Where a large inrush current and high starting torque can be permitted, the motor can be started across the line, by applying full line voltage to the terminals direct-on-line, DOL. This exploits a feature of the machine where it consumes power at a leading power factor when its rotor is over excited. In this section, the discussion will be limited to those types most common to integral-horsepower motor ratings of 1 hp and higher.
The optimum performance that can be achieved in a two-value capacitor, single-phase motor is a function of the economic factors as well as the technical considerations in the design of the motor. Electronically commutated EC motors are electric motors powered by direct-current DC electricity and having electronic commutation systems, rather than mechanical commutators and brushes. This technique is more common in Europe than in North America. Which of the following motors is preferred for tape-recorders? Single-phase motors have appreciable rotor current and rotor losses at no load. The general characteristics of these types of single-phase induction motors are as follows.
3-phase AC induction motors
The rotor current and rotor losses are insignificant at no load in a three-phase motor. The RS-IR motor has a centrifugal switch that shorts all segments of the commutator so that the motor operates as an induction motor once it is close to full speed. In applications where the motor requires a fixed rotation, one end of the start circuit is permanently connected to the main winding, with the contacts making the connection at the other end. A universal motor is one which a can be operated either on D.
The most common design is the two-speed motor, which has three windings: This initial design change also shows the shortest payback period. Where it is necessary to limit the starting inrush current where the motor is large compared with the short-circuit capacity of the supply , the motor is started at reduced voltage using either series inductors, an autotransformer , thyristors , or other devices.
The starting capacitor is usually an electrolytic type, which provides high capacitance per unit volume. They require a secondary magnetic field that causes the rotor to move in a specific direction. The poles are rather narrow, and between the poles leading from one end of the coil are an identical set leading from the other end.
Representative are low-torque synchronous motors with a multi-pole hollow cylindrical magnet internal poles surrounding the stator structure. The excitation is adjusted until a near unity power factor is obtained often automatically. Until the s they dominated electric traction electric, including diesel-electric railway and road vehicles ; many traction power networks still use special low frequencies such as It should be recognized that, with this type of motor, the speed change is obtained by letting the motor speed slip down to the required low FIGURE 1. Because of AC's advantages in long distance high voltage transmission, there were many inventors in the United States and Europe during the late 19th century trying to develop workable AC motors.
This is similar to a transformer, where the primary's electrical load is related to the secondary's electrical load. It is typically cast aluminum or copper poured between the iron laminates of the rotor, and usually only the end rings will be visible. The Fundamentals of Nuclear Power Generation:
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A universal motor can run on a A. They are referred to as synchronous condensers. PSC motors may operate within a wide range of speeds, much lower than the motor's electrical speed. Most common AC motors use the squirrel-cage rotor , which will be found in virtually all domestic and light industrial alternating current motors. In all, this creates a repeating sequence of four poles, unshaded alternating with shaded, that creates a circumferential traveling field to which the rotor's magnetic poles rapidly synchronize. For a given capacitor-start motor, the effect of adding a running capacitor in the auxiliary winding circuit is as follows:
Unless something else limits the current or cuts it off completely overheating and destruction of the winding insulation is the likely outcome. The synchronous motor can also be used as an alternator. The Fundamentals of Nuclear Power Generation: After the starting winding of a single phase induction motor is disconnected from supply, it continues to run only on a running winding b rotor winding c field winding d compensating winding Ans: Which of the following single-phase induction motors is generally used in time phonographs? Representative are low-torque synchronous motors with a multi-pole hollow cylindrical magnet internal poles surrounding the stator structure.
- One use for this type of motor is its use in a power factor correction scheme. Several types of repulsion motors have been manufactured, but the repulsion-start induction-run RS-IR motor has been used most frequently. In repulsion motor direction of rotation of motor a is opposite to that of brush shift b is the same as that of brush shift c is independent of brush shift Ans: A single-phase motor has no revolving field at standstill and therefore develops no locked-rotor torque.
- Where it is necessary to limit the starting inrush current where the motor is large compared with the short-circuit capacity of the supply , the motor is started at reduced voltage using either series inductors, an autotransformer , thyristors , or other devices. A shaded pole motor does not possess a centrifugal switch b capacitor c commutator d all of the above Ans: In all, this creates a repeating sequence of four poles, unshaded alternating with shaded, that creates a circumferential traveling field to which the rotor's magnetic poles rapidly synchronize. AC asynchronous induction IM. Views Read Edit View history.
Hysteresis refers to how the magnetic flux in the metal lags behind the external magnetizing force; for instance, to demagnetize such a material, one could apply a magnetizing field of opposite polarity to that which originally magnetized the material. In a hysteresis motor, the rotor must have a retentivity b resistivity c susceptibility d none of the above Ans: This causes a time lag in the flux passing through the shading coil, so that the maximum field intensity moves higher across the pole face on each cycle. The typical performance data for integral-horsepower, rpm, capacitor-start, induction-run motors are shown in Table 1. In universal motors the stator and rotor of a brushed DC motor are both wound and supplied from an external source, with the torque being a function of the rotor current times the stator current so reversing the current in both rotor and stator does not reverse the rotation. The brushless wound-rotor doubly fed synchronous motor system has an independently excited rotor winding that does not rely on the principles of slip-induction of current.
An AC motor is an electric motor driven by an alternating current AC. Small single-phase AC motors can also be designed with magnetized rotors or several variations on that idea; see Hysteresis synchronous motors below. Consequently, the major application of the permanent split capacitor motor has been to direct-driven fans and blowers. As the rotor picks up speed the torque builds up to its full level as the principal magnetic field is rotating relative to the rotating rotor. The speed of a universal motor is usually reduced by using a gearing b belts c brakes d chains Ans: For V power service, a common connection of the windings is called the T connection.
An unloaded squirrel-cage motor at rated no-load speed will consume electrical power only to maintain rotor speed against friction and resistance losses. This magnetic field passes down perpendicularly through the disc, inducing circular eddy currents in the plane of the disc centered on the field. The pole faces were parallel and relatively close to each other, with the disc centred between them, something like the disc in a watthour meter. Repulsion motors are wound-rotor single-phase AC motors that are a type of induction motor. Still widely used, universal traction motors have been increasingly displaced by polyphase AC induction and permanent magnet motors with variable-frequency drives made possible by modern power semiconductor devices.