BEE2123 ELECTRICAL MACHINES Mohd Rusllim Mohamed Ext : 2080 Mohd Rusllim Mohamed Upon completion of the chapter the student should be able to: ◦ State the principle by which machines convert mechanical energy to electrical energy. ◦ Understand the principle of DC generator as it represents a logical behavior of dc motors. ◦ Discuss the operating differences between different types of dc motors Mohd Rusllim Mohamed Direct-current generators are not as common as they used to be, because direct current, when required, is mainly produced by electronic rectifiers. These rectifiers can convert the current of an ac system into direct current without using any moving parts as shown in Fig. 2.1. Yet, still the DC generator is used in many plants to produce power needed to operate large dc motor. Generator Motor Fig 2.1 Full wave rectifier Mohd Rusllim Mohamed A generator is a machine that converts mechanical energy into electrical energy by using the principle of magnetic induction (Fig. 2.2) In this example, the ends of the wire loop have been connected to two sliprings mounted on the shaft, while brushes are used to carry the current from the loop to the outside of the circuit. (Details will be discussed later) Fig 2.2 Principle of magnetic induction in DC machine Mohd Rusllim Mohamed Recap : Whenever a conductor is moved within a magnetic field in such a way that the conductor cuts across magnetic lines of flux, voltage is generated in the conductor. The AMOUNT of voltage generated depends on: i. the strength of the magnetic field, ii. the angle at which the conductor cuts the magnetic field, iii. the speed at which the conductor is moved, and iv. the length of the conductor within the magnetic field Mohd Rusllim Mohamed The POLARITY of the voltage depends on the direction of the magnetic lines of flux and the direction of movement of the conductor. To determine the direction of current in a given situation, the LEFT-HAND RULE FOR GENERATORS is used as shown in Fig. 2.3. •thumb in the direction the conductor is being moved •forefinger in the direction of magnetic flux (from north to south) •middle finger will then point in the direction of current flow in an external circuit to which the voltage is applied Fig 2.3 Left Hand Rules Mohd Rusllim Mohamed The simplest elementary generator that can be built is an ac generator. Basic generating principles are most easily explained through the use of the elementary ac generator. For this reason, the ac generator will be discussed first. The dc generator will be discussed later. An elementary generator (Fig. 2.4) consists of a wire loop mounted on the shaft, so that it can be rotated in a stationary magnetic field. This will produce an induced emf in the loop. Sliding contacts (brushes) connect the loop to an external circuit load in order to pick up or use the induced emf. Fig 2.4 Elementary Generator Mohd Rusllim Mohamed The pole pieces (marked N and S) provide the magnetic field. The pole pieces are shaped and positioned as shown to concentrate the magnetic field as close as possible to the wire loop. The loop of wire that rotates through the field is called the ARMATURE. The ends of the armature loop are connected to rings called SLIP RINGS. They rotate with the armature. The brushes, usually made of carbon, with wires attached to them, ride against the rings. The generated voltage appears across these brushes. (These brushes transfer power from the battery to the commutator as the motor spins – discussed later in dc elementary generator). Mohd Rusllim Mohamed In Fig. 2.5, the armature winding has been left out so that it is easier to see the rotor action. The key thing to notice is that as the armature passes through the horizontal position, the poles of the electromagnet flip. Because of the flip, the north pole of the electromagnet is always above the axle so it can repel the field magnet's north pole and attract the field magnet's south pole. It's this magnetic attraction and repulsion that causes the armature to rotate Armature winding has been left out N s Fig. 2.5 Rotor being rotated Mohd Rusllim Mohamed In Figure 2.6, an end view of the shaft and wire loop is shown. At this particular instant, the