Speed control of separately excited DC motor

American Journal of Applied Sciences, March, 2008 by Moleykutty George

INTRODUCTION

Direct current (DC) motors have been widely used in many industrial applications such as electric vehicles, steel rolling mills, electric cranes, and robotic manipulators due to precise, wide, simple, and continuous control characteristics. Traditionally rheostatic armature control method was widely used for the speed control of low power dc motors. However the controllability, cheapness, higher efficiency, and higher current carrying capabilities of static power converters brought a major change in the performance of electrical drives. The desired torque-speed characteristics could be achieved by the use of conventional proportionalintegral-derivative (PID) controllers. As PID controllers require exact mathematical modeling, the performance of the system is questionable if there is parameter variation. In recent years neural network controllers (NNC) were effectively introduced to improve the performance of nonlinear systems. The application of NNC is very promising in system identification and control due to learning ability, massive parallelism, fast adaptation, inherent approximation capability, and high degree of tolerance.

A constant-power field weakening controller based on load-adaptive multi-input multi-output linearization technique has been proposed to effectively operate a separately excited dc motor in the high-speed regimes (1). A single-phase uniform PWM ac-dc buck-boost converter with only one switching device able to produce a controllable dc voltage ranging from zero to more than the maximum value of input ac voltage has been used for armature voltage control method of a separately excited dc motor (2). In (3) a general simulation method based on an estimation of the average value of voltages and currents on each PWM period, to improve the simulation speed has been proposed, analyzed and tested for an efficient computation of the torque-speed characteristics of the drives using poly-phase brushless DC motors fed by a PWM inverter with current regulation. An open loop control system which can predict the dynamic behavior of systems involving mechanic and electronic modules has been successfully designed and implemented to control the speed of a DC motor (4). Several other speed control techniques using conventional controllers have been reported in (5), (17). Recently, the superior performance of artificial intelligence (AI) based controllers urged power system and power electronic engineers to replace conventional speed control circuit with intelligent speed controllers (18)-(30).

In this paper, NARMA-L2 controller has been proposed for the speed control of separately excited dc motor in the constant torque region. The novelty of this paper lies in the application of NARMA-L2 controller for the speed control of separately excited dc motor. This paper also discusses speed control of a SEDM using chopper circuit. The speed control techniques of SEDM are detailed in the second part of this paper. Simulation results in the third part demonstrate the successful application of NARMA-L2 controller to control the speed of a separately excited dc motor.

Speed control techniques of separately excited dc motor: The speed of a separately excited dc motor could be varied from zero to rated speed mainly by varying armature voltage in the constant torque region. Whereas in the constant power region, field flux should be reduced to achieve speed above the rated speed. The motor drives a mechanical load characterized by inertia J, friction coefficient [D.sub.m], and load torque TL. The specifications of the dc motor are detailed as follows:

Shaft power                       - 5 hp
Rated voltage                     - 240 V
Armature resistance               - 0.6 [omega]
Armature inductance               - 0.012 H
Field resistance                  - 240 [omega]
Field inductance                  - 120 H
Total inertia (J)                 - 1 [kgm.sup.2]
Viscous friction coefficient (B)  - 0.02 Nms
Coulomb friction torque (Tf)      - 0 Nm

a) Modeling and control of SEDM using MATLAB/SimPowerSystems: Fig. 1 shows the speed control circuit of an armature controlled separately excited dc motor using chopper circuit, and in Fig. 2 its MATLLAB/SimPowerSystems model (31) is shown. It consists of a separately excited dc motor fed by a DC source through a chopper circuit. A single GTO thyristor with its control circuit and a free-wheeling diode form the chopper circuit. The motor drives a mechanical load characterized by inertia J, friction coefficient B, and load torque TL. The control circuit consists of a speed control loop and a current control loop. A proportional-integral (PI) controlled speed control loop senses the actual speed of the motor and compares it with the reference speed to determine the reference armature current required by the motor. One may note that any variation in the actual speed is a measure of the armature current required by the motor. The current control loop consists of a hysteresis current controller (HCC). The block diagram of a hysteresis current controller is shown in Fig. 3. HCC is used to generate switching patterns required for the chopper circuit by comparing the actual current being drawn by the motor with the reference current. A positive pulse is generated if the actual current is less than reference armature current, whereas a negative pulse is produced if the actual current exceeds reference current.