What are DC Drives? Working & Operation of DC Drives

DC drive technology is efficient, reliable, cost effective, operator friendly and relatively easy to implement. DC drive provides many advantages over AC drives, especially for regenerative and high power applications. DC drives have been widely used in industrial drive applications in order to offer very precise control.

• Also read:

What are Electrical Drives? Working & Operation of AC Drives

Of course, variable frequency drives (VFDs) and AC motors are now offering an alternative to DC drives and motors, but there are many other applications where DC drives are extensively used including crane and hoists, elevators, spindle drives, winders, paper production machines, crushers, etc. due to the advantages of DC drives.

What are DC Drives?

DC drive is basically a DC motor speed control system that supplies the voltage to the motor to operate at desired speed. Earlier, the variable DC voltage for the speed control of an industrial DC motor was generated by a DC generator.

By using an induction motor, the DC generator was driven at a fixed speed and by varying the field of the generator, variable voltage was generated. Soon after this Ward Leonard set was replaced by a mercury arc rectifier and later by thyristor converters. Nowadays, the thyristor family of devices is used widely to control the speed of the DC motor.

Components of a DC Drive?

The main components of a DC drive system are shown in figure below.DC Drive Input: Some thyristor based DC drives operate on a single phase supply and use four thyristors for full wave rectification. For larger motors, three phase power supply is needed because the waveforms are much smoother. In such cases, six thyristors are needed for full wave rectification.

Rectifier Bridge: The power component of a controlled DC drive is a full wave bridge rectifier which can be driven by three phase or single phase supply. As mentioned above the number of thyristor may vary depends on the supply voltage.

A six-thyristor bridge (in case of three phase converter) rectifies the incoming AC supply to DC supply to the motor armature. The firing angle control of these thyristors varies the voltage to the motor.

Field Supply Unit: The power to be applied to the field winding is much lower than the armature power, so, most often single phase supply is provided. A separate thyristor bridge or diode rectifier is used for supplying the power to the field winding of the motor.

In many cases a two-phase supply is drawn from the three phase input (that supplies power to the armature) and hence the field exciter is included in the armature supply unit.

The function of the field supply unit is to provide a constant voltage to the field winding to create a constant field or flux in the motor. In some cases, this unit is supplied with thyristors to reduce the voltage applied to the field so as to control the speed of the motor above the base speed.

In case of permanent magnet DC motors, the field supply unit is not included in the drive.

Speed Regulation unit: It compares the operator instruction (desired speed) with feedback signals and sends appropriate signals to the firing circuit. In analog drives, this regulator unit consists of both voltage and current regulators. The voltage regulator accepts the speed error as input and produces the voltage output which is then applied to the current regulator.

The current regulator then produces required firing current to the firing circuit. If more speed is required, additional current is called from the voltage regulator and hence thyristors conducts for more periods. Generally, this regulation (both voltage and current) is accomplished with proportional-integral-derivative controllers.

The field current regulator is also provided where speed greater than the base speed is required.

In modern digital microprocessor based drives, the speed control is achieved with a lookup table to determine the current for the firing circuit with additional digital circuitry.

Firing Circuit: It supplies the gate pulses to thyristors so that they turned ON for particular periods to produce variable armature voltage. Isolation is also provided in this gate drive circuit.

Working Principle of DC Drives

In DC motors, the speed is proportional to the armature voltage and inversely proportional to the field current. And also, the armature current is proportional to the motor torque. Therefore, by increasing or reducing the applied voltage, the speed of the motor is varied. However, it is possible up to the rated voltage. If the speed greater than the base speed is required, the field current of the motor has to be reduced.By reducing the field current, the flux in the motor reduces. The reduction of field current reduces the armature counter emf. The more armature current flows if there is less counter armature emf. Further, this armature current increases the motor torque and hence the speed. These are the two basic principles employed in DC drives to control the speed of the motor.

In armature controlled DC drives, drive unit provides a rated current and torque at any speed between zero and the base of the motor. By varying the armature voltage, variable speed is obtained as shown in figure.