loop of wire (the black and white conductors of the loop) is parallel to the magnetic lines of flux, and no cutting action is taking place. Since the lines of flux are not being cut by the loop, no emf is induced in the conductors, and the meter at this position indicates zero. This position is called the NEUTRAL PLANE. Fig. 2.6 00 Position (Neutral Plane) Mohd Rusllim Mohamed In Figure 2.7, the shaft has been turned 900 clockwise, the conductors cut through more and more lines of flux, and voltage is induced in the conductor. at a continually increasing angle , the induced emf in the conductors builds up from zero to a maximum value or peak value. Observe that from 00 to 900, the black conductor cuts DOWN through the field. At the same time the white conductor cuts UP through the field. The induced emfs in the conductors are series-adding. This means the resultant voltage across the brushes (the terminal voltage) is the sum of the two induced voltages. The meter at position B reads maximum value. Fig. 2.7 900 Position Mohd Rusllim Mohamed After another 900 of rotation (Fig. 2.8), the loop has completed 1800 of rotation and is again parallel to the lines of flux. As the loop was turned, the voltage decreased until it again reached zero. Note that : From 00 to 1800 the conductors of the armature loop have been moving in the same direction through the magnetic field. Therefore, the polarity of the induced voltage has remained the same Fig. 2.8 1800 Position Mohd Rusllim Mohamed As the loop continues to turn, the conductors again cut the lines of magnetic flux (Fig. 2.9). This time, however, the conductor that previously cut through the flux lines of the south magnetic field is cutting the lines of the north magnetic field, and vice-versa. Since the conductors are cutting the flux lines of opposite magnetic polarity, the polarity of the induced voltage reverses. After 270' of rotation, the loop has rotated to the position shown, and the maximum terminal voltage will be the same as it was from A to C except that the polarity is reversed. Fig. 2.9 2700 Position Mohd Rusllim Mohamed After another 900 of rotation, the loop has completed one rotation of 3600 and returned to its starting position (Fig. 2.10). The voltage decreased from its negative peak back to zero. Notice that the voltage produced in the armature is an alternating polarity. The voltage produced in all rotating armatures is alternating voltage. Fig. 2.10 3600 Position Mohd Rusllim Mohamed Observes ◦ The meter direction the conductors of the armature loop Direction of the ◦ current flow ◦ ◦ Fig. 2.11 Output voltage of an elementary generator during one revolution Mohd Rusllim Mohamed Since DC generators must produce DC current instead of AC current, a device must be used to change the AC voltage produced in the armature windings into DC voltage. This job is performed by the commutator. The commutator is constructed from a copper ring split into segments with insulating material between the segments (See next page). Brushes riding against the commutator segments carry the power to the outside circuit. The commutator in a dc generator replaces the slip rings of the ac generator. This is the main difference in their construction. The commutator mechanically reverses the armature loop connections to the external circuit. Fig. 2.12 Commutator Mohd Rusllim Mohamed The armature has an axle, and the commutator is attached to the axle. In the diagram to the right, you can see three different views of the same armature: front, side and endon. In the end-on view, the winding is eliminated to make the commutator more obvious. You can see that the commutator is simply a pair of plates attached to the axle. These plates provide the two connections for the coil of the electromagnet. Fig 2.13 Armature with commutator view Mohd Rusllim Mohamed The diagram at the right shows how the commutator and brushes work together to let current flow to the electromagnet, and also to flip the direction that the electrons are flowing at just the right moment. The contacts of the commutator are attached to the axle of the electromagnet, so they spin with the magnet. The brushes are just two pieces of springy metal or