A Guide to Electric Drives and DC Motor Control August 1, 2011 | Guides An electric drive is an electromechanical system that employs an electric motor as the prime mover instead of a diesel engine, steam or gas turbines, hydraulics, etc. to control the motion and processes of different machines and mechanisms. 1 Typical applications of electric drives include fans, ventilators, compressor pumps, hoists, cranes, conveyors, excavators, escalators, electric locomotives and cars. 2 An electric drive has several advantages 3 over other types of drives systems, such as:
Control characteristics can be adapted to application requirements. Simple and easy speed control methods. Electric braking can be applied easily. Pollution free. Wide range of speed, power, torque ratings. Efficiency is higher. Short time overload capacity. Function a variety of work environments, such as explosive, radioactive and submerged. Self-starting–no need of external starting equipment.
Compared to hydraulic and diesel prime movers, its operation is cleaner, less noisy and less maintenance.
A typical electric drive system includes a controller, a transmission, an electric motor and a driven load (e.g., fans, pumps, conveyors and others previously cited.) A key difference in different types of electric drive systems is the type of controller: (A) distinct D.C. motor control 4 components, such as motor starters, switches and operator controls, or (B) electronic motor controllers, called drive controls, that use semiconductors with electronic circuitry and software to perform the same functions of distinct D.C. motor control components.
Types of D.C. Motor Control There are three general types of D.C. motor control: manual, semi-automatic and automatic. Manual control directly connects a D.C. motor to the input power line or mains. Operator intervention is required. 5 Semi-automatic control uses switches or sensors (.e.g., limit, pressure, temperature, float level, flow, proximity, timing and photo-sensitive switches) 6 to control a magnetic contactor or starter which, when enabled or closed, will connect the motor to the input power line. 7 In semi-automatic operation, an operator is needed to start or stop the motor but the rest of the operation is controlled by the sensors or switches. Automatic control is similar to semi-automatic control with one important difference: no operator intervention is required. For example, a thermostat in an air conditioning system or a refrigerator will turn a compressor motor on or off to maintain the setpoint temperature automatically.
Functions of D.C. Motor Control Whether a D.C. motor is controlled manually, semi-automatically or automatically, the control system will perform a variety of common functions 8, which include:
Starting Stopping Jogging/Inching Plugging Speed Control Reversing Braking Protection
Starting There are several types of starting functions. Across-the-line starting is the most basic starting method. The motor is connected directly to the power line via an operator control switch. Closing the switch connects the motor to the line. However, for some D.C. motor types, applying full voltage line during starting would generate huge amounts of inrush current, which could be beyond the capacity of the power supply as well as possibly cause damage to the motor. In these cases, a starting resistance is used to limit the inrush current. 9 This is accomplished by
temporarily inserting a starting resistor in series with the motor’s armature winding. The resistor drops some of the line voltage to limit the starting current. The motor will gradually accelerate; this is commonly called soft starting. When the motor comes up to full speed, the starting resistor is removed.
Stopping There are three forms of stopping: coasting, braking or a deceleration ramp. When power is removed from a motor, it begins to coast to a stop and is time-dependent upon the inertia and the load. Coasting is impractical in some applications because the motor would take too long to stop. In these cases, a brake can be used to stop the motor quickly. There are four types of brakes: mechanical, magnetic clutch/eddy current, dynamic and regenerative brakes. Dynamic braking is accomplished by dissipating the kinetic energy in the armature across a braking resistor. During stopping, while the armature is rotating, it acts as a generator. The dynamic braking resistor becomes this generator’s load, thus, a transfer of energy occurs from the armature (acting as a generator) into the resistor, which dissipates the energy in the form of heat, causing the motor to slow down. 10
Jogging/Inching Jogging/inching moves the motor very short distances for positioning or alignment purposes. Inching uses a reduced voltage to move a motor while jogging uses full voltage. 11
