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Mechanical Coupling of Two Motor Drives

Introduction

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

Speed Regulated AC4 with Torque Regulated DC2

To begin with, the AC4 model operates as a speed regulated motor loaded by the DC2 model operating as a torque regulated generator. This setup, contained in the cs_coupling_1 file, allows the testing of the AC4 model speed ramps and load torque disturbance responses. Note that in steady state, the signs of the AC4 electric torque and speed should be the same, confirming that AC4 operates as a motor. The DC2 electric torque and speed should be of opposite signs, confirming that DC2 operates as a generator. This is in line with the sign of the reference torque applied to the DC2 motor drive that is opposite to the speed sign.

Speed Ramp and Load Disturbance Torque Responses of the AC4 Motor Drive shows the results of an AC4 motor drive startup at nearly full load followed by the application of load disturbance torques. You can see that the AC4 motor speed is exactly superposed to the reference ramp of +400 rpm/s since the AC4 electric torque maximum limit is high enough. The AC4 motor speed reaches the demanded value of 400 rpm at t = 1.0 s. At that moment, the AC4 electric torque drops down to 10 N.m. Then at t = 1.4 s, a reference torque of 0 N.m is applied to DC2; the AC4 electric torque immediately drops down to zero in order to maintain the regulated speed. At t = 1.9 s, a reference torque of +10 N.m is applied to the DC2 drive, forcing AC4 to operate as a generator and DC2 as a motor (look at the speed and torque signs of the two drives). Finally, a negative reference speed ramp of -400 rpm/s is applied to AC4 at t = 2.3 s. Note that, again, AC4 precisely follows the demanded ramp. A new steady state is reached at t = 2.8 s, and the AC4 electric torque stabilizes at -10 N.m. Speed Ramp and Load Disturbance Torque Responses of the AC4 Motor Drive also shows the mechanical torque transmitted by the shaft, which is similar to the AC4 electric torque but contains less ripple.

Speed Ramp and Load Disturbance Torque Responses of the AC4 Motor Drive

Torque Regulated AC4 with Speed Regulated DC2

This time, AC4 operates as a torque regulated motor loaded by the DC2 drive that is speed regulated. The complete system is shown in SPS Diagram of the Two Interconnected Drives and is contained in the cs_coupling_2 file. The interconnection of the mechanical shaft model with the two drives remains unchanged with respect to Interconnections of the Mechanical Shaft Model. All the regulator gains of both drives are the same as in the previous case. The setup is tested in the same conditions as before.

SPS Diagram of the Two Interconnected Drives

Speed Ramp and Load Disturbance Torque Responses of the DC2 Motor Drive shows the results of a DC2 motor drive startup at nearly full load followed by the application of load disturbance torques. Note that the DC2 motor speed follows the reference ramp of 400 rpm/s with some overshoot and undershoot. The DC2 motor speed reaches the demanded value of 400 rpm at t = 1.0 s and stabilizes completely at t = 1.2 s. Then at t = 1.4 s, a reference torque of 0 N.m is applied to AC4; observe how fast the AC4 torque responds. At t = 1.9 s, a reference torque of +10 N.m is applied to the AC4 drive, forcing DC2 to operate as a generator and AC4 as a motor (look at the speed and torque signs of the two drives). Observe that the DC2 speed overshoots each time the load torque changes. Finally, a negative reference speed ramp of -400 rpm/s is applied to DC2 at t = 2.3 s. The DC2 speed follows well but presents a small overshoot and a small undershoot. A new steady state is reached at t = 2.8 s, and the DC2 electric torque stabilizes at -10 N.m. Speed Ramp and Load Disturbance Torque Responses of the DC2 Motor Drive also shows the mechanical torque transmitted by the shaft, which is very similar to the negative of the DC2 electric torque but with more ripple.

You can see from the results shown in Speed Ramp and Load Disturbance Torque Responses of the AC4 Motor Drive and Speed Ramp and Load Disturbance Torque Responses of the DC2 Motor Drive that the speed ramp responses are more precise and the load torque disturbance more efficiently rejected with the AC4 drive than with the DC2 drive. This is essentially due to the fast dynamics of the AC4 electric torque. Recall that the AC4 drive consists of a direct torque controller based on hysteresis comparators and high-frequency switching, while the DC2 drive relies entirely on naturally commutated thyristor converters. However, the torque ripple magnitude of the AC4 drive is higher than for the DC2 drive.

Speed Ramp and Load Disturbance Torque Responses of the DC2 Motor Drive

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