carbon that make contact with the contacts of the commutator. Through this process the commutator changes the generated ac voltage to a pulsating dc voltage which also known as commutation process. Fig 2.14 Brushes and commutator Mohd Rusllim Mohamed The loop is parallel to the magnetic lines of flux, and no voltage is induced in the loop Note that the brushes make contact with both of the commutator segments at this time. The position is called neutral plane. Fig. 2.15 00 Position (DC Neutral Plane) Mohd Rusllim Mohamed As the loop rotates, the conductors begin to cut through the magnetic lines of flux. The conductor cutting through the south magnetic field is connected to the positive brush, and the conductor cutting through the north magnetic field is connected to the negative brush (Fig 2.16). Since the loop is cutting lines of flux, a voltage is induced into the loop. After 900 of rotation, the voltage reaches its most positive point. Fig. 2.16 900 Position (DC) Mohd Rusllim Mohamed As the loop continues to rotate, the voltage decreases to zero. After 1800 of rotation, the conductors are again parallel to the lines of flux, and no voltage is induced in the loop. Note that the brushes again make contact with both segments of the commutator at the time when there is no induced voltage in the conductors (Fig 2.17). Fig. 2.17 1800 Position (DC) Mohd Rusllim Mohamed During the next 900 of rotation, the conductors again cut through the magnetic lines of flux. This time, however, the conductor that previously cut through the south magnetic field is now cutting the flux lines of the north field, and vice-versa. (Fig 2.18). Since these conductors are cutting the lines of flux of opposite magnetic polarities, the polarity of induced voltage is different for each of the conductors. The commutator, however, maintains the correct polarity to each brush. The conductor cutting through the north magnetic field will always be connected to the negative brush, and the conductor cutting through the south field will always be connected to the positive brush. Since the polarity at the brushes has remained constant, the voltage will increase to its peak value in the same direction. Fig. 2.18 2700 Position (DC) Mohd Rusllim Mohamed As the loop continues to rotate (Fig 2.19), the induced voltage again decreases to zero when the conductors become parallel to the magnetic lines of flux. Notice that during this 3600 rotation of the loop the polarity of voltage remained the same for both halves of the waveform. This is called rectified DC voltage. The voltage is pulsating. It does turn on and off, but it never reverses polarity. Since the polarity for each brush remains constant, the output voltage is DC. Fig. 2.19 00 Position (DC Neutral Plane) Mohd Rusllim Mohamed Observes ◦ The meter ◦ direction the conductors of the armature loop ◦ Direction of the current flow Fig. 2.20 Effects of commutation http://www.sciencejoywagon.com/physicszone/lesson/otherpub/wfendt/generatorengl.htm Mohd Rusllim Mohamed To increase the amount of output voltage, it is common practice to increase the number of turns of wire for each loop (Fig 2.20). If a loop contains 20 turns of wire, the induced voltage will be 20 times greater than that for a single-loop conductor. The reason for this is that each loop is connected in series with the other loops. Since the loops form a series path, the voltage induced in the loops will add. In this example, if each loop has an induced voltage of 2V, the total voltage for this winding would be 40V (2V x 20 loops = 40 V). Fig. 2.21 Effects of additional turns Mohd Rusllim Mohamed When more than one loop is used, the average output voltage is higher and there is less pulsation of the rectified voltage. Since there are four segments in the commutator, a new segment passes each brush every 900 instead of every 1800. Since there are now four commutator segments in the commutator and only two brushes, the voltage cannot fall any lower than at point A. Therefore, the ripple is limited to the rise and fall between points A and B on the graph. By adding more armature coils, the ripple effect can be further reduced. Decreasing ripple in this way increases the