DC Machine - Implement separately excited DC machine

Library

Machines

Description

The DC Machine block implements a separately excited DC machine.

An access is provided to the field terminals (F+, F-) so that the machine model can be used as a shunt-connected or a series-connected DC machine. The torque applied to the shaft is provided at the Simulink input T_L.

The armature circuit (A+, A-) consists of an inductor La and resistor Ra in series with a counter-electromotive force (CEMF) E.

The CEMF is proportional to the machine speed.

K_E is the voltage constant and ω is the machine speed.

In a separately excited DC machine model, the voltage constant K_E is proportional to the field current I_f:

where L_af is the field-armature mutual inductance.

The electromechanical torque developed by the DC machine is proportional to the armature current Ia.

where K_T is the torque constant. The sign convention for T_eand T_Lis

The torque constant is equal to the voltage constant.

The armature circuit is connected between the A+ and A- ports of the DC Machine block. It is represented by a series Ra La branch in series with a Controlled Voltage Source and a Current Measurement block.

Mechanical part:

The field circuit is represented by an RL circuit. It is connected between the F+ and F- ports of the DC Machine block.

The mechanical part computes the speed of the DC machine from the net torque applied to the rotor. The speed is used to implement the CEMF voltage E of the armature circuit.

The mechanical part is represented by Simulink blocks that implement the equation

where J = inertia, B_m = viscous friction coefficient, and T_f = Coulomb friction torque.

Dialog Box and Parameters

Configuration Tab

Preset model

Provides a set of predetermined electrical and mechanical parameters for various DC machine ratings of power (HP), DC voltage (V), rated speed (rpm), and field voltage (V).

Select one of the preset models to load the corresponding electrical and mechanical parameters in the entries of the dialog box. Select No if you do not want to use a preset model, or if you want to modify some of the parameters of a preset model, as described below.

When you select a preset model, the electrical and mechanical parameters in the Parameters tab of the dialog box become unmodifiable (grayed out). To start from a given preset model and then modify machine parameters, you have to do the following:

Select the desired preset model to initialize the parameters.
Change the Preset model parameter value to No. This will not change the machine parameters. By doing so, you just break the connection with the particular preset model.
Modify the machine parameters as you wish, then click Apply.

Mechanical input

Allows you to select either the torque applied to the shaft or the rotor speed as the Simulink signal applied to the block's input.

Select Torque TL to specify a torque input, in N.m, and change labeling of the block's input to TL. The machine speed is determined by the machine Inertia J and by the difference between the applied mechanical load torque TL and the internal electromagnetic torque Te. The sign convention for the mechanical torque is the following: when the speed is positive, a positive torque signal indicates motor mode and a negative signal indicates generator mode.

Select Speed w to specify a speed input, in rad/s, and change labeling of the block's input to w. The machine speed is imposed and the mechanical part of the model (Inertia J) is ignored. Using the speed as the mechanical input allows modeling a mechanical coupling between two machines and interfacing with SimMechanics and SimDriveline models.

The next figure indicates how to model a stiff shaft interconnection in a motor-generator. The speed output of machine 1 (motor) is connected to the speed input of machine 2 (generator), while machine 2 electromagnetic torque output Te is applied to the mechanical load torque input TL of machine 1. The Kw factor takes into account speed units of both machines (rad/s) and gear box ratio. The KT factor takes into account torque units of both machines (N.m) and machine ratings. Also, as the inertia J2 is ignored in machine 2, J2 referred to machine 1 speed must be added to machine 1 inertia J1.

Parameters Tab

Armature resistance and inductance [Ra La]: The armature resistance Ra, in ohms, and the armature inductance La, in henries.
Field resistance and inductance [Rf Lf]: The field resistance Rf, in ohms, and the field inductance Lf, in henries.
Field armature mutual inductance Laf: The field armature mutual inductance, in henries.
Total inertia J: The total inertia of the DC machine, in kg.m².
Viscous friction coefficient Bm: The total friction coefficient of the DC machine, in N.m.s.
Coulomb friction torque Tf: The total Coulomb friction torque constant of the DC machine, in N.m.
Initial speed: Specifies an initial speed for the DC machine, in rad/s, in order to start the simulation with a specific initial speed. To start the simulation in steady state, the initial value of the input torque signal T_L must be proportional to the initial speed.

Advanced Tab

Sample time (-1 for inherited): Specifies the sample time used by the block. To inherit the sample time specified in the Powergui block, set this parameter to -1.

Inputs and Outputs

TL

The block input is the mechanical load torque, in N.m.

w

The alternative block input (depending on the value of the Mechanical input parameter) is the machine speed, in rad/s.

m

The output of the block is a vector containing four signals. You can demultiplex these signals by using the Bus Selector block provided in the Simulink library.