Electrical Troubleshooting 101

Question: There is an electrical problem in our plant: 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.
MA 1302522-01A
MOTOR, ABB 90, 1.5KW, 3/60/400
EEX NA, T3, TEFC, 1P55

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.

Why does motor RPM slow down on a furnace when…

Question: Why does motor RPM slow down on a furnace when

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.

What do you call the crud that collects on my motor?

Machine Enemy Number One: 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 a 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?

When should I grease or replace a bearing?

Replacing or Greasing Bearing Affects Motor Efficiency

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.

What is Motor Circuit Analysis (MCA)?

Multiple tests are required before and after repairs.

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!