Plugging Plugging has the dual function of stopping or reversing a motor It is accomplished by reversing the supply polarity to the armature while the motor is still running. As a result, a counter torque is developed and the motor slows down quickly. When the motor reaches zero speed and begins a reverse rotation, the supply to the motor is disconnected by closing a zero speed plugging switch. 12
Reversing Reversing controls the direction of the motor’s rotation (CW or CCW). It is accomplished through a control switch or via an electronic drive controller and entails reversing the polarity of the armature connections, which can be done with a push button control of mechanically interlocked forward and reverse buttons or programming the electronic drive controller for reverse operation 13
Speed Control A D.C. motor’s speed can be varied throughout the continuous speed range by varying the armature current by using a resistor or an electronic drive controller. 14
Protection
Protective circuits will protect the motor with 15, fuses, circuit breakers, overload relays, open field protection, overspeed protection, current sensors and metal oxide varistors (MOV) for surge protection. 16
Introduction to Electronic D.C. Drives An electronic D.C. drive, sometimes called a semi-conductor drive, is a subset of all the various electric drive systems used to control the motion and vary the speed of a D.C. motor. Early electric drive types, such as the Ward-Leonard system, controlled the motors indirectly. The Ward-Leonard system is an AC motor-DC generator set that feeds a variable voltage to the armature of a shunt wound DC motor to vary the motor’s speed. 17 While the Ward-Leonard system has good speed and torque control with a speed range of 25:1, it was phased out due to the excessive cost of purchasing three separate rotating machines as well as the considerable maintenance necessary to keep the brushes and commutators of two D.C. machines in proper operating conditions. 18 (A similar fate happened to the eddy current clutch. 19) Today’s electronic D.C. drives have numerous advantages over previous electrical drive systems, such as the Ward-Leonard drive. 20 They include:
Large range of power availability. Capable of full torque at standstill without a clutch. Very large speed range without needing gearboxes. Clean operation. Safe operation in hazardous environments. Immediate use (no warm up time) Low no-load losses. Low acoustic noise. Excellent control ability. Four-quadrant operation: forward motoring, forward braking, reverse motoring and reverse braking.
While the advantages are numerous, electronic D.C. drives have some disadvantages, 21, such as:
Very complex and require highly skilled technicians to maintain. Introduce harmonics/electrical noise into the power line 22
Electronic D.C. Drives: Control Methodology and Characteristics An electronic D.C. drive is an electronic thyristor AC/DC converter/rectifier or a DC/DC converter, called a D.C. chopper. A converter is a complex electronic control that can precisely control a D.C. motor’s rotation, torque and speed characteristics. AC/DC converters come in several configurations: (A) full-wave, 12-pulse bridge, (B) full-wave, 6-pulse bridge, or (C) halfwave, 3-pulse bridge. 23 The most common configuration is the full-wave, 6-pulse bridge because it produces less distortion on the DC side of the converter and has lower losses in the
DC motor than a 3-pulse bridge. (12-pulse bridges are typically used on larger drives to reduce harmonics on the AC power line.) The efficiency of the converter is usually greater than 98% and the overall efficiency of the D.C. drive plus the D.C. motor is about 90%. In addition, AC/DC converters can be built for applications up to several megawatts with good control and performance characteristics. 24 The other type of D.C. drive controller is a DC-to-DC converter or a D.C. chopper. While an AC/DC converter is powered from an A.C. supply, the D.C. chopper is powered from a D.C. power source. Both electronic controls produce a variable D.C. voltage that when applied to the D.C. motor’s armature varies the armature current, hence, the motor speed. The AC/DC converter produces this variable D.C. voltage by controlling the firing angle of its SCR bridge rectifier, while a D.C. chopper varies the voltage by controlling the varying angle to vary the duty cycle. 25 The output voltage of the chopper is in the form of pulses. The time ratio of the chopper can be controlled to vary the average voltage. Voltage variation at the load can be obtained by either current limit or time ratio control. For instance, in current-limit control, when current reaches the upper limit, the chopper is turned off to disconnect the motor from supply. Load current freewheels through the freewheeling diode and decays. When it falls to the lower limit, the chopper is turned on and connected to supply, thus, an average current is maintained. 26
