Your questions answered: Motor management and troubleshooting techniques for ac induction motors

Courtesy: Advanced Energy Corp.

Recently, Plant Engineering and Control Engineering together presented a well-attended webcast featuring Advance Energy Corp.’s Michael Lyda, motor lab director, and Ronnie Alford, motor lab coordinator.

Following their presentations, Michael and Ronnie fielded questions on motor management and troubleshooting techniques from audience members. Those questions and their answers are included in the following.

To view the webcast in its entirety, please visit the archived webcast section on the Plant Engineering or Control Engineering website. The webcast’s learning objectives included:

• Understand motor management basics
• Identify the most common issues associated with ac induction motors
• Review common induction motor maintenance procedures
• Learn what it takes to get optimum, long-term performance from ac induction motors.

Question: How many times can a motor be rewound?

Answer: There are many factors at play in deciding the number of times a motor could be rewound. The specific number of times depends on how much degradation the interlaminar insulation in the core has encountered. It is best practice to perform pre-burn and post-burn core loss test during each rewind. This allows you to capture and see the actual amount of degradation that is being encountered during the burn out and stripping process.

Question: Is there a general acceptable HP rating at which a motor should be replaced versus rewound?

Answer: This depends on your motor population. It also depends on the age, efficiency rating, run-hours, etc. of the motor. It is great to have motor repair and motor purchase specifications so that you don’t have to make these decisions on the fly. For example, a repair/purchase specification could state to replace all motors under 25HP, and for motors above 25HP replace if the repair is to be 60% or higher of the replacement cost.

Question: Can variable frequency drives (VFDs) cause bearing failure on ac motors?

Answer: Short answer is yes. There are a lot of factors that figure into the failure. VFDs supply a different type of voltage signal to motors than the grid. It’s called a pulse-width modulated (PWM) signal, which is more like a square wave than the sine wave a motor sees from grid power. A motor controlled by a VFD is seeing much higher peak voltages at its terminals than a motor controlled by grid power. If the motor doesn’t have inverter duty windings and no shaft grounding or other type of device is used, then those higher voltages end up getting dissipated through the shaft and into the bearings and lubrication.

Question: What type of motors have regulated efficiency levels in the United States?

Answer: There are some general criteria to describe efficiency regulated motors. Single-speed induction motors, motors rated for continuous duty, squirrel-cage rotor, operate on polyphase 60Hz ac power, rated 600 V or less, have a 2-, 4-, 6-, or 8-pole configuration, built in a 3- or 4-digit NEMA frame or IEC equivalent, produce at least 1 horsepower but not greater than 500 horsepower, and meet all the performance requirements of a NEMA Design A, B or C motor or IEC Design N or H. In more recent years the Department of Energy expanded the scope of regulated motors to include brake motors, close-coupled pump motors, motors with a separately powered blower, immersibles, vertical hollow shaft motors and others. Some smaller electric motors, single-phase motors and three-phase open motors with 56 NEMA frame were also added in 2014. As a matter of fact, there are other motors are on the docket to be regulated at this time including air-over motors, submersible motors and synchronous motors. A public commenting period just ended for a notice of proposed rule-making around those newly proposed motors just a couple of weeks ago.

Question: What type of meters should we use when testing motors controlled by VFDs?  Including measuring current on all phases?

Answer: Standard multi-meters won’t register correctly when measuring the PWM voltage signal (as mentioned above). When measuring current of a VFD controlled motor, you could use a multi-meter or clamp-on ammeter. However, a standard multi-meter isn’t typically capable of measuring harmonic currents outside of the fundamental. Current is best measured with a power analyzer that captures harmonic content.

Question: Have there been any explanation how utilities can have a standard of 3% unbalance allowed while that results in a 10% derating of motors and NEMA states that less than 1% is needed to operate at the motor’s rating?

Answer: 3% is just an estimate, your specific Power Purchase Agreement may be different. Voltage unbalance at the substation could very well be minimal, but if you have a lot of single-phase loads unevenly spread out in your facility then one phase may sag lower than the other two. It’s important that you keep the loading across all three phases of your power system balanced so as to not self-induce voltage unbalance on “your side of the meter”.

Question: What is the best method of grounding Induction motors?

Answer: Follow manufacturer guidelines and attach ground wires only in specified locations on the frame or in the terminal box.

Question: What type of motor data presents evidence that a motor has experienced damage due to being controlled by VFDs?

Answer: Damage may show up in a surge or hi-pot test if there were voltage spikes in the winding that caused damage to the winding insulation. Bearing damage such as fluting may also be apparent. These indications may show up in a vibration spectrum analysis or in a physical analysis of the motor bearings.

Question: Can you grease a motor too much?

Answer: Too much grease can be just as detrimental as too little. Not only can it create excess friction and heat in the bearings, but excess grease can bypass the bearing and build up on the winding end turn and create a blanket effect that causes the winding to run hotter and shorten motor-life.

Question: Should vibration sensors be added to all motors or only to motors of a large horse power? 

Answer: It is best practice to monitor vibration on any critical applications regardless of horsepower range. The data is used to prevent unexpected motor failure and down time.

Question: Does megering degrade insulation across time?

Answer: If recommended test voltages are used, the insulation resistance test is a non-destructive test procedure.

