Machines Quiz Reviewer

What Is SINGLE-PHASE INDUCTION MOTORS?

single-phase motor's stator winding produces a pulsating magnetic field that doesn't rotate. Due to this, the alternating flux cannot cause rotation in a stationary squirrel-cage rotor. However, if the rotor is initially rotated in one direction, it will continue to run in that direction. It accelerates until it reaches a speed slightly below the synchronous speed and then continues to rotate even with single-phase current flowing through the stator winding. This starting method is not practical for large motors or motors located in inaccessible areas.

How many percent that the Starting winding of Capacitor-Start Motor is opened?

when the motor attains about 75% of synchronous speed. The motor then operates as a single-phase induction motor and continues to accelerate till it reaches the normal speed.

What are the Characteristics of HYSTERESIS MOTOR?

(i) A hysteresis motor can synchronize any load which it can accelerate, no matter how great the inertia. It is because the torque is uniform from standstill to synchronous speed. (ii) Since the rotor has no teeth or salient poles or winding, a hysteresis motor is inherently quiet and produces smooth rotation of the load. (iii) The rotor takes on the same number of poles as the stator field. Thus by changing the number of stator poles through pole-changing connections, we can get a set of synchronous speeds for the motor.

Characteristics of CAPACITOR-START MOTOR?

(i) Although starting characteristics of a capacitor-start motor are better than those of a split-phase motor, both machines possess the same running characteristics because the main windings are identical. (ii) The phase angle between the two currents is about 80° compared to about 25° in a split-phase motor. For the same starting torque, the current in the starting winding is only about half that in a split-phase motor. (ii). The starting winding of a capacitor start motor heats up less quickly and is well suited to applications involving either frequent or prolonged starting periods. (iii) Capacitor-start motors are used where high starting torque is required and where the starting period may be long e.g., to drive: (a) compressors (b) large fans (c) pumps (d) high inertia loads

What is the Operation of Shaded- Pole Motor?

(i) During the portion OA of the alternating- current cycle, the flux begins to increase and an e.m.f. is induced in the shading coil. The resulting current in the shading coil will be in such a direction (Lenz's law) so as to oppose the change in flux. Thus the flux in the shaded portion of the pole is weakened while that in the unshaded portion is strengthened as shown. (ii) During the portion AB of the alternating-current cycle, the flux has reached almost maximum value and is not changing. (ii) As the flux decreases (portion BC of the alternating current cycle), current is induced in the shading coil so as to oppose the decrease in current. (iii) The effect of the shading coil is to cause the field flux to shift across the pole face from the unshaded to the shaded portion. (iv) The rotor is of the squirrel-cage type and is under the influence of this moving field.

Types of DOUBLE-FIELD REVOLVING THEORY

(i) ROTOR AT STANDSTILL (i) ROTOR RUNNING

what are some changes made in a d.c. motor that is to operate satisfactorily on a.c. supply. The changes effected are:

(i) The entire magnetic circuit is laminated in order to reduce the eddy current loss. Hence an a.c. series motor requires a more expensive construction than a d.c. series motor. (ii) The series field winding uses as few turns as possible to reduce the reactance of the field winding to a minimum. This reduces the voltage drop across the field winding. (iii) A high field flux is obtained by using a low-reluctance magnetic circuit. (iv) There is considerable sparking between the brushes and the commutator when the motor is used on a.c. supply. It is because the alternating flux establishes high currents in the coils short- circuited by the brushes. When the short-circuited coils break contact from the commutator, excessive sparking is produced. This can be eliminated by using high-resistance leads to connect the coils to the commutator segments..

What are the Characteristics of REPULSION-INDUCTIONMOTOR?

(i) The no-load speed of a repulsion-induction motor is somewhat above the synchronous speed because of the effect of repulsion winding. (ii) The speed regulation of the motor is about 6%. (iii) The starting torque is 2.25 to 3 times the full-load torque; the lower value being for large motors. The starting current is 3 to 4 times the full-load current. This type of motor is used for applications requiring a high starting torque with essentially a constant running speed. The common sizes are 0.25 to 5 H.P.

What are the Characteristics of SINGLE-PHASE REPULSION MOTOR?

(i) The repulsion motor has characteristics very similar to those of an a.c. series motor i.e., it has a high starting torque and a high speed at no load. (ii) The speed which the repulsion motor develops for any given load will depend upon the position of the brushes. (iii) In comparison with other single-phase motors, the repulsion motor has a high starring torque and relatively low starting current.

What are the Characteristics of Shaded Pole Motor?

