Tracking down excellence

If you've never visited a motor rewind service center, it's time you did

By Chuck Yung

Plant Services magazine

Whether you're new to the job or a veteran, vendor choice is critical to successful plant management -- especially for repairing electric motors and drives. Selection can be tricky, given the variables to consider, including price, delivery, service, expertise, reliability and quality. No one wants to pay too much, but buying on price alone can backfire. The cost of downtime, idled workers, lost production, damaged product, missed deadlines and dissatisfied customers can quickly offset whatever you might have saved on a cheaper repair.

So what's the best way to choose? Begin by listing the types of equipment you have, your basic requirements, and any desired value-added extras that can save time, money and aggravation. Then, evaluate prospective service centers to see how they measure up. If one service center can't provide everything you need, it might make sense to use two or more, based on your requirements and their expertise.

Narrow the choice
Service centers come in many sizes, with varying capabilities and expertise. To narrow down the field, qualify candidates based on their experience in the type of work you expect of them. If most of your motors are 460V and less than 500 hp, it probably doesn't matter that a service center can rewind form-coil motors or rebuild DC machines.

Take a tour
Touring the facilities of several repairers to inspect work-in-process provides an opportunity to compare capabilities, expertise and workmanship. The best-equipped facility isn't always the most capable or the best fit for your needs. It takes skilled people to troubleshoot and repair motors, drives and controls. Look for a well-trained, stable workforce. Probe to determine if the technicians have the expertise and application knowledge to determine the root cause of failure. If not, they may just keep replacing that bearing that seems to fail every six months without ever solving the real problem.

A walk-through inspection allows you to investigate obvious things, such as crane capacity, general cleanliness (especially in critical areas like the rewind department) and overall organization. Specialized equipment is helpful, but repairs often can be accomplished in more than one way. The equipment outlined below is essential, and it should be in good working order.

Essential equipment
The test panel should handle a useful range of voltages. Some panels feature a variable-voltage supply, rated 0V to 600V, with additional steps for higher voltages. Others incorporate multi-tap transformers. If your plant uses special voltages (e.g. 208, 575, 950, 7,200, or 13,200), verify that the service center has those voltage capabilities.

The test bed should be able to support motors rigidly during test runs and vibration analysis. Ideally, the baseplate should be grouted to a foundation having a total mass of at least 15 times that of the largest motor it can support. Using a T-slotted base helps simulate actual operating conditions, a factor that becomes important for motor speeds of 3,600 rpm and above (see Figure 1). If your plant uses vertical motors, ask how the service center will support them during testing.

Massive stability

Figure 1. A test bed having a T-slotted base helps simulate operating conditions. The base should have a mass at least 15 times the mass of the largest motor it can handle.


Vibration analysis equipment may be stand-mounted or portable. Portable equipment is important for final inspection of repaired rotating equipment, as well as for field analysis of problem machines. A stand-mounted vibration analyzer is often an integral part of the balancing stand.

A balancing stand allows each rotating element to be precision balanced, a measure that becomes more important at higher motor speeds. Expect most service centers to have a balancing stand because subcontracting balancing likely results in delay. Although most commercial balancing stands have a 100% overload capacity, they should be sized to handle work without dipping into that reserve. To protect shafts from scoring, balancing stand rollers must be kept clean and should be covered when not in use.

Standard electrical test equipment should include a surge tester of appropriate size and an AC or DC high-potential tester. The latter should have a voltage rating suitable for testing new windings, as specified by NEMA standard MG1-2003. This is generally twice the rated voltage plus 1,000 for the AC test and 1.7 times rated voltage for the DC test. A megohmmeter is important as a first line test, but isn't adequate by itself.

A core loss tester is an important tool that verifies the stator core wasn't damaged by winding failure or removal, and for checking the condition of repaired or restacked cores. Commercial core testers are easy to use and they document results with a printout. A core test also can be performed using a power supply and multiple loops of insulated cable. See EASA Tech Note 17 for more information.

The controlled-temperature burnout oven should be fitted with a chart recorder to monitor the stator core temperature during the burnout process. A flame-suppression system is important to prevent excess temperatures, while an afterburner reduces volatile emissions. The service center also should follow proper procedures for loading the oven. For more information, download "Guidelines for Maintaining Motor Efficiency During Rebuilding," from www.easa.com [no hyphens]. Figure 2 shows a controlled-temperature burnout oven.