Ward-Leonard Motor Generation System

Introduction:
A control system generally controls, regulates, and updates its output continuously based on present inputs, nature of the system, and the past outputs. In other words, there is a feedback mechanism that is inherent in the system, and is called a closed loop control system. For example, adding an emitter resistance to a common emitter amplifier, results in a negative feedback mechanism in the amplifier system – if the output current increases, the input voltage decreases accordingly so as to reduce the output current; if the output current decreases, then the input voltage increases in order to raise the output current level. Negative feedback thus ensures that the output is always controlled to stay within an optimal range. Similarly other systems may exhibit a positive feedback mechanism. In either case, a control system can be represented as follows: Figure 1: Control System representation
In Figure 1, G represents the forward loop function and Fb represents the feedback function. In case the Fb function is absent, the system is reduced to an open loop system.

The most popular representation of control systems is using the ‘Block Diagram’ approach (Balakrishnan, 1988). Each component in the system is represented as a block, with its role represented as a mathematical function. From the block diagram representation, the overall function of the system, called the transfer function, is obtained. The transfer function of a system is defined as (O/p(s))/(I/p(s)), where O/p(s) and I/p(s) denote the output and input functions of the system respectively, denoted in Laplace form. It is important to note that for Transfer system analysis, the initial conditions need to be zero. Further, the transfer function model does not support analysis of Multiple Input Multiple Output (MIMO) systems, where modelling becomes very complicated.

The disadvantages associated with the transfer function model led to the state space model, where the system is represented by means of the smallest number of variables associated with the system, which can completely describe it at any instant. For a linear, time invariant system, the general state space representation is as follows: x ̇(t)=A x(t)+B u(t) y(t)=C x(t)+D u(t)
Where x(t)is the state vector (column vector) that comprises the state variables x_1 (t), x_2 (t) and so on; u(t) is the input vector, and y(t) is the output vector; and x ̇(t)represents the differentiation of each component of the sate vector, with respect to time (Kamaraju, Narasimham, 2009). All automated systems are control systems with complex feedback and regulatory mechanisms. The design of such systems involves a clear understanding of representation of various parameters associated with the system, how they affect the outputs, and the effect of presence of any external disturbances to the system performance. These parameters contribute to the stability of the system, which is the most important design factor.

Essentially, the physical components in a control system depend on the nature of the system, in terms of functionality and stability, and in terms of its surroundings. Depending on the nature of the components in the system, the system can be classified as electrical, mechanical, electromechanical, digital, and other control systems. For example, a complex system consisting of motors and other electrical drives to carry our electrical operations, using mechanical components such as shafts can be termed as an electromechanical system. Most of the motion control systems are electromechanical systems. An important part of a motion control system, is speed control. Unlike fully mechanical or hydraulic speed controls, electric speed controls are capable of varying the speed of the electric motor itself. Majority of electrical variable speed drives, also known as VSDs are compatible with the 3-phase AC power supply. The two most popular electrical VSDs that were originally designed as an AC commutator motor and DC motor generator were the Schrage motor and the Ward-Leonard motor respectively (Gottlieb, 1994).

Schrage motor
A Schrage motor has the primary winding on its rotor. Movable brushes accomplish the task of variable speed control by their relative position, using a servo-motor or a hand wheel. This type of motor has become redundant – it is too expensive and complicated for use in today’s technologically advanced world which has micro chips that can perform similar functions (Tooley, 2010).

Ward-Leonard Motor
The basic principle behind designing a DC VSD is that the speed of an independent DC motor is directly proportional to the armature voltage of the motor. As per the original design, the Ward-Leonard DC motor has an AC induction motor; a DC generator; and a DC motor. The induction motor has a fixed speed, and it drives the excited DC generator, which feeds a varying voltage to the DC motor (shunt wound), resulting in a variable speed drive (DC). The overall working of the system is as shown in Figure 2.

Figure 2: Ward-Leonard motor generator system

The motor generator set consists of a motor M2, which has a fixed speed, and a generator G as shown in fig. y. The motor-generator combination supplies the required variable armature voltage, which in turn controls the speed of the motor M1. The input to the main motor M1 is fed from the output (voltage) of the generator. The magnitude and the direction of the voltage generated can be controlled using a filed regulator and a reversing switch respectively. This in turn affects the magnitude of speed and direction of rotation in M1.

A ward-Leonard motor-generator system is typically used in high sensitivity applications, and when a wide range of speed magnitudes are required. For example, it is commonly used in hoists and elevators. Since it used field control (which is a vector based control), the output rotation can be in both directions as required. However, a major disadvantage associated with the system in Figure 2 is that two extra machines are required, which increase the capital cost involved. Further, the output machine is generally large and bulky – hence the system is not very easy to handle (Sivanagaraju et al., 2009).
The next step was to modify the system for better stability, ensure protection against overload conditions and power fluctuations. These objectives led to a modified Ward Leonard system, also popularly known as Ward-Leonard-Ilgner system, where a flywheel is used to reduce fluctuations in the supply circuit, and to protect against overload as follows: The flywheel stores energy during optimal operation. When there is a sudden increase in the load, more current is drawn from the generator, resulting in a jump or fluctuation in the power supply mains. However, the presence of the flywheel avoids such a scenario, by providing the required kinetic energy from its stored energy, so as to balance the needs of the load, without resulting in a fluctuation in the supply. Further, the feedback mechanism ensures that the system is relatively more stable than in the original design (Toncich, 1994).

