Induction Motor:
Induction motors are probably the simplest and most rugged of all electric motors. They consist of two basic electrical assemblies: the wound stator and the rotor assembly.
The rotor consists of laminated, cylindrical iron cores with slots for receiving the conductors. On early motors, the conductors were copper bars with ends welded to copper rings known as end rings. Viewed from the end, the rotor assembly resembles a squirrel cage, hence the name squirrel- cage motor is used to refer to induction motors. In modern induction motors, the most common type of rotor has cast-aluminum conductors and short-circuiting end rings. The rotor turns when the moving magnetic field induces a current in the shorted conductors. The speed at which the magnetic field rotates is the synchronous speed of the motor and is determined by the number of poles in the stator and the frequency of the power supply.
Synchronous speed is the absolute upper limit of motor speed. At synchronous speed, there is no difference between rotor speed and rotating field speed, so no voltage is induced in the rotor bars, hence no torque is developed. Therefore, when running, the rotor must rotate slower than the magnetic field. The rotor speed is just slow enough to cause the proper amount of rotor current to flow, so that the resulting torque is sufficient to overcome windage and friction losses, and drive the load.
Polyphase motors — NEMA classifies polyphase induction motors according to locked rotor torque and current, breakdown torque, pull up torque, and percent slip
**Locked rotor torque is the minimum torque that the motor develops at rest for all angular positions of the rotor at rated voltage and frequency.
**Locked rotor current is the steady state current from the line at rated voltage and frequency with the rotor locked.
**Breakdown torque is the maximum torque that the motor develops at rated voltage and frequency, without an abrupt drop in speed.
**Pull up torque is the minimum torque developed during the period of acceleration from rest to the speed that breakdown torque occurs. Figure 4 illustrates typical speedtorque curves for NEMA Design A, B, C, and D motors.
**Design A motors have a higher breakdown torque than Design B motors and are usually designed for a specific use. Slip is 5%, or less.
**Design B motors account for most of the induction motors sold. Often referred to as general purpose motors, slip is 5% or less.
**Design C motors have high starting torque with normal starting current and low slip. This design is normally used where breakaway loads are high at starting, but normally run at rated full load, and are not subject to high overload demands after running speed has been reached. Slip is 5% or less.
**Design D motors exhibit high slip (5 to 13%), very high starting torque, low starting current, and low full load speed. Because of high slip, speed can drop when fluctuating loads are encountered. This design is subdivided into several groups that vary according to slip or the shape of the speed-torque curve. These motors are usually available only on a special order basis.
Synchronous motors:
Without complex electronic control, synchronous motors are inherently constant-speed motors. They operate in absolute synchronism with line frequency. As with squirrel-cage induction motors, speed is determined by the number of pairs of poles and the line frequency.
Synchronous motors are available in sub fractional self-excited sizes to high-horsepower direct-current excited industrial sizes. In the fractional horsepower range, most synchronous motors are used where precise constant speed is required. In high-horsepower industrial sizes, the synchronous motor provides two important functions. First, it is a highly efficient means of converting ac energy to work. Second, it can operate at leading or unity power factor and thereby provide power-factor correction.
There are two major types of synchronous motors: nonexcited and direct- current excited.
Non excited motors — Manufactured in reluctance and hysteresis designs, these motors employ a selfstarting circuit and require no external excitation supply.
- Reluctance designs have ratings that range from subfractional to about 30 hp. Sub fractional horsepower motors have low torque, and are generally used for instrumentation applications. Moderate torque, integral horsepower motors use squirrel- cage construction with toothed rotors. When used with an adjustable frequency power supply, all motors in the drive system can be controlled at exactly the same speed. The power supply frequency determines motor operating speed.
- Hysteresis motors are manufactured in sub fractional horsepower ratings, primarily as servomotors and timing motors. More expensive than the reluctance type, hysteresis motors are used where precise constant speed is required.
DC-excited motors — Made in sizes larger than 1 hp, these motors require direct current supplied through slip rings for excitation. The direct current can be supplied from a separate source or from a dc generator directly connected to the motor shaft.
Synchronous motors, either single or polyphase, cannot start without being driven, or having their rotor connected in the form of a self-starting circuit. Since the field is rotating at synchronous speed, the motor must be accelerated before it can pull into synchronism. Accelerating from zero rpm requires slip until synchronism is reached. Therefore, separate starting means must be employed.
Dc motors:
Industrial applications use dc motors because the speed-torque relationship can be varied to almost any useful form -- for both dc motor and regeneration applications in either direction of rotation. Continuous operation of dc motors is commonly available over a speed range of 8:1. Infinite range (smooth control down to zero speed) for short durations or reduced load is also common.
Dc motors are often applied where they momentarily deliver three or more times their rated torque. In emergency situations, dc motors can supply over five times rated torque without stalling (power supply permitting).
Dynamic braking (dc motor-generated energy is fed to a resistor grid) or regenerative braking (dc motor-generated energy is fed back into the dc motor supply) can be obtained with dc motors on applications requiring quick stops, thus eliminating the need for, or reducing the size of, a mechanical brake.
Dc motors feature a speed, which can be controlled smoothly down to zero, immediately followed by acceleration in the opposite direction -- without power circuit switching. And dc motors respond quickly to changes in control signals due to the dc motor's high ratio of torque to inertia.
Stepper motors:
Stepper motors offer many advantages. Although feedback is not usually required, stepper motors are compatible with feedback signals, either analog or digital. Error is noncumulative as long as pulse-to-step integrity is maintained by the stepper motor. A stream of pulses can be counted into stepper motors, and the stepper motor's final position will be known within a small percentage of one step.
Since maximum dynamic torque occurs at low pulse rates, stepping motors can easily accelerate a load. When the desired position is reached and command pulses cease, the stepper motor shaft stops and there is no need for clutches or brakes. The stepper motor is generally left energized at a stop position. Once stopped, the stepper motor resists dynamic movement up to the value of the holding torque. An additional feature of the PM stepper motor is that when all power is removed, it is magnetically detented in the last position. A wide range of step angles are available -- 1.8 to 80°, for example -- without logic manipulation. Stepper motors have inherent low velocity without gear reduction. A typical stepper motor driven at 500 pps turns at 150 rpm. The stepper motor's rotor inertia is usually low. Multiple stepper motors driven from the same source maintain perfect synchronization.
But the stepper motor's efficiency is low; much of the input energy must be dissipated as heat. Load must be analyzed carefully for optimum stepper motor performance. And inputs must be matched to the stepper motor and load. Damping may be required when load inertia is exceptionally high to prevent oscillation.
DC Servo Motors:
Dc servo motors are normally used as prime movers in computers, numerically controlled machinery, or other applications where starts and stops are made quickly and accurately. Servo motors have lightweight, low-inertia armatures that respond quickly to excitation-voltage changes. In addition, very low armature inductance in these servo motors results in a low electrical time constant (typically 0.05 to 1.5 msec) that further sharpens servo motor response to command signals.
Servo motors include permanent-magnetic, printed-circuit, and moving-coil (or shell) dc servo motors. The rotor of a shell dc servo motor consists of a cylindrical shell of copper or aluminum wire coils which rotate in a magnetic field in the annular space between magnetic pole pieces and a stationary iron core. The servo motor features a field, which is provided by cast AlNiCo magnets whose magnetic axis is radial. Servo motors usually have two, four, or six poles.
Dc servo motor characteristics include inertia, physical shape, costs, shaft resonance, shaft configuration, speed, and weight. Although these dc servo motors have similar torque ratings, their physical and electrical constants vary.
RSS Feed (xml)
No comments:
Post a Comment