A three-phase transformer bank can be easily created by using three single-phase transformers. The two sides of these three transformers can be either connected in a wye or a delta configuration, thus allowing four possible types of connections. These are: • WYE WYE: With the wye-wye (Y-Y) connection, the secondary side is in phase with the primary circuit, and the ratio of primary to secondary voltage is the same as the ratio of turns in each of the phases. A possible connection is shown in Figure 1. Power distribution circuits supplied from a wye-wye bank often create series disturbances in communication circuits (e.g., telephone interference) in their immediate vicinity. One of the advantages of this connection is that when a system is changed from a delta to a four-wire wye to increase system capacity, existing transformers can be used. Figure 1 Y-Y transformer with 0° phase shift between the primary and the secondary sides. • WYE-DELTA: In the Y-Δ connec...
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WHAT IS POWER FACTOR Power factor (PF) is simply the relationship between the active and reactive power on an electricity distribution network and a measure of whether the system’s voltage and current are ‘in phase’. Take, for example, a frothy latte. The coffee body is the ‘active power’ that you can use to do work. The froth on the top is ‘reactive power’; some is useful, but too much is simply a waste the same as the foam you leave behind in your glass. If a network is 100% efficient (i-e if no reactive power is present) its power factor (PF) is 1 or unity. This is the ideal for power transmission, but is practically impossible to attain. Variation in power factor is caused by different types of electrical devices connected to the grid that consume or generate reactive power. Unless this variation is corrected, higher currents are drawn from the grid, leading to grid instability, higher costs and reduced transmission capacity A poor PF results in additional costs for the ele...
The stator winding of a synchronous motor is similar to that of a 3 phase induction motor. The rotor consists of salient poles excited by dc field windings like that of inward-projecting poles of a dc motor. The rotor field windings are energized by direct current passed through slip rings from an external source or from a dc generator, mounted on the same rotor shaft. When the stator winding is energized from a 3 phase supply, a revolving magnetic field, the speed of which is given by N s =120f/P is produced. This speed is called synchronous speed. To enable the synchronous motor to run at the above mentioned synchronous speed the rotor field winding is energized and at the same time brought near to the synchronous speed, by some other means. The rotor poles, which are always equal to that of the stator poles, are pulled to synchronous speed and the two set of poles lock with each other and the rotor starts rotating at synchronous speed. Thus, to run a synchronous motor, the rotor ...
Single phase motors are manufactured in fractional kilowatt range to be operated on single phase supply and for use in numerous applications like ceiling fans, refrigerators, food mixers, hair driers, portable drills, vacuum cleaners, washing machines, sewing machines, electric shavers, office machinery etc. Single phase motors are manufactured in different types to meet the requirements of various applications. Single phase motors are classified on the basis of their construction and starting methods employed. The main types of single phase motors are: (a) Induction motors (b) Synchronous motors (c) Commutator motors The various types of motors under each class are shown as under: Repulsion, repulsion induction and reluctance start motors are not used these days, they have been largely replaced by split phase motors with special capacitors which can be designed to perform equally well as repulsion types. In addition they offer such advantages as lower cost and trouble fr...
The direction of rotation of a universal motor can be changed by either: (i) Reversing the field connection with respect to those of armature; or (ii) By using two field windings wound on the core in opposite directions so that the one connected in series with armature gives clockwise rotation, while the other in series with the armature gives counterclockwise rotation. The second method, i.e, the two field method is used in applications such as motor operated rheostats and servo systems. This method has somewhat simpler connections than the first method. For simple applications like portable drills etc. manual switches are frequently used for reversing the direction of rotation of the motor. Figure 1 (a and b) shows how a DPDT (Double Pole Double Throw) switch and a three position switch may be used for reversing the direction of rotation of single field and double field type of motors respectively. Figure 1 Reversing of a universal motor (a) Armature re...
The stator of a standard split phase motor has two windings viz., a main winding and an auxiliary winding. These two windings have different ratios of resistance to inductive reactance. The windings are connected in parallel across the single phase ac supply. The line current is thus split into two parts; one part flowing through the main winding and the other part flowing through the auxiliary windings. Because of different ratio of resistance to inductive reactance of these two windings current flowing through them will have a time phase difference of 30 electrical degrees or more. In some motors, both the windings are energized continuously, while in most of them, the auxiliary winding is used only during the starting period along with the main winding to develop the required starting torque. When the motor reaches final speed the auxiliary winding is disconnected from the supply. The auxiliary windings can be disconnected by using a centrifugal switch in series with it. During star...
