Welcome to my Question & Answer blog on electric motors. If you don't see your topic listed below, feel free to contact me and start the conversation. I am here to answer your questions, address any electric motor issues, or find the root cause of problems that your facility may have regarding reliability maintenance, rotating machinery, or other troubleshooting. For our team at Dreisilker, it's all in a day's work. Bring it on!

CLICK HERE TO START THE CONVERSATION--

The Motor Doc is here to help!


Links to our Social Web 2.0:

LinkedIn Company:

LinkedIn Group:

Twitter:

YouTube:

More questions, conversations and solutions about electric motors to come! Follow us on the web!

Typical motor armature removed from stator during rewinding process.

The use of multiple technologies is extremely important when condition monitoring or troubleshooting equipment. Additionally, your own senses are exceedingly important in ensuring an accurate identification of problems.

During a recent evaluation of a special armature (AC commutator motor armature), it was noticed that the insulation system had overheated visibly and some of the insulation appeared brittle. Technicians immediately noted that standard surge, micro-ohm, and insulation to ground testing, including AC hi pot, did not detect any defects.

As Dreisilker Electric Motors, Inc. has been using motor circuit analysis (MCA) for ten years in the shop and field, an MCA test was performed with the following results:

Impedance in Ohms at 800 Hz: 14, 14, 12

Phase Angle (Fi): 76, 76, 66

I/F: -49, -50, -46

First, the motor had appeared single phased; second, the instrument identified a bad set of slip rings; and, finally, the difference in Fi and I/F identified a turn to turn short. The Z identified the unbalance.

Does this mean that the other tests had no value? Not by any means! There are other instances in which MCA will not detect a problem that other technologies will, which is why the use of multiple technologies is extremely important when evaluating a machine.

In another example, a stator was pre-cleaned for testing and passed standard testing (as above). The use of MCA identified that the stator was bad. How could this be? As it turned out, some metal particles from the customer operation had become embedded into some of the insulation system and were not accessible during inspection cleaning. Amazingly, the condition made it through a surge test at 2x voltage plus 1000 volts and a 2000 volt AC highpot test.

What is particularly scary is that more than 50% of motor repair shops do not perform any testing on electric motors through the repair process.

Gives you something to think about!

Typical ball bearing used on motors and other machinery.

Did you know that the simple act of replacing bearings or greasing an electric motor will directly impact the efficiency of your electric motor?

Following the passing of EPAct 92 in the U.S., and similar energy initiatives around the globe, a significant number of energy-efficiency projects were initiated relating to electric motor repair. The BC Hydro motor repair study, performed in 1993, covered 11 energy-efficient 20 hp electric motor models. One of each was held as a "standard" for dynamometer testing, and two more of each model were shorted and sent blind to various repair shops across a large geographical area. Findings from the BC Hydro study showed the lowest decrease in efficiency to be 0.5%, with the most significant being around a 4% loss of efficiency.

The cause of the highest losses? Bearing replacement. An increase in friction and windage because contact sealed bearings were used resulted in an average loss of 3% of efficiency. This was a surprise as the researchers expected the most significant losses to be the result of core and I2R from rewinding. Instead, rewind losses accounted for an average of 1% per rewind, while mechanical problems resulted in much higher values. The solution? Use non-contact sealed bearings when such applications are required.

Other areas that can increase friction loading include over-greasing and improper mechanical fits through repair. Your average electric motor (C3) bearing is not designed to be packed full of grease. In conditions where you see an increase in bearing temperature after greasing the bearings, you are identifying the lost efficiency (heat) due to increased friction. Through repair, improper mechanical fits—including the use of peening or fillers instead of proper machining practices —will also increase the friction in your bearings.

The lesson? Follow proper greasing practices and ensure that your repair facility is performing quality machining.

Typical machine failure due to swarf.

In the grinding and machining industry the residue of the process: chips; particles; and fluids (cutting, moisture and other) is referred to as ‘swarf.’ This insidious material is known to be aggressive to the reliability of the equipment, machines, is a skin irritant and hazardous to electrical systems and components. Swarf is considered an hazardous material.

The type of swarf depends on the material being ground, the type of grinding, the machine, fluids involved, and other factors. As many of the cutting fluids can be aggressive with insulation materials coupled with any conductive materials being cut, the impact should swarf enter a machine can be devastating and reduce equipment life significantly.

In the picture shown above, swarf was dripping from the blower directly onto the commutator of a DC motor. The blower took air in directly from the environment and the resulting thick syropy material collected between the brushes and commutator. In addition to the the heat that developed due to poor contact, the materials in the swarf were conductive. The material was made up of cutting fluid and included (in ppm): Iron – 413; Chromium – 19; Aluminum – 94; Copper – 1858; Lead – 47; Nickel – 66; Silicon – 165; Sodium – 1349; Boron – 169; Magnesium – 751; Calcium – 693; Phosphorus – 54; Zinc – 1914.

In addition to the heating effects, which caused bars to raise, the silicon and other abrasives are known to aggressively attack carbon brushes. There are a number of solutions to such a problem, including: changing the cutting materials, determining where the leaks are coming from, and/or bringing in outside air to the blower, and sealing the motor (or at least a positive pressure within the machine). Have you run into similar situations?

GE X-D Ultra High-Efficiency Motor

the access door to the compartment is removed relieving the static pressure at the inlet of the furnace?
Bob F.