Generally, a fixed field supply is provided in these DC drives. As the torque is constant (which describes a load type) over the speed range, the motor output horsepower is proportional to the speed (HP = T × N / 525). The motor characteristics of this drive are shown below.In case of armature and field controlled drives, the armature voltage to the motor is controlled for constant torque-variable HP operation up to the base speed of the motor. And for the above base speed operation, drive switches to the field control for constant HP- reduced torque operation up to maximum speed as shown in figure below. In this case, reducing the field current increases the speed of the motor up to its maximum speed as shown in figure.

Digital and Analog DC Drives

Nowadays, digital implementations have replaced analog circuitry of electric drivesystem in all forms of industrial control. Digital controllers offer greater flexibility to produce the precise control, self-tuning, and ease of interfacing with host computers and other drives. However, a basic understanding of analogue version DC drive makes less difficult to understand its digital equivalent. Let us look on both of these DC drives.

Analog DC Drives

A standard analog DC drive with speed and current control is shown in figure below. The objective of this system is to provide speed control and hence the speed reference becomes the input to the system and speed of the motor is the output of the system which is measured by the tachometer.

The working of this drive goes like this; consider that motor is running at a set speed. Now, the speed reference signal has increased to somewhat greater than the actual speed. So there will be an error speed signal at left-hand summing junction as shown in figure. This speed error indicates the required acceleration by the motor, which means the torque and hence more current.

The error is amplified by the speed controller (which is basically a speed-error amplifier) and its output is given as current input reference to the inner control system. As the current reference increases, the inner current controller drives the more current to the motor thereby extra torque is provided.The inner current loop is responsible for maintaining the zero current error between the actual motor current and current reference signal which means to make actual motor current to follow the reference current. The amplified current error signal from the current controller controls the firing angle of the bridge and hence the output voltage of the converter. The current feedback is achieved either by DC transformer or by AC transformer (with rectifier) in the main supply lines.

This entire operation is performed by a current error amplifier with a high gain. In most cases, this amplifier is of proportional plus integral control (PI) type circuit that maintains the actual and desired currents exactly equal under steady-state conditions. This current controller also limits the current through the motor by considering the minimum and maximum currents of the motor.

The outer loop provides the speed control by comparing the actual speed obtained by the DC tachogenerator with desired or required speed from the speed reference. These two inputs are fed into the speed-error amplifier, and then resulted error is amplified and applied as an input to the current controller.

The speed amplifier produces the current output proportion to the speed error. For this amplifier also a PI control is employed (by using analog electronics) in order to achieve zero steady state error. Using this, the actual speed of the motor is maintained exactly at reference speed for all loads.

Digital DC Drives

With the advancements in digital control, DC drives become more flexible and faster (due to faster response times) compared with analog drives. A schematic arrangement of digital DC drive is shown in below figure; of course it is similar to the analog scheme, but here analog circuit (analog amplifiers) is replaced by digital circuitry.A speed reference signal given as the drive’s input compared with the feedback speed in the summing circuit. If the output of the summing circuit is positive error, indicating that a speed increase is required and if it generates a negative error, indicating that a speed decrease is required (because motor is operating at faster than desired speed).

The error speed is given to the speed controller in the microprocessor which determines output voltage to operate the motor at desired speed. At the same time, current controller in the microprocessor determines the firing signals to the SCRs in the bridge converter. SCRs then convert the three phase supply to DC supply in relation to the desired speed.

This drive can operate in open loop without any feedback and can achieve a speed regulation of 5-8%. However, a speed regulation less than 5% is required in many applications. In such cases, the speed measuring/scaling unit switches to the EMF feedback measuring circuit.

This feedback circuit measures the armature voltage, scales it in proportion to the output voltage (scaling function in microprocessor) and gives to the summing circuit. Further, it is transformed into a speed error signal in speed controller.

If the speed regulation less than 1% is required, tachometer generator feedback is used. So the speed measuring/scaling circuit then switches to the tachometer feedback. This feedback achieves very precise control compared with EMF feedback.

Also for field control (above rated speeds), this drive includes a separate field exciter. A field current regulator in the microprocessor determines the voltage to the field windings by accepting the flux/field reference signal from the operator. This regulator provides the firing signals required by the field converter unit to produce the required DC voltage proportional to the speed.

What’s inside of Power Conversion make SCR DC Drives?

Silicon Controlled Rectifiers (SCRs) are widely used thyristors for large DC motor drives in its power conversion unit. An SCR conducts when a small voltage applied to its gate terminal. Its conduction continues till the starting of negative cycle and it turned OFF automatically once the voltage across the SCR goes through natural zero till next gated signal.

