In a hysteresis motor, the rotor must have a retentivity b resistivity c susceptibility d none of the above Ans: 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. At each end of the coil are a pair of circular plates with rectangular teeth on their edges, formed so they are parallel with the shaft. The running performance of this type of motor in terms of efficiency and power factor is the same as a two-value capacitor motor.
In a shaded pole motor, the shading coil usually consist of a a single turn of heavy wire which is in parallel with running winding b a single turn of heavy copper wire which is short-circuited and carries only induced current c a multilayer fine gauge copper wire in parallel with running winding d none of the above Ans: The vast majority of the rotor currents will flow through the bars rather than the higher-resistance and usually varnished laminates. Other variations of the same design are used for polyphase e. For low-speed operation, the intermediate winding is connected in series with the main winding and with the auxiliary circuit as shown in Fig. Transistorized inverters with variable-frequency drive can now be used for speed control, and wound rotor motors are becoming less common.
Which of the following motors is used in tape-recorders? When the AC polarity reverses, the eddy currents in the disc and the direction of the magnetic flux from the current coils both change, leaving the direction of the torque unchanged. Compared with induction motors, universal motors do have some drawbacks inherent to their brushes and commutators:
Anauxiliary winding is necessary to produce the rotating field required for starting. Most of the fractional horsepower motors have either a hard and annealed bearings b ball or roller bearings c soft and porous bearings d plain or sleeve bearings Ans: These motors were used in industrial and scientific devices. One of the pair of discs distributes the coil's flux directly, while the other receives flux that has passed through a common shading coil. For a given capacitor-start motor, the effect of adding a running capacitor in the auxiliary winding circuit is as follows: 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.
Applying AC to the coil created a field that progressed in the gap between the poles. Froehlich; Allen Kent The disc is braked by a permanent magnet so that speed is proportional to torque and the disc mechanically integrates real power. 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 Compared to the shaded pole motor, these motors provide much greater starting torque. Huge numbers of three phase synchronous motors are now fitted to electric cars.
It had a single field coil, and two principal poles, each split halfway to create two pairs of poles. Some of these motors also lift the brushes out of contact with source voltage regulation. The load curve of a typical fan load is superimposed on the motor speed-torque curves to show the speed reduction obtained on the low-speed connection. The typical performance of integral-horsepower, two-value capacitor motors is shown in Table 1. In a hysteresis motor, the rotor must have a retentivity b resistivity c susceptibility d none of the above Ans: Synchronous motors are occasionally used as traction motors ; the TGV may be the best-known example of such use.
3-phase AC induction motors
Each pole face was split, and had a shading coil on one part; the shading coils were on the parts that faced each other. Direction of rotation of a split phase motor can be reversed by reversing the connection of a running winding only b starting winding only c either a or b d both a and b Ans: For V power service, a common connection of the windings is called the T connection. For a given capacitor-start motor, the effect of adding a running capacitor in the auxiliary winding circuit is as follows: Electronically commutated EC motors are electric motors powered by direct-current DC electricity and having electronic commutation systems, rather than mechanical commutators and brushes.
In a two value capacitor motor, the capacitor used for running purposes is a air capacitor b paper spaced oil filled type c ceramic type d a. To illustrate this, Table 1. The torque is thus proportional to the instantaneous line voltage times the instantaneous load current, automatically correcting for power factor. Which of the following single-phase motors has relatively poor starting torque? In a shaded pole motor, the shading coil usually consist of a a single turn of heavy wire which is in parallel with running winding b a single turn of heavy copper wire which is short-circuited and carries only induced current c a multilayer fine gauge copper wire in parallel with running winding d none of the above Ans: Which of the following statements regarding reluctance-start motor is incorrect?
By transformer action, the stator induces currents in the rotor, which create torque by repulsion instead of attraction as in other motors. Which of the following statements regarding reluctance-start motor is incorrect? From Wikipedia, the free encyclopedia. When the AC polarity reverses, the eddy currents in the disc and the direction of the magnetic flux from the current coils both change, leaving the direction of the torque unchanged.
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The schematic diagram for a two-value capacitor motor is shown in Fig. There are a number of versions and methods of winding motors. The high-resistance rotor is used to improve stable speed operation and to increase the starting torque. Which of the following motors has two separate windings on the motor?
The speed of a universal motor is generally reduced by using a gear trains b V-belts c brakes d chains Ans: A single-phase induction motor is a inherently self-starting with high torque b inherently self-starting with low torque c inherently non-self-starting with low torque d inherently non-self-starting with high torque Ans: The repulsion-start induction-run motor is used because of a good power factor b high efficiency c minimum cost d high starting torque Ans:
Burning out of winding is due to a short circuited capacitor b capacitor value hiving changed c open circuiting of capacitor d none of the above Ans: Another common single-phase AC motor is the split-phase induction motor ,  commonly used in major appliances such as air conditioners and clothes dryers. These are used for applications where extreme mechanical positioning accuracy and high cycling capability are needed. A two-value capacitor motor is a capacitor motor with different values of capacitance for starting and running.
The axial component overcomes the spring force, releasing the brake; while the radial component causes the rotor to turn. The purpose of stator winding in the compensated repulsion motor is to a provide mechanical balance b improve power factor and provide better speed regulation c prevent hunting in the motor d eliminate armature reaction Ans: Single-phase motors have appreciable rotor current and rotor losses at no load. There is no additional brake control required.
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. Your email address will not be published. Single-phase motors do not have a unique rotating magnetic field like multi-phase 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. Other variations of the same design are used for polyphase e.
Applying AC to the coil created a field that progressed in the gap between the poles. Very low voltages at very high currents are typical in the bars and end rings; high efficiency motors will often use cast copper to reduce the resistance in the rotor. A single-phase motor has no revolving field at standstill and therefore develops no locked-rotor torque.