Tech Updates


Choosing a motor isn’t an easy thing. To choose the best motor, you need to know certain things, some of which are listed below.


There are three general categories in which line-operated motors fall: constant torque, torque that changes abruptly, and torque that change gradually over time. Bulk material conveyors, extruders, positive displacement pumps, and compressors without air unloaders run at relatively steady levels of torque.

In order to size a motor for these applications, the torque (or horsepower) should be known. In choosing the best motor, consider the highest continuous load point, which typically occurs at the highest speed. Load demands by elevators, compactors, punch presses, saws, and batch conveyors change abruptly from low to high in a short time, often in a fraction of a second whereas loads from centrifugal pumps, fans, blowers, compressors with unloaders, and similar equipment tend to be variable over time.


Selecting the horsepower of the motor is the nest task. There is only one rule of thumb to select a motor with the right horsepower, which is, to select only what you need, and avoid the temptation to oversize or undersize.


Another thing to consider while buying a motor is its inertia. Consider the inertia particularly during the startup of the motor. Every load represents some value of inertia, but punch presses, ball mills, crushers, gearboxes that drive large rolls, and certain types of pumps require high starting torques due to the huge mass of the rotating elements. Motors are rated as per one of their various design types for their ability to endure the heat of that starting and pull up.


A motor must handle load over the period when it starts, runs, and stops. This is called duty cycle. There are different kinds of duty, as given below–

Continuous duty: Continuous duty is considered by far the simplest and most efficient application. The continuous duty cycle begins with startup, which is then followed by long periods of steady operation where the heat in the motor can stabilize as it runs.

In a motor with continuous duty, the temperature has a chance to stabilize and thereafter, it can be operated safely at or near its rated capacity. because the temperature has a chance to stabilize.

Intermittent duty: Intermittent duty is more complicated than continuous duty as the life of commercial airplanes is measured by the number of their landings. It is in the same way as the life of a motor is proportional to the number of starts it makes. Frequent starts in a motor can shorten its life because inrush current at startup heats the conductor rapidly, therefore, challenging it. This results in the production of a lot of heat. Because of this heat, the motor can only have a limited number of starts and stops that they can make in an hour. This can cause an obvious problem which can be hard to deal with.


Jeffery D. Silvers
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