Mechanical Coupling of Two Motor Drives

In order to test a motor drive under various load conditions, you must provide a variable and bidirectional load at the motor shaft. Moreover, an ideal load should also allow returning the absorbed energy from the motor back to the power grid as electric energy. Such a load can be implemented using a four-quadrant motor drive such as the DC2 or DC4 models. Either of these two motor drives can be conveniently coupled to the motor drive model being tested by the use of the mechanical shaft model.
Therefore this case study will consist of coupling the AC4 motor drive model to the DC2 motor drive. The AC4 motor drive is a DTC three-phase induction motor-based drive. The DC2 motor drive is a single-phase dual-converter DC motor drive. In such a system, one drive is speed regulated while the other is torque regulated, but each drive can operate either as a motor or as a generator, as will be seen later. The DC2 motor drive is rated 3 hp, 240 V, 1800 rpm, and the AC4 motor drive is rated 3 hp, 380 V, 60 Hz, 4 poles.

Note   It is also possible to couple two motor drives using the Mechanical input menu located in the lower part of the GUI. The next figure indicates how to model a stiff shaft interconnection in a motor-generator configuration. The speed output of drive 1 (mechanical input is load torque) is connected to the speed input of drive 2 (mechanical input is motor speed), while drive 2 electromagnetic torque output Te is applied to the mechanical torque input Tm of drive 1. The Kw factor represents the speed reduction ratio. Also, because inertia J2 and viscous friction F2 are ignored in the machine of drive 2, they have to be added in the machine tab of drive 1.

System Description

The complete system consisting of two motor drives mechanically coupled together is shown in SPS Diagram of the Two Interconnected Drives. The mechanical shaft model is contained in the third block of the diagram. If you open this block, you will see, as in Interconnections of the Mechanical Shaft Model, that the AC4 and DC2 motor speed signals are connected respectively to the Nm and Nl inputs of the mechanical shaft model. The output Tl of the mechanical shaft model represents the mechanical torque transmitted from the AC4 motor to the DC2 generator. Therefore, this output is connected directly to the mechanical torque input of AC4, and is also sign inverted and then connected to the mechanical torque input of DC2, as can be seen in  SPS Diagram of the Two Interconnected Drives.

SPS Diagram of the Two Interconnected Drives

Interconnections of the Mechanical Shaft Model