shared the periodic maintenance schedule in our article on the electric motor maintenance and periodic inspection schedule.
Supplying the Right Motor With the Right Check
The insulation resistance test is an extension of the purchasing process: it is the way to ensure that the motor you bought will run healthily on site. The asynchronous motors in the HEM Motor range are produced with class F insulation, IP55 protection and 100% copper winding standards; these features increase the winding's resistance to moisture and temperature. Nevertheless, we recommend a pre-commissioning insulation check on every motor that has waited a long time, because this check protects both the motor and the connected system. You can examine pole, phase and starting decisions in asynchronous motor selection in our articles on the asynchronous motor buying guide: 2, 4, 6 poles and starting AC asynchronous motors.
You can access our entire efficient electric motors range and our other products from our products page, and our corporate information from our home page. Receiving the right motor with the right checks secures your production continuity.
Frequently Asked Questions
Why should I perform a megger test before commissioning a stock motor?
The winding of a motor that has waited a long time in storage can absorb moisture without any visible damage; this moisture causes the insulation to break down and permanent winding damage at first energization. Measuring the insulation resistance with a megger reveals this risk before connecting the motor to the grid. If the value is below the acceptance limit, the motor returns to health by drying. This simple check protects both the motor and the connected system and prevents warranty disputes.
How many MΩ should the insulation resistance be?
The acceptable value depends on the motor's voltage class and winding temperature; in low-voltage motors, a healthy winding is expected to be at the level of tens, often hundreds, of megaohms. The value dropping to a few megaohms is a sign of moisture or degradation. Rather than focusing on a single absolute value, it is a more accurate approach to monitor the trend of the same motor over time and to note the winding temperature during measurement. At a low value, the motor must be dried before being energized.
What are the PI and DAR tests for?
The polarization index (PI) and dielectric absorption ratio (DAR) measure the change in insulation resistance over time to show whether the insulation is dry and clean. In healthy insulation, resistance rises over time, giving high PI and DAR values; in damp or dirty insulation, resistance does not rise much and the values stay low. Because these tests provide a more reliable picture than a single instantaneous measurement, they are recommended especially for stock motors that have waited a long time.
Get a Quote
Contact the HEM Motor expert team for our asynchronous motors produced with class F insulation, IP55 protection and 100% copper winding standards. We quote the motor in the right power, speed and efficiency class for your application, together with stock and delivery time information, and also guide you on pre-commissioning check matters. Call us now at +90 (532) 345 49 86 or request a quote via our contact page.
