Load types

When the motorshaft torque is equal to the torque exerted by the load, the motor is stationary. In such cases the torque and the speed are constant.

The characteristics for motor and machine are stated as the ratio between speed and torque or output. The torque characteristics have already been discussed. The characteristics of the machine can be divided into four groups.

Types of motor load

The first group (1) consists of machines for winding material under tension. This group includes, for example veneer cutting machines and machine tools.

Group (2) consists of conveyor belts, cranes, positive displacement pumps, compressors as well as machine tools.

Group (3) consists of machines such as hydraulic pumps, rollers, smoothing machines, and other processing machines.

Group (4) comprises machines operating by centrifugal force, such as centrifuges, centrifugal pumps and fans.

The stationary state occurs when motor and machine torque are identical (see below). The graphs cross each other at point B. When a motor is sized for a given operating machine, the intersection point should be as close as possible to point N for the rated motor data.

A surplus torque should be available throughout the range, from standstill to the intersection point. If that is not the case, operation becomes unstable and the stationary state may change if the speed is too low. One of the reasons for this is that the surplus torque is required for acceleration.

In particular for machines in groups 1 and 2, it is necessary to take account of this starting condition. These types of load may have an initial starting torque which is same size as the starting torque of the motor. When the starting torque of the load is higher than the starting torque of the motor, the motor cannot start.

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Source: Danfoss - Facts Worth Knowing About 3 ph Inverters