VIEWS: 39 PAGES: 7 POSTED ON: 12/10/2011
1 DC motor drives ..................................................................................................................... 1 Fields of Application .............................................................................................................. 1 DC motor types ...................................................................................................................... 1 Permanent magnet DC motor ............................................................................................. 1 DC motor with separately excited field winding ............................................................... 1 Power Supplies of the DC motor Drives ................................................................................ 1 Diode Rectifiers.................................................................................................................. 2 Controlled rectifiers ............................................................................................................ 2 DC/DC converters .............................................................................................................. 3 Characteristics of the DC motors ........................................................................................... 3 Permanent-magnet DC motors ........................................................................................... 3 DC motors with a separately excited field winding ........................................................... 4 Transfer functions .................................................................................................................. 6 DC motor drives Fields of Application Traditionally, dc motor drives have been used for sped and position control applications. In the past few years, the use of ac motor servo drives in these applications is increasing. In spite of that, in applications where an extremely low maintenance is not required, dc drives continue to be used because of their low initial cost and excellent drive performance. DC motor types Permanent magnet DC motor Often in small dc motors, permanent magnets on the stator produce a constant field flux. The torque-speed characteristics of this type can be shifted along the speed axis by controlling the applied terminal voltage Vt. Therefore the speed of a load with an arbitrary torque-speed characteristics can be controlled by controlling Vt in a permanent-magnet dc motor with a constant field flux. They have limited ratings of a few horsepower and also have a maximum speed limitation. DC motor with separately excited field winding The limitations of the previous type can be overcome if the field flux is produced by means of a field winding on the stator, which is supplied by a dc current If. At this type of dc motor both the terminal voltage Vt and the field flux f can be controlled to yield the desired torque and speed. Power Supplies of the DC motor Drives There are some devices, which can be used as DC motor power supplies. 2 Diode Rectifiers The trend is to use the inexpensive rectifiers with diodes to convert the input AC into DC in an uncontrolled manner. Fig. 5.1 Block diagram of a rectifier. In such diode rectifiers, the power flow can only be from the utility ac side to the dc side. They are used in switching dc power supplies, ac motor drives, dc servo drives and so on. These rectifiers draw highly distorted current from the utility. Now and in the future, harmonic standards and guidelines will limit the amount of current distortion allowed into the utility, and the simple diode rectifiers may not be allowed. Controlled rectifiers As the name of these converters implies, the line-frequency voltages are present on their ac side. In these converters, the instant at which a thyristor begins or ceases to conduct depends on the line-frequency ac voltage waveforms and the control inputs. They are used in dc motor and ac motor drives, where it is necessary or desirable to be able to control the power flow in both directions between the ac and dc sides. (regenerative capabilities) A fully controlled converter is shown in Fig. 6-1a in a block diagram form. Fig. 6.1a Line-frequency controlled converter. For given ac line voltages, the average dc-side voltage can be controlled from a positive maximum to a negative minimum value in a continuous manner. The converter dc current Id 3 (or id on an instantaneous basis) cannot change direction. Thus, a converter of this type can only operate in two quadrants (of the Vd and Id plane). In some applications, such as in reversible-speed dc motor drives with regenerative braking, the converter must be capable of operating in all four quadrants. This is accomplished by connecting two two-quadrant converters in antiparallel or back to back. DC/DC converters The dc-dc converters are widely used in regulated switch-mode dc power supplies and in dc motor drive applications. As shown in Fig. 7.1, often the input to these converters is an unregulated dc voltage, which is obtained by rectifying the line voltage, and therefore it will fluctuate due to changes in the line-voltage magnitude. Dc-to-dc converters are used to convert the unregulated dc input into a controlled dc output at a desired voltage level. Fig. 7.1 A dc-dc converter system. These converters are very often used with an electrical isolation transformer in the switch- mode dc power supplies and almost always without an isolation transformer in case of dc motor drives. Characteristics of the DC motors Permanent-magnet DC motors Often in small dc motors, permanent magnets on the stator as shown in Fig. 13-1a produce a constant field flux f. In steady state, assuming a constant field flux f: Tem = kTIa (13-10) Ea = kEm (13-11) Vt = Ea+RaIa (13-12) where kT = ktf and KE = Kef. Equations 13-10 through 13-12 correspond to the equivalent circuit of Fig. 13-4a. From the above equations, it