effective voltage of the output. Fig. 2.22 Effects of additional coils Mohd Rusllim Mohamed Q.1Generators convert mechanical motion to electrical energy using what principle? Q.2 What rule should you use to determine the direction of induced emf in a coil? Q.3 What is the purpose of the slip rings? Q.4 Why is no emf induced in a rotating coil when it passes through the neutral plane? Q.5 What component causes a generator to produce dc voltage rather than ac voltage at its output terminals? Q.6 At what point should brush contact change from one commutator segment to the next? Q.7 An elementary, single coil, dc generator will have an output voltage with how many pulsations per revolution? Q.8 How many commutator segments are required in a two-coil generator? Mohd Rusllim Mohamed The actual construction and operation of a practical dc generator differs somewhat from our elementary generators Nearly all practical generators use electromagnetic poles instead of the permanent magnets used in our elementary generator The main advantages of using electromagnetic poles are: (1) increased field strength and (2) possible to control the strength of the fields. By varying the input voltage, the field strength is varied. By varying the field strength, the output voltage of the generator can be controlled. Fig. 2.23 Four-pole generator (without armature) Mohd Rusllim Mohamed If you ever have the chance to take apart a small electric motor, you will find that it contains the same pieces described above: two small permanent magnets, a commutator, two brushes, and an electromagnet made by winding wire around a piece of metal. Almost always, however, the rotor will have three poles rather than the two poles as shown in this article. There are two good reasons for a motor to have three poles: ◦ It causes the motor to have better dynamics. In a two-pole motor, if the electromagnet is at the balance point, perfectly horizontal between the two poles of the field magnet when the motor starts, you can imagine the armature getting "stuck" there. That never happens in a three-pole motor. ◦ Each time the commutator hits the point where it flips the field in a twopole motor, the commutator shorts out the battery (directly connects the positive and negative terminals) for a moment. This shorting wastes energy and drains the battery needlessly. A three-pole motor solves this problem as well. It is possible to have any number of poles, depending on the size of the motor and the specific application it is being used in. Mohd Rusllim Mohamed More loops of wire = higher rectified voltage In practical, loops are generally placed in slots of an iron core (Fig 2.24) The iron acts as a magnetic conductor by providing a low-reluctance path for magnetic lines of flux to increase the inductance of the loops and provide a higher induced voltage. The commutator is connected to the slotted iron core. The entire assembly of iron core, commutator, and windings is called the armature (Fig 2.25). The windings of armatures are connected in different ways depending on the requirements of the machine. Fig. 2.25 DC machine armature Fig. 2.24 loops of wire are wound around slot in a metal core Mohd Rusllim Mohamed Lap Wound Armatures ◦ are used in machines designed for low voltage and high current ◦ armatures are constructed with large wire because of high current ◦ Eg: - are used is in the starter motor of almost all automobiles ◦ The windings of a lap wound armature are connected in parallel (Fig 2.26). This permits the current capacity of each winding to be added and provides a higher operating current ◦ No of current path, C=2p ; p=no of poles Mohd Rusllim Mohamed Fig. 2.26 Lap wound armatures Wave Wound Armatures ◦ are used in machines designed for high voltage and low current ◦ their windings connected in series (Fig 2.27) ◦ When the windings are connected in series, the voltage of each winding adds, but the current capacity remains the same ◦ are used is in the small generator in hand-cranked megohmmeters ◦ No of current path, C=2 Fig. 2.27 Wave wound armatures Mohd Rusllim Mohamed Frogleg Wound Armatures ◦ the most used in practical nowadays ◦ designed for use with moderate current and moderate