Simplifying motor selection for aggregate producers
Aligning the torque produced by the motor with the application demand torque will allow the motor to run efficiently. This means if the application requires 250 ft.-pounds of torque to keep the load moving, users should calculate the ideal nameplate horsepower in order to prevent over or undersizing the motor:
Horsepower = torque x speed / constant 5252
Horsepower = 250 ft.-pounds x 1,750 rpm / 5252 = 83 ideal nameplate horsepower
In this case, because 83 hp does not exist in standard motor offerings, select either 75 hp, 100 hp, 125 hp, 150 hp or another offering. Users often think oversizing is better, but this isn’t always the case. If only 83 hp is required to run the load and the conveyor is run by a 150-hp motor, then the motor is mathematically running at 55 percent load. And the motor, then, operates at less-than-optimal efficiency.
In the example, the target motor should be 100 hp. This will run at 83 percent load and with the motor running at an optimal efficiency.
Today, every motor has a nameplate service factor. This value is the extra allowable intermittent overload, meaning a 100-hp motor with 1.15 service factor will be sufficient. Keep in mind, though, that overheating can occur. The nameplate service factor is built-in protection for a specific motor.
Remember that when a motor is overloaded, heat is produced and a motor will begin to breakdown. Exceeding the motor nameplate power continuously can result in equipment damage and additional expense that is unnecessary.
A delicate balance exists between the power needed on a continuous basis versus how much inefficiency or overloading may occur with a system. So relying on service factor capabilities for carrying overloads on a continuous basis is not recommended.
Recommended service factors can also vary depending on the type of motor. It is not a requirement to add service factor to a nameplate if the motor does not have the capacity to exceed 1.0 service factor. This can apply to motors used in hazardous locations that are not recommended to be used over the standard 1.0 service factor.
Mechanical features
The type of operating environment must be considered when selecting a motor, as well.
Will the motor operate outdoors? Be exposed to weather and temperature changes? Will dust be present? Will the motor need to withstand a beating from rocks falling onto the motor?
Mechanical features can extend operating life by protecting the motor from the environment in which it operates. While there is no industry standard for “heavy duty” or “severe duty,” some manufacturers use these phrases loosely to cover entire families of products. Looking carefully at each feature can make a difference in overall cost and motor performance.
Sealing and bearing design, frame material, cooling systems, insulation and conduit box protection, rotor balancing and other aspects of a motor can make a difference in motor performance.
Bearings and seal design
Bearings are a critical component within a motor, and they’re often the reason for premature motor failure.
Bearings have several important functions, from supporting the rotor of the motor and driven equipment, to helping to reduce heat generated throughout the motor. Depending on the conveyor, going from a traditional bearing to an oversized ball bearing – and even to roller bearing designs for large radial loads – can help with motor performance in high-torque applications.
Another phenomenon with motors and VFDs is shaft currents. Simply put, current needs to find grounding. If a motor isn’t grounded properly, it is common for a current to travel through the shaft and, potentially, damage bearings.
When shaft currents constantly travel through bearings, small indentations are made in the bearing raceways. This is called brunnelling, which is not much different than the rumble strips on the shoulder of the interstate.
Over time, these marks can result in premature bearing failure. To protect the motor bearings, upgrade and use insulated bearings or other shaft-grounding devices.
Additionally, multiple seal options are available throughout the motor industry. Some are patented designs that positively lubricate bearings to extend motor bearing life, while others are maintenance-free and sealed for life.
Frame material
From rolled steel to aluminum and stainless steel to cast iron, there are many material types that can be used for motor frames. Most motors used in heavy-duty conveyors are made of cast iron. Cast iron is not only used for the frame itself, but also for end bells, conduit boxes and fan covers – with each having specific purposes in the overall design and motor life.
Does the motor weight matter? If yes, weight can be reduced by selecting alternate materials for components. If the motor weight is irrelevant, selecting all-cast-iron construction will help the motor withstand the beating it may take in severe-duty operations.
Consider, too, that optional coatings can be added to extend the life of a motor frame. In some cases, these come at a large expense and may not provide adequate payback. If a motor operates in a high-moisture environment or is in an extremely corrosive application, epoxy or C3 paint may provide additional life.
Cooling systems
When a motor is energized, heat is created. A motor design needs to take this into account and be able to dissipate heat.
Totally-enclosed-fan-cooled motors are most common due to exposure to contaminants such as dust and water. These motors feature a fan on the opposite drive end, which rotates with the shaft and results in air being pushed over the housing and between the fins.
Motor fins can help dissipate heat on cast-iron motors. For that reason it is important to keep the exterior of a motor free from contaminants to allow it to run cooler. This, in turn, will extend its operating life.
Final thoughts
Three key areas must be considered early in the process of selecting a motor:
1. Know your torque. Balancing the torque needed and the motor ability is the best way to choose the correct electric design.
2. Motor service factor should only be used intermittently.
3. The construction of the motor must be able to withstand the environment. For a minimal upfront investment, some mechanical features can greatly increase the overall life of the motor.
Shelley Gager is global segment manager of general industries at ABB.
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