Match the motor to the load

Analyze the application to eliminate frequent failures.

By James Taylor

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There’s a machine in your facility whose motor fails every couple of years. Your job is to find out why. Conducting a root cause analysis of the failures is where to start. You need an understanding of the factors to consider in choosing the correct motor for the application so you can determine if the motor is the right one. If you know the major factors that could cause the motor to fail, you can check each in your application as you search for a solution.

Motor characteristics

Figure 1. This conceptual graphic shows that torque exists, whether or not there is rotation.
Figure 1. This conceptual graphic shows that torque exists, whether or not there is rotation.

We need to understand some of the characteristics of motors — torque, horsepower, current, service factor and load profile, among others. Motor horsepower is what we think of first, torque is next. The difference can be illustrated by imagining a wrench turning a shaft (Figure 1).

Torque is the turning force applied to the shaft. It doesn't matter if the shaft actually turns. Torque is measured in foot-pounds (ft-lb or lb-ft) and is independent of rotating speed. Now, if we let the torque turn the shaft, we introduce speed into the equation. This leads us to horsepower, which is how fast we are injecting work into the shaft. If we double the turning speed while keeping torque constant, we double the horsepower. By definition, the relationship between torque and horsepower is:

1 hp = 33,000 lb-ft/min.

Figure 2. There are four important points on a motor’s speed-torque curve.
Figure 2. There are four important points on a motor’s speed-torque curve.

Figure 2 is a typical motor speed-torque curve. The one shown corresponds to a NEMA Design B motor, probably the most common in industry. There are several points on this curve we need to understand if we are to select a motor properly.

Full-load torque is the torque necessary to produce rated horsepower at full-load speed. It's equal to:

FLT = hp x 5,252/FLS

Where FLT = full-load torque (in ft.-lb.)
hp = rated horsepower
FLS = full-load speed (in rpm)

The locked-rotor torque, also called starting torque, is the torque applied to the shaft when power is first applied but before the motor starts to turn. Locked-rotor torque usually is expressed as a percentage of full-load torque.

Pull-up torque, also called accelerating torque, is the minimum torque developed during the period from locked-rotor condition to breakdown. It’s the minimum torque needed to accelerate the load from zero speed to running speed. It’s also the first critical selection factor. Pull-up torque is usually expressed as a percentage of full-load torque.

Breakdown torque occurs at the point where increasing load causes a rapid speed decrease. Consider a motor with a brake. As we apply the brake tighter and tighter, we reach a point where the motor slows to a stop. That's breakdown and it’s expressed as a percentage of full-load torque.

Figure 3. This is a graphical representation of the inrush current that flows when a motor first starts turning.
Figure 3. This is a graphical representation of the inrush current that flows when a motor first starts turning.

Motor current also is related to speed (Figure 3). There are two points of interest here. The first is full-load current, which is the steady-state current that flows when the motor is operating at full-load torque and speed. Locked-rotor current is the steady-state current through a motor with the rotor locked and with rated voltage applied. Operating at locked-rotor condition for more than 20 seconds can result in insulation damage because of excessive heat generation.

Service factor

Service factor is the permissible amount of overload a motor can withstand while remaining within defined temperature limits. When voltage and frequency are maintained at nameplate-rated values, the motor tolerates overload to the horsepower obtained by multiplying the rated horsepower by the service factor shown on the nameplate.

However, locked-rotor torque, locked-rotor current and breakdown torque are unchanged. Operating under these conditions will shorten motor life because of the higher temperatures, which causes the insulation to deteriorate more rapidly.

Motor temperature

Heat is a major consideration when selecting a motor. If the motor runs too hot, the insulation breaks down much faster unnecessarily. A common guideline states that each 10°C temperature rise above rated temperature cuts insulation life in half. High temperatures also can degrade the grease in the motor’s bearings, causing early bearing failure. Bearing or gear lubricant life is reduced by half for every 25°F (approximately 14°C) increase in temperature. The motor is subjected to two sources of heat: internal and external. The ambient temperature for the application should be expressed in degrees Celsius (°C). Most motors are designed to operate in an ambient of 40°C. If the ambient temperature is greater than that or the application requires frequent starting and frequent overloads, you should consider special motors to compensate for the increase in total temperature.

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