“ANILLO TURÍSTICO DE CARTAGO”
Y SUS REFORMAS
General Principles of Operation—Stepper
A step or stepper motor is one in which electrical pulses are converted into mechanical movements. A standard DC motor, for example, rotates con- tinuously; but a stepper motor rotates in fixed increments whenever it is pulsed on. A standard DC motor would be considered an analog device, while a stepper motor would be considered digital.
The size of the step, or the step angle, is determined by the motor con- struction or by the type of controller connected. (Note: The step angle is determined in fractions of 360º, which is one complete shaft rotation.) For example, the step resolution of 90º would be four steps per rev (revolu- tion). A 15º resolution would indicate 12 steps per rev, and 1.8º would indicate 200 steps per rev. Microstep motors are capable of thousands of steps per rev.
Because of their exactness of rotation, stepper motors are used, “open- loop” in control systems where position is critical. In many high-accuracy applications, an encoder or position feedback device is used to confirm the actual position of the motor shaft.
Stepper motors require a drive package with an electronic controller, power supply, and feedback device, if needed. Figure 3-42 indicates the principle of stepper motor design.
The stepper motor is a two-phase type of motor. The indexer provides step and direction pulses to the drive controller (amplifier). The amount of cur- rent for each phase is determined by the controller, which is then used as an output to the stepper motor. The stepper motor is operated by pulses, which determine the “steps” of the motor shaft. The frequency of these steps determines the speed of the motor.
The most common types of stepper motors are probably the permanent magnet (PM) and the variable reluctance (VR). The diagram in Figure 3- 42 is one type of a PM stepper motor. Is could be considered a design simi- lar to the synchronous induction motor.
The rotor moves in step with the stator windings when the windings are energized. If the windings are continuously energized from the two-phase supply, then the motor would essentially act as a low-speed synchronous motor. As seen in Figure 3-42, the PM rotor is surrounded by the two- phase stator. The rotor sections are offset by 1/2 tooth pitch (180º) from
120 Motors and Drives
each other. As the voltage rotates clockwise, from phase A to phase B, a set of rotor magnets will align themselves with the stator magnetic field. The rotor will therefore turn one step. If for some reason, both phases are energized simultaneously, the rotor will establish a location midway between the stator poles. If that were to happen, the motor would be con- sidered half stepping.
The VR type stepper motor is basically constructed the same way as a PM motor. The difference is that the VR type does not have magnets in the rotor. It would contain, however, 2-, 3- or 4-phase stator windings. The motor would operate similar to an induction motor, with the rotor align- ing itself with a stator pole that is energized.
The stepper motor is essentially a brushless motor. It can deliver high torque at zero speed, with no drifting of the shaft position. The direction of the motor can be reversed by reversing the direction of the pulses from the controller. The device has low inertia, similar to a servomotor, a result of the windings in the stator and a permanent magnet rotor.
There are several application considerations that come with stepper motors. Periodically, possibly at low speeds, this type of motor exhibits oscillations with every step. This is caused by poles in the rotor seeking the next available magnetic field. Many times, the magnetic fields of the rotor and stator do not match up, typically upon power-up. Also, the motor, controller, and load must be somewhat matched to minimize the oscilla- tions. Stepper motors tend to run hotter than standard induction motors. This is due to the pulse waveform from the controller, especially at low
Figure 3-42. PM stepper motor diagram
Position Of Rotor (Rotates Clockwise) S N B + B_ N S A+ A- N S N S Phase A is at 100%
Phase B is At 0 Phase B is at 100%Phase A is At 0 Indexer (Amplifier)Controller Stepper
speed, with high current levels present (a product of high torque response at low speed).
AC Vector Motors
This type of motor is a specific type that would be applied to an AC vector or flux vector drive. Principles of operation for this motor are basically identical to the standard AC induction motor. Because this motor is oper- ated from a flux vector drive, special design characteristics are required.
Vector control basically means the requirement of full torque at zero speed. In applications such as elevators, hoists, and ski lifts, the motor usu- ally is started while under rated load. If the device is an elevator car, the position of the device cannot change when the motor is started. If a stan- dard induction motor were used, the motor would have to “slip back” for torque to be developed. During the process of developing “motor slip,” the elevator car may have dropped several feet before the motor could
develop enough torque to move it upward. The vector motor is specially designed to operate at extremely low slip and be able to handle the heat generated by providing full torque at zero speed.
The general principle of operation lies with analyzing the motor in terms of voltage and flux vectors. The rotor is divided into 360º of rotation, which is one complete rotation. A vector would be the direction and amount of a certain quantity in the motor circuit—in this case, rotor flux or stator flux. The relationship between rotor and stator flux is indicated in Figure 3-43.
The torque in an induction motor is developed by the relationship of rotor and stator flux. The physical torque developed is a byproduct of the mag- nitude of the stator and rotor flux vectors. Stator flux is a function of the input voltage to the motor. (The voltage vectors are indicated by U1 to U6 in the figure.) We could consider the dashed curve set the torque span
Figure 3-43. Vector motor relationships - stator and rotor flux (Courtesy of ABB Inc.)
y s r x U U U U U U2 1 3 4 5 6 T = c ( x ) Where: T = Torque Produced c = Constant s = Stator Flux r = Rotor Flux s r
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developed in the motor. The device that would control the amount of sta- tor and rotor flux generated would be considered a vector or flux vector AC drive.
The vector motor, in most cases, must have provisions for the mounting of a feedback device on the shaft end. The feedback device (encoder or resolver) sends information back to the drive control, indicating exactly where the rotor position is located. The drive control needs this informa- tion to calculate and generate V/Hz. The V/Hz waveform is then used by the motor to generate the flux vectors shown in the figure.
Vector control, drive control, and feedback devices will be discussed in Chapter 4 (AC Drives section) and Chapter 5 (Closed Loop Control sec- tion). This type of technology is definitely in high demand throughout industry today. The use of motor vector control (torque control) allows manufacturing systems to increase accuracy and productivity. The basic design of the AC induction motor has not changed much in the last few decades. Magnetism is magnetism. However, the ratings are now more precise than they were a few decades ago. The efficiencies are definitely higher than a few decades ago. There are AC drive manufacturers that require a flux vector drive and motor combination—a matched set. The direction of industry, however, is to be able to use a combination of vendor equipment to achieve the desired results.