Step 1: Know the load characteristics
For line-operated motors, loads fall into three general categories: 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.
Sizing a motor for these applications is simple once the torque (or horsepower) for the application is known. 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.
Loads from centrifugal pumps, fans, blowers, compressors with unloaders, and similar equipment tend to be variable over time. In choosing a motor for these conditions, consider the highest continuous load point, which typically occurs at the highest speed.
Step 2: Get a handle on horsepower
The rule of thumb for motor horsepower is: Select only what you need, and avoid the temptation to oversize or undersize. Calculate the required horsepower from this formula:
Where torque is in lb-ft and speed is in rpm.
Step 3: Getting started
Another consideration is inertia, particularly during startup. 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 one of four “design types” for their ability to endure the heat of that starting and pull up.
In ascending order of their ability to start inertial loads, NEMA designates these as design type A, B, C, and D. Type B is the industry standard and is a good choice for most commercial and industrial applications.
Step 4: Adjust for duty cycle
Duty cycle is the load that a motor must handle over the period when it starts, runs, and stops.
Continuous duty is by far the simplest and most efficient application. The duty cycle begins with startup, then long periods of steady operation where the heat in the motor can stabilize as it runs.
A motor in continuous duty can be operated safely at or near its rated capacity because the temperature has a chance to stabilize.
Intermittent duty is more complicated. The life of commercial airplanes is measured by their number of landings; in the same way, the life of a motor is proportional to the number of starts it makes. Frequent starts shorten life because inrush current at startup heats the conductor rapidly.
Because of this heat, motors have a limited number of starts and stops that they can make in an hour.
Step 5: The last consideration, motor hypoxia
If your motor is going to operate at altitudes that are substantially above sea level, then it will be unable to operate at its full service factor because, at altitude, air is less dense and does not cool as well. Thus, for the motor to stay within safe limits of temperature rise, it must be derated on a sliding scale.
Up to an altitude of 3,300 ft, SF = 1.15; at 9000 ft, it declines to 1.00.
Should You Buy New or To Rewind?
When you have a motor failure you’ll need to decide if you should buy a new motor or fix the old one. A common cause of motor failure is problems with the motor windings, and the solution often is to rewind the old motor. Because it is economical in terms of initial cost, rewinding of motors is very common particularly for motors of more than 10 horsepower.
However, the motor rewinding process often results in a loss of motor efficiency.
The quality of the rewind has a big impact on operating cost.
A poorly rewound motor may lose up to 3% in efficiency. A 100 HP motor may use several hundred dollars more in electricity each year due to this drop in efficiency, compared to its original efficiency. The operating cost may be even more compared to a new high efficiency motor.