CUSTODIADAS EN LOS DEPOSITARIOS ADUANEROS Y EN BODEGAS DE LAS ADUANAS

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. Figure 3-47 indicates a linear stepper motor.

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

Figure 3-47. Linear stepper motor design

Slider Movement Phase A

Coils Phase BCoils

Slider S

N _Teeth

Air

Gap PermanentMagnet

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.

Chapter 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 wind- ing 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 mainte- nance 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 wind- ing 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 protec- tion against vertical falling materials and also allow for the ventilation of cool outside air. Totally enclosed motor frames provide a higher degree of

128 Motors and Drives

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 gen- erating ability, altitude, heat capability, vibration, and electrical specifica- tions. 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: stan- dard 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 com- mercial 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 rela- tionship 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 wind-

ings. 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.

Check Your Knowledge

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 indi- cate 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?

130 Motors and Drives

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?