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Servomotors: General Principles of Operation
Introduction
The servomotors used in industry today are used in a closed-loop servo system. To understand how the servomotor is used in the system, it is first necessary to review the entire system. FIG. 44 indicates a block diagram of a typical servo system.
FIG. 44. Typical servo system block diagram
A reference input (typically called a velocity input) is sent to the servo amplifier, which controls the speed of the servomotor. Directly mounted to the machine (or to the servomotor) is a feedback device (either an encoder or resolver). This device changes mechanical motion into electrical signals and is used as a feedback loop. This feedback loop is then sent to the error detector, which compares the actual operation with that of the reference input. If there is an error, that error is fed directly to the amplifier, which makes the necessary corrections.
In many servo systems, both velocity and position are monitored. (Note: In servo systems, the word "velocity" is often used to describe speed control. Velocity indicates a rate of change of position, with respect to time. It also indicates a rate of motion in a particular direction, with respect to time.) The velocity loop control may take its command from the velocity loop feedback device-a resolver or tachometer mounted directly to the motor. The position loop control may take its command from the position feedback device-an encoder. Depending on the system, both devices may be mounted to the actual machine or controlled device.
The stability of the entire system is dependent upon the tuning of the components in the system and how well those components are matched.
Tuning the system involves working with a PID (proportional integral derivative) control. This type of closed loop control is standard on all high accuracy systems. The main factors in this closed loop system are the gain, integration time, and derivative time of the loop.
The amplifier gain must be set satisfactorily. The gain sets how responsive the amplifier will be during changes in error signal. A high gain will cause the motor to overshoot the intended speed target. Too low of a gain may mean that the target is reached late in the cycle, or possibly not at all.
The integration time allows the amplifier to respond to changes in the error signal, mostly at zero speed. The zero speed error signal is multiplied by the gain setting, and results in increased motor responsiveness (stiffness) and accuracy.
The derivative function is the most difficult to accurately adjust. This controls the dampening or oscillations of the system. This function basically dictates the amount of correction given per unit of error. The error signal can be corrected immediately (in milliseconds), or throughout a longer period of time (seconds).
If there is a difficult part to the tuning task, it would be during the derivative setup. The gain and integration time is interactive. One setting affects the other. Proper setup of the derivative function involves multiplying the position error by the position error rate (how much correction should take place per unit of time). If the system components are not matched, oscillations, overshoot, or undershoot of velocity can result, which means unstable operation.
Servomotors are special electromechanical devices that operate in precise degrees of rotation. This type of motor quickly responds to positive or negative signals from a servo amplifier. Fast and accurate speed, torque, and direction control are the mark of a servomotor's characteristics. Very high starting torque must be obtained from the servomotor. The standard AC induction motor's torque is measured in pound-feet. By contrast, the servomotor's torque is measured in inch-pounds.
In today's servo systems, three basic types of servomotors are used: AC, DC, and AC brushless. As one might expect, the AC design is based on AC induction motor characteristics. The DC design is based on the design of a DC motor. The brushless DC design is based on that of a synchronous motor. The basic principles of the DC and brushless DC servomotor have already been reviewed. We will therefore review the general characteristics of the AC servomotor. Linear devices will also be reviewed, since most of the position systems operate on linear technology.
AC Servomotors
This type of motor is basically a two-phase induction motor, capable of reverse operation. To achieve the dynamic requirements of a servo system, the servomotor must have a small diameter, low inertia, and high-resistance rotors. The low inertia allows for fast starts, stops, and reverse of direction. The high-resistance rotor provides for almost linear speed/ torque characteristics and accurate control.
An AC servomotor is designed with two phases set at right angles to each other. A fixed or reference winding is excited by a fixed voltage source.
The control winding is excited by a variable voltage source, usually the servo amplifier. Both sets of windings are usually designed with the same voltage per turns ratio (meaning that with equal voltage applied to each winding, the same magnetic flux will be produced). This allows for maxi mum control of speed, with very little speed drift. In many cases, the design of the AC servomotor offers only reasonable efficiency, at the sacrifice of high starting torque and smooth speed response. FIG. 45 indicates a typical AC servomotor design.
Linear Stepper Motor Systems
Linear stepper motor systems are based on the principles of the stepper motors previously presented. The stepper drive system is basically a servo system, but often without the velocity feedback loop. Without the feed back loop, some sacrifice in accuracy is made. However, the cost of a step per motor system is less than that of servomotor system.
