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The identification and real time speed control, without reverse motion, for a series DC motor is presented. The identification is performed using the transient response analysis of the mechanical and electrical subsystems of a series DC motor. A linearized model which does not include the magnetic saturation around the operating conditions is considered. Based on this model, a PI speed controller is designed. A well-known problem arising in this type of electrical motors is the singularity at zero speed. It is shown that, in spite of this inconvenience, the PI controller, together with an antiwindup scheme, presents adequate regulation and tracking performance. It is also shown that the control system can compensate for varying loads and the counter-electromotive force with acceptable levels of current consumption.

Series DC motors, as well as series universal motors, are electric motors with one voltage supply and a field winding connected in series with the rotor winding. This series connection results in a motor with very high starting torque. However, torque decreases as the speed builds up due to an increment of the back or counter-electromotive force (EMF). This is why series DC motors have poor speed regulation. That is, increasing the motors load tends to slow its speed which in turns reduces the back EMF and increases the torque to accommodate the load. A limitation of these motors is that the sense of rotation is fixed for most of their applications. In order to change the direction of torque and rotation, it is necessary to change the polarity of the current flow.

This work is comprised of an investigation of the design of a control system which takes advantage of the high torque properties of series DC motors. By maintaining the high torque generation with low current consumption, these motors may have a greater range of applications.

Different control strategies have been applied in order to improve the speed performance of series DC motors. In [

Considering the complexity of the approaches described above, the main objective of this paper is to present a simple and effective controller for the speed control of series DC motors. That is, the paper shows a solution to a nonsolved problem based in well-known techniques suitable to an engineering context which are easy to implement and analyze, contrary to more complex methodologies whose results are limited to digital simulations. A second objective is to show a methodology for the identification of series DC motors, measuring only the motor rotor speed and the current consumption, which is likewise based on the well-known linear transient response analysis. It is shown that adequate design of PI schemes can improve the robustness and performance of these electrical machines resulting in an appropriate engineering design easy to implement and test.

The paper is organized as follows: Section

A series wound DC motor similar to the shunt wound DC motor or compound wound DC motor is a self-excited DC motor. It gets its name because the field winding is connected internally in series to the armature winding as shown in Figure

Series connection of a DC motor.

The electric diagram of the series wound DC motor is shown in Figure

Electric diagram of a series DC motor.

Based on the electric diagram of Figure

The EMF

The flux

Therefore, the differential equations of the series DC motor without magnetic saturation result in

The identification of the monophasic universal-motor Koblenz model HC8825M110 is presented. The nominal maximum speed and power are 24000 RPM and 0.815 HP.

In particular, these machines must operate loaded in order to avoid damage. Hence, a steel disc load was added as shown in Figure

Series DC motor.

From (

Experimental setup.

The experimental setup consists of a voltage source with a maximum voltage of 50 [V] and a maximum current of 3 [A], the series DC motor, and a power driver with a USB communication. The power driver was designed considering the electric demand of the motor. Hence, the power driver is based on the MOSFET 12N65, which operates with a maximum current of 12 [A] and a maximum input voltage of 650 [V]. An equally important element is the ACS711LC circuit used to monitor the power consumed by the motor.

Inductive loads together with pulsed excitation signals generate reverse currents that may damage switching elements such as MOSFETs. Although the MOSFETs used in the implementation have an internal protection diode, two transistors FR307 of rapid recovery were added in order for additional protection.

The electric diagram of the power driver is shown in Figure

Power driver.

From (

The steady state gain and the steady state time of

Current step response (rotor locked).

The mutual inductance

Rearranging (

In Figures

Current step response.

Rotor’s speed step response.

From (

Figure

Estimating the rotor inertia ^{2} was closer to the actual response of the system.

The résumé of the estimated parameters of the series DC motors is shown in the following list:

^{2}.

A comparison of the responses of the real motor with that of the nonlinear model is shown in Figures

In [

Restructuring (

the nonlinear state space representation of the series DC motor is given by

The equilibrium point

The linear approximation of (

If the load torque is assumed zero,

These calculations lead to a model, which can be considered as a system with one input, voltage

The transfer function

The poles of the transfer function (

The design of the PI controller for the series DC motor was obtained using the validated linear representation (

One of the advantages of classical control is that stabilization, robustness, and performance of the closed-loop system are inherent to the process design. Thus, by Bode shaping the open loop systems it is possible to guarantee the design specifications. For instance, the objectives of design are bandwidth of

The PI controller

The Bode plots of the open loop control system

Bode plots of

Recalling that the series DC motors cannot reverse the direction of rotation unless the input voltage polarity is likewise reversed, the minimum output of the controller of equation (

Antiwindup PI controller.

To verify the performance of the speed control system depicted in Figure

In Figure

Rotor’s speed response to a square reference signal from 0 to 630 [rad/sec].

The range of variation of the reference signal covers the full range of operation of the control system, which, as mentioned in the experimental setup description, is limited by the maximum supply voltage of 50 [V]. The input voltage

Input voltage

Current consumption

Figure

Figures

System responses to a sinusoidal signal reference with frequency 33 rad/sec.

System responses to a sinusoidal signal reference with frequency 66 rad/sec.

Figure

Figure

However, in the periods when the speed is increased the control system presents an excellent performance.

In this experiment the control system is affected by an unknown constant torque load perturbation when the reference signal is

System responses to an unknown torque load perturbation.

A speed control system for a series DC motor based on a classical PI controller was designed and implemented. It is proved that despite the nonlinearities and the non-well-defined relative degree of this kind of motors it is possible to design classical controllers, like the PID, based on linear models resulting in control systems with excellent performance along a wide range of operation. Moreover, by means of several real time experiments it is verified that the control system has excellent performance under tracking and torque load perturbations conditions. It is also found that in order to fully exploit the capabilities of this kind of motors, high torque with low current consumption, it is necessary to improve the power driver so that it can reverse the direction of the magnetic torque and therefore the direction of rotation of rotor. The design of the controller here proposed was based on linear models obtained through an identification process based on the step response of the real motor and the nonlinear model without magnetic saturation.

The authors declare that they have no conflicts of interest.