Jonathan Robinson
Technical Education Committee Member
Burford Electric Service, Inc. 

When repairing electrical apparatus of any type, a common task is to ensure that machined fits are within acceptable limits of Geometric Dimensioning and Tolerancing (GD&T). This ranges from the simple, such as checking motor foot coplanarity, to the complex, including verifying the straightness of a stator bore as well as its concentricity with itself and with the bearings. The complexity of the task, of course, adds time, and time is never in surplus at a motor service center. Skipping these GD&T checks to save time can lead to mistakes and failures, with costs ranging from a few days of additional work to complete motor replacement and possibly lost production time for both the customer and the service center. In recent years, research has been conducted and tools have been developed that significantly reduce the time required to perform these tasks, enabling service centers to save time and ensure that all fits are within tolerance.

What is a Coordinate Measuring Machine (CMM Measuring System)? 
A Coordinate Measuring Machine, or CMM, is a three-dimensional measuring tool used to measure, record and compare the exact size and position of different features on a part or series of parts. Instead of using calipers or micrometers, a CMM enables the collection of highly accurate three-dimensional measurements by touching points on the part with a probe or by using a laser scanning head. The CMM records three-dimensional data points and uses these points to measure the object as well as determine its form and shape. Once these data points are stored in the CMM software, the software can compare them to assess the parts’ geometric relationships to each other, such as coplanarity or concentricity. By making this comparison, a user can determine how aligned different fits are with one another on a given machine. The accuracy of these measurements typically varies based on the CMM model, brand and arm reach. Accuracy may also be affected by the temperature stability of the environment in which the measurements are recorded. 

There are many different brands and types of CMMs available on the market, including both stationary and portable models. Our repair center utilizes a portable arm-style CMM manufactured by FARO. This device, often referred to as a “FARO Arm,” can be moved around the repair center and used directly on motors or other components. It's beneficial for QUICKLY checking things like: 

• If a bearing seat is centered correctly on the stator 
• If surfaces are in the same plane with one another 
• If the stator core is still cylindrical (two-pole stators are notorious for having cylindricity issues) 
• Whether mechanical fits and features are aligned properly 
• If a surface is tilted (Did you know that a tilted standpipe in a vertical motor can cause oil leaks?) 

Using a CMM like this helps catch problems early, during incoming assessment or before assembly. The FARO Arm has become one of our most valuable tools for ensuring precision and avoiding costly mistakes, especially on large motors where geometric location between parts is not easily measured. (See Figure 1.) 

Trained Hands and Engineering Oversight
The operation of the FARO Arm is performed exclusively by technicians who have completed formal factory training. The primary operators are based in the machine repair department and work under the supervision of our quality control supervisor, a mechanical engineer. This ensures that measurements are taken properly and interpreted with engineering insight rather than guesswork. 

A More Antiquated Approach
Before the availability of CMM measuring equipment, checking the geometric location between the parts on a machine was a very time-consuming process. A couple of options were available to most repair centers, none of which could be performed quickly. If the repair center was attempting to ascertain the cylindricity of a motor’s stator core, the most common check was to mount the stator core on a large mill and true up the rabbet fits. (See Photo 1.) Once trued-up, the bore would be swept with dial indicators mapping out “highs” and “lows” in the core iron to determine if the iron was round and cylindrical. (See Photo 2.) Many years ago, we needed to evaluate the stator core geometry of a two-pole motor and determine how centered the core was about the motor's babbitt bearings. We machined a dummy shaft and supported it in the babbitt bearings, with the shaft running through the bore of the stator. Numerous dial indicators were put on the dummy shaft to "sweep and map" the stator bore. Both processes are very time-consuming. These traditional methods require multiple days of recording and then interpreting the recorded data to begin formulating a conclusion about the motor's condition. In a few hours, a service center can now perform the same checks using a CMM.