Question: Is Wye/delta motor starting good for starting high-torque motors? Please explain the advantages and disadvantages.

Answer: Wye/delta starting is helpful in lowering in-rush current (by reducing motor incoming voltage) and is a cost-effective way to deal with starting a high-torque application. It will not be available on certain motors and will require more wiring as well as adding in the starter itself. For example, wye/delta starting will not work for motors with only three leads.

Question: What is the cost difference between a soft starter and a VFD?

Answer: Pricing can change substantially based on manufacturer, so we are unable to provide a direct price comparison at this time. Soft starters will generally be cheaper than VFD’s. If you have a single-speed application that needs reduced in-rush current, then a soft starter may be your best bet. If you need variable speeds and lower in-rush current, then VFD would be the better option.

Question: Does NEMA MG1 provide motor testing recommendations?

Answer: NEMA MG-1 provides guidance on many motor related specifications including limits for performance-based metrics, safety, sound, vibration, efficiency and many others. It assists users in the proper selection and application of motors and generators and contains practical information on the construction and manufacture of ac and dc motors and generators.

Question: Are there any motor management and troubleshooting techniques for ac permanent magnet motors?

Answer: Many of the same techniques for standard induction motors apply here. One specific metric to keep an eye on for permanent magnet motors is heating. If a permanent magnet motor runs overloaded or at elevated temperatures for too long, the magnets in the rotor could permanently de-magnetize rendering the motor unusable.

Question: What is the typical life expectancy of induction motors?

Answer: This is a very broad question than merits an even broader answer. Motor life is relative to application and depends on build quality, environment, proper maintenance techniques, and incoming power quality. Motors could fail after days or weeks, if not properly operated and environmental conditions are not considered. Under ideal conditions, a motor could last for decades with proper operations and maintenance techniques.

Question: Can circulating ground currents in a motor be identified while a motor is operating?

Answer: There are devices on the market that will measure circulating shaft currents while the motor is in operation.

Question: Do you advice checking motor efficiency once a motor has been repaired?

Answer: It may be difficult and costly to verify motor efficiency following every repair. If your motor repair shop has the capability to perform load testing, this could be valuable data to capture. For your motor repairs, following a repair specification that ensures high-quality repairs is more important than completing efficiency testing following the repair.

Question: How do you determine burn-out temps? What does this mean relative to repair and how is it used?

Answer: There are standards in place for motor repair. The EASA AR100 standard states that a motor core temperature should not exceed 700F to avoid degradation of the inter-laminar insulation. When using a burn-off oven to complete this task a water suppression system is used to maintain the temperature.

Question: Can you comment on motor sizing and efficiency and load matching?

Answer: A lot of times engineers oversize motors, which may adversely affect the operating efficiency and therefore the life-cycle costs. An ac induction motor will typically be most efficient, around 75-80% of its rated load. If a motor is oversized for an application, it may be running at 50% or less of rated load. Efficiency of the motor will likely be lower and lifecycle costs increased.

Question: It was mentioned that slightly elevated voltage may increase efficiency. At what point is over-voltage at the motor detrimental?

Answer: It depends on the motor and loading. The testing referenced in the presentation from our lab in Raleigh showed a peak efficiency at 103% of rated voltage for two different machines, while running at full-load.

Question: How often should a motor be inspected?

Answer: This is really up to the facility, but a recommendation is a daily or weekly test for any visible, smell, or noise issues. Preventative maintenance would be more appropriate monthly or quarterly, i.e., capturing vibration analysis, infrared thermography and measuring line currents.

Question: How do you determine greasing volume when greasing motor bearings?

Answer: We defer to manufacturer specifications for this. General recommendation is to grease every 2000 hours of run-time. You can also take this question to your motor distributor or repair shop.

Question: What are the electrical protection concerns related to high-efficiency motors?

Answer: Follow manufacturer catalog recommendations for any type of motor wiring and setup. Standard efficiency or high efficiency designs will advise similar wiring and setup.

Question: Are there any readily available resources provided by Advanced Energy for motor management for manufacturing corporations?

Answer: Please visit our website to learn more:

Please see our Motor Survey:

Our Horsepower Bulletin:

Question: Are there any free troubleshooting documents for motors available for manufacturers?

Answer: The EASA website could be valuable place for information like this (www.easa.com). Advanced Energy also posts tech tips and articles to our blog. Here is a recent blog post about motor troubleshooting.

Question: What’s the effect of powering a 60Hz motor on a 50Hz system?

Answer: A standard ac induction motor rated for 60Hz, could still be operated on a 50Hz power supply (in most cases). The motor output power should be de-rated by 5/6. The motor will be spinning slower so keep an eye out for over-heating since there is less cooling from the fan (assuming it is a fan-cooled machine).

Question: How important is service factor for a motor? How does it affect motor life?

Answer: Service factor is short term overload factor. By including the service factor on the nameplate, the manufacturer is letting you know that you could operate the motor at that percentage of rated load and the motor will operate continuously at a thermally stabilized temperature below that of the insulation class rating. However, short-term overload is a very relative term. Additional heat will lead to shorter motor life in the long run so keeping the motor at or below rated load for its lifespan should maximize motor life, assuming you follow proper maintenance techniques discussed.

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