(i) The salient features of this motor are extremely simple construction and absence of centrifugal switch. (ii) Since starting torque, efficiency and power factor are very low, these motors are only suitable for low power applications e.g., to drive: (a) small fans (b) toys (c) hair driers (d) desk fans etc.

what is the Characteristic of an A.C. SERIES MOTOR OR UNIVERSAL MOTOR?

(i) The speed increases to a high value with a decrease in load. In very small series motors, the losses are usually large enough at no load that limit the speed to a definite value (1500 - 15,000 r.p.m.). (ii) The motor torque is high for large armature currents, thus giving a high starting torque. (iii) At full-load, the power factor is about 90%. However, at starting or when carrying an overload, the power factor is lower.

CHARACTERISTICS of SPLIT-PHASE INDUCTION MOTOR.

(i) The spinning torque is 15 to 2 times the full-loud torque mid (lie starting current is 6 to 8 times the full-load current. (ii) Due to their low cost, split-phase induction motors are most popular single phase motors in the market. (iii) Since the starting winding is made of fine wire, the current density is high and the winding heats up quickly. If the starting period exceeds 5 seconds, the winding may burn out unless the motor is protected by built-in-thermal relay. This motor is, therefore, suitable where starting periods are not frequent. (iv) An important characteristic of these motors is that they are essentially constant-speed motors. The speed variation is 2-5% from no-load to full-load. (v) These motors are suitable where a moderate starting torque is required and where starting periods are infrequent e.g., to drive: (a) fans (b) washing machines (c) oil burners (d) small machine tools etc.

What are Characteristics of REPULSION-START INDUCTION-RUN MOTOR?

(i) The starting torque is 2.5 to 4.5 times the full-load torque and the starting current is 3.75 times the full-load value. (ii) Due to their high starting torque, repulsion-motors were used to operate devices such as refrigerators, pumps, compressors etc.

What are the Characteristics of CAPACITOR-START CAPACITOR-RUN MOTOR?

(i) The starting winding and the capacitor can be designed for perfect 2- phase operation at any load. The motor then produces a constant torque and not a pulsating torque as in other single-phase motors. (ii) Because of constant torque, the motor is vibration free and can be used in: (a) hospitals (b) studios and (c) other places where silence is important.

What are the Construction of REPULSION-INDUCTIONMOTOR?

(i) The stator carries a single distributed winding fed from single-phase supply. (ii) The rotor of this motor design consists of two separate windings. The inner winding is a squirrel-cage winding with permanently short-circuited rotor bars. Over the squirrel cage winding, there is a repulsion commutator armature winding.

What are the Characteristics of the Reluctance Motor?

(i) These motors have poor torque, power factor and efficiency. (ii) These motors cannot accelerate high-inertia loads to synchronous speed. (iii) The pull-in and pull-out torques of such motors are weak.

what Is the Operation of SPLIT-PHASE INDUCTION MOTOR?

(i) When the two stator windings are energized from a single-phase supply, the main winding carries current Im while the starting winding carries current Is . (ii) Since main winding is made highly inductive while the starting winding highly resistive, the currents Im and Is have a reasonable phase angle a (25° to 30°) between them. Consequently, a weak revolving field approximating to that of a 2-phase machine is produced which starts the motor. (iii) When the motor reaches about 75% of synchronous speed, the centrifugal switch opens the circuit of the starting winding. The motor then operates as a single-phase induction motor and continues to accelerate till it reaches the normal speed. The normal speed of the motor is below the synchronous speed and depends upon the load on the motor.

SINGLE-PHASE INDUCTION MOTORS?

- Such a motor inherently does not develop any starting torque and, therefore, will not start to rotate if the stator winding is connected to single-phase a.c. supply. -However, if the rotor is started by auxiliary means, the motor will quickly attain the final speed. -Single-phase induction motor having a squirrel cage rotor and a single phase distributed stator winding.

SPLIT-PHASE INDUCTION MOTOR

- The stator of a split-phase induction motor is provided with an auxiliary or starting winding S in addition to the main or running winding M. The starting winding is located 90° electrical from the main winding and operates only during the brief period when the motor starts up. The starting winding S has a high resistance and relatively small reactance while the main winding M has relatively low resistance and large reactance. The starting winding S has a high resistance and relatively small reactance while the main winding M has relatively low resistance and large reactance. The currents flowing in the two windings have reasonable phase difference

What are the two design used in CAPACITOR-START CAPACITOR-RUN MOTOR?

1. In one design, single capacitor C is used for both starting and running as shown. This design eliminates the need of a centrifugal switch and at the same time improves the power factor and efficiency of the motor. 2. In the other design, two capacitors C1 and C2 are used in the starting winding as shown. The smaller capacitor C1 required for optimum running conditions is permanently connected in series with the starting winding.

what are Methods of Single Phase Induction Motor to make them Self- starting Motor.?

