How you define a Heavy Duty Motor?

Most modern motors are built to standards and frame sizes, efficiencies, power factor are very close from manufacturer to manufacturer. The definition depends on what you mean by "Heavy Duty". Examples would be "Punch Press" or a "Rock Crusher", anything with a high inertia or heavy intermittent load. If the load is a high inertia load the first thing to look as is the design letter of the motor. The standard motor is Design "B" and a motor for a high inertial load should be Design "C". A motor for punch press should be Design "D" which is high torque and high slip.

When someone asks me to build them a heavy duty motor I look at it in two ways. 1) Mechanically and 2) Electrically. If you are not sure how you would build a motor that needs to withstand heavy mechanical stress in a standard frame. I would build it in an oversized cast iron frame with oversized bearings and shaft. Mechanical stress creates heat in the motor and in the motor winding. Heat is the enemy of an electric motor. The oversized frame allows you to oversize the conductor, reduce that heat and add in some much needed insulation to ground and between phases. Another important process in any winding is proper consolidation of the winding. Windings want to move and it is important to minimize that movement. There are other things to consider: The Ambient and the environment.
Induction motor
The service factor is something to consider, and although an argument could be made otherwise, I have always considered the service factor as a temporary thing to withstand an occasional overload. If you run an induction motor 100% in its service factor it will burn out prematurely. Remember that rule of thumb. "FOR EVERY !0 DEGREES THAT A MOTOR RUNS ABOVE ITS RATED TEMPERATURE, YOU CUT THE LIFE OF THE MOTOR IN HALF. I know that many motors are classed as "heavy duty". In most cases they look beefy on the outside but not much changes on the inside. Certain motors will be available in two frame sizes. You should chose the larger as there will be more metal work. The specification sheets will still state the required values but more steel, more cooling.

Some suppliers will de-rate the motor from say 30kW for a 22kW installation. This is not true heavy duty as it just means that the motor is running light. There are ramifications for this in that the efficiency and power factor change with load, generally for the worse. Here is my example.

22kW, IE2, 4 pole, 400v, induction motor. Selected to run on a 20kW mechanical load.
Supplier offers a 30kW motor to act as a heavy duty motor. 20/30 x 100 = 66.67% loaded.

From some data sheets:

  • 30kW motor at 100% load, power factor = 0.82 and efficiency of 91.0%
  • 30kW motor at 50% load, power factor = 0.63 and efficiency of 90.5%

You can get a more accurate figure if you go to the graphs but this will show the problem.

If you have not taken the worse power into consideration, you could be incurring costs on your electrical utility bill that you did not expect from the power factor (kVA) and if this was done for many motors, then incomer cables could become an issue. You will also be wasting 0.5% or 1kW per hour run.

Other significant factors in defining "heavy duty" are things like mechanical robustness and bearings. A motor installed to drive some severely vibrating SAG (Semi-Autonomous-Grinding) mill, or some scrap metal compactor will be subject to a quite different kind of heavy duty than for instance a motor for an underground mining elevator, which simply needs to be designed for frequent stop-start cycles.

It is always of interest to consider the long term economy behind any choice. Unfortunately this is often forgotten when the final design simply caries on from some simplified initial FEED study, without spending the required time on any thorough detail design.

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