3.2 Load torques as a function of angle
These characteristics appear in machinery with reciprocating motion, for example,
in table motors. They are also present in piston machinery (compressors in
heat pumps) due to intermittent loading. The electric input current of the drive
motor follows this motion cycle and can generate a rhythmically fluctuating
voltage drop in the line. Generally a so-called torque force diagram is plotted in
the planning of these applications.
3.3 Load torques as a function of path
They are typical, for example, in vehicles, or in table motors, cableways and conveyor
belts.
3.4 Load torques as a function of time
These motors are loaded intermittently or periodically. Examples are:
- punches
- hoists
- conveyor systems
- rock crushers
- ball mills
3.5 Breakaway torque
Another important concept is the so-called breakaway or static torque which is
caused by static friction. In order for a motor to start reliably, this value should
be known as accurately as possible and the starting torque MA of the motor
should exceed the load torque. In large machines with slide bearings it may significantly
exceed the rated torque Mn.
Figure 3.4.1 shows certain torque characteristics of common driven machines.
Comparison with Figure 3.2.1 shows that most of them have a typical characteristic
and thus classification is possible.
Example: The speed of an induction motor operated with a load controller can be
infinitely adjusted between 50% and 100%. How does this affect the delivery
rate of a piston or centrifugal pump?
• Piston pump: The torque demand is almost independent of speed as shown in
Figure 3.2.1 a, and the torque remains almost constant. The delivery output is
therefore proportional to the speed. At half speed it also falls accordingly to
P' = P . 0.50 = 50%