The purpose of using these SCRs in DC drives is to convert the fixed AC supply to variable DC supply that controls the motor speed.As discussed earlier, some SCR DC drives are supplied from single phase AC supply and use four SCRs in the form of bridge for the DC rectification. In case of high power DC drives, a three phase supply with six SCRs is used for DC rectification.

In case four quadrant operation (forward motoring, forward braking, reverse motoring and reverse braking) of the DC drive, a bridge rectifier of consisting of 12 SCRs with a three phase incoming supply is used. During each quadrant operation, SCRs are triggered at a phase angle in order to provide required DC voltage to the motor.

The connection of SCRs (for four quadrant operation of the drive) from incoming three phase AC supply to the DC output is shown in figure below. In this, the motoring SCR bridge and regeneration SCR bridge achieve the drive four quadrant operation by receiving the appropriate gate signals from (analog or digital) controller.

If the SCRs were gated with a phase angle of zero degrees, then the drive function as a rectifier which feds the full rectified rated DC supply to the motor and by varying the firing angle to the SCRs, a variable DC supply is applied to the motor.The DC output voltage waveform in relation to the AC waveform for above circuit is shown below. This average DC output voltage is obtained for 40⁰, 32⁰ and 24⁰ firing phase angles. By this way, the average output is controlled by varying the firing phase angles to the SCRs.

As the field winding also requires the regulated DC supply, only four SCRs are used in the field bridge converter. This is because field never requires a negative current and hence another set of SCRs is not required, which were used in armature for reversing the motor.

In modern DC drives, SCRs are completely replaced by MOSFETs and IGBTs in order to achieve high speed switching so that distortion to the AC incoming power and currents during switching is eliminated. Hence, the drive becomes more efficient and accurate.

As discussed in AC drives article, DC drives are also available in modular units that consisting analog and digital I/Os, multifunctional keypads, remote operator panels in addition to the software programming and configuring capabilities, from various manufacturers such as ABB, Siemens, Rockwell automation, Emerson, etc. These can be connected to the other drives or a computer host via communication cables.Programming macros of these drives enables to implement any control structure to an application. These are also capable of receiving the remote control signals from remoteprogrammable logic controllers via field bus communication systems.

Hall Effect

Brief History:

It was Prof. Edwin Hall an American physicist while experimenting in his Lab; Hall used a thin Gold foil and in 1879 discovered for the first time that an electric potential acting perpendicularly to both the current and the magnetic field. The effect has since been known as the Hall Effect. Hall discovered that charge carriers moving along the conductor experience a transverse force and tend to drift to one side and this movement of charge carriers induces a voltage on the conductor known as Hall voltage. VH

What is the Hall Effect Sensor/Device

A device which converts magnetic or magnetically encoded information into electrical signals is called HALL EFECT SENSOR. A Hall Effect device/sensor is a solid state device that is becoming more and more popular because of its many uses in different types of applications. Hall Effect devices are immune to vibration, dust and water.

A Hall effect sensor is a transducer that varies its output voltage in response to a magnetic field. Hall effect sensors are used for proximity switching, positioning, speed detection, and current sensing applications. In its simplest form, the sensor operates as an analog transducer, directly returning a voltage.

Conventional Hall effect Current Sensor :

How Hall Effect Sensor works

The Basic Principle of Hall Effect is the activation by an external magnetic field. As we are familiar that there are two important characteristics of a magnetic field.

 Flux density, (B) and polarity (North & South Poles).

When the magnetic flux density around the sensor exceeds a certain preset threshold, the sensor detects it and generates an output voltage called the Hall Voltage, VH. The following diagram shows a basic function of a Hall Effect Sensor. 

Why use the Hall effect? 

The reasons for using a particular technology or sensor vary according to the application. Cost, performance and availability are always considerations. The features and benefits of a given technology are factors that should be weighed along with the specific requirements of the application in making this decision.

