Developments of high performance motor drives are very essential for industrial applications. A high performance motor drive system must have good dynamic speed command tracking and load regulating response. DC motors provide excellent control of speed for acceleration and deceleration. The power supply of a DC motor connects directly to the field of the motor which allows for precise voltage control, and is necessary for speed and torque control applications. DC drives, because of their simplicity, ease of application, reliability and favorable cost have long been a backbone of industrial applications. DC drives are less complex as compared to AC drives system. DC drives are normally less expensive for low horsepower ratings. DC motors have a long tradition of being used as adjustable speed machines and a wide range of options have evolved for this purpose. Cooling blowers and inlet air flanges provide cooling air for a wide speed range at constant torque. DC regenerative drives are available for applications requiring continuous regeneration for overhauling loads. AC drives with this capability would be more complex and expensive. Properly applied brush and maintenance of commutator is minimal. DC motors are capable of providing starting and accelerating torques in excess of 400% of rated. D.C motors have long been the primary means of electric traction. They are also used for mobile equipment such as golf carts, quarry and mining applications. DC motors are conveniently portable and well fit to special applications, like industrial equipments and machineries that are not easily run from remote power sources.
D.C motor is considered a SISO (Single Input and Single Output) system having torque/speed characteristics compatible with most mechanical loads. This makes a D.C motor controllable over a wide range of speeds by proper adjustment of the terminal voltage. Now days, Induction motors, Brushless D.C motors and Synchronous motors have gained widespread use in electric traction system. Even then, there is a persistent effort towards making them behave like dc motors through innovative design and control techniques. Hence dc motors are always a good option for advanced control algorithm because the theory of dc motor speed control is extendable to other types of motors as well
Speed control techniques in separately excited dc motor
A chopper is a static power electronic device that converts fixed dc input voltage to a variable dc output voltage. A Chopper may be considered as dc equivalent of an ac transformer since they behave in an identical manner. As chopper involves one stage conversion, these are more efficient. Choppers are now being used all over the world for rapid transit systems. These are also used in trolley cars, marine hoist, forklift trucks and mine haulers. The future electric automobiles are likely to use choppers for their speed control and braking. Chopper systems offer smooth control, high efficiency, faster response and regeneration facility. The power semiconductor devices used for a chopper circuit can be force commutated thyristor, power BJT, MOSFET and IGBT.GTO based chopper are also used. These devices are generally represented by a switch. When the switch is off, no current can flow. Current flows through the load when switch is “on”. The power semiconductor devices have on-state voltage drop of 0.5V to 2.5V across them. For the sake of simplicity, this voltage drop across these devices is generally neglected. As mentioned above, a chopper is dc equivalent to an ac transformer, have continuously variable turn’s ratio. Like a transformer, a chopper can be used to step down or step up the fixed dc input voltage.
If motoring and regenerating operation are required with both directions of rotation then the full bridge converter is required. Using this configuration allows the polarity of the applied voltage to be reversed, thus reversing the direction of rotation of the motor. Thus in a full bridge converter the motor current and voltage can be controlled independently. The motor voltage Va is given by:
Va = V12 - V34
Where V12 is controlled by switchingS1 and S2 as described above, andV34 by switching S3 and S4. The usual operating mode for a full bridge converter is to group the switching devices so that S1 and S3 are always on simultaneously and that S2 and S4 are on simultaneously. This type of control is then referred to as bipolar control.
The four-quadrant H-bridge dc chopper is shown in figure 1 where the load current and voltage are referenced with respect to T1, so that the quadrant of operation with respect to the switch number is persevered. The H-bridge is a flexible basic configuration where its use to produce single phase ac is considered , while its use in smps applications is considered. It can also be used as a dc chopper for the four quadrant control of a dc machine. With the flexibility of four switches, a number of different control methods can be used to produce four-quadrant output voltage and current (bidirectional voltage and current). All practical methods should employ complementary device switching in each leg (either T1 or T4 on but not both and either T2 or T3 on, but not both) so as to minimize distortion by ensuring current continuity around zero current output. One control method involves controlling the H-bridge as two virtually independent two-quadrant choppers, with the over-riding restriction that no two switches in the same leg conduct simultaneously. One chopper is formed with T1 and T4 grouped with D2 and D3, which gives positive current io but bidirectional voltage ±vo (QI and QIV operation). The second chopper is formed by grouping T2 and T3 with D1 and D4, which gives negative output current -io, but bi-direction voltage ±vo (QII and QIII operation).
Considerations for converter driven DC motors
Device current rating
The power electronic converter must be matched to the requirements of the motor and the load. DC motor drives can be used to provide torques in excess of the maximum continuous rated torque of the motor for short intervals of time. This is due to the long thermal time constants of the motor. The peak torque requirement of the motor will determine its peak current demand, and hence the peak current requirement for the power switches.
The current rating of a PowerMOS device is limited by the maximum junction temperature of the device, which should not be exceeded even for short periods of time due to the short thermal time constant of the devices. The devices must therefore be rated for this peak current condition of the drive. Operation at maximum current usually occurs during acceleration and deceleration periods necessary to meet the performance requirements of DC servo systems.
The voltage rating of the power switches will be determined by the power supply DC link voltage and the motor emfs, including those which occur when the motor is operating in its constant power region at above rated speed but below rated torque.
The armature current supplied to the motor by the switching converter is not constant. The presence of ripple current in addition to the normal DC current affects the performance of the motor in the following ways:
Ripple in the motor current waveform will cause a corresponding ripple in the motor output torque waveform. These torque pulsations may give rise to speed fluctuations unless they are damped out by the inertia of the mechanical system. The torque pulsations occur at high frequencies where they may lead to noise and vibration in the motor laminations and mechanical system.
Winding losses in a DC motor are proportional to iRMS2, whereas the torque developed by the motor is proportional to I DC. Ripple in the motor current will increase the RMS current and thus give rise to additional losses and reduce the system efficiency.
If the ripple current is large then the peak device current will be significantly higher than the design DC value. The devices must then be rated for this higher current. Current ripple will also increase the current which must be handled by the motor brushes possibly increasing arcing at the brush contacts.
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