Stepper motors, as you recall, are two-phase stator-type motors. The cur rent is carried in the stator, which allows for maximum heat dissipation.
Current that is switched on and off creates an electromagnetic field that produces rotation. The position of the motor shaft is determined by which phase is at maximum strength. The pole pair that is at maximum strength will interact with the permanent magnet rotor, and rotation will occur.
The linear stepper system could be of several different designs, but several common types for stepper and servo systems are the rack and pinion system and the leadscrew system. FIG. 46 indicates these two types of systems.
FIG. 45. AC servomotor design (Rockwell Automation, Inc.)
FIG. 46. Linear stepper motor systems
In the rack and pinion design, the linear table is moved by the rotating motion of the stepper motor. The stepper motor shaft is fitted with a cylindrical gear that mates with linear table sprockets. When the stepper motor engages, the linear table moves in the forward or reverse direction, depending on the signal from the amplifier.
In the leadscrew design, the moveable table contains an integral nut that is threaded to the specifications of the long machine threaded screw. The stepper motor shaft is directly connected to the leadscrew by means of a coupling. When the motor engages, the leadscrew rotates in the direction dictated by the amplifier, thereby setting the table into motion.
A limiting factor to this system is the backlash that can occur, causing some sloppiness of motion. Backlash is the measured play or looseness between the gear and linear table sprocket (rack and pinion) or between leadscrew and moveable table (leadscrew). Since motion is transferred from the motor shaft to another device, this type of looseness is inevitable.
Some backlash can be reduced by preloading the nut or linear device. This means that the linear device may be spring-loaded, keeping it in tight con tact with the leadscrew nut, so there is no play in the system.
Linear Motors
Linear motor systems operate basically the same as rotating motors. The difference, of course, is that linear motion is produced, rather than clock wise or counterclockwise motion.
There are two main components of the linear stepper motor-the platen and the slider (sometimes referred to as the forcer). The platen could be considered the stator of the motor. The slider could be considered a linear rotor. FIG. 47 indicates a linear stepper motor.
FIG. 47. Linear stepper motor design
The electromagnetic "teeth" extend over the entire length of the platen.
The slider also contains "teeth" and has both permanent magnets and coils that are electrically charged. It should be noted that the platen and slider have tooth structures that almost match. The slight offset is what causes the slider to be attracted to the next available magnetic field in the platen.
The slider, in many cases, will contain air bearings that assist in developing a slight air gap. This air gap is where the magnetic flux is developed, and would be considered common to any standard AC induction motor.
When the slider coils are energized, the linear stepper motor moves in 1/4 tooth steps. Extremely fine resolution can be obtained from this type of motor, in some cases up to 25,000 steps per inch. This type of motor is well suited for applications where fast acceleration and high-speed movements are required, but where low mass or weight is needed.
Speeds of up to 100 ips (inches per second) are possible, with movements in increments down to 0.00005 inches. The linear stepper motor system has its advantages in precision open-loop control, mechanical simplicity, reliability, applications where space is limited, and the ability for multiple motion (more than one slider can be applied onto one platen). In addition, this type of system is an alternative in applications where leadscrews (with backlash issues), belts, pulleys, and gears are not practical.
Section Review
AC and DC motors are the two major types in use today that are related to the industrial and HVAC applications. These motors provide the speed, torque, and horsepower necessary to operate the application. The motor changes one form of energy (electrical) to rotational or linear motion (mechanical).
The two major components of a DC motor are the armature and field winding. The armature is the rotating part that is physically connected to the shaft and develops magnetic flux around its windings. The field winding is the part of the stationary frame and provides the flux necessary to interact with the armature flux to produce rotation. The commutator acts as an electrical switch and always ensures that a repelling force is present between the armature and field flux circuits. This repelling force against the field winding flux causes rotation of the armature. Brushes are the devices that physically connect the voltage supply to the armature circuit.
Brushes are constructed of carbon material and require routine maintenance or replacement to reduce arching at the commutator segments.
Two separate voltage supplies are connected to the DC motor, one for the armature (variable DC voltage armature supply) and one for the field winding (fixed-voltage field exciter). Speed of the DC motor is directly controlled by the magnitude of the armature supply voltage. Speed is also inversely proportional to the magnitude of the field flux. If the field winding flux is reduced, the motor speeds up and could continue to infinite speed unless safety circuits are not implemented.
