Operating behaviour of speed variable DC machines under load

Direct current machines are no longer so popular among controlled and regulated drives. The high purchase price and maintenance costs arising due to the brushes mean these have largely been replaced by BLDC machines.

Despite the aforementioned disadvantages, this drive is still used in drive technology because of its highly dynamic control behaviour.

Objectives

• Energy requirement for starting under load
• Energy requirement for change of direction of rotation
• Behaviour of working machines
• Torque is constant as with lifts; cranes (T_L = const.)
• Torque rises lineally with speed; extruder, flexing work (T_L ~ n)
• Torque rises quadratically with speed; turbo machines, fans, pumps (T_L ~ n²)
• Torque decreases inversely proportional to speed; lathes and milling machines, winding machines.(T_L = 1/n)
• Flywheel mass

The learner will get to know the behaviour of working machines on a drive with direct current motor. In the first few experiments, the efficiency of a speed-variable drive is investigated. In later experiments, the different behaviours of working machines are presented.
Characteristics diagrams and time diagrams are recorded and analysed for evaluation purposes.

Highlights:

• The measurements are carried out with the CASSY Machine Test.
• All measurement channels are potential-free and can therefore be used freely.
• All measurements can be carried out with or without a computer.
• In order to protect against overheating, the stator windings of the direct current motor are equipped with thermal detectors.
• The direct current motor has an educationally designed terminal board with the winding configuration printed on it.

All functions of the Converter Controller CASSY and the Machine Test CASSY 0.3 can be operated quickly and directly via the display, the rotary wheel and the buttons on the unit. All settings and measurement results can be saved on the unit and quickly called up again later or simply downloaded.

In addition, the Converter Controller CASSY and the Machine Test CASSY 0.3 is fully controllable in real time via RJ45 Ethernet, W-LAN and USB-C interfaces.

These interfaces can be used by the following software:

• CASSY Lab 2 for drives and energy systems,
• MATLAB® and LabVIEW ™
• Lab Docs Editor Advanced

Several integrated servers are available in Converter Controller CASSY and the Machine Test CASSY 0.3 for the local media connection of at least four end devices simultaneously.

For more details, see product data 7735290 or 7735291 Converter Controller CASSY and the 7731900 or 7731901 Machine Test CASSY 0.3.

The equipment is equally suitable for pupil and student experiments in the laboratory with low voltage (DC, AC and three-phase current) and - with a mobile trainer - for teacher demonstrations in the classroom or lecture hall. The experiments are carried out according to the manual.

The target group is made up of commercial apprentices and students of electrical machine construction. The course offers experiments at an intermediate level and also allows for the necessary insight into machine behaviour for scientific interpretation at undergraduate level. The media connection makes the experiments suitable for demonstration in the classroom or a lecture hall.

In the following configurations there are further extended experiments performed with the educational frequency converter and a direct current parallel machine:

• E2.5.2.2 Four-quadrant operation of a direct current machine with an IGBT-H bridge circuit
• E2.6.2.1 DC servo with industrial direct current machine

Topics

• Energy efficiency of drives
• Work machines with:
• • T_L = const.
  • T_L ~ n
  • T_L ~ n²
  • T_L ~1/n
  • Flywheel mass
• real work machine