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Oversized Electric Motors: Should They Be Replaced?

Oversized Electric Motors: Should They Be Replaced?

Motor oversizing has become an increasingly common practice these days. To oversize a motor basically means to select a higher horsepower than necessary. There are various reasons why many people choose to oversize. For one, oversizing is believed to make up for disparities in system operations. Others also cite preventive measures, as they believe that the cost of a larger motor is easier to deal with than the cost of a mechanical mishap.

However, many people are also not fond of motor oversizing because they think that oversized electric motors usually cost more to run and only lead to energy waste. For them, more appropriate motors can minimise energy use and subsequently reduce the operation costs. They essentially believe that oversized motors are inefficient and should be replaced right away with appropriately sized units that are energy-efficient.

However, in reality, replacement is not always the best solution for an oversized electric motor. There are various pieces of information that must be obtained first to come up with an accurate assessment of energy savings. These include the load of the motor, the efficiency of the motor at a specific load point, and the operating speed of the motor.

Load of the Motor

Electric motors are basically machines that convert electrical energy into mechanical energy to act upon a mechanical load. The load is the burden placed on the motor when this mechanical process occurs. Generally, having the motor load properly match the motor is necessary, as it prevents damage to the motor and avoids inefficient and costly operations.

Using a motor that is significantly oversized compared to the motor load usually results in unnecessary expenses both with regards to the initial cost and the continued operation of the motor. However, sometimes, a significantly oversized motor may also be required when substantially larger peak loads are expected.

Nonetheless, even though some situations may demand oversizing for peak loads, it is otherwise always best to select a motor that operates efficiently in the 75 to 100 percent load range. In some cases, downsizing the motor may generate energy demand savings.

Efficiency of the Motor At Specific Load Point

Motor efficiency is essentially the ratio between the amount of mechanical work that the motor carries out and the electrical power that it consumes to do the work. This measure is usually determined in percentage. Naturally, as the load becomes higher, the efficiency of the motor also increases. This is mainly because frictions play a gradually smaller role in the overall motor efficiency as the load increases.

Generally, oversized electric motors are considered less efficient compared to the appropriately sized ones. These motors are at their optimal working point when operating between 30 percent and 100 percent of rated load.

However, in some instances, motors in larger sizes can also operate at loads down to 25 percent of the rated load with a significantly high efficiency.

In essence, the efficiency of an electric motor is going to be dependent on several factions, such as its design, materials, rating, load, power quality, and operating conditions.

Operating Speed of the Motor

The speed of an electric motor is usually determined by the power supply frequency as well as the number of poles in its magnetic field. The actual operating speed of a motor is generally less than its synchronous speed. This difference between the actual and the synchronous speed is called a “slip.” Slip and operating speed usually depend on applied load, and the load imposed upon a motor depends in turn on its size.

As the load on a motor increases, it starts to rotate slower until it operates at the full-load speed. Hence, oversized motors have a tendency to operate at speeds that approach synchronous. In most cases, oversized motors tend to operate at a higher speed than appropriately sized, energy-efficient motors. This should be considered when determining energy and demand savings.


There are usually various concerns and characteristics that determine whether oversizing is right for specific systems. On one hand, oversizing can increase operating costs and call for electric motor rewinds or replacement when strains occur due to the inability of the system to handle extra power. When facing motor issues, crucial questions should be asked before deciding whether to rewind or replace the motor.

On the other hand, oversizing can compensate for lack of maintenance, especially when generator overhauling is not done regularly. Even though it is not maintained, a motor with a higher power can still run longer under the same conditions. Overall, the advantages and disadvantages of motor oversizing cannot be generalised.