Pendulum machine

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A pendulum machine is an electrical machine whose stator ( stator ) is rotatably mounted in axial bearings and connected to a force measuring device. It serves as a mechanical load and allows torque measurement in engine test stands.


The testing of engines (electric, hydraulic or internal combustion engines ) takes place on engine test stands . The shaft of the drive to be tested is coupled with a power brake (loading device). This performance brake must be able to absorb the entire drive power. In the past, water vortex brakes were mainly used for this . However, if the specimen must also be dragged (examination of generators, pumps and brakes) are direct-current machines used whose stator ( stator ) is movably mounted to the torque to be measured. The machine housing “swings”, held by the force measurement, in two external bearings. The DC machine consumes the engine power (generator operation); the speed can be controlled via its excitation current.

Internal combustion engine on a test bench with a DC pendulum machine

With the DC machine is of the test engine on the drive shaft , a propeller shaft or, if necessary, a gear connected.

The picture shows:

  1. Torque balance
  2. DC pendulum machine
  3. Tachometer generator
  4. Opposed piston engine (test item)


This so-called pendulum machine simultaneously fulfills the function of a loading device and a torque measuring device . The structure and mode of operation correspond to that of the separately excited DC machine . Depending on the load , the excitation field can be supported by an additional auxiliary series winding. The pendulum machine works as a generator (positive torque from the point of view of the drive machine ) or as a motor (negative torque) at an adjustable speed. The torque is measured by determining the reaction torque. The stand is supported against the solid base via a lever arm of a scale . The power supply of the stator takes place via free-hanging in the machine movable cable. The counterforce of the balance prevents further rotation of the pendulum machine housing. For very simple test stands , constructions similar to beam scales with a weighing pan and weights were also used. The balance thus measures a force that can be converted into the effective torque if the length of the lever arm is known . Electrically, this reaction torque of the pendulum machine can also take place via the force measurement with a bending beam or load cell .

With the simultaneous measurement of the speed ( tachometer generator , incremental encoder ), there is the possibility of determining the performance for both types of pendulum machine :

P - mechanical power
M - torque - angular frequency 1 / s = n - speed, 1 / min F - force on the torque balance r - radius or lever length of the force measurement


Until the 1970s, the Leonardsatz dominated the speed setting of a pendulum machine ; these were replaced by four-quadrant converters ; these are smaller and lighter.

With the development of powerful frequency converters from around 1970, asynchronous pendulum machines were increasingly used.

In general, the electric pendulum machine has the advantage of energy recovery compared to mechanical loading devices and also compared to the eddy current brake and the possibility of being able to test hydraulic pumps, brakes and generators.