armatures voltage ◦ the windings are connected in series parallel (Fig2.28). ◦ Most large DC machines use frogleg wound armatures. Fig. 2.28 Frogleg wound armatures Mohd Rusllim Mohamed Mohd Rusllim Mohamed Most DC machines use wound electromagnets to provide the magnetic field. Two types of field windings are used : ◦ series field, and ◦ the shunt field. Mohd Rusllim Mohamed Series field windings ◦ are so named because they are connected in series with the armature ◦ are made with relatively few windings turns of very large wire and have a very low resistance ◦ usually found in large horsepower machines wound with square or rectangular wire. The use of square wire permits the windings to be laid closer together, which increases the number of turns that can be wound in a particular space (Fig 2.29) ◦ Square and rectangular wire can also be made physically smaller than round wire and still contain the same surface area (Fig 2.30) Fig. 2.29 square wire permits more turns than round wire in the same area Fig. 2.30 Square wire contains more surface than round wire Shunt field windings ◦ is constructed with relatively many turns of small wire, thus, it has a much higher resistance than the series field. ◦ is intended to be connected in parallel with, or shunt, the armature. ◦ high resistance is used to limit current flow through the field. Mohd Rusllim Mohamed When a DC machine uses both series and shunt fields, each pole piece will contain both windings (Fig 2.31). The windings are wound on the pole pieces in such a manner that when current flows through the winding it will produce alternate magnetic polarities. Fig. 2.31 Both series and shunt field windings are contained in each pole piece S – series field F – shunt field Mohd Rusllim Mohamed Winding armature field Self excited Wave C=2 Lap C=2p Separately Excited Frogleg series shunt compound Mohd Rusllim Mohamed Electric motors are everywhere! In a house, almost every mechanical movement that you see around you is caused by an DC (direct current) electric motor. An electric motor is a device that transforms electrical energy into mechanical energy by using the motor effect. Mohd Rusllim Mohamed DC motors consist of rotor-mounted windings (armature) and stationary windings (field poles). In all DC motors, except permanent magnet motors, current must be conducted to the armature windings by passing current through carbon brushes that slide over a set of copper surfaces called a commutator, which is mounted on the rotor. Parts of an electric motor The commutator bars are soldered to armature coils. The brush/commutator combination makes a sliding switch that energizes particular portions of the armature, based on the position of the rotor. This process creates north and south magnetic poles on the rotor that are attracted to or repelled by north and south poles on the stator, which are formed by passing direct current through the field windings. It's this magnetic attraction and repulsion that causes the rotor to rotate. Mohd Rusllim Mohamed The greatest advantage of DC motors may be speed control. Since speed is directly proportional to armature voltage and inversely proportional to the magnetic flux produced by the poles, adjusting the armature voltage and/or the field current will change the rotor speed. Today, adjustable frequency drives can provide precise speed control for AC motors, but they do so at the expense of power quality, as the solid-state switching devices in the drives produce a rich harmonic spectrum. The DC motor has no adverse effects on power quality. Mohd Rusllim Mohamed Power supply, initial cost, and maintenance requirements are the negatives associated with DC motors Rectification must be provided for any DC motors supplied from the grid. It can also cause power quality problems. The construction of a DC motor is considerably more complicated and expensive than that of an AC motor, primarily due to the commutator, brushes, and armature windings. An induction motor requires no commutator or brushes, and most use cast squirrelcage rotor bars instead of true windings — two huge simplifications. Mohd Rusllim Mohamed Series motors connect the field windings in series with