Torque is a direct result of the interaction of armature and field winding flux. If the armature windings are constantly energized, as well as the field windings, constant torque will result, as well as very high torque at zero speed.
Various types of enclosures are constructed to safeguard the DC motor against harm. For example, drip-proof motors provide a degree of protection against vertical falling materials and also allow for the ventilation of cool outside air. Totally enclosed motor frames provide a higher degree of protection, but are not practical for large frame motors because of the inability to remove heat.
Motors are listed with many types of ratings that indicate the torque generating ability, altitude, heat capability, vibration, and electrical specifications. DC motors are constructed in several different types, related to the field winding circuit: series wound, shunt (parallel wound), and com pound wound. In addition, several armature styles are also available: standard armature windings and permanent magnet armatures. Specialty DC motors include the PM (permanent magnet) servomotor and the brushless DC servomotor.
AC motors are in widespread use today, both in the industrial and commercial HVAC markets, but also in the residential and consumer markets.
AC motors are listed with one of two ratings: NEMA or IEC. NEMA ratings reflect the U.S. market demands, where IEC has its roots in the European marketplace, mostly in the union of European Common Market countries.
All motors can be classified into single-phase or polyphase categories.
Three-phase motors are the motor of choice in industry because of their relatively low cost, high efficiency, and ability for simple direction control.
The main components of the AC motor are the rotor and stator. The rotor is the rotating part and the stator is the stationary part connected to the frame. Only one power source is required to set the rotor into motion. The stator windings create magnetic flux that causes a magnetic field (flux) to be induced in the rotor. The attracting forces of the rotor and stator flux produce torque and rotation of the rotor.
Speed of an AC induction motor is related to the frequency applied and the number of pole pairs. The number of pole pairs causes an inverse relationship in speed, but the frequency applied has a direct relationship to speed. The AC motor will always operate at a slower speed than synchro nous. This is due to the requirement of magnetic flux in the rotor to be attracted to the rotating magnetic flux in the stator. Various torque values are associated with an AC motor connected across the line. Locked rotor, peak, and rated torque are the three most common values needed to apply an AC induction motor.
AC motors typically draw 600% inrush current upon start-up. Once the speed has increased to near synchronous, the current draw drops closely in line with the torque being produced. All AC motors are designed with a specific torque producing characteristic in mind-V/Hz. If the volts per hertz relationship is kept constant, the motor will develop the rated torque it was designed to produce.
A common rating scale for AC induction motors is that of a NEMA design classification: A, B, C, D, and E. Each classification indicates a different motor torque-producing category. AC induction motor nameplates have similar designs to DC motors, only referring to AC input power. A major indication of motor durability is the temperature class of the stator windings. IEC ratings differ with NEMA in most categories. NEMA tends to include a certain amount of overload in its ratings, while IEC rates the motor exactly to its capability.
AC motor types range from the standard induction motor to wound rotor, synchronous, and multiple pole motors. Specialty motors include stepper, AC vector, servomotors, linear stepper, and linear motors.
QUIZ
1. What are the two main parts of a DC motor and what is the purpose of each?
2. What is the purpose of the brushes?
3. Why are the laminations in the armature skewed?
4. What is the purpose of the commutator?
5. What is the purpose of the commutation windings?
6. What is the purpose of compensation windings?
7. How is speed controlled in a DC motor?
8. How is torque controlled in a DC motor?
9. What is the difference between a DPFG and a TEFC motor?
10. Identify eight ratings listed on the DC motor nameplate and briefly indicate their meanings.
11. What is the difference between a series wound and shunt wound DC motor?
12. How is a permanent magnet DC motor different from the other standard DC motors?
13. How does a DC servomotor differ from a standard DC motor?
14. What are the main components of an AC induction motor, and what is the purpose of each?
15. How is speed determined in an AC induction motor?
16. What determines the horsepower of a motor?
17. What is the definition of base speed?
18. What is the V/Hz ratio?
19. What NEMA design class would provide the highest amount of starting torque, when connected across line power?
20. When considering inrush current draw, which NEMA motor type requires the highest amount of current upon start-up?
21. When reviewing NEMA frame sizes, how is the centerline shaft to foot distance determined?
22. What is the difference between IEC and NEMA ratings?
23. How does a synchronous motor differ from a standard AC induction motor?
24. What is the principle of operation behind an AC vector motor?
25. How do stepper motors differ from standard AC induction motors?
ANSWERS-- Section 3
1. Armature: rotating part of the motor, contains windings and magnetic flux that repels the flux developed in the field winding. Field winding: stationary part of the motor that develops magnetic flux that interacts and repels the armature flux. The interaction of both magnetic fields produces rotation.