 General features of Hall effect based sensing devices are:

 • True solid state

 • Long life (30 billion operations in a continuing keyboard module test program) 

• High speed operation - over 100 kHz possible

 • Operates with stationary input (zero speed)

 • No moving parts • Logic compatible input and output

 • Broad temperature range (-40 to +150°C) • Highly repeatable operation

What are Electrical Drives? Working & Operation of AC Drives

Electrical drives are integral part of industrial and automation processes, particularly where precise control of speed of the motor is the prime requirement. In addition, all modern electric trains or locomotive systems have been powered by electrical drives. Robotics is another major area where adjustable speed drives offer precise speed and position control.

Even in our day-to-day lives, we can find so many applications where variable speed drives (or adjustable speed drives) have been using for fulfilling a wide range of functions including control of electric shavers, computer peripheral control, automatic operation of washing machines, and so on.

What is an Electric Drive? Why It is Needed?

A drive operates and controls the speed, torque and direction of moving objects. Drives are generally employed for speed or motion control applications such as machine tools, transportation, robots, fans, etc. The drives used for controlling electric motors are known as electrical drives.

The drives can be of constant or variable type. The constant speed drives are inefficient for variable speed operations; in such cases variable speed drives are used to operate the loads at any one of a wide range of speeds.

Why Electrical Drives are needed?

The adjustable speed drives are necessary for precise and continuous control of speed, position, or torque of different loads. Along with this major function, there are many reasons to use adjustable speed drives. Some of these include

• To achieve high efficiency: Electrical drives enable to use wide range of power, from milliwatts to megawatts for various speeds and hence the overall cost of operating the system is reduced
• To increase the speed of accuracy of stopping or reversing operations of motor
• To control the starting current
• To provide the protection
• To establish advanced control with variation of parameters like temperature, pressure, level, etc.

The advancement of power electronic devices, microprocessors and digital electronics led to the development of modern electric drives which are more compact, efficient, cheaper and have higher performance than bulky, inflexible and expensive conventional electric drive system that employs multi-machine system for producing the variable speed.

Block Diagram of an AC Electric Drive

The components of a modern electric drive system are illustrated in below figure.In the above block diagram of an electric drive system, electric motor, power processor (power electronic converter), controller, sensors (e.g PID Controller) and the actual load or apparatus are shown as the major components included in the drive.

The electric motor is the core component of an electrical drive that converts electrical energy (directed by power processor) into mechanical energy (that drives the load). The motor can be DC motor or AC motor depends on the type of load.

• Also read: What is Motor Efficiency & How to improve it?

Power processor is also called as power modulator which is basically a power electronic converter and is responsible for controlling the power flow to the motor so as to achieve variable speed, reverse and brake operations of the motor. The power electronic converters include AC-AC, AC-DC, DC-AC and DC-DC converters.

The controller tells the power processor, how much power it has to generate by providing the reference signal to it after considering the input command and sensor inputs. The controller could be a microcontroller, a microprocessor, or a DSP processor.

A variable speed drive used to control DC motors are known as DC drives and the variable speed drives used to control AC motors are called as AC drives. In this article we are going to discuss about the AC drives.

Classification of AC Drives

AC drives are used to drive the AC motor especially three phase induction motors because these are predominant over other motors in most of the industries. In industrial terms, AC drive is also called as variable frequency drive (VFD), variable speed drive (VSD), or adjustable speed drive (ASD).

Though there are different types of VFDs (or AC drives), all of them are works on same principle that converting fixed incoming voltage and frequency into variable voltage and frequency output. The frequency of the drive determines the how fast motor should run while the combination of voltage and frequency decides the amount of torque that the motor to generate.

A VFD is made up of power electronic converters, filter, a central control unit (a microprocessor or microcontroller) and other sensing devices. The block diagram of a typical VFD is shown below.

Block Diagram of AC Drive (Typical VFD)

Construction and Parts of a Typical VFD AC Drive

The various sections of the variable frequency drive (VFD) include

Rectifier and Filter section converts the AC power into DC power with negligible ripples. Mostly, the rectifier section is made with diodes that produce uncontrollable DC output. The filter section then removes ripples and produces the fixed DC from pulsating DC. Depends on the type of supply number of diodes is decided in the rectifier. For example, if it is three phase supply, a minimum of 6 diodes are required and hence it is called as six pulse converter.