1. Split-phase motors - started by two phase motor action through the use of an auxiliary or starting winding. 2. Capacitor motors - started by two-phase motor action through the use of an auxiliary winding and a capacitor. 3. Shaded-pole motors - started by the motion of the magnetic field produced by means of a shading coil around a portion of the pole structure.

What is the Equivalent Circuit of SINGLE-PHASE INDUCTION MOTOR? 1. At STANDSTILL?

1. at stand still, motor is simply a transformer with its short circuited. 2. double field revolving theory suggest characteristics associated with each revolving field will just 1/2 with actual total flux. 3. core loss has been neglected.

What is the Equivalent Circuit of SINGLE-PHASE INDUCTION MOTOR? 1.ROTOR RUNNING ?

1. motor is spinning at some speed in the direction of forward revolving field, the slip being s. 2. rotor current produced by the forward field.

Type of Single phase motor

1Single-phase induction motors (i) split-phase type (ii) capacitor type (iii) shaded-pole type 2. A.C. series motor or universal motor 3. Repulsion motors (i) Repulsion-start induction-run motor (ii) Repulsion-induction motor 4. Synchronous motors (i) Reluctance motor (ii) Hysteresis motor

What is A.C. SERIES MOTOR OR UNIVERSAL MOTOR?

A d.c. series motor will rotate in the same direction regardless of the polarity of the supply. One can expect that a d.c. series motor would also operate on a single-phase supply. It is then called an a.c. series motor.

What is SINGLE-PHASE REPULSION MOTOR?

A repulsion motor is similar to an a.c. series motor except that: (i) brushes are not connected to supply but are short-circuited. Consequently, currents are induced in the armature conductors by transformer action. (ii) the field structure has non-salient pole construction. By adjusting the position of short-circuited brushes on the commutator, the starting torque can be developed in the motor.

What is REPULSION-START INDUCTION-RUN MOTOR?

A repulsion-start induction-run motor combines the characteristics of a repulsion motor and a single-phase induction motor.

DOUBLE-FIELD REVOLVING THEORY

Consider two rotating magnetic fluxes f1 and f2 each of magnitude Φm/2 and rotating in opposite directions with angular velocity ω. Therefore, an alternating field can be replaced by two relating fields of half its amplitude rotating in opposite directions at synchronous speed. When the rotating flux vectors are in phase, the resultant vector is Φ = Φm; when out of phase by 180° , the resultant vector Φ = 0.

HYSTERESIS MOTOR APPLICATIONS?

Due to their quiet operation and ability to drive high-inertia toads, hysteresis motors are particularly well suited for driving (i) electric clocks (ii) Timing devices (iii) tape-decks (iv)from-tables and other precision audio-equipment.

If ROTOR IS STAND STILL?

If the rotor of a single-phase induction motor is stationary and the stator winding is connected to a single-phase power supply, the alternating flux produced can be represented as the sum of two rotating fluxes, f1 and f2 , rotating in opposite directions. Each of these fluxes generates torque, T1 in the anti-clockwise direction and T2 in the clockwise direction. At standstill, these torques cancel each other out, resulting in a net torque of zero. Consequently, a single-phase induction motor is not self-starting in this scenario.

What it is operates? REPULSION-START INDUCTION-RUN MOTOR?

Initially, it operates as a repulsion motor, providing high starting torque. Once the motor reaches a certain speed, a centrifugal device triggers the short-circuiting of the commutator, transforming the motor into a single-phase induction motor. This motor has a similar construction to a repulsion motor but is equipped with a centrifugal device on the armature shaft. When the motor reaches 75% of its full speed, the device shorts all the commutator bars, converting the rotor into a squirrel-cage type.

A single phase induction motor is very similar to a 3-phase squirrel cage induction motor.

It has (i) a squirrel-cage rotor identical to a 3-phase motor and (ii) a single-phase winding on the stator. It is not self-starting but requires some starting means.

What is Hysteresis motor?

It is a single-phase motor whose operation depends upon the hysteresis effect i.e., magnetization produced in a ferromagnetic material lags behind the magnetizing force.

What is Reluctance Motor?

It is a single-phase synchronous motor which does not require d.c. excitation to the rotor. A reluctance motor (also called synchronous reluctance motor) consists of: (i) a stator carrying a single-phase winding along with an auxiliary winding to produce a synchronous-revolving magnetic field. (ii) a squirrel-cage rotor having unsymmetrical magnetic construction.

What is CAPACITOR-START MOTOR?

