Don't allow motor repair practices to degrade motor efficiency

In the 1970s and 1980s, efficiency loss through core losses and repair practices became a concern in a number of industries. Trade associations and equipment manufacturers studied the relationship between core loss and temperature. This resulted in proposed recommendations for improving efficiency by reducing certain losses and several companies reviewed the potential of reducing losses through alternate coil removal practices.

The studies continued in the 1990s. The Canadian Electrical Association (CEA) studied a variety of losses incurred when motor repair processes are tightly controlled. Work on the mechanical aspects of traditional repair practices continued with research and published papers related to changes in soft foot and air gap.

Conclusions of the various studies were published and some repair facilities misinterpreted them as allowing for even more stressful repair practices.

Figure 1. The 150-hp motor being prepared for testing on a 400-hp dynamometer.

Figure 2. Collecting dynamometer and instrumentation data using Bluetooth technology.

In particular, the reference to 680° F and 750° F for high-grade core steel in burnout is related to the core temperature of the machine, not the oven temperature. The cited studies documented that the core temperature is an average of 120° F higher than the oven temperature. Therefore, setting the temperatures at 650° F and 750° F exceed the acceptable limits as did the practice of loading more than one stator at a time in an oven.

Of concern here is the relationship of repair outcome to the IEEE Standard 1068-20103, traditional repair practices, observations and precision motor repair (PMR) practices. The three electric motors used in these examples were evaluated for efficiency using IEEE Standard 112-20044 Method B (Segregated Losses). The changes applied to losses and evaluated efficiency are based upon allowable limits by standards and observed changes to machines. The various effects can be related to greenhouse gas emissions.

Evaluated machines

The three machines in question were 150 hp, 3 ph, 460 VAC, 167 A, 1,780 rpm that were measured for efficiency at load points 25%, 50%, 75%, 100% and 125% in accordance with IEEE Standard 112 Method B (Figures 1 and 2).

Motor losses are broken down as follows (Tables 1,2 and 3, Figures 3, 4 and Figure 5):

• Core losses consist of eddy-current and hysteresis losses. This is considered a constant loss in a constant-frequency environment.
• Friction and windage losses: fans, bearings and the machine’s surface friction. This is considered a constant loss.
• Stator I2R losses are the result of current passing through the stator’s DC resistance. This loss varies with current flow (load).
• Rotor I2R losses are the result of current in the rotor as with stator I2R losses.
• Stray load losses are the other losses, including magnetic fringing that weren’t included in the previous losses.