the armature. Series motors lack good speed regulation, but are well-suited for high-torque loads like power tools and automobile starters because of their high torque production and compact size. Ra ia M Rf Ea VT (dc supply) VT Ea ia ( Ra R f ) note : ia iL Ea K1K 2 I a K Mohd Rusllim Mohamed Pcf=ia2Rf Pin= VTiL P Pm Pca=ia2Ra P is normally given Pin = Pout + total losses Where, Pca =armature copper loss Pcf =field copper loss P=stray, mech etc Pm= Ea ia Pout 60 P 2 N 60 Po for output / load torque , o 2 N 60 Pm for mechanical torque, m 2 N P Efficiency , out Pin Mohd Rusllim Mohamed Example 1: A dc machine in Figure 1 is consumed a 6.5kW when the 12.5 A of armature current is passing thru the armature and field resistance of 3.3 and 2.0 respectively. Assume stray losses of 1.2kW. Calculate a) terminal voltage, VT b) back emf, Ea c) net torque if the speed is at 3560rpm d) efficiency of the machine [520V, 453.75V, 12N-m, 68.8%] M Rf Ra ia Ea VT (dc supply) Figure 1 Mohd Rusllim Mohamed Pin 6.5kW , I a 12.5 A, Ra 3.3, R f 2.0, P 1.2kW a ). Pin VT I L 60 Po 2 N Po Pm P , Pm Ea I a c). o sin ce, I L I a Pm 453.73(12.5) 5672W Pin 6.5k VT 520V I L 12.5 Po 5672 1.2k 4472W b). Ea VT I a ( Ra R f ) 520 12.5(3.3 2.0) 453.75V 60(4472) o 12 Nm 2 (3560) d ). % Po 100% Pin 4472 100% 6.5k 68.8% Abu Zaharin Ahmad Example 2: A 600V 150-hp dc machine in Figure 2 operates at its full rated load at 600 rpm. The armature and field resistance are 0.12 and 0.04 respectively. The machine draws 200A at full load. Assume stray losses 1700W. Determine a) the armature back emf at full load, Ea b) developed power and developed torque c) assume that a change in load results in the line current dropping to 150A. Find the new speed in rpm and new developed torque. {Hint: Ea=K} M Rf Ra ia Ea VT (dc supply) Figure 2 [Ea=568V, P=113.6kW, T=1808Nm, n=608.45rpm, T=1017Nm] Mohd Rusllim Mohamed VT 600V , I a 200 A, Ra 0.12, R f 0.04, P 1700W , n 600rpm a). Ea VT I a ( Ra R f ) 600 200(0.12 0.04) 568V 60 Pm b). m 2 N Pm Ea I a 568(200) 113.6kW 60(113.6k ) o 1808 Nm 2 (600) c). Ea 2 K 2 Ea1 K 1 Since K is cons tan t Ea 2 Ea 2 2 1 or n2 n1 Ea1 Ea1 Ea 2 600 150(0.12 0.04) 576V 576 n2 600 608.5rpm 568 m Ea 2 I a 2 576(150) 86.4 Nm Abu Zaharin Ahmad Shunt motors use highresistance field windings connected in parallel with the armature. Varying the field resistance changes the motor speed. Shunt motors are prone to armature reaction, a distortion and weakening of the flux generated by the poles that results in commutation problems evidenced by sparking at the brushes. Installing additional poles, called interpoles, on the stator between the main poles wired in series with the armature reduces armature reaction. ia iL Ra if M Ea Rf VT (dc supply) VT Ea ia ( Ra ) note : iL ia i f VT i f R f Mohd Rusllim Mohamed Pcf=if2Rf Pin=VTiL P Pm Pca=ia2Ra P is normally given Pin = Pout + total losses Where, Pca =armature copper loss Pcf =field copper loss P=stray, mech etc Pm= Ea ia Pout 60 P 2 N 60 Po for output / load torque , o 2 N 60 Pm for mechanical torque, m 2 N P Efficiency , out Pin Mohd Rusllim Mohamed Example : ◦ A voltage of 230V is applied to armature of a machines in Fig 3 results in a full load armature currents of 205A. Assume that armature resistance is 0.2. Find the back emf, net power and torque by assuming the rotational losses are 1445W at full load speed of 1750rpm. [189V, 37.3kW, 203.5Nm] Mohd Rusllim Mohamed VT 230V , I a 205 A, Ra 0.2, P 1445W , n 1750rpm a). Ea VT I a Ra 230 205(0.2) 189V 60 Po c). o 2 N 60(37.3k ) o 203.5 Nm 2 (1750) b) Po Pm P , Pm Ea I a Pm 189(205) 38.745kW Po 38.745k 1445 37.3kW Abu Zaharin Ahmad the concept of the series and shunt designs are combined. ia Ra Rf2 iL if M Ea VT Ea ia ( Ra R f 2 ) Rf1 VT (dc supply) note : iL ia i f VT i f R f 1 Mohd Rusllim Mohamed Pca=ia2Ra Pcf2=ia2Rf2 Pin=VTiL Pm P Pout Pcf1=if2Rf1 60 P P is normally given Pin = Pout + total losses Where, Pca =armature copper loss Pcf =field copper loss P=stray, mech etc Pm= Ea ia 2 N 60 Po for output / load torque , o 2 N 60 Pm for mechanical torque, m 2 N P Efficiency , out Pin Mohd Rusllim Mohamed There is no direct connection between the armature and field winding resistance DC field