The generator-motor drive system (F-D system), also known as the Ward-Leonard system, is a traditional DC drive arrangement used in rolling mills. It uses a dedicated DC generator, usually driven by an AC motor, to supply adjustable DC voltage to the main DC drive motor of the rolling mill.
The system controls the output voltage of the generator by adjusting its field excitation current. This allows the speed and torque of the DC motor to be regulated over a wide operating range, supporting the low-speed entry of billets, normal rolling, and rapid reversal required by rolling mill operations.
Main components of the F-D system
The F-D system mainly consists of three electrical machines: an AC driving motor, a DC generator, and a DC motor. Excitation circuits and control equipment are also required to regulate the generator output and motor operation.
AC driving motor
The AC driving motor acts as the prime mover for the generator. It is usually a three-phase induction motor or synchronous motor and runs at an approximately constant speed.
Its main function is to mechanically drive the DC generator. By maintaining a stable generator speed, the system provides a consistent basis for adjusting the generator output voltage through excitation control.
DC generator (G)
The DC generator is driven by the AC motor and supplies DC power to the armature circuit of the rolling mill's main DC motor.
Its output voltage depends on the generator speed and magnetic field strength. By changing the generator field current (Ifg), the control system adjusts the magnetic flux and changes the generated voltage (Ug).
This arrangement allows a relatively small excitation current to regulate the electrical power supplied to the main drive motor. Depending on the configuration, the system may also include an auxiliary exciter to provide power to the field circuits.
DC motor (D)
The separately excited DC motor receives adjustable DC voltage from the generator and converts electrical energy into mechanical power.
In a rolling mill, this motor drives the main rolling stand through the transmission system, including the gearbox and connecting shafts. Its speed and output torque depend mainly on the armature voltage, armature current, and field excitation.
Operating principle of the F-D system
The main control principle is to regulate the excitation current of the DC generator. This changes its output voltage and, in turn, controls the speed of the DC motor.
Speed control through generator excitation
The generator field current (Ifg) determines the strength of the generator's magnetic field. With generator speed held constant, changing the field current changes the generated voltage (Ug).
When the generator field current increases in the same excitation direction, the generated voltage generally increases until magnetic saturation becomes significant. The higher armature voltage supplied to the DC motor allows its speed to increase.
For a separately excited DC motor, the approximate speed relationship is:
n \approx \frac{U_g-I_aR_a}{C_e\Phi_m}
Where:
(n) is the motor speed.
(U_g) is the generator output voltage applied to the motor armature.
(I_a) is the motor armature current.
(R_a) is the motor armature circuit resistance.
(C_e) is the motor back EMF constant.
(\Phi_m) is the motor field flux.
When the motor field flux remains constant, its speed is mainly controlled by changing the generator output voltage. The equation neglects brush voltage drop and other minor losses.
Armature voltage regulation and field weakening
The F-D system uses two main methods to control the speed of the DC motor.
Armature voltage control
Below the motor's base speed, the motor field current is generally maintained at its rated value. The generator field current is adjusted to change the generator output voltage from zero to its rated value.
As the armature voltage increases, motor speed rises. With appropriate armature current regulation, the motor can maintain approximately constant torque throughout this speed range.
This method provides smooth, stepless speed adjustment and is suitable for the low-speed and normal rolling conditions of a rolling mill.
Motor field weakening
When the motor needs to operate above its base speed, the armature voltage is generally maintained at its rated value while the motor field current is reduced.
Reducing the field flux increases motor speed, but the available torque decreases as speed rises. This operating region is commonly referred to as constant-power operation, provided the armature current remains within its rated limit.
It is important to distinguish these two methods: generator excitation controls the motor armature voltage, while motor field excitation controls the magnetic flux and enables operation above base speed.
Torque control under changing rolling loads
The electromagnetic torque of a separately excited DC motor is proportional to its armature current and field flux:
T_e=C_t\Phi_m I_a
Where:
(T_e) is the electromagnetic torque.
(C_t) is the motor torque constant.
(\Phi_m) is the motor field flux.
(I_a) is the armature current.
During rolling, the load on the motor changes with the deformation resistance of the material and the rolling conditions.