Literature Survey:
The earliest practical application of the Ward Leonard System can be traced back to the late 19th century. At the time, a lot of effort and research was going on in the improvement in the functioning and efficiency of Electric elevators. The Otis Electric Company, co founded by the Otis brothers and General Electric in 1892, was started to make electric elevator motors based on Eickemeyer’s electric motor design, by obtaining the rights to the patents of the same. However a major drawback of electric elevators during this period was the fact that most elevator motors were essentially single speed motors. This was hugely disadvantageous as the elevator’s speeds could not be altered while leaving or reaching a landing, whereas ideally, the elevator would need to slow down during these conditions.
Otis Electric Co. was greatly benefitted by the breakthroughs achieved by Ward Leonard, an electrical engineer from MIT, in this field. Ward Leonard developed a control system which used a constant shunt field at the electric motor, and could hence control the motor speeds by varying the field currents. Thus the Ward Leonard System could be used by elevator operators to indirectly control the elevator motor’s speeds by varying the generator field resistance to control the armature current in the motor (Gray, 2002).

The next direct application of the Ward Leonard system came in the form of electric railways. In the late 19th century, electric trains were controlled by the Sprague series motor founded by Frank Sprague, a former navy officer. The Sprague motor was a constant speed, non sparking motor with fixed bushes. These motors were primarily used in Electric street railway systems, and subsequently in more diverse applications such as cable cars.
Ward Leonard however possessed the acumen to realize that the electric railway system in America would undergo monumental expansion in the coming years, and believed the Ward Leonard Control System was better suited for the same.

One of the major drawbacks of the Sprague series motor was the huge current drawn during starting of the electric streetcars, and also while crawling up gradients or slopes. This resulted in huge fluctuations in the loading of the power stations servicing these streetcars. Another reason for the inefficiency of the Sprague series motor was the power losses from the rheostats used during the starting of the streetcars. It was generally believed by many engineers during this time that variable transmission ratios between the motor and the axles of the electric streetcars would help in significantly reducing the power losses during the starting of the streetcar, as well as during motion in low speeds.

Ward Leonard proposed the use of his variable speed motors to control electric trains. He reasoned that the efficiency of electric motors could be increased by varying the torques and speeds of the streetcars in different operating conditions. For example, during starting, a high torque would be supplied to the axle of the streetcar, without creating a high demand from the power station, thanks to the variable speed motor. He also argued that his control system would lead to regeneration of current whenever trains decelerated or moved down gradients. He concluded that, from a holistic point of view, taking into account the savings in power in the power stations, his system would be more economical than the existing Sprague system. This sparked off a debate between Ward Leonard and Frank Sprague, and even though Leonard successfully exhibited the working of his control system in a working model during a paper presentation in 1894 (Duffy, 2008).

In the 20th century, the application of the Ward Leonard System was extended to the field of mining. Around 1925, machine shovels powered by steam were replaced by electrically powered shovels. These electrical shovels were driven by the Ward Leonard Control System. One of the major drawbacks and difficulties associated with the electric steam shovels was the presence of mechanical brakes. Mechanical brakes are inefficient in comparison to electrical braking systems and also needed more complex design and construction of the shovels for their accommodation. The Ward Leonard drive made the use of mechanical brakes in shovels redundant, and used the motor as a brake instead. Even though the Ward Leonard system functioned slower than the conventional Steam based system, its higher efficiency and growing research in electrical systems during the 20th century helped the Ward Leonard system become preferable system to operate excavators.

Also, in the early 1900s many South African factories which were functioning based on the Ward Leonard system were concerned about their factories becoming outdated and less sophisticated. Most of these companies tried to become more modern by digitalizing their existing systems as they felt that this would be a cheaper alternative to adopting completely new technology. The digitalization of the systems was done by using thyristor bridges to replace the exciter generator. Two PID controllers are also used in the system for achieving variable speed control. One is to control the DC motor armature voltage and another is used to control the EMF of the motor. Other than this Ward Leonard systems have also been extensively used in Ice Breaker Ships and Fast Breeder Nuclear Reactors (Ylitolva, 2011).

Closed Loop Control of Ward Leonard Systems is being used in recent times and leading electronic companies like Siemens have developed products like the SINAMICS DCM DC Converter for the same. The main aim of these DC converters are to facilitate the closed loop control of a motor generator block by controlling the generator field, armature current, and motor speed. The motor field can also be controlled by using an additional SINAMICS DCM. Three closed loop control loops are used to control the generator field, generator current and the motor speed.

REFERENCES:

Balakrishnan, A.V. (1988) State Space Theory of Systems: An Introduction, New Delhi: Optimization Software

Bay, J.S. (1999) Fundamentals of Linear State Space Systems, New York: McGraw Hill College

Duffy, M.C. (2008) Electric Railways 1880-1990, London: The Institution of Engineering and Technology

Gottlieb, I. (1994) Electric Motors and Control Techniques, New York: McGraw Hill Professional

Gray, L.E. (2002) From Ascending Rooms to Express Elevators: A History of the Passenger Elevator in the 19th Century, U.S.: Edwards Brothers

Kamaraju, V., Narasimham, R.L. (2009) Linear Systems: Analysis And Applications, New Delhi: I.K. International Publishing House.

Shinners, M. (1998) Modern Control System Theory and Design, New York: John Wiley & Sons

Sivanagaraju, S., Reddy, M.B., Prasad, A.m. (2009) Power Semiconductor Drives, New Delhi: PHI Learning Pvt. Ltd.

Toncich, D.J. (1994) Computer Architecture and Interfacing to Mechatronic Systems, Brighton: Chrystobel Engineering.

Tooley, M. (2010) Plant and Process Engineering 360°, Oxford: Elsevier

Ylitolva, K. (2011) Ward Leonard Control System, Thesis: Department of Electrical Engineering, Central Ostrobothnia University of Applied Sciences