current is supplied by an independent source ◦ (such as battery or another generator or prime mover called an exciter) Circuit analysis: 2 pnZ Ea K f if n K f n 60C If Rf Lf Vf Where p= no of pole pair n= speed (rpm) Z=no of conductor =Flux per pole (Wb) C= no of current/parallel path =2p (lap winding) =2 (wave winding) ia KVL: note : ia iL M Ra La Ea Vf i f Rf VT Ea ia Ra VT PMDC is a dc motor whose poles are made of permanent magnets. Do not require external field circuit, no copper losses No field winding, size smaller than other types dc motors Disadvantage: cannot produce high flux density, lower induce voltage Torque –speed characteristic for shunt and separately excited dc motor E I Developed torque, a a E I a a 2n V Ea Ea R 2 n a same as separately excited , VK f I f 2Ra 2 2 K f I f n 2Ra m VK f I f c 2Ra Starting torque K f 2 I f 2n slope 2Ra =0 n=0 n Mohd Rusllim Mohamed nNL n By referring to the Torque –speed characteristic for shunt and separately excited dc motor VK I f f 2Ra 2 2 K f I f n 2Ra note that, there are three variables that can influence the speed of the motor, V Variables If Ra Thus, there are three methods of controlling the speed of the shunt and separately excited dc motor, i. Armature terminal – voltage speed control ii. Field speed control iii. Armature resistance speed control Mohd Rusllim Mohamed i. Armature resistance speed control - Speed may be controlled by changing Ra The total resistance of armature may be varied by means of a rheostat in series with the armature The armature speed control rheostat also serves as a starting resistor. From -n characteristic, VK f I f start c 2 R a K f 2 I f 2n slope 2Ra Will be changed Mohd Rusllim Mohamed Torque –speed characteristic m Ra1 Ra1 < Ra2 < Ra3 Ra2 Ra3 n3 n2 n1 nNL n Mohd Rusllim Mohamed Advantages armature resistance speed control: i. Starting and speed control functions may be combined in one rheostat ii. The speed range begins at zero speed iii. The cost is much less than other system that permit control down to zero speed iv. Simple method Disadvantages armature resistance speed control : i. Introduce more power loss in rheostat ii. Speed regulation is poor (S.R difference nLoaded & nno loaded) iii. Low efficiency due to rheostat Mohd Rusllim Mohamed ii. Field Speed Control - Rheostat in series with field winding (shunt or separately ect.) If field current, If is varied, hence flux is also varied Not suitable for series field Refer to -n characteristic, - Slope and nNL will be changed Mohd Rusllim Mohamed Torque –speed characteristic m If1 < If2 < If3 1 < 2 < 3 Base speed n1 n2 n3 nNL1 nNL2 nNL3 Mohd Rusllim Mohamed n Advantages field speed control: i. Allows for controlling at or above the base speed ii. The cost of the rheostat is cheaper because If is small value Disadvantages field speed control : i. Speed regulation is poor (S.R difference nLoaded & nno loaded) ii. At high speed, flux is small, thus causes the speed of the machines becomes unstable iii. At high speed also, the machines is unstable mechanically, thus there is an upper speed limit Mohd Rusllim Mohamed iii. Armature terminal – voltage speed control - Use power electronics controller - - AC supply rectifier DC supply chopper Supply voltage to the armature is controlled Constant speed regulation From -n characteristic, - C and nNL will be change - Slope constant Mohd Rusllim Mohamed Torque –speed characteristic m V1 < V2 < V3 n3 n2 n1 nNL1 nNL1 n nNL3 Mohd Rusllim Mohamed Advantages armature terminal voltage speed control: i. Does not change the speed regulation ii. Speed is easily controlled from zero to maximum safe speed Disadvantages armature terminal voltage speed control : i. Cost is higher because of using power electronic controller Mohd Rusllim Mohamed Mohd Rusllim Mohamed A 120v shunt motor has the following parameters: Ra = 0.40 Ω, Rf= 120 Ω and rotational 240W. On full load the input current is 19.5A and the motor runs at 1200rpm. Determine: a) b) c) d) Developed power Output power Output torque Efficiency Mohd Rusllim Mohamed Given VT = 120 V, Ra = 0.40 Ω, Rf= 120 Ω , Pu= 240 W, IL= 19.5 A, nr=1200 rpm a) Developed power • 1st find If =VT/Rf • Ia=IL- If • Ea= VT – IaRa • Pm= EaIa