Term Paper: AC Theory

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AC Theory

Which connection gives better power factor and efficiency on light loads for a three-phase Cage motor "Star" or "Delta"?

Star connection refers to one end of the coils being connected to one single point and the other ends of the coils are connected directly to three phase power phases. However, in delta connection, coils are directly wired between two mains phases. A motor with similar coiling can work at lower voltages if it is connected in delta configuration. (Re: star-Delta) at the start of a motor, star connection is used; here though the motor is connected for a higher operating voltage, it receives only a low voltage to start. After the start, the delta connection is switched on. And the motor gets the nominal voltage. Hence, delta connection gives better power factor and efficiency as it can enable the motor to operate at lower voltages. (Star-/delta connection)

What are the advantages of a three-phase Cage motor compared with its single phase counterpart?

A single-phase motor is actually a two-phase machine with main and auxiliary windings and a squirrel-cage rotor. (1-Phase AC Induction Motor) Three-phase squirrel cage motor is a rotating machine that operates with a three-phase source of alternating voltage. The stator here has winding displaced by 120°. The advantages of a three-phase Cage motor compared with single-phase cage motor are: it operates at medium efficiency at with low speed and operates at high efficiency at high speeds; it has higher start torque; it is easy to reverse motor; here inverter 'shoot-through' is possible for which 'dead-time' circuits & compensation are required. (3-Phase AC Induction Motor)

3) Why is a Star-Delta Starter often used with a three phase cage motor?

To reduce the high starting current of rotary current asynchronous motors, star/delta connections are used. The starting connection is the star connection. After the start, the delta connection is switched on either manually or automatically. (Star-/delta connection) the concept used in reduced voltage starting is that motor torque is proportional to the square of the terminal voltage. A star/delta starter contains three contactors and a time switch and it changes the motor winding configuration from an initial star connecting to a delta as the motor speed increases. The time switch controls the transition point and this point preferably occurs when the motor speed is at 80% of its full speed. A star start enables the alteration of voltage across each stator winding to 58% of its normal speed. This leads to a one third reduction of starting torque compared to locked rotor torque or LRT. This in turn leads to reduction in starting currents and acceleration forces. (the Principles of Fixed-Speed Induction Motor Control)

4) What are the disadvantages of a three-phase cage motor, compared with its three phase slip-ring motor counterpart?

The difference between a three-phase cage motor and a slip-ring motor exists in the construction of the rotor. In 3-phase cage motor, the rotor is made up of laminated steel sheets assembled around a shaft. The rotor winding consists of copper or aluminum bars. In slip-ring motor, the cylindrical core of the rotor is made up of steel laminations, slotted to hold the formed coils of the three single-phase windings. These windings are placed 120 electrical degrees apart. The advantages of slip-ring motor over cage motor are: its receptiveness to speed control by adapting rotor resistance; its high initiating torque of 200-250% of full load torque; and its comparatively low starting current from 250 to 350% of the full load current in comparison to a squirrel-cage motor. This would have a starting current in the order of 600% of its full load current. (Electricity and Motors)

The starting torque is low relative in comparison to the current taken by the stator. The initial values are 150% of full-load torque and 600% of full-load current. The stator current has a low power factor of about 0.35 at the stop. These values have changes between small and large machines, with the smaller one behaving in a better manner. This is because the natural resistance of the windings is proportionally at higher levels. (Three Phase induction Motors)

5) How can speed control of this cage motor be affected?

By varying ac voltage, the variable voltage controllers have control over the speed and torque of a cage motor. This method is cheap and it offers a readily acceptable solution for small and medium-power machines. This could be witnessed in the case of fans, centrifugal pumps and electric hoists. By merely varying the stator voltage, the speed of a 3-phase squirrel-cage induction motor can also be brought to have variations. This is useful particularly for a motor which is driving a blower or centrifugal pump. (Chapter 4: Motor Drives)

6) How does the efficiency of a synchronous motor compare with the three phase cage and wound rotor induction motors?

The two types of three-phase induction motors used are Squirrel-cage induction motor and Wound-rotor or slip ring induction motor. The motors are made up of parts which consist of the Stator; and the Rotor. For the purpose of both these motors, the stator construction and winding arrangement are nearly the same in capacity and usage. The losses in an induction motor comprises of the stray losses and the copper losses. The stray power losses consist of mechanical friction losses, winding losses and iron losses. These losses are almost constant at all loads and are frequently known as fixed losses. The copper losses include the I2R losses in the windings of the motor. An increase in load enhances the current in the motor windings and the I2R losses results. The percent efficiency is low at light loads because the fixed losses occupy a major part of the input power. As the load enhances, the fixed losses become a very small part of the input power. Thus, the efficiency enhances with load. but, as the rated capacity of the motor is surpassed, the copper losses become too much and the efficiency decreases. (Electricity and Motors) synchronous motor contains a DC field winding on its rotor, a three-phase winding on its stator and a method to take it to speed which is typically a squirrel cage winding placed in the salient poles on the rotor. A synchronous motor is initiated as an induction motor or by a separate induction motor. When the motor reaches the speed, the DC excitation is supplied to the field winding and the motor pulls into synchronism. During synchronous operation, no voltage is formed in the auxiliary rotor winding. The DC excitation is supplied by an exciter provided either from the motor's shaft or by a separate motor. Brush-less synchronous motors are presently manufactured to reduce maintenance. An alternator which is mounted on the motor shaft substitutes the exciter. The AC from the alternator is converted to DC by means of silicon diodes and then supplied directly to the field winding. This facility eliminates the exciter, commutator and the field slip rings. The capacity of a synchronous motor to function at leading power factor makes it appropriate to be used for power factor enhancement. When a synchronous motor is used particularly for power factor improvement and not for influencing any mechanical load, it is called a synchronous condenser. Hence, efficiency of a synchronous motor is better in efficiency than that of induction motors. (Electricity and Motors)

7) List the advantages and disadvantages of the synchronous motor, compared with its three-phase motor counterparts.

Advantages of synchronous motor compared to induction motors are: Synchronous motors have the feature of constant speed between no load and full load. They are able to correct the low power factor of an inductive load when they are operated under selected backgrounds. They are frequently used to drive dc generators. Synchronous motors are devised in sizes up to thousands of horsepower. They may be devised as either single-phase or multiphase machines. The disadvantages of a synchronous motor compared to induction motors are: Synchronous motors cannot be started from an idle position by applying three-phase ac power to the stator. When ac is applied to the stator, a high-speed rotating magnetic field appears straight away. This rotating field runs past the rotor poles so quickly that the rotor does not have an opportunity to get started. In effect, the rotor is prevented first in one direction and then the other. A synchronous motor in its simple form has no starting torque. It has torque only when it is in operation at synchronous speed. A squirrel-cage type of winding is supplemented to the rotor of a synchronous motor to enable it to start. (Synchronous Motors)

8) Discuss the main uses for the Synchronous Motor.

A synchronous motor comprises of a DC field winding on its rotor, a three-phase winding on its stator and a way to bring it to speed like a squirrel cage winding placed in the salient poles on the rotor. A synchronous motor is developed as an induction motor or by a separate induction motor. When the… [END OF PREVIEW]

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