Part # AN380 datasheet

Part Manufacturer: ST Microelectronics

ST Microelectronics


Part Details:

AN380 APPLICATION NOTE HOW TO DRIVE DC MOTORS WITH SMART POWER ICS by Herbert Sax There are many ways to control DC motors. Open-loop current control acts directly on torque andthus protects the electronics, the motor and the load. Open-loop variable voltage control makessense if the motor and electronics are not overloaded when the motor stalls. Open-loop variablevoltage control with a current limiting circuit constitutes the simplest way of varying speed.However, a closed-loop system is needed if precision is called for in selecting speeds. No other motor combines as many positive characteristics as the direct current design: high efficiency,ease of control & driving, compactness without sacrificing performance and much more. And DC motorscan be controlled in many ways --open loop current control, variable voltage control or closed-loop speedcontrol -- providing great flexibility in operational characteristics.Before we turn to a detailed discussion of the various methods of control, it is worthwhile recalling a fewbasics. DC MOTOR BASICSGenerally speaking, the electric equivalent circuit of a motor (figure 1) consists of three components: EMF,L and RM. Figure 1. Electrical equivalent circuit of a DC motor, consisiting of EMF, the winding inductance L and the winding resistance RM. The EMF is the motor terminal voltage, though the motor is always a generator, too. It is of no significancewhether the unit operates as a motor or a generator as far as the terminal voltage is concerned. The EMFis strictly proportional to the speed and has an internal resistance of zero. Its polarity represents the direc-tion of motion, independent of the motor voltage applied.The winding inductance, L, is the inevitable result of the mechanical design of the armature. Since it hin-ders the reversal of current flow in the armature, to the detriment of torque as speed increases, the windinginductance is an interference factor for the motor. It also obstructs rapid access to the generator voltage(EMF).Motors of coreless, bell armature or pancake design are considerably less susceptible to winding induc-tance. The smaller mass of these motors improves their dynamic performance to a significant extent. Onthe positive side, the winding inductance can be used to store current in pulse-width modulation (PWM)drive systems.The winding resistance, RM, is purely an interference variable because losses that reduce the degree ofefficiency increase as the load torque on the motor shaft increases, the latter being proportional to the cur-rent IM. It is also due to the winding resistance that the speed of the motor drops as load increases while December 2003 1/14 AN380 APPLICATION NOTE the terminal voltage Vs remains constant.Some of the mathematical relationships are shown below in simplified form: EMF = VS - (IM.RM) Motor current IM = (VS - EMF)/RM EMF I P Efficie ncy M = ------------ ou t = ------ V I P S M IN The drive torque at the motor shaft is proportional to the motor current IM. Figure 2 shows the relationshipsgraphed in a form commonly used for DC motors. It is because of bearing and brush friction that the effi-ciency tends towards zero at low load torques. Figure 2. Relationship between speed, efficiency and motor current of a DC motor. These basics show that essentially there are only two parameters governing how an electrical change canbe made to act on the motor shaft:a) with the current to vary the torqueb )with the mapping of the EMF on the speedOn account of the winding resistance RM, open loop variable voltage control exercises no more than anindirect effect on torque and speed and can therefore be used only for simple functions (speed variation).A number of sample applications using smart power ICs and illustrating open-loop variable voltage or cur-rent control and closed loop speed control are discussed here. All of these circuits permit the motor to runin both directions. The modifications needed for unidirectional operation are slight and generally involve asimplification of the design. OPEN-LOOP VARIABLE VOLTAGE CONTROLIn technical terms variable voltage control is the simplest to implement. Its main scope of application is insimple transport or drive functions where exact speed control is not essential. Applications of this kind arefound, for example, in the automobile industry for driving pumps, fans, wipers and power window lifts.The circuit shown in figure 3 is an example of a variable speed motor with digital direction control. Themotor voltage can be controlled via an analog input. If the polarity of the control signal is the variable thatdetermines the motor s direction of rotation -- as is usually the case in servo systems, for example -- thedesign shown in figure 4 can be used.One of the operational amplifiers is responsible for the V/VIN voltage and the other has an gain of 1, sothat the voltage losses Vs - VM are divided evenly between the two parts of the bridge.Equivalents to the circuits in figures 3 and 4 are shown in figure 5 and figure 6; these latter circuits, how-ever, are switchmode and their efficiency is thus improved to a considerable extent. 2/14 AN380 APPLICATION NOTE Figure 3. Circuit for driving a variable-speed motor. Where the enable function is needed, the type L6242 can be used. Figure 4. A typical circuit for driving servo system. Figure 5. Equivalent circuit to that in Figure 3, but using PWM. 3/14

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