Matching Motor Torque To Your Load

Mar 1, 1999 12:00 PM, By Robert J. Lawrie, Senior Editorial Consultant

You must consider steady-state, acceleration, and starting torque to ensure the lowest motor operating costs.

There's torque, and then there's torque. There's torque, as required by your load, and torque, as available at your motor output. Both are critically important when you're selecting and designing a drive for maximum efficiency.

There are three major application aspects of motor torque you should consider: steady-state; acceleration; and starting (breakaway). Before you can effectively apply these aspects, you should understand motor characteristic curves, such as those shown in Fig. 1 (original article). These curves of speed versus torque (and current) are typical for a NEMA Design B standard motor, and show the motor torque developed at any given motor speed. (We'll discuss energy-efficient motors in an upcoming article.)

Steady-state load torque. This is what a load continuously demands. It establishes the thermal demand the motor must be able to continuously support. The motor's nameplate horsepower rating guarantees it can power this value of load continuously, without exceeding the standard temperature rise for the motor's particular class of insulation. This means the motor losses at nameplate loading will be thermally dissipated without adversely affecting the life expectancy of the motor.

However, a nameplate rating tells little about other values of loading. The steady-state load torque you need for an application will seldom match the rating of the motor exactly. For this reason, NEMA gives its standard motors a service factor rating (usually 1.15 for drip proof motors). This is for infrequent overloads that may occur. This service factor assures the motor's operating temperature will not exceed "safe temperature rise." At 115% of nameplate loading, this permits you to select a more economical motor for those cases where continuing overloads are infrequently required. By using the service factor properly, you make a small compromise in ultimate life expectancy to gain energy savings and first cost benefits. We commonly interpret this compromise with a rule-of-thumb stating: "Every hour at 15% overload shortens a motor's life expectancy by two to three hours."

This can be an effective trade-off in cases where overload demands are not frequent. You encounter such cases with machine tools that occasionally must handle unusually large workloads. In these instances, consider the trade-off as a definite possibility, because the alternative is to oversize the motor-a counterproductive practice.

Acceleration torque. Acceleration of most loads isn't a common problem. However, when driven machinery has a high inertia, the acceleration time may become critical, particularly if it exceeds 10 sec.

You can see the reason for this by looking at Fig. 1 (original article) again. It shows the very high current drawn by the motor until it nears rated speed. If this high current persists too long, the attendant losses will cause serious overheating of the motor or nuisance tripping of its protective device-both of which are unacceptable conditions.

Acceleration time depends primarily on the inertia of the rotating system. However, it's also dependent on the net difference between load torque demand and motor torque capability at each speed. This is shown in Fig. 2, on page 70, for a NEMA B standard motor with a typical constant torque load demand. Note: The crosshatched area indicates the net torque available for accelerating the system. You may find another type of load, such as a fan or blower, easier to accelerate without excessive heating. In Fig. 3 (left), we see curves for the same NEMA B motor, but it's now driving a fan with a typical variable torque load demand. This results in a much greater net acceleration capability, as shown by the increased crosshatched area.

Of course, oversizing the motor could provide more accelerating torque to solve the problem. But then, the first cost would be greater and steady-state operation would be at a lower percentage of rated torque, resulting in poorer operating efficiency.

Starting (breakaway) torque. Certain loads may present difficult torque demands at initial starting or breakaway. Such equipment typically shows the constant-torque load characteristic of Fig. 2 (original article), mostly due to mechanical friction of the entire rotating system.

Depending on the type of bearings used, the static friction can be much higher than the rolling friction. This occurs most frequently with sleeve bearings that cannot establish proper oil film lubrication until having made one or two revolutions. Until the system breaks away, the motor torque must overcome a higher value of torque than shown by the dynamic starting torque.

Once again, oversizing the motor could resolve this problem, but not without defeating energy-saving objectives. Instead, you could consider using other types of motors with characteristics better suited to the problem. Some of these alternatives may provide a much better solution than the general-purpose motor.

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