Understanding Service Factor in Electric Motors


What is Service Factor? 

The service factor (SF) is a measure of the periodically overload capacity, or the amount of extra load a motor can handle for short periods without sustaining damage. The SF is usually expressed as a decimal or a percentage above the motor's rated power or horsepower (HP). For example, if a motor is rated at 10 HP and has a service factor of 1.15, it can handle 11.5 HP for short periods without damage.

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The National Electrical Manufacturers Association (NEMA) sets standards for electrical equipment, including motors. The NEMA standard service factor for totally enclosed motors is 1.0, meaning that these motors are designed to operate at their rated nameplate horsepower and should not be overloaded beyond that point.

Figure 1. NEMA Service Factor at Synchronous Speed (rpm) for drip proof motors

When a motor is operated continuously at a service factor greater than 1, it will experience increased stress and heat generation, which can cause premature wear and failure of its components, such as bearings, windings, and insulation. Therefore, running a motor at a service factor greater than 1 can result in reduced life expectancy and increased maintenance and repair costs.


Design Power vs. Motor Power and Service Factor can be expressed as

PD = SF P    (1)


PD = design power (hp, W)

SF =service factor

P = motor power (hp, W)

Example - Design Power vs. Service Factor

A 1 HP motor with a Service Factor - SF = 1.15 can operate at

PD = (1 hp) x 1.15 = 1.15 hp

However, it is important to note that even if a motor is designed to operate at a service factor greater than 1, it should only be operated at that level for short periods and not continuously. The manufacturer's recommendations and guidelines should be followed to ensure the motor's reliable and safe operation. Additionally, regular maintenance and inspections are essential to detect and address any issues before they lead to motor failure.

When a motor is operated at a service factor greater than 1, it can experience several negative effects. One of the primary effects is increased heat generation due to the increased current flowing through the motor's windings. This heat can lead to thermal expansion of the motor's components, causing them to warp or deform, which can result in increased friction and reduced efficiency. Additionally, the increased heat can cause insulation breakdown, which can lead to short circuits or ground faults.

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Another effect of operating a motor at a service factor greater than 1 is increased mechanical stress on the motor's components, such as bearings and shafts. The increased stress can cause premature wear, leading to increased friction, vibration, and noise. The increased stress can also cause the motor to become unbalanced, which can result in additional wear and damage to the motor and the connected equipment.

How Long a Motor Can Run When Service Factor is Applied? 

NEMA did not provide specific time that the motor can run with service factor applied. However, the length of time that a motor can run with its service factor depends on several factors, including the motor's design, size, operating environment, and load characteristics. The service factor is intended to provide a safety margin for occasional overloading of the motor without causing damage, but the motor should not be operated continuously at the service factor rating as this can lead to overheating and premature failure.

The motor manufacturer's specifications and guidelines provide specific information on the appropriate use and duration of the motor's service factor. In general, motors can run with their service factor for short periods of time, typically ranging from a few minutes to a few hours, depending on the motor's size and design.

However, it is important to note that prolonged operation at the service factor rating can lead to increased motor heating, which can reduce the motor's lifespan and increase the risk of failure. Therefore, it is recommended to limit the use of the service factor to occasional overloading and to operate the motor at or below its rated capacity whenever possible.

Important Points to Consider in Electrical Design

The service factor of a motor can be used in protective device coordination studies to ensure that protective devices such as fuses, circuit breakers, and overload relays are properly sized and coordinated to provide adequate protection for the motor.

The protective device coordination study involves analyzing the characteristics and settings of the protective devices in a power distribution system to ensure that they operate in a coordinated and sequential manner to protect the equipment and minimize downtime in the event of a fault. The study considers various factors such as the size and type of the motor, the expected load and operating conditions, and the protective devices' characteristics and settings.

In this context, the service factor is used as a safety margin to account for the occasional overloading of the motor, which can cause temporary increases in the motor's current draw. By considering the motor's service factor in the protective device coordination study, the protective devices can be appropriately sized and coordinated to provide adequate protection for the motor under both normal and overload conditions.

For example, if a motor has a service factor of 1.15, this means that it can handle a temporary overload of up to 15% above its rated capacity. The protective devices can be coordinated to allow for this temporary overload without tripping or interrupting the power supply to the motor. However, the protective devices should be sized to provide adequate protection in the event of sustained overloads or faults that exceed the motor's service factor rating.

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