2. The brushes are carbon devices that transfer voltage from the armature supply to the actual armature circuit inside the motor.
3. To allow for smooth rotational action when operating at low speeds.
4. The commutator acts as an electric switch to cause the armature flux polarity to be in opposition to the field winding flux.
5. Commutation windings are constructed in the armature circuit to straighten out the magnetic flux generated through the armature unit.
Increased motor torque is the result.
6. Compensation windings are additional poles installed to the magnetic poles of the field winding (stator). They tend to smooth the flux developed across the pole.
7. Speed is the direct result of the magnitude of the armature voltage applied. A constant field flux must be present for the magnetic interaction to take place.
8. Torque is a direct result of the interaction of the field winding flux and the armature current (flux). When more load is applied to the DC motor, more current is drawn in the armature circuit and more torque is developed.
9. A DPFG motor has no means of external cooling. It does have vents that allow outside air into the motor housing, but reduce the input of airborne materials. A TEFC motor contains an integral fan that circulates the air around the motor housing. It does not allow the entry of air or particles into the unit.
10. Ambient temperature (maximum ambient), insulation class (available temperature rise above the ambient), duty cycle (continuous or intermit tent), armature voltage and current (rated voltage and current to be connected to the armature), field voltage and current (rated voltage and current to be connected to the field), enclosure type (indicated degree of protection against incoming particles or fluids), base speed/maximum speed (indicated speed at rated field and armature voltage and current, and rated load).
11. A series wound DC motor includes a series field winding within the armature circuit. One supply voltage is required. A shunt wound DC motor has two separate circuits-one for the armature and one for the field winding.
12. A permanent magnet motor does not have windings in the armature circuit. Permanent magnets replace the armature windings.
13. A DC servomotor is longer and narrower than a standard DC motor. The narrow design and low inertia make the servomotor ideal for quick acceleration and direction changes. The narrow size also allows for higher speed operation, compared with a standard DC motor.
14. The rotor is the part that rotates and develops a magnetic field by induction from the stator. The stator is the stationary part (connected to the frame) that develops a rotating magnetic field. This field causes the rotor to try to catch up to the rotating magnetic field of the stator, thereby producing rotation.
15. Speed is determined by either the frequency applied (Hz) or by the number of pole pairs constructed in the motor. As applied frequency increases, so does the motor speed. The number of pole pairs has an inverse affect on speed.
16. Horsepower is determined by the design of the motor, and also by the product of motor torque and speed, divided by a constant.
17. Base speed is the nameplate speed in rpm, when the motor is operating at rated voltage, rated frequency (Hz), and rated load applied.
18. The V/Hz ratio is the proportion of rated voltage to frequency. Every motor requires this proportion in order to develop rated torque (e.g., 7.67 V/Hz for a 460-V line-operated motor).
19. NEMA design D, per the classification curves.
20. NEMA design E, per the rated current/synchronous speed graph.
21. The first two digits, divided by 4, determine the shaft centerline height.
22. NEMA typically conservatively rates motors and includes horsepower overload capability through an S.F. number. IEC designates more frame sizes than NEMA and rates motors exactly to its capability. IEC also lists more enclosure classifications.
23. A standard AC induction motor uses an induced magnetic field in the rotor, caused by the stator, to produce rotation. A synchronous motor has two separate circuits-rotor and stator. A rotor power supply is used to create magnetic flux. The stator field is energized by another power sup ply (three-phase) that creates a rotating magnetic field. The rotor exactly matches the rotating magnetic field of the stator, and the speed synchronizes.
24. AC vector motors are used in conjunction with a flux vector drive. The drive causes the motor to create exact amounts of torque, depending on the application requirements. The motor typically has a shaft-mounted feedback device to allow the drive to accurately assess where the rotor is physically located. Vector motors develop a specific amount of torque, given a precise voltage and frequency from the vector drive.
25. Stepper motors operate on a similar principle to that of synchronous motors. The permanent magnet rotor rotates in lock-step with the two phase stator field. A stepper "controller" is required to input the proper amount of voltage to the stator, which in turn creates the appropriate steps of rotor rotation.