The inverter takes the DC power from the rectifier section and then converts back to the AC power of variable voltage and variable frequency under the control of microprocessor or microcontroller. This section is made with series of transistors, IGBTs, SCRs, or MOSFETs and these are turned ON/OFF by the signals from the controller. Depends on the turn ON of these power electronic components, the output and eventually the speed of the motor is determined.

The controller is made with microprocessor or microcontroller and it takes the input from sensor (as speed reference) and speed reference from the user and accordingly triggers the power electronic components in order to vary the frequency of the supply. It also performs overvoltage and under voltage trip, power factor correction, temperature control and PC connectivity for real time monitoring.

Principle Operation of Variable Frequency Drive (VFD)

We know that the speed of an induction motor is proportional to the frequency of the supply (N = 120f/p) and by varying the frequency we can obtain the variable speed. But, when the frequency is decreased, the torque increases and thereby motor draw a heavy current. This in turn increases the flux in the motor. Also the magnetic field may reach to the saturation level, if the voltage of the supply is not reduced.

Therefore, both the voltage and frequency have to be changed in a constant ratio in order to maintain the flux within the working range. Since the torque is proportional to the magnetic flux, the torque remains constant throughout the operating range of v/f.The above figure shows the torque and speed variation of an induction motor for voltage and frequency control. In the figure, voltage and frequency are changed at a constant ratio up to the base speed. Thus the flux and thereby torque remain almost constant up to the base speed. This region is called as a constant torque region.

Since the supply voltage can be changed up to the rated value only and hence the speed at rated voltage is the base speed. If the frequency increased, beyond the base speed, the magnetic flux in the motor decreases and thereby torque begins falling off. This is called flux weakening or constant power region.

This type of control is called constant v/f control method used in variable frequency drives (VFDs) and it is the most popular type of control in industries. Suppose the induction motor is connected to a 460V, 60Hz supply, then the ratio will be 7. 67 V/Hz (as 460/60 = 7.67). As long as this ratio maintained in proportion, the motor will develop a rated torque and variable speed.

Control Schemes of VFD

There are different speed control techniques implemented for variable frequency drives. The major classification of control techniques used in modern VFDs is given below.

• Scalar Control
• Vector Control
• Direct Torque Control

Scalar Control

In this, the magnitudes of voltage and frequency are controlled by maintaining v/f ratio constant and hence called as scalar control (scalar values determines the speed and torque). The motor is fed with variable voltage and frequency signals generated by the PWM control from an inverter.

The inverter can be controlled with a microcontroller, microprocessor, or any other digital controller depending upon the type of manufacturer. This control scheme is widely used because it requires a little knowledge of the motor to perform the speed control. The scalar control can be implemented in a number of ways and some of the popular schemes include

Sinusoidal PWM

In this method, the frequency of the switch is varied depending on the sped reference input and the average or RMS value of the voltage for that frequency is determined by number of pulses and width of the pulses. If the width of the pulse is varied, the voltage across the motor is also varied. This voltage creates the sinusoidal current through motor which is much closer to true sine wave.

Only little calculations are needed to achieve this method. However, this method has disadvantages that it includes harmonics at PWM switching speed and also the magnitude of fundamental voltage is less than 90%.In this method, sinusoidal weighted values are stored in the microcontroller or microprocessor and are made to available at the output port at user defined intervals which are then applied to the inverter in order to produce a variable supply to the motor.

Six-Step PWM

In this method, the inverter of the VFD has six distinct switching states and they are switched in a specific order so as to produce the variable voltage and frequency to the motor. The direction reversal of the motor is readily accomplished by changing the inverter output phase sequence by means of the firing angle.

This method can easily be implemented as there is no intermediate calculations are required and also the magnitude of fundamental voltage is more than the DC bus. However the low order harmonics are high in this method which cannot be filtered by the motor inductance and hence it results more losses, motor jerky operation and high torque ripple.

Space Vector Modulation PWM (SVPWM)

In this technique, three phase voltage vectors of an induction motor are converted into a single rotating vector. The inverter of the VFD can be driven to eight unique states. The PWM voltage to the load is accomplished by properly selecting the switch states of the inverter and by calculating appropriate time period for each state.

By using space vector transformation, three phase sine waves are generated for each state, which are then applied to the motor.

The main advantage of this technique is that the harmonic magnitude is less at the PWM switching frequency. However, more calculations are required for employing this technique.