When the rolling load suddenly increases, motor speed may drop temporarily. The resulting reduction in back EMF allows armature current to increase, which raises the motor's electromagnetic torque when the field flux remains constant.
This increase in torque helps the motor respond to the higher rolling load. However, the response is limited by the motor's rated current, overload capacity, and protection settings. If the load exceeds the available torque for too long, the motor may slow down excessively or stall.
The control system can also use current feedback and speed regulation to adjust the generator excitation and maintain the required operating conditions.
Operating characteristics of the F-D system
The F-D system has several operating characteristics that made it suitable for heavy-duty rolling mill drives.
Drooping mechanical characteristic
A separately excited DC motor has a mechanical characteristic in which its speed decreases slightly as load torque increases, when the applied voltage and field flux remain unchanged. This is known as a drooping mechanical characteristic.
Under certain operating conditions, this characteristic can help distribute loads between mechanically coupled or parallel drives. However, it cannot by itself guarantee load balance between rolling stands. Tandem rolling mills still require coordinated speed and current control to maintain the required operating relationship between stands.
Four-quadrant operation
The F-D system can support four-quadrant operation, covering forward motoring, forward braking, reverse motoring, and reverse braking.
The direction of the generator output voltage can be changed by reversing the generator field current. With the DC motor field direction maintained, reversing the armature voltage reverses the motor's direction of rotation.
The system can also support regenerative braking. During braking, the DC motor operates as a generator and returns electrical energy through the motor-generator system toward the supply side, depending on the drive configuration.
Four-quadrant operation requires the generator, motor, and control circuits to support the necessary voltage and current directions. It is not achieved simply by changing the polarity of the generator field without considering the complete power circuit.
Advantages and limitations of the F-D system
The F-D system was widely used in rolling mills because it provided smooth speed regulation, good overload performance, and flexible forward and reverse operation.
Advantages
Wide stepless speed range: Generator excitation allows continuous adjustment of the DC motor armature voltage. Under suitable design and operating conditions, the speed ratio can reach 20:1 or even higher.
Smooth speed regulation: The system provides controlled acceleration and deceleration, which is useful for rolling processes involving frequent speed changes.
Good overload performance: DC motors can deliver high torque during short-term overload conditions, subject to their electrical and thermal limits.
Four-quadrant operation: The system supports forward and reverse operation, as well as braking in both directions.
Low-power excitation control: A relatively small field current can regulate the generator output and control a high-power DC drive.
Limitations
Large equipment footprint: The system uses three main rotating electrical machines, requiring considerable installation space and a suitable foundation.
Energy losses: Power passes through the AC motor, DC generator, and DC drive motor, resulting in additional conversion and mechanical losses. Overall efficiency is generally lower than that of modern static converter-based drives. The actual efficiency depends on equipment ratings, loading conditions, and maintenance status.
High maintenance requirements: Rotating machines, bearings, commutators, and carbon brushes require regular inspection and maintenance.
Operating noise: The multiple rotating machines generate mechanical and electromagnetic noise.
Slower dynamic response: Compared with modern semiconductor drive systems, the rotating generator set has greater mechanical inertia and generally responds more slowly to rapid speed changes.
Current applications and replacement technologies
The F-D system was widely used in the previous generation of steel rolling mills, particularly where DC motors required a wide speed adjustment range and four-quadrant operation.
With the development of power electronic converters, rotating motor-generator sets have gradually been replaced in new and upgraded installations. Early replacements included static thyristor converters for DC motors. Modern rolling mill drive systems increasingly use AC motors with variable-frequency control.
A common configuration is the AC-DC-AC variable-frequency drive. It converts the incoming AC power into DC through a rectifier, then uses an inverter to supply variable-frequency AC power to the motor.
Compared with the traditional F-D arrangement, modern converter-based drives generally have:
Higher overall energy efficiency.
Faster dynamic response.
More precise speed and torque regulation.
Smaller installation space.
Lower routine maintenance requirements due to fewer rotating electrical components.
The specific drive solution depends on the mill's power requirements, rolling process, existing equipment, and modernization budget. F-D systems can still be found in older rolling mills that continue to operate with their original DC drive equipment.