Capacitor type split phase motors are generally similar to the standard split phase construction except for the addition of capacitor and a slightly modified switching arrangement for the two value capacitor type motors. The three types of capacitor split phase motors have been shown in Figure. Figure: Three types of capacitor type split phase motors 1) CAPACITOR START SPLIT PHASE MOTORS These motors are used where high starting torque is required. To accomplish this, capacitors of large values are to be used. Electrolytic capacitors designed for short duty service are available and used in these motors. The motor should come upto its speed quickly and disconnect the capacitor from the windings, otherwise the capacitor will get damaged. The centrifugal switch or the relay that function in auxiliary winding should be very reliable. 2) PERMANENT SPLIT CAPACITOR MOTORS These motors are mainly used for low starting torque loads where they are generally shaft mounted...
We know that direction of a split phase motor can be reversed if the connection of one of the windings is reversed with respect to the other. In an automatic starter this can be achieved by using two contactors i.e., one for forward direction of rotation and the other for reverse direction of rotation. The connections for control circuit and the power circuit diagrams have been shown in Figure. The circuit does not need any further explanation as it is a simple forward reverse starter scheme. Figure: Forward/reverse starter for a permanent split capacitor motor
This type of split phase motors have two capacitors of different values in the auxiliary winding. The motors develop a high starting torque with one capacitor of a higher value and gives a quite running performance with the other capacitor of comparatively lower value. The circuit for forward / reverse operation of a two value capacitor motor has been shown in Figure. Figure Forward/reverse starter for a two value capacitor motor In the circuit shown an electrolytic capacitor has been used for starting purpose and an oil filled capacitor has been used for continuous running operation. During starting both the capacitors are in the circuit. When the motor picks up speed the electrolytic capacitor gets disconnected due to opening of a relay contact. The circuit operation is as follows: When the START-push button is pressed the M contactor is energized which in turn energizes both the main and the auxiliary windings (with both the capacitors in the auxiliary winding circuit) due...
This type of starter uses a current relay in series with the main winding and an auto-transformer in the auxiliary winding to increase the effective value of the capacitors in the auxiliary winding circuit. The circuit has been shown in Figure. In this circuit, when contactor M is energized, a high inrush current flows through the main winding which energizes relay R. Its contact R 1 closes and contact R 2 opens. Due to closing of contact R 1 , the auto transformer winding comes in circuit and the effective value of capacitor is increased. The current relay is designed to pick up at 3 times the full load current and to drop at twice the full-load value. The motor therefore develops high starting torque during starting due to high effective capacitance. When the motor picks up speed, the current drops and the relay R drops out. Due to opening of contact R 1 and closing of contact R 2 , the auto-transformer gets open circuited. The auxiliary winding then gets connec...
The principle of dynamic breaking discussed earlier in connection with poly phase induction motors, also applies well to split phase motors. The procedure, as already explained, is to disconnect the stator windings from ac supply while the motor is running and instead, connect dc supply across the windings. Full wave rectifier is used to obtain dc supply from the available ac source. Precaution must be taken to avoid, simultaneous energization of the stator windings from both ac as well as dc supply. This is taken care of by providing electrical and/or mechanical interlocking of the contactors. A rheostat is provided in the dc circuit to control the time required for the motor to stop. Less is the value of rehostatic value of resistance, more quickly will the rotational energy get converted to electrical energy and get dissipated as heat and hence less will be the time required for the motor to stop. The control diagram for dynamic breaking of a split phase motor is shown in Figure. ...
The capacitor start motors suffer from the disadvantage that they are not easily reversible due to the centrifugal switch connected in the auxiliary winding. The motor cannot be instantaneously reversed by simple control as the auxiliary winding remains disconnected till the motor comes near to zero speed. However, by proper design of the control circuit the motor can be made instantly reversible. This is accomplished by using an electromagnetic relay along with a special two contact centrifugal switch as shown in Figure. The circuit shown in Figure is for a small hoist using a capacitor start motor. The upper and lower limits of travel are controlled by two limit switches viz. LSU and LSD. When UP-push button is pressed, contactor U gets energized. Its contacts U 1 and U 2 energies the main winding. Closing of its contact U 3 causes relay R to get energized through the centrifugal switch contacts A—B. After the energization of relay R auxiliary winding gets ene...
Speed control of split phase motors (without capacitors) is obtained by either pole changing method or by using special winding arrangements. These methods will, however, increase the size of such motors and make them expensive. To obtain two speed operation of a split phase motor, the motor will have two main windings and two auxiliary windings, each set wound for different number of poles. However centrifugal switch is set to open at the lower of the two speeds. In case of fast speed operation, therefore, the rotor has to accelerate to full speed on the main winding only. No problem would arise on this account if the load on motor is moderate. Figure shows the control circuit for a two speed motor with two main and two auxiliary windings. The control circuit permits starting of the motor at slow or fast speed depending upon the setting of the selector switch. During running condition, transfer can be made from one speed to the other speed. Figure: Control circuit for a two speed s...
