What is DC Drive?
DC drive technology is efficient, dependable, low-cost, user-friendly, and relatively simple to implement. DC drives have numerous advantages over AC drives, particularly in regenerative and high-power applications. DC drives have become popular in industrial drive applications due to their ability to provide extremely precise control.
Block Diagram of D.C. Drive
Working Principle:
The speed of a direct current motor is proportional to the armature voltage and inversely proportional to the field current. In addition, the armature current is proportional to the motor torque. As a result, changing the applied voltage changes the speed of the motor. However, this is possible up to the rated voltage. If a speed greater than the base speed is required, the field current of the motor should be reduced.
By reducing the field current, the flux in the motor decreases. Decreasing the field current reduces the armature counter EMF. The lower the counter-armature EMF, the more armature current flows. In addition, this armature current increases motor torque, and therefore speed. These are the two basic principles used in DC drives to control the speed of the motor.
On armature-controlled DC drives, the drive unit provides rated current and torque at any speed between zero and the bottom of the motor. By varying the armature voltage, a variable speed can be obtained as shown in the figure.
Typically, these DC drives provide a fixed field distribution. Since the torque is constant (describing a load type) at the speed range, the motor output is proportional to the horsepower (HP = T × N / 525). The motor features of this drive are shown below.
In the case of armature and field controlled drives, the armature voltage to the motor is regulated for continuous torque-variable HP operation up to the basic speed of the motor. Additionally, for the base speed operation mentioned above, the drive switches to field control for continuous HP-reduced torque operation at maximum speed, as shown in the figure below. In this case, the decrease in field current causes the motor to accelerate to its maximum speed as shown in the figure.
Components of D.C Drive
The components of D.C drive system are
- D.C Drive input
- Rectifier Bridge
- Field Supply Unit
- Speed Regulation Unit
- Firing Circuit
DC drive input :
Some thyristor-based DC drives operate on a single-phase supply and use four thyristors for full-wave correction. For larger motors, a three-phase power supply is required because the waveforms are much smoother. In such cases, six thyristors are required for full-wave correction.
Rectifier Bridge:
The power component of a controlled DC drive is a full-wave bridge rectifier that can be driven through a three-phase or single-phase distribution. As mentioned above, the number of thyristors depends on the supply voltage.
The six-thyristor bridge (in the case of a three-phase converter) adjusts the incoming AC supply to the DC supply to the motor armature. The firing angle control of these thyristors varies to the voltage motor.
Field Supply Unit:
The force to be applied on the field winding is much less than the armature power, so it often provides a single phase supply. A special thyristor bridge or diode rectifier is used to power the field winding of the motor.
In many cases, a two-phase supply is obtained from the three-phase input (which supplies power to the armature) so the field exciter is included in the armature supply unit.
The function of the field supply unit is to apply a constant voltage to the field winding to create a constant field or flux for the motor. In some cases, thyristors are supplied to this unit to reduce the voltage applied to the field to control the speed of the motor above the base unit.
In the case of fixed magnet DC motors, the field distribution unit is not included in the drive.
Speed Regulation Unit:
It compares the operator instruction (required speed) with the feedback signals and sends the appropriate signals to the firing circuit. On analog drives, this regulator unit contains the voltage and current regulators. The voltage regulator accepts the speed error as input and produces a voltage output that is applied to the current regulator.
The current regulator generates the required firing current to the firing circuit. If more speed is required, additional current is called in from the voltage regulator, so the thyristors run longer. In general, this control (voltage and current) is performed using proportional-integral-derivative controllers.
Field current regulators are also provided where speeds greater than the base speed are required.
On modern digital microprocessor-based drives, speed control is achieved using an additional digital circuitry with a lookup table to determine the current for the firing circuit.
Firing circuit:
It supplies gait pulses to thyristors so that they can be turned on at specific intervals to produce variable armature voltage. Isolation is also provided in this gate drive circuit.
Advantages and Disadvantages of DC Drive
Advantages
- Less complex and less expensive
- Adjustable speed
- DC motors can deliver starting and accelerating torques in excess of 400 percent of their rated capacity.
- Cooling blowers and inlet air flanges supply cooling air over a wide speed range while maintaining constant torque.
- Flanges and kits for mounting feedback tachometers and encoders as accessories.
Disadvantages
- The presence of a mechanical commutator that limits the maximum power rating and speed