Most facilities need to get maximum life and efficiency out of their HVAC systems. HVAC operational costs no longer are assumed—even that budget item now is scrutinized. Taken for granted, HVAC systems can be costly to operate and even more expensive to replace. Even new or newly upgraded systems are subject to the same degradation rules as other equipment.
The best way to keep a system running efficiently is to follow best maintenance practices. Baselines should be established at installation, equipment needs to be inspected regularly according to manufacturer recommendations, and action must be taken before failure. This kind of proactive maintenance improves operational efficiency and prolongs equipment life span.
Electrical, insulation-resistance, and thermal-measurement testing are three ways to inspect an HVAC system’s motors and associated controls. Used on a regular basis, thermal imagers can detect potential problems, while insulation-resistance and electrical tests can determine their cause.
Hand-held thermal imagers can collect heat signatures from a range of HVAC motors, from 5 to 1,000 hp. Thermal imagers are useful for spot checks and troubleshooting; they can show which motors and associated panels and controls are operating too hot and help find the failed component at fault. They also can check for phase imbalance, bad connections, and abnormal heating on an electrical supply.
An insulation multimeter can perform many of the other tests needed to troubleshoot and maintain motors—as well as motors submersed in liquid. When a motor is having problems, checklist items should include:
- Verifying the supply voltage.
- Using insulation testing to check the starter and control contacts.
- Measuring the insulation resistance of the line and load circuits to ground.
- Measuring winding resistance phase to phase and phase to ground.
THERMAL MEASUREMENTS
A motor’s heat signature will tell you a lot about its quality and condition. If a motor is overheating, windings will deteriorate rapidly. In fact, a motor can lose half of its life for every 18˚F rise over its maximum-rated temperature because of insulation failure.
Using a thermal imager to inspect motors often can indicate abnormal heating patterns and help pinpoint the source of the problem (e.g., short circuits, winding problems, bearings, coupling misalignments, convection issues, etc.)
There are many causes of abnormal thermal patterns in electrical systems. Most typically are the result of a high-resistance connection. These usually will appear warmest at the spot of high resistance and seem to cool farther away from that spot.
Load imbalances, whether normal or out of specification, appear equally warm throughout the phase or part of the circuit that is undersized/overloaded. Harmonic problems can create a similar pattern. If an entire conductor is warm, it could be undersized or overloaded. To determine the problem, the conductor’s rating and actual load should be verified.
Failed components typically look cooler than similar, normally functioning ones. The most common example is a blown fuse. In a motor circuit, this can result in a single-phase condition and, possibly, costly damage to a motor.
A top-down approach is useful when implementing thermal-imaging maintenance. A facility manager or engineer can determine the priority and frequency of inspection, task technicians with capturing thermal images, and analyze the images.
Three things are needed to interpret a thermal image:
- A sensitive radiometric thermal imager. Radiometric thermal imagers capture a discrete temperature for every single pixel in an image. The thermal imager may not show all of the values on its screen, but when computer software is utilized, the captured data can provide opportunities to adjust temperature alarms, consider the thermal impact of neighboring items, and otherwise analyze the image.
- Good focus. This is one thing that cannot be adjusted later with software.
- A basic understanding of what is being inspected.
Using software, a manager or engineer can compare the image against previous inspections or baselines and change temperature alarms and the like to analyze thermal patterns in detail. Level I thermal training is recommended to obtain maximum benefits from thermal-image analysis.
INSULATION RESISTANCE AND ASSOCIATED ELECTRICAL TESTING
Insulation problems on motors and drives usually are caused by improper installation, environmental contamination, mechanical stress, or age. To calculate and display insulation resistance, testers can apply a direct-current (DC) voltage across an insulation system and measure the resulting current. Typically, the test verifies high insulation resistance between a conductor and ground. Insulation-resistance testing can check coil or winding resistances, heating-element resistances, thermistor resistance values, and so on. All of these measurements can occur through circuits within the insulators, except when checking for a short to ground. Once a short to ground is detected, a catastrophic failure of the device has occurred, and it is too late for preventive maintenance or proactive remedies. A catastrophic motor failure within a hermetic or semihermetic compressor containing oil and refrigerant at best involves extensive cleanup and at worst requires equipment, rather than component, replacement and results in lost production time and revenues. It is better to check insulation values regularly and record them for comparison during the next visit so that changes are readily apparent.
There is no hard and fast pass/fail rule on how to interpret insulation-resistance values, but manufacturers and agencies seem to agree that insulation-resistance trending can be a clear indicator of a motor’s projected health. The Institute of Electrical and Electronics Engineers (IEEE) Standard 43, Recommended Practice for Testing Insulation Resistance of Rotating Machinery, gives a minimum acceptable value of 1 megohm plus 1 megohm per kilovolt of motor operating voltage. For a 460-v motor, the pass/fail threshold value would be 1.46 megohms, or a current leakage rate of 500 v DC divided by 1,460,000 ohms equals 342 microAmps.
However, this standard is for motors that are not hermetically sealed with oil and refrigerant. A motor submersed in liquid may need to use lower values as recommended by the manufacturer. A motor submersed in a liquid may be acceptable at 600,000 ohms with 500-v DC applied, or a current leakage rate of 500 v divided 600,000 ohms equals approximately 833 microAmps. Some insulations in use since 1975 have improved insulating values that may not readily allow leakage current, may have insulation-resistance-testing values near 20,000 megohms (20 gigohms), and may be unacceptable for use if insulation-resistance-testing values are below 100 megohms, regardless of whether or not the windings have surface contaminants.
Applying insulation-resistance testing to hermetic compressors is a two-step procedure because of the nature of the compressor motor’s operating environment. First, insulation-resistance testing should be conducted to check for degredation of motor-winding insulation. Second, insulation-resistance-testing results should be checked to ensure they have not been affected by the presence of contaminants.
One test should be performed with a compressor that has been off for a period of time. This test is more likely to expose contaminants within the oil or refrigerant. Another test should be performed after the compressor has been operating for 5 or 10 min. While still affected by contaminants, this test is more of a “true” motor insulation-resistance test because it drives the bulk of refrigerant, oil, and moisture off of the windings.
Insulation testing can be combined easily with regular motor maintenance to identify degradation before failure and during installation procedures to verify system safety and performance. During troubleshooting, insulation-resistance testing can be the missing link that enables a motor to return to operation easily with a simple cable replacement. The only complicating factor is that most insulation tests require a de-energized system.
CONCLUSION
A machine begins its final state of failure from the moment it is put into production. As maintenance and attention to detail increase, so does the probable life expectancy of a product. As the costs of failure or inefficiency increase, so do the advantages of regular testing and tracking of measurements.
A senior product-marketing manager for Fluke Corp., Michael Stuart manages thermography products for the company and has previous experience in electrical and insulation-resistance testing. He is a practicing T/IRT Level II thermographer certified in compliance with American Society of Nondestructive Testing standards. He can be reached at [email protected].
Testing influences
Factors affecting insulation resistance and compressor lifespans:
- Failure to properly dehydrate a system.
- Failure to ream tubing prior to assembly.
- Failure to displace oxygen with an inert gas, such as nitrogen or argon, during the brazing process.
- Refrigerant leaks.
- Temperature.