is possible to obtain the steady-state speed m as a function of Tem for a given Vt: 1 R m Vt a Tem (13-13) kE kT The plot of this equation in Fig. 13-4b shows that as the torque is increased, the torque-speed characteristic at a given Vt is essentially vertical, except for the droop due to the voltage drop IaRa across the armature-winding resistance. This droop in speed is quite small in integral horsepower dc motors but may be substantial in small servo motors. More importantly, 4 however, the torque-speed characteristics can be shifted horizontally in Fig. 13-4b by controlling the applied terminal voltage Vt. Fig 13-4 Permanent-magnet dc motor: (a) equivalent circuit; (b) torque-speed characteristics: Vt5>Vt4>Vt2>Vt1, where Vt4 is the rated voltage; (c) continuous torque-speed capability. Therefore, the speed of a load with an arbitrary torque-speed characteristic can be controlled by controlling Vt in a permanent-magnet dc motor with a constant f. In a continuous steady state, the armature current Ia should not exceed its rated value, and therefore, the torque should not exceed the rated torque. Therefore, the characteristics beyond the rated torque are shown as dashed in Fig. 13-4b. Similarly, the characteristic beyond the rated speed is shown as dashed, because increasing the speed beyond the rated speed would require the terminal voltage Vt to exceed its rated value, which is not desirable. This is a limitation of permanent-magnet dc motors, where the maximum speed is limited to the rated speed of the motor. The torque capability as a function of speed is plotted in Fig. 13-4c. It shows the steady-state operating limits of the torque and current; it is possible to significantly exceed current and torque limits on a short-term basis. Figure 13-4c also shows the terminal voltage required as a function of speed and the corresponding Ea. DC motors with a separately excited field winding 5 Permanent-magnet dc motors are limited to ratings of a few horsepower and also have a maximum speed limitation. These limitations can be overcome if f is produced by means of a field winding on the stator, which is supplied by a dc current If. To offer the most flexibility in controlling the dc motor, the field winding is excited by a separately controlled dc source vf, as shown in Fig. 13-5a. The steady-state value of f is controlled by If (= Vf/Rf), where Rf is the resistance of the field winding. Fig 13-5 Separately excited dc motor: (a) equivalent circuit; (b) continuous torque-speed capability. Since f is controllable, Eq. 13-13 can be written as follows: 1 Vt Ra Tem (13-14) m k e f k t f recognizing that kE = kef and kT = ktf. Equation 13-14 shows that in a dc motor with a separately excited field winding, both Vt and f can be controlled to yield the desired torque and speed. As a general practice, to maximize the motor torque capability, f (hence If) is kept at its rated value for speeds less than the rated speed. With f at its rated value, the relationships are the same as given by Eqs. 13-10 through 13-13 of a permanent-magnet dc motor. Therefore, the torque-speed characteristics are also the same as those for a permanent- magnet dc motor that were shown in Fig. 13-4b. With f constant and equal to its rated value, the motor torque-speed capability is as shown in Fig. 13-5b. where this region of constant f is often called the constant-torque region. The required terminal voltage Vt in this region 6 increases linearly from approximately zero to its rated value as the speed increases from zero to its rated value. The voltage Vt and the corresponding Ea are shown in Fig. 13-5b. To obtain speeds beyond its rated value, Vt is kept constant at its rated value and f is decreased by decreasing If. Since Ia is not allowed to exceed its rated value on a continuous basis, the torque capability declines, since f is reduced. In this so-called field-weakening region, the maximum power EaIa (equal to mTem) into the motor is not allowed to exceed its rated value on a continuous basis. This region, also called the constant-power region, is shown in Fig. 13-5b, where Tem declines with m and Vt, Ea, and Ia stay constant at their rated values. It should be emphasized that Fig. 13-5b is the plot of the maximum continuous capability of the motor in steady state. Any operating point within the regions shown is, of course, permissible. In the field-weakening region, the speed may be exceeded by 50- 100% of its rated value, depending on the motor specifications. Transfer functions Figure 13-6 shows a dc motor operating in a closed loop to deliver controlled speed or controlled position. Fig 13-6 Closed-loop position/speed dc servo drive To design the proper controller that will result in high performance, it is important to know the transfer function of the motor. It is then combined with the transfer function of the rest of the system in order to determine the dynamic response of the drive for changes in the desired speed and position or for a change in load. The equations for the motor-load combination can be represented by transfer function blocks, as shown in Figure 13-7. Fig 13.7 Block diagram representation of the motor and load (without any feedback). 7 The inputs to this system are the terminal voltage Vt(s) and the load torque TWL(s). If we consider just the motor without the load, then the transfer function will be: (s) 1 G1 ( s ) m Vt ( s ) k E ( s m 1)( s e 1) where R J m a m = mechanical time constant, and kT k E La e = electrical time constant Ra The electrical time constant determines how quickly the armature current builds up, in response to a step change vt in the terminal voltage, where the rotor speed is assumed to be constant. The mechanical time constant determines how quickly the speed builds up in response to a step change vt in the terminal voltage, provided that the electrical time constant is assumed to be negligible and, hence, the armature current can change instantenously.