February 1996 Volume 2 Issue 3
WHAT TO DO WHEN A MOTOR BURNS OUT
Most production facilities use large (1 to 500 horsepower) three-phase electric motors to perform important tasks such as conveying materials and driving refrigeration compressors. The roles these motors play are often so critical that when one burns out it creates a crisis — sometimes even shutting down the plant. The purpose of this article is to help you think through the motor-replacement situation before the next crisis arrives, and thus handle it more effectively.
One by one, let’s go over the three most common ways of dealing with a motor-burnout crisis.
Option 1: Have the burned-out motor rewound at a local motor rewinding shop.
(TEMPTING, BUT NOT USUALLY A GOOD IDEA)
This course of action is popular because it is often the cheapest and fastest way to get the plant up and running again. Unfortunately, it is often the most expensive long-run approach. The problem is that rewound motors tend to have lower efficiency than new motors. In order to remove the old windings, the motor is placed in a “burnout oven” where it is heated to a high temperature. Heating loosens the windings but also tends to damage the magnetic core. Unless the burnout process is thermostatically controlled at 260°C (500°F) or lower, core damage results, and this damage reduces the efficiency of the motor. Some P.E.I. motor repair shops do winding burnouts at temperatures much higher than 260°C, and with no thermostatic control of oven temperature.
Tests have shown that cores baked at 454°C result in rewound motors having an average efficiency 4.6 per cent lower than the original motors. A difference of less than five per cent may not sound significant, but where large motors are concerned, a small difference in efficiency can result in a large difference in operating cost. For example, if the cost of electricity is 8.0 cents per kWh, a one per centdifferencein the efficiency of a 50 HP motor running at full load for 8,000 hours produces a $284 difference in the annual electricity bill.
Many experts maintain that there are only a few situations where motor rewinding makes sense. Two of these might be:
· if the motor operates less than a thousand hours per year, or
· if there is no replacement motor available that meets the physical (space and mounting) requirements.
Option 2: Replace the motor with a new, standard motor of the same horsepower.
(BETTER THAN REWINDING, BUT NOT ALL NEW MOTORS ARE GOOD CHOICES)
Except in circumstances such as those just mentioned, purchasing a new replacement motor is almost always preferable to rewinding the old one. The catch is, however, that there is no “standard” replacement motor. “Standard” motors differ in efficiency from make to make and model to model, and one should compare efficiency figures for all likely candidates. Beware of “bargain” motors. They tend to have lower efficiency than average motors of the same horsepower rating. On the other hand, some average-price motors have well above
As this graph indicates, motor efficiency differs widely, even among motors with the same horsepower rating. Shop around for a model that gives high efficiency at a reasonable price. Do not buy low-price, low-efficiency “bargain” motors.
Option 3: Replace the motor with a new energy efficient (EE) motor of the same horsepower.
(USUALLY THE BEST OPTION)
Many motor manufacturers have a line of energy-efficient EE motors of the same horsepower, size, and shape as their standard-efficiency motors. These EE motors are generally priced somewhat higher, but in situations where the motor runs for several thousand hours per year, the lower operating cost soon pays back the difference. When a motor burns out and forces the business to make a large expenditure anyway, it’s a perfect time to upgrade that part of the plant and reduce its future operating cost.
GETTING MOTOR INFORMATION AND SOFTWARE
If your whole plant’s functioning relies on one or more of its large electric motors, it makes obvious sense to look into motor replacement options and line up likely models and sources before the next panic hits. Two software tools that can help you make motor replacement decisions are the MotorMaster and Power Wise Calculator programs available from the Energy and Minerals Section. MotorMaster is a U.S.-developed program that contains performance data and U.S. list prices for 9,000 motors ranging in energy efficiency from average to very high.
The printed literature supplied by Canadian motor manufacturers (and available from motor distributors) usually contains efficiency figures. Using these figures, the Power Wise Calculator program allows you to compare one option against another. You enter efficiency and power factor figures from the manufacturer’s literature, plus information about motor operating conditions and your electrical rate. The program then calculates the annual cost of running the motor. In addition, the software will compare two options, and will show the kW, kWh, and dollar advantage of one over the other.
If your facility uses one or more large electric motors we suggest that you call Mike Proud or Ron Estabrooks at 368-5010. Request the software mentioned above, and Energy Efficient Motors For Industry, Commerce, and Agriculture on Prince Edward Island, a free booklet that gives an informative overview of the motor efficiency issue.
AN IMPORTANT FINAL POINT: WHY DID THE MOTOR FAIL?
Because you don’t want the replacement motor to fail, too, it is important to understand why the first motor failed. Was the supply voltage too low or too high? Did one of the three supply phases fail? Was the motor’s load excessive — perhaps because the equipment the motor was driving was being operated improperly, or because that equipment has mechanical problems? Did something interfere with motor cooling? Or was this motor simply at the end of its useful life?
Unfortunately, ready answers may not be available. To understand what happened it is often necessary to run system tests using a functioning motor — exactly what is not available when a motor has burned out! If you must replace a motor without knowing why it failed, it is especially important to make sure that the replacement motor is operating within its voltage, current and load ratings. This can be done by making voltage, current, and slip (speed-under-load) measurements an integral part of the new-motor installation procedure.