Avoiding Costly Mistakes in Vertical Motors

One critical application for the FARO Arm is inspecting vertical motors, particularly those with adjustable, insulated top bearing seats. This adjustable end bell configuration is widely used in one manufacturer's medium-voltage vertical motor design. To clarify, there is nothing inherently wrong with the design; however, we have identified instances where the top bearing seats were not concentric with the rabbet fit on the stator. These adjustable bearing seats in top end bells typically have as much as 0.060” (.1524 cm) of radial clearance in how they can move side to side. You can see these adjustment bolts in Photo 3. If there is misalignment between these parts, the motor is likely to experience numerous issues, the least of which would be electrically related vibration due to an asymmetrical air gap. A more catastrophic outcome would be the rotor contacting the stator's core iron, as shown in Photo 4. An event of this magnitude on a large machine could easily cost either the repair center or the customer more than the replacement cost of a new motor, depending on the extent of the damage.

High-Speed Motors and Hidden Problems
Another high-value use for the CMM is inspecting bearing-to-stator core alignment, especially on two-pole machines. In our experience, an asymmetrical air gap is one of the leading causes of vibration issues, especially on two-pole machines. We go to considerable lengths to make sure the air gap on two-pole machines is as even as possible. Most repair centers recognize that any two-pole motor needs all odds in its favor to operate reliably. 

According to EASA AR100, the air gap of a two-pole machine should not vary from the average by more than 5%. An airgap variation of up to 10% is permissible in slower-speed machines. 

Some of the consequences of an asymmetrical air gap are: 

1. Unbalanced magnetic pull over 
2. Increased noise and vibration 
3. Circulating currents and heating 
4. Rotor to stator core iron poling – especially on startup under load! 
5. Reduced power factor and efficiency 

Maintaining a balanced air gap on two-pole motors is especially crucial, as these motors often feature long shafts with narrow bearing journals. Repair centers must remember that the magnetic force acting on the air gap in all electric motors varies as the square of the ratio of the air gap distance. (See Figure 2.) At minimum, an asymmetric air gap in a two-pole motor will result in a noisy motor. If the motor’s air gap is imbalanced enough, the motor may experience a poling event when the motor starts across the line under load.

The FARO Arm is used to confirm that the bearing seats are concentric with the stator core and to verify that the core iron itself is cylindrical, not bowed or banana-shaped. (See Photo 5.) Keep in mind, you CANNOT detect a banana-shaped stator core by measuring the ID of the bore. The stator bore can be relatively the same diameter across its length while also being banana-shaped. A curved or banana-shaped core is difficult to detect with traditional methods, such as feeler gauges, on a long, narrow-bored motor, like a two-pole motor. 

Several years ago, we assisted a customer in diagnosing a vibration issue in a new, medium-voltage two-pole motor. The motor was sent back to the manufacturer for root cause analysis. During the investigation, it was determined that the motor’s stator core was banana-shaped due to excessive wear on the stator core iron’s stacking mandrel used by the manufacturer. The point in bringing this up is that everyone makes honest mistakes. Service centers need to have the technology to verify the quality of both manufacturers and vendors. Failure to catch a mistake early can result in a far more expensive outcome. 

Catching Issues Early = Real Savings
The cost benefit of investing in a CMM has been a logical choice for our repair center. 

The CMM helps to avoid: 

• Customer downtime 
• Warranty claims - One catastrophic failure avoided covers the cost of the CMM equipment. 
• Installation delays 
• The ability to quality control check newly manufactured parts, like a re-stacked stator core, before and after winding. 

The FARO Arm also enables a repair center to provide dimensional reports when needed, which builds customer confidence in the integrity of our repairs, especially when high-speed or high-horsepower motors are involved. 

Conclusion
The CMM isn't just a high-tech tool; it has become a key part of the quality process. From checking the cylindrical integrity of stator cores to verifying concentricity in complex bearing assemblies, it's helped us deliver more accurate repairs, avoid costly errors and maintain our customers' trust. In today's competitive motor repair industry, measuring up matters—and with a quality CMM, motor repair centers can do just that!

Categories: Technical topics, Service center equipment

Tags: Service center equipment

Rate this article:

No rating

Print