Jasper Fisher, 
Chair Technical Education Committee 
Rexel United (Motor Repair) 
Alton, Illinois And 
Chuck Yung 
EASA Senior Technical Support Specialist 

One of the most significant changes to occur in our industry over the past several decades has been the shift from simply repairing equipment to solving the customer’s problem. By that, we mean determining why it failed, and improving the suitability and reliability of the equipment for the application. 

Doing so requires knowledge about what the motor is doing, where it is operating, and under what conditions. Our customers appreciate results and are increasingly aware of the value a good service center adds above and beyond a simple repair. 

Internalize Systems Approach 
The same approach can be used within each service center to improve our efficiency and quality. By internalizing a systems approach to apparatus repair, we can help focus our attention on those areas where we get the greatest return on our investment. 

There are several key steps, starting with initial inspection through assembly and testing to commissioning (when the machine is reinstalled), that are critical paths to an effective repair. Perhaps the most neglected opportunity is to participate in the steps immediately prior to the startup of repaired equipment. This provides a valuable opportunity to head off problems, by verifying alignment, coupling condition, the condition of driven equipment, and special features such as blowers on DC machines. 

Consider this example: A squirrel cage blower often uses a 3-phase motor. Operating backwards, that blower only delivers 1/3 to 1/2 the intended volume of air. The electrician has only a 50% chance of getting the rotation correct the first time, but the blower moves air in the correct direction even when running in reverse. It is very possible for someone to connect the blower so it runs in reverse without realizing it. 

Premature Failure 
A DC machine supplied with only half the required airflow will fail prematurely. If you repaired that motor and it fails prematurely, the customer’s expectation is that it should be covered under warranty. That means we are at great risk each time a DC machine with an auxiliary blower is installed. We have a nearly 100% certainty of being blamed for something the customer has only a 50% chance of getting right. This sounds like a place we should focus our attention. 

A worthy goal, then, is to treat the repair process as a system and identify those critical areas where mistakes are more likely and focus attention on them to reduce errors. 
Considering the repair flow through the service center, key steps might be:

Initial inspection 

• Journal and housing measurement 
• Shaft runout 
• Coupling and coupling fit, keyway condition 
• Electrical tests such as insulation resistance, surge comparison, hipot and growler 
• Rotor squirrel cage evaluation using a growler and/or single phase test 

Machine shop 

• Verification of final dimensions 
• Shaft runout 
• Balance quality 
• Keyway condition

Winding department 

• Accuracy of reported data 
• Winding tests (surge, hipot) 
• Winding treatment method 
• Loop or core test (before and after burnout)

Assembly 

• Completion of required repairs before starting assembly 
• All required parts in stock 
• End play (thermal expansion, and vertical thrust) 
• Electrical tests 

Test run 

• Vibration checks 
• Current balance and percent of full load amps (FLA) 
• Shaft runout, coupling condition 
• Is it assembled correctly? 

These are key steps most of us would identify as either a place where we see errors, or a quality checkpoint at which we can discover problems. That makes each of them important to check. Of those, there are certain places where a little extra effort results in a lot of extra quality. 

A simple example is dynamic balancing. When a person sets a rotor up and balances it, it is not uncommon to see a good technician balance a rotor to well below the acceptable (e.g. ISO 1940) tolerance. The additional adjust­ment adds very little time and the fact that the motor runs smoothly on the test deck is a source of pride for the technician who balanced the rotor, the mechanic who assembled the motor, and the supervisor – not to mention the cus­tomer. The total time charged to balance a typical rotor can be broken 
down as follows:

Task	Percent
Find rotor	6%
Transport rotor	7%
Set up balancing stand	24%
Trial run	20%
Verify balance correction	30%
Remove and store rotor	7%
Paperwork	6%

Note: The net effectiveness and reliability of our repairs are proportional to the precision with which each task is completed. 

Winding Data 
Likewise with winding data: One of the most productive steps is to verify the flux densities after taking winding data and before making any coils. This is especially true today with the base price for copper at $5/lb (3 Euros/kg). If an error was made during a previous repair, it is far better to identify it before labor has been expended making coils, much less winding, connecting and assembling the motor. 

Yet we all know that once in a while a problem is discovered while running a motor on the test panel. Many of those problems could have been revealed by using the EASA AC Motor Verification and Redesign Program, or a similar program, to calculate the airgap density and compare it to established values. 

There is little benefit to using a micrometer to measure every step of a shaft, but the bearing journal dimension is certainly critical. By measuring the journal to the greatest accuracy possible, in at least three directions to check for eccentricity (which can shorten bearing life), we can improve quality. The same is true of the bearing housing. The use of tri-gage micrometers yields better results than using a conventional micrometer to measure the dimension in three directions. 

Double-Check Critical Dimensions 
When a machinist makes a new shaft or repairs a bearing fit, it is a good idea for someone (another machinist, the foreman or yourself) to double-check the critical dimensions before the rotor is balanced and the motor is assembled. A common error is to measure the fit immediately after machining, before the part has cooled to room temperature. The increased temperature results in an erroneous oversize reading. 

On the test panel, when the motor is being test run, the no-load current is one of the most important items to check. Does the motor draw approximately 1/3 of the rated full-load current? If not, why not? Very low speed machines might draw higher current. But for the majority of 4-pole machines, it is reasonable to expect about 1/3 of FLA. 

Involve Everyone! 
Lastly, it only requires a few minutes for the supervisor or truck driver to compare the incoming photo to the finished product. Did it have any parts visible in the photo that are not on the motor? Are the leads positioned on the correct side? By making the driver part of the quality control process, we can catch those rare mistakes instead of shipping them. 

Most technicians in our industry are rightfully proud of their work. By helping them focus on the areas where they can improve quality, we can empower people to improve our collective repair quality. By identifying those areas where we see mistakes made (or where we made them) we improve the odds of heading off those problems at the pass. 

Categories: Miscellaneous, Repair procedures & tips

Tags: Motor rewind/repair Quality

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