b) Output power Pout = Pm- Pu c) Output torque To=60Po /2n d) Efficiency Pin = VTIL =(Po/Pin) *100 Mohd Rusllim Mohamed 460 V dc motor drives as 50 hp and at 900 rmp. The stunt field resistance as 57.5 Ω and the armature resistance is 0.24 Ω of the motor efficiency is 82%, determine a) the rotational loss b) the new developed power if the motor speed is reduced to 750 rpm using armature resistance speed control Note : 1 horse power = 746watt. Mohd Rusllim Mohamed Given VT = 460 V , Po= 50 hp (due to word of drives as otherwise rated output) Ra = 0.24 Ω , Rf= 57.5 Ω , nr= 900 rpm, = 82% a) Pu? • Po = 50hp x 746 W • =(Po/Pin) *100, hence Pin • Pin = VTIL, hence IL • If =VT/Rf • Ia=IL- If • Ea= VT – IaRa • Pm= EaIa • Pu = Pm- Pout Mohd Rusllim Mohamed Ea=KKn ◦ Ea1 Kn1 ◦ Ea2 Kn2 ◦ For Ea1 / Ea2 Kn1/Kn2 ,then Ea2 Note that for armature resistance speed control Torque remains the same 𝜏1 = 𝜏2 60𝑃𝑚 2𝜋𝑛1 = 60𝐸𝑎1𝐼𝑎1 2𝜋𝑛1 60𝑃𝑚2 2𝜋𝑛1 = 60𝑃𝑚2 2𝜋𝑛2 Mohd Rusllim Mohamed There are two factors affecting the performance of dc machine 1. Armature reaction 2. Armature inductance Definition of armature reaction: 1. It is the term used to describe the effects of the armature mmf on the operation of a dc machine as a "generator" no matter whether it is a generator or motor. 2. It effects both the flux distribution and the flux magnitude in the machine. 3. The distortion of the flux in a machine is called armature reaction Two effects of armature reaction: 1. Neutral Plane Shift 2. Flux Weakening Effect on flux distribution: Neutral plane shift ◦ When current is flowing in the field winding, hence a flux is produced across the machine which flows from the North pole to the South pole. ◦ Initially the pole flux is uniformly distributed and the magnetic neutral plane is vertical Effect on flux distribution: Neutral plane shift ◦ effect by the air gap on the flux field causes the distribution of flux is no longer uniform across the rotor. ◦ There are two points on the periphery of the rotor where B= 0. Effect on flux distribution: Neutral plane shift ◦ when a load connected to the machines a resulting magnetic field produced in the armature ◦ If the armature is rotated at a speed by an external torque each armature coil experiences a change in flux t as it rotates. ◦ A voltage is generated across the terminals of each winding according to the equation e = t Effect on flux distribution: Neutral plane shift ◦ Both rotor and pole fluxes (flux produced by the field winding and the flux produced by the armature winding) are added and subtracted together accordingly ◦ The fields interact to produce a different flux distribution in the rotor. ◦ Thus, the flux on the middle line, between the two field poles, is no longer zero. Effect on flux distribution: Neutral plane shift The combined flux in the machine has the effect of strengthening or weakening the flux in the pole. Neutral axis is therefore shifted in the direction of motion. The result is current flow circulating between the shorted segments and large sparks at the brushes. The ending result is arcing and sparking at the brushes. Solution to this problem: ◦ placing an additional poles on the neutral axis or mid-point that will produce flux density component, which counter-acts that produced by the armature. Effect on flux magnitude: Flux Weakening Most machine operate at saturation point When the armature reaction happen, at location pole surface: ◦ The add of rotor mmf to pole mmf only make a small increase in flux ◦ The subtract of rotor mmf from pole mmf make a large decrease in flux. ◦ The result is the total average flux under entire pole face is decreased. ◦ This is called Flux Weakening d –flux decrease under subtracting section of poles When rotor turns, thus we have inductance value, e1 = L(di/dt). Lat say current ia1. That means, we have ability to store energy If the machine is turn ‘off’, thus, e1 will decreased. This will affect the current as well. Say ia2. When the machine is turn ‘on’ again, it will produce e2 while e1 is still inside. The current now is reversed direction from previous (decreasing) current. Thus, it will cause sparking resulting the same aching problem caused by neutral plane shift.