Motor Doc: When the door is closed and the machine is running, there is a balanced load - a restriction on the inlet side of the fan which reduces the amount of air to flow, so there is a limited amount of air. When the door is open, there is an unlimited amount of air that has to be moved (i.e., system pressure increases). You can also feel this if you were to feel the air movement on the output side of the fan before and after the door was opened. I hope this was helpful.

What you have said is understood, the question that needs answering is: why does the rpm decrease as witnessed by a stroboscope?

When the load increases, it slows the shaft down. This results in an increased slip between the rotor and stator and an increase in rotor current. This is then seen as an increase in measured current at the motor connections. Induction motors slow down as the load increases, that is how the current increases with load. The nameplate RPM is the RPM where the motor is at full load. If the load is lighter, then it speeds up.

Motor Diagnostics Evaluate Existing Conditions

An auxiliary lube oil pump trips frequently due to high current. So, could you please show me how I can start the troubleshooting? Actually, I have no experience in electrical. I am a new engineer. Your help is highly appreciated.
Abdullah K.

Motor Doc: Thank you for your question. If possible, could you supply the nameplate information and also, when does it trip on high current? On startup or when it is operating normally?

There is a thermal overload frequently tripping the “Lube Oil Pump for Wash Oil Pump Motor” 3P-2875-OPM. As per the motor solo-run test with a no-load current, the result was 1.3 amperes (motor-rated current in accordance to the LV MCC is 3.26 amperes).
Pump was checked by mechanic and found no problem at all. On the other hand, the mechanic ran a test with a load current result of 4.8 amperes. So, that high load current causes a thermal overload; then it will trip the pump motor. The load current is more than the motor rated current. According to the operation personnel, the lube oil pump will be running for a minimum of 30 seconds and maximum of 3 minutes to lubricate the main pump (3P-2875) and then stops after building-up pressure.
So, what we can do to avoid the thermal overload tripping of the pump motor. Please help us to solve this problem. See the specification of the motor below.
MOTOR:
AUX LUBE
MA 1302522-01A
MOTOR, ABB 90, 1.5KW, 3/60/400
EEX NA, T3, TEFC, 1P55
IM B35 MOUNTING

Motor Doc: In fact, the motor is operating at 160% load, well over the load rating of a standard motor, so the fact that it is tripping is not surprising.
The motor rating, per your attachment, is actually 3amps. The motor seems to be performing within it's operating specifications with the no load current being 1.3amps per the attachment.
If this problem has been with the system since it's installation, then the motor may not have been selected correctly, or there may be a problem in the pump system. Otherwise, it is a problem in the pump system.
Check filters and strainers, valves or other restrictions in the pumping system.

CLICK HERE TO START THE CONVERSATION--

The Motor Doc is here to help!

You can download Motor Doc white papers from the Dreisilker "College of Motor Knowledge": our archive of information in convenient PDF format (Adobe® Reader required). Articles include detailed studies of electric motor repair; field services; turnkey projects; motors, drives; specialty motor design; motor controls; forensic analysis of equipment; controls; and much, much more! This archive will be updated frequently so check back from time to time to take a refresher course from The Motor Doc. Enjoy!

Motor Circuit Analysis has long been defined within industry as the de-energized testing of electrical systems. In this article, we will discuss the use of motor circuit analysis as a method to test assembled synchronous motor stators and rotors and how to determine insulation breakdown in the rotating fields of a synchronous machine.

• Download PDF: Synchronous Machine Testing with Motor Circuit Analysis Instrumentation

A fair amount of research had been performed on the impacts of thermal stripping on motor cores including work by General Electric, the Canadian Electrical Association and the Electrical Apparatus Association over the years. However, in each case, representatives from the motor repair industry had objected to an evaluation of the mechanical aspects of the system. Due to a number of instances identified through field service work, an independent research project was performed by DREISILKER Electric Motors Inc. and several partners. This presentation is a summary of the 1997 IEEE EIC conference paper on the impact of thermal stripping on electric motor reliability.

• Download PDF: Electric Motor Repair Soft Foot Study

Over the past four decades interest in the impact of electric motor repair and its effect on energy efficiency and reliability has increased. A majority of the studies have dealt with efficiency and losses with very little attention on the mechanical issues and continued reliability of the motor. Studies performed in the 1990s were performed by third party groups such as the Canadian Electrical Association (CEA). The first were blind studies performed by Ontario Hydro, BC Hydro and Hydro Quebec. The motors sent out in 1991 were standard efficiency motors that were sent out blind with all resulting in increased losses by motor repair shops that use burnout ovens.

• Download PDF: Mechanical Impact of High Temperature Stripping

The time for startup of chillers is upon a great many commercial properties and the challenge that has existed for decades returns: how do you verify the condition of your chiller motor before startup?

Common testing for electric motors includes vibration analysis for mechanical components and insulation resistance (Meg-Ohm) testing for winding insulation. However, vibration analysis is challenging and may miss critical findings due to the location of bearings and mechanical components and insulation resistance and high voltage tests are frowned upon by a number of hermetic chiller manufacturers. Improper information on the interpretation of results and understanding test standards creates additional challenges to determining the condition of your chiller motor.
To download the entire article as a PDF, click below.

• Download PDF: Cooling Season Startup Considerations