Control Methodology The basic D.C. drive is a variable speed, closed-loop system. The advantages 27 of a closed-loop system are:
More accurate and reliable Responds to environmental changes Effects due to non-linearity and distortion are reduced Preferred when disturbances and variations are unpredictable
A closed-loop, speed-control system 28 consists of reference circuit (the speed setpoint), a differential (error) amplifier, a firing pulse generator, an SCR bridge and a speed feedback signal from a tachometer or an encoder. The reference speed is set by adjusting the reference error (differential) amplifier to a voltage proportional to reference speed. An error signal (the difference between speed setpoint and actual shaft speed) is applied to the firing pulse generator which generates firing pulses which then set the firing angles of the thyristors of the SCR bridge. The output of the SCR bridge is a variable D.C voltage that is applied to DC motor’s armature. The speed of the D.C. motor is dependent and directly proportional is to this variable voltage. The speed encoder or tachometer senses the actual speed of motor in rpm and converts it to a feedback signal proportional to shaft speed. This signal closes the speed-control loop. The feedback is compared with reference setpoint and fed to the differential error amplifier, ad infinitum. Motor speed will remain constant until the speed setpoint changes. 29
Characteristics The torque/speed characteristics of an electronic D.C. variable speed drive with a shunt wound D.C. motor 30 are:
Motor speed, N, is proportional to armature back EMF voltage (Ve) and inversely proportional to field flux (Flux Φ): N α Ve/Flux Φ Output torque, T, is proportional to the product of Iarmature x Flux Φ: T α (Iarmature) Flux Φ Output power, P, is proportional to the product of torque (T) and Speed (N): P α (T)N In general, the speed of a D.C. motor can be controlled by varying the armature voltage or the field flux (but not both). Since the field flux is kept constant below base speed, the motor speed can be varied by increasing or decreasing the armature voltage. This is done by adjusting the variable voltage produced at the output of the SCR bridge drive (phase controlled rectifier) or the average variable voltage produced by that SCR bridge that controls the duty cycle of the D.C. chopper. When the maximum output voltage of the converter is reached, additional speed can be achieved by reducing the field flux. This is called field weakening. In field weakening, the speed range is usually limited to about 3:1 to ensure stability and good motor commutation. The motor has full torque over the normal speed range and even at standstill. In field weakening, torque falls in proportion to speed but the output power remains constant. 31
Factors Affecting D.C Drive Selection Selecting or sizing an electronic D.C. drive depends on a variety of factors 32 that include:
Ratings and capital cost Speed range Efficiency Speed regulation Controllability Braking requirements Reliability Power-weight ratio Power factor Load factor and duty cycle Availability of supply Effect of supply variations Loading of the supply Environment Operating costs
Capital costs vary with rating of the drive. Larger drives with more performance features will cost more. By way of comparison, A.C. drive controllers cost more than D.C. drive controllers, but A.C. motors cost less and have lower maintenance costs than brushed D.C. motors. 33 While D.C. drives have a wide speed range, they may require thyristors with higher ratings due to the higher currents associated with low speed operation. Efficiency is the ratio of output power to
input power. Low efficiency has two disadvantages: (1) costly wasted energy and (2) excessive heating of the drive’s components and the motor. D.C. drives typically have high efficiencies; however, at low speeds, they have a low power factor. 34 D.C. drives have a power-to-weight ratio advantage over Ward-Leonard electric drives but less so compared to A.C. drives. 35 Most industrial loads are inductive and operate at a lagging power factor (less than unity) and so do variable speed motor drives. But, controlled SCR rectifiers or D.C. choppers reduce the drive’s power factor further. Power factor correction capacitors on the power line input or the drive’s output can be added to compensate for lagging power factors. 36 Thyristor drives can accept limited voltage and current variations. However, for low impedance power supplies, input chokes or isolating transformers will need to be added on the D.C. drive’s input. 