The capacitor-start motor is identical to a split-phase motor except that the starting winding has as many turns as the main winding. A capacitor C is connected in series with the starting winding. The value of capacitor is so chosen that Is leads Im by about 80° which is considerably greater than 25° found in split-phase motor. Starting torque (Ts = k Im Is sin α) is much more than that of a split-phase motor

what is the construction of A.C. SERIES MOTOR OR UNIVERSAL MOTOR?

The construction of en a.c. series motor is very similar to a d.c. series motor except that above modifications are incorporated. Such a motor can be operated either on a.c. or d.c. supply and the resulting torque-speed curve is about the same in each case. For this reason, it is sometimes called a universal motor.

If ROTOR IS STAND STILL?

The flux rotating in the clockwise direction is the forward rotating flux (Φf) and that in the other direction is the backward rotating flux (Φb). The slip w.r.t. the forward flux will be: -The rotor rotates opposite to the rotation of the backward flux. Therefore, the slip w.r.t. the backward flux will be: Sb = 2-S As speed increases, the forward flux increases, increasing the driving torque while the backward flux decreases, reducing the opposing torque. The motor-quickly accelerates to the final speed.

what are the Application of A.C. SERIES MOTOR OR UNIVERSAL MOTOR?

The fractional horsepower a.c. series motors have high-speed (and corresponding small size) and large starting torque. They can, therefore, be used to drive: (a) high-speed vacuum cleaners (b) sewing machines (c) electric shavers (d) drills (e) machine tools etc.

what is the Power rating of SPLIT-PHASE INDUCTION MOTOR?

The power rating of such motors generally lies between 60 W and 250 W.

What is the Power Rating of SHADED-POLE MOTOR?

The power rating of such motors is up to about 30 W.

what is the Power rating of CAPACITOR-START MOTOR?

The power rating of such motors lies between 120 W and 7-5 kW.

What is REPULSION-INDUCTIONMOTOR?

The repulsion-induction motor produces a high starting torque entirely due to repulsion motor action. When running, it functions through a combination of induction-motor and repulsion motor action.

What is Shaded Pole Motor?

The shaded-pole motor is very popular for ratings below 0.05 H.P. (~ 40 W) because of its extremely simple construction. It has salient poles on the stator excited by single-phase supply and a squirrel cage rotor as shown. A portion of each pole is surrounded by a short-circuited turn of copper strip called shading coil.

What is the construction of SINGLE-PHASE REPULSION MOTOR?

The single-phase commutator motor consists of a stator winding connected directly to a single-phase source and a rotor with a drum-type winding connected to a short-circuited commutator. This configuration effectively turns the rotor into a squirrel cage.

What is CAPACITOR-START CAPACITOR-RUN MOTOR?

This motor is identical to a capacitor-start motor except that starting winding is not opened after starting so that both the windings remain connected to the supply when running as well as at starting.

How to make Single-Phase Induction Motor Self- Starting?

To make a single-phase induction motor self-starting, we should somehow produce a revolving stator magnetic field which may be achieved by converting a single-phase supply into two-phase supply through the use of an additional winding. When the motor attains sufficient speed, the starting means (i.e., additional winding) may be removed depending upon the type of the motor.

What is Single Phase SYNCHRONOUS MOTORS?

Very small single-phase motors have been developed which run at true synchronous speed. They do not require d.c. excitation for the rotor. Because of these characteristics, they are called unexcited single-phase synchronous motors. The most commonly used types are: (i) Reluctance motors (ii) Hysteresis motors The efficiency and torque-developing ability of these motors is low; The output of most of the commercial motors is only a few watts.

what is the operation of A.C. SERIES MOTOR OR UNIVERSAL MOTOR?

When a single-phase motor is connected to an AC supply, the field and armature windings carry the same alternating current. The field winding generates an alternating flux f, which interacts with the current in the armature to produce a torque. As both the armature current and flux reverse simultaneously, the torque always acts in the same direction. It's important to note that this type of motor does not produce a rotating flux; its operation is similar to that of a DC series motor.

What is the Operation of Reluctance Motor?

Whenever a piece of ferromagnetic material is located in a magnetic field; a force is exerted on the material, tending to align the material so that reluctance of the magnetic path that passes through the material is minimum.

What is DOUBLE-FIELD REVOLVING THEORY?

explain no torque at start and yet torque once rotated. This theory is based that an alternating sinusoidal flux (Φ = Φm cos ωt) can be represented by two revolving fluxes, each equal to one-half of the maximum value of alternating flux (i.e., Φm /2) and each rotating at synchronous speed (Ns = 120 f/P, ω = 2πf) in opposite directions.