The speed of a permanently split capacitor motor can be adjusted by connecting it to a variable voltage source such as an auto-transformer. The limitation of this method is that the starting torque developed is very low, especially when the motor is started on low speed. Another limitation is that speed is sensitive to voltage changes on low speed connections. Further speed varies considerably for different loading conditions of the motor. The motor can be started in three different ways using an auto-transformer as shown in Figure 1. In Figure 1 (a) the voltage across the main winding and the auxiliary winding are varied simultaneously resulting in low starting torque for low speed operation. This shortcoming, however, can be overcome by first starting the motor on high speed and then stepping it down to low speed as shown in Figure 1 (a). Good starting torque at all speeds can be obtained if the auxiliary winding is connected across the mains supply and voltage adjustment is p...
There are various methods of controlling the speed of a universal motor. A wide range of speed control is possible by inserting a rheostat in the line circuit which causes variable voltage to appear across the motor terminals resulting in reduced motor speed. Another method of speed control, not very commonly used is by brush shifting mechanism. The speed of the motor increases when the brushes are moved backward relative to the direction of rotation. However, only a limited range of speed control is possible by this method. This is because when the brushes are moved further from the magnetic neutral, commutation worsens. Another speed control method makes use of a tapped field winding. Universal motors are always bipolar. The number of turns on the two poles need not always be the same as the air gap flux is created by series combination of mmfs of the two pole windings. As shown in Figure 1 the field winding having larger number of turns is tapped at three points thus making p...
The construction and principle of working of a universal series motor are similar to a dc series motor. To enable the motor to work satisfactorily on ac supply also, some modifications are required in its construction. The important modification required are: (i) Field structure should be completely laminated to avoid losses due to eddy currents: (ii) To combat effects of armature reaction and resulting poor commutation the armature is to be designed to have lower voltage gradient between adjacent commutator segments than in an equivalent dc motor; (iii) Poor commutation with ac (due to emf induced by the alternating main field flux in a coil undergoing commutation) is improved by using distributed field windings and compensating field windings that are placed in a slotted stator core. When a universal motor is used with ac supply the armature reactance drop exerts a speed lowering effect with increased loading. At the same time at increased loading the effective flux per ac a...
DC motors are classified into three types depending on the way their field windings are excited. Field winding connections for the three types of dc motors have been shown in Figure. Brief description of different types of dc motors are given as follows: SHUNT MOTOR In this type of motor, the field winding is connected in parallel with armature as shown in Figure (a). There are as many number of field coils as there are poles. When connected to supply, constant voltage appears across the field windings (as they are connected in parallel with armature). The field current is therefore constant and is independent of the load current. Shunt field winding usually are designed to have large number of turns of fine wire. Its resistance, therefore, is high enough to limit the shunt field current to about 1 to 4 percent of the rated motor current. A shunt motor operates at nearly constant speed over its normal load range. It has a definite stable no-load speed. The motor is adaptable to ...
Control of a machine can be semi-automatic or fully automatic. There are probably more machines operated by semi-automatic control than by manual or fully automatic controls. Consider, for example, an over-head tank which supplies drinking water to a factory. If we provide a manual switch near the pump motor and depute an operator to switch it ON when water level falls, then this is classified as manual control. Here, the operator has to go to the pump site to fill the tank. For the same pump if a magnetic starter is provided near the pump motor and for its starting, a switch is provided near foreman’s desk it may be classified as a semi-automatic control. A lamp indication or a bell can also be provided near the desk to indicate if the tank is full. The foreman can switch ON the pump from his desk without going to the pump site. Over-flow can also be avoided by switching OFF the pump when the lamp glows or the bell rings. If a float switch is provided in the tank to switch ON the pu...
Having discussed the starting and stopping of a motor by using control devices like push buttons, contactors and over-load relays, we are in a position to discuss the advantages of magnetic control over the manual control. The various advantages are listed as follows: 1) Magnetic control permits installation of power contacts close to motor whereas the actuating control device i.e., a push button switch could be located away from the motor in a position most convenient to the operator. 2) Magnetic control provides safety to the operator as remote operation described above minimizes the danger to the operator of coming into accidental contact with live parts or being exposed to power arc and flashes at the main contacts. 3) The most important advantage of magnetic control is the elimination of dependence on operators’ skill for control of motor performance. Current and torque peaks could be limited thus resulting in less wear and less maintenance. 4) Magnetic control also mak...