Vector Control

This method is also called flux oriented control, field oriented control, or indirect torque control. In this, three phase current vectors are converted to a two-dimensional rotating reference frame (d-q) from a three-dimensional reference frame using Clarke-Park transformation. The ‘d’ component is the flux producing component of the stator current and the ‘q’ component is the torque producing component.The two components are controlled independently through separate PI controller and then the outputs of PI controllers are transformed back to three dimensional stationary reference plane using inverse of the Clarke-Park transformation.

Using space vector modulation technique, the corresponding switching is pulse width modulated. The different types of vector control techniques include stator flux oriented control, rotor flux oriented control and magnetizing flux oriented control.

The vector control gives better torque response and accurate speed control compared to scalar control. But, it requires complex algorithm for speed calculations and it is costlier compared to scalar control due to feedback devices.

Direct Torque Control

This method has no fixed switching pattern as compared with traditional vector control. It switches the inverter according to the need of the load. This technique achieves high response particularly during changes of the load due to the absence of fixed switching pattern. It eliminates the use of any feedback, although it ensures the speed accuracy up to 0.5%. This technique uses the adaptive motor model which is based on the mathematical expressions of basic motor theory.This model requires the basic parameters of the motor such as stator resistance, saturation co-efficiency, mutual inductance, etc. and the algorithm captures this data without rotating the motor. This model calculates the actual torque and flux of the motor by considering inputs like DC bus voltage, current switch position and line currents. Then these values are given to the two level comparators of the torque and flux.

The output of the comparators is the torque and flux reference signals and is given to the switch selection table, wherein selected switch position is applied to the inverter without any modulation. Hence the name direct torque control as the motor torque and flux become direct controlled variables.

Real Time AC Drives at Glance

Several advanced features of AC drives or (VFDs) make them as a cost-effective choice in variable speed applications. The features like package designs, analog I/Os, digital I/Os, multi-functional keypads, and IGBT technology make VFDs easy to set up for any application.

Nowadays, Most AC drive designs are of more compact, because of the use of microprocessors, IGBTs and also the use of surface mount technology ( e.g SMD Resistors) for assembling components. These units can be wall-mounted or freestanding drives. There are various drives from different manufacturers including ABB, AB, Siemens, Delta and so on. Various packages of ABB AC drives are shown below.

Basically, a setup of AC drive for an application includes three major steps, namely control wiring, power wiring and software programming. Once the power and control wiring is done, we have to configure the AC drive parameters appropriate to the application requirements through software programming, removable keypad, or remote operator panel.

There is no need of rewiring the drive, if the application is altered. The setup for new applications is performed simply by changing the drive functions in the program.

AC drives are provided with analog inputs (like speed reference), analog outputs (for auxiliary metering), digital inputs (like start, stop, reverse, etc.), and relay outputs (speed relays, fault relays, etc.) in the control wiring section. This section is monitored by dedicated software called I/O status that monitors and displays the drive inputs and outputs.

In conventional drives, programming panels or touch keypads are attached to the drive itself. Modern drives consist of removable programming panels that allow the user to program, navigate various functions and configure the drive appropriate to the application requirement.

Apart from hand tools, every AC drive comes with a dedicated software which makes easy to start up and maintenance tool. This tool consists of setup wizards for setting of parameters. The software tool allows viewing, editing, saving and downloading parameters into the drive. It also provides the graphical and numerical signal monitoring.

During the design, manufacturers programs the AC drive parameters to default values. So the operator need to load the motor data values and values to customize the drive to the application. In addition to default values, manufacturers also provide macros which are nothing but a preprogrammed set of values.

The user or operator can set up and configure all the parameters included in macros in a few seconds rather than setting all parameters individually which could take several minutes. These macros include three-wire control, hand-auto, PID control and torque control.

Proportional-integral-derivative (PID) control macro allows the drive to control the speed automatically by receiving the control inputs such as pressure, temperature, or tank level. With proper programming of analog and digital I/O parameters with PID control macro, the closed loop operation of the drive is achieved.

AC drives are built with plug-in field bus control option in order to make a connection with major automated systems like PLCs, PCs, PACs, SCADA systems, etc. They can support a wide variety of communication field bus systems including DeviceNet, PROFIBUS DP, ControlNet, MODBUS, PROFINET, Ethernet/IP, etc.