37 Corrosive or explosive environments will require totally enclosed motors because D.C. motors are sensitive to “corrosive and dust-laden environments.” 38 1. U.A.Bakshi and M.V.Bakshi. Electrical Drives And Control. 1st ed. Technical Publications Pune, 2009. Page 1-1 ↩ 2. N. K. De and P. K. Sen. Electric Drives. Prentice Hall of India, 2006. Page 1 ↩ 3. U.A.Bakshi and M.V.Bakshi. Electrical Drives And Control. 1st ed. Technical Publications Pune, 2009. Page 1-1 ↩ 4. Herman, Stephen L. Industrial Motor Control. 6th ed. Delmar Cengage Learning, 2010. Page 1 ↩ 5. Herman, Stephen L. Industrial Motor Control. 6th ed. Delmar Cengage Learning, 2010. Page 1 ↩ 6. Herman, Stephen L. Electric Motor Controls. 9th ed. Delmar Cengage Learning, 2010. Page 20 ↩ 7. Herman, Stephen L. Industrial Motor Control. 6th ed. Delmar Cengage Learning, 2010. Page 1 ↩ 8. Herman, Stephen L. Industrial Motor Control. 6th ed. Delmar Cengage Learning, 2010. Page 7 ↩ 9. Herman, Stephen L. Electric Motor Controls. 9th ed. Delmar Cengage Learning, 2010. Page 7 ↩ 10. Jeffrey J. Keljik. Electricity Four: AC/DC Motors, Controls and Maintenance. 9th ed. Delmar Cengage Learning, 2009. Page 85 ↩ 11. Herman, Stephen L. Electric Motor Controls. 9th ed. Delmar Cengage Learning, 2010. Page 7 ↩ 12. S.K. Bhattacharya, Brijinder Singh, S K Bhattacharya. Control of machines. New Age Unternational Ltd Publishers 2006. Page 146 ↩ 13. Jeffrey J. Keljik. Electricity Four: AC/DC Motors, Controls and Maintenance. 9th ed. Delmar Cengage Learning, 2009. Page 83 ↩ 14. Herman, Stephen L. Electric Motor Controls. 9th ed. Delmar Cengage Learning, 2010. Page 11 ↩ 15. Herman, Stephen L. Electric Motor Controls. 9th ed. Delmar Cengage Learning, 2010. Page 7 ↩ 16. Herman, Stephen L. Electric Motor Controls. 9th ed. Delmar Cengage Learning, 2010. Page 12 ↩ 17. Malcolm Barnes. Practical Variable Speed Drives and Power Electronics. Elsevier Lincare House: IDC Technologies, 2003. Page 21 ↩
18. Malcolm Barnes. Practical Variable Speed Drives and Power Electronics. Elsevier Lincare House: IDC Technologies, 2003. Page 21 ↩ 19. G. K. Dubey. Fundamentals of Electrical Drives. 2nd ed. Alpha Science International, 2001 Page 356 ↩ 20. André Veltman, Duco W. J. Pulle, R. W. A. A. De Doncker. Fundamentals of Electrical Drives. Springer Science + Business Media, 2007. Page 2 ↩ 21. André Veltman, Duco W. J. Pulle, R. W. A. A. De Doncker. Fundamentals of Electrical Drives. Springer Science + Business Media, 2007. Page 4 ↩ 22. J. B. Dixit, Amit Yadav. Electrical Power Quality. Laximi Publications, 2010. Page 34 ↩ 23. Malcolm Barnes. Practical Variable Speed Drives and Power Electronics. Elsevier Lincare House: IDC Technologies, 2003. Page 21 ↩ 24. Malcolm Barnes. Practical Variable Speed Drives and Power Electronics. Elsevier Lincare House: IDC Technologies, 2003. Page 21 ↩ 25. Vedam Subrahmanyam. Thyristor control of electric drives. Tata McGraw-Hill, 2008. Page 11 ↩ 26. Vedam Subrahmanyam. Electric Drives: Concepts & Applications. 2nd ed. Tata McGraw-Hill. 2011. Page 395 ↩ 27. V.S.Bagad and A.P.Godse. Mechatronics and Microprocessor. 1st ed. Technical Publications Pune, 2009. Page 1-18 ↩ 28. V.S.Bagad and A.P.Godse. Mechatronics and Microprocessor. 1st ed. Technical Publications Pune, 2009. Page 3-27 ↩ 29. V.S.Bagad and A.P.Godse. Mechatronics and Microprocessor. 1st ed. Technical Publications Pune, 2009. Page 3-27 ↩ 30. U.A.Bakshi and M.V.Bakshi. Electrical Drives And Control. 1st ed. Technical Publications Pune, 2009. Page 2-9 ↩ 31. Malcolm Barnes. Practical Variable Speed Drives and Power Electronics. Elsevier Lincare House: IDC Technologies, 2003. Page 21 ↩ 32. William Shepherd, Lance Norman Hulley, D. T. W. Liang. Power Electronics and Motor control. 2nd ed. Cambridge University Press. 1995. Page 129 ↩ 33. William Shepherd, Lance Norman Hulley, D. T. W. Liang. Power Electronics and Motor control. 2nd ed. Cambridge University Press. 1995. Page 126 ↩ 34. Bela G. Liptak, Editor-in-Chief. Instrument Engineers’ Handbook: Process control and optimization. 4th ed. Taylor and Francis. 2006. Page 2118 ↩ 35. H. Bülent Ertan. Modern Electrical Drives. Kluwer Academic Publishers. 2000. Page 688 ↩ 36. Timothy L. Skvarenina, Ed. The Power Electronics Handbook. CRC Press. 2002. Page 17-17 ↩ 37. Bill Drury. The Control Techniques drives and controls handbook. The Institution of Electrical Engineers. 2001. Page 243 ↩ 38. U.S. Environmental Protection Agency. Guide to Industrial Assessments for Pollution Prevention and Energy Efficiency. U.S. EPA. 2001. Page 199 ↩
Published by Ohio Electric Motors: http://www.ohioelectricmotors.com/a-guide-to-electricdrives-and-dc-motor-control-688#ixzz20Coy4eF1