When electric motors were first introduced, simple manual switches were used to start and stop the motor. The only protective device used was the fuse. Progress was subsequently made along the lines of improving the reliability, flexibility and make-break performance of the manual switches. In those days one large motor was used to drive a line shaft through a belt pulley arrangement. Individual machines were then connected to the line shaft through belt and pulley arrangements. This system of driving a number of individual loads from a common line shaft and manual switching of motors had many disadvantages as listed below: 1) Starting, stopping and speed control of motor had to be performed by hand every time. 2) The operator had to move a manual switching device from one position to another. 3) Switching of large motors required great physical effort. 4) Operator had to remain continuously alert to watch indicators so as to adjust motor performance according to drive requi...
Most of the large synchronous motors are provided with damper windings, in order to nullify the oscillations of the rotor whenever the synchronous machine is subjected to a periodically varying load. Damper windings are special bars laid into slots cut in the pole face of a synchronous machine and then shorted out on each end by a large shorting ring, similar to the squirrel cage rotor bars. When the stator of such a synchronous machine is connected to the 3-Phase AC supply, the machine starts as a 3-Phase induction machine due to the presence of the damper bars, just like a squirrel cage induction motor. Just as in the case of a 3-Phase squirrel cage induction motor, the applied voltage must be suitably reduced so as to limit the starting current to the safe rated value. Once the motor picks up to a speed near about its synchronous speed, the DC supply to its field winding is connected and the synchronous motor pulls into step i.e. it continues to operate as a Synchronous motor r...
The second method of starting a synchronous motor is to attach an external starting motor (pony motor) to it and bring the synchronous machine to near about its rated speed (but not exactly equal to it, as the synchronization process may fail to indicate the point of closure of the main switch connecting the synchronous machine to the supply system) with the pony motor. Then the output of the synchronous machine can be synchronized or paralleled with its power supply system as a generator, and the pony motor can be detached from the shaft of the machine or the supply to the pony motor can be disconnected. Once the pony motor is turned OFF, the shaft of the machine slows down, the speed of the rotor magnetic field B R falls behind B net , momentarily and the synchronous machine continues to operate as a motor. As soon as it begins to operates as a motor the synchronous motor can be loaded in the usual manner just like any motor. This whole procedure is not as cumbersome as it ...
If the rotating magnetic field of the stator in a synchronous motor rotates at a low enough speed, there will be no problem for the rotor to accelerate and to lock in with the stator’s magnetic field. The speed of the stator magnetic field can then be increased to its rated operating speed by gradually increasing the supply frequency f up to its normal 50- or 60-Hz value. This approach to starting of synchronous motors makes a lot of sense, but there is a big problem: Where from can we get the variable frequency supply? The usual power supply systems generally regulate the frequency to be 50 or 60 Hz as the case may be. However, variable-frequency voltage source can be obtained from a dedicated generator only in the olden days and such a situation was obviously impractical except for very unusual or special drive applications. But the present day solid state power converters offer an easy solution to this. We now have the rectifier- inverter and cyclo-converters, which can be us...
Basically there are three methods that are used to start a synchronous motor: 1) To reduce the speed of the rotating magnetic field of the stator to a low enough value that the rotor can easily accelerate and lock in with it during one half-cycle of the rotating magnetic field’s rotation. This is done by reducing the frequency of the applied electric power. This method is usually followed in the case of inverter-fed synchronous motor operating under variable speed drive applications. 2) To use an external prime mover to accelerate the rotor of synchronous motor near to its synchronous speed and then supply the rotor as well as stator. Of course care should be taken to ensure that the direction of rotation of the rotor as well as that of the rotating magnetic field of the stator are the same. This method is usually followed in the laboratory- the synchronous machine is started as a generator and is then connected to the supply mains by following the synchronization or paralleling pr...
Even though the basic functions of the oil used in transformers are (a) heat conduction and (b) electrical insulation, there are many other properties which make a particular oil eminently suitable. Organic oils of vegetative or animal origin are good insulators but tend to decompose giving rise to acidic by-products which attack the paper or cloth insulation around the conductors. Mineral oils are suitable from the point of electrical properties but tend to form sludge. The properties that are required to be looked into before selecting an oil for transformer application are as follows: INSULTING PROPERTY : This is a very important property. However most of the oils naturally fulfill this. Therefore deterioration in insulating property due to moisture or contamination may be more relevant. VISCOSITY : It is important as it determines the rate of flow of the fluid. Highly viscous fluids need much bigger clearances for adequate heat removal. PURITY : The oil mu...