10. Starting Method for Induction Motors

1 3rd Class / AC MachinesDr. Inaam Ibrahim3410. Starting Method for Induction MotorsA 3-phase Induction motor is theoretically self stator of an Induction motor consists of 3-phase windings,which when connected to a 3-phase supply creates a rotatingmagnetic field. This will link and cut the rotor conductorswhich in turn will induce a current in the rotor conductors andcreate a rotor magnetic field. The magnetic field created by therotor will interact with the rotating magnetic field in the statorand produce , 3-phase Induction Motors employ a startingmethod not to provide a Starting torque at the rotor, butbecause of the following reasons;1) Reduce heavy Starting currents and prevent motor ) Provide overload and no-voltage are many methods in use to start 3-phase inductionmotors. Some of the common methods are; Direct On-Line Starter (DOL) Star-Delta Starter Auto Transformer Starter3rd Class / AC MachinesDr.

2 Inaam Ibrahim35 Rotor Impedance Starter Power Electronics StarterDirect On-Line Starter (DOL)The Direct On-Line (DOL) starter is the simplest and themost inexpensive of all Starting methods and is usually usedfor squirrel cage Induction Motors . It directly connects thecontacts of the motor to the full supply voltage. The startingcurrent is very large, normally 6 to 8 times the rated Starting torque is likely to be to 2 times the full loadtorque. In order to avoid excessive voltage drops in the supplyline due to high Starting currents, the DOL starter is used onlyfor Motors with a rating of less than 5 KWThere are safety mechanisms inside the DOL starterwhich provides protection to the motor as well as the operatorof the power and control circuits of Induction motorwith DOL starter are shown in figure(1).* K1M Main contactor3rd Class / AC MachinesDr. Inaam Ibrahim36 Power CircuitControl CircuitFig.(1): power and control circuits of with DOL starterThe DOL starter consists of a coil operated contactorK1M controlled by start and stop push buttons.

3 On pressing the startpush button S1, the contactor coil K1M is energized from line three mains contacts (1-2), (3-4), and (5-6) in fig. (1) are motor is thus connected to the supply. When the stop push3rd Class / AC MachinesDr. Inaam Ibrahim37button S2 is pressed, the supply through the contactor K1M isdisconnected. Since the K1M is de-energized, the main contacts (1-2), (3-4), and (5-6) are opened. The supply to motor is disconnectedand the motor StarterThe star delta Starting is a very common type of starterand extensively used, compared to the other types of the Method used reduced supply voltage in Starting . Figure(2)shows the connection of a 3phase Induction motor with a star delta Method achieved low Starting current by firstconnecting the stator winding in star configuration, and thenafter the motor reaches a certain speed, throw switch changesthe winding arrangements from star to delta connecting the stator windings, first in star and then indelta, the line current drawn by the motor at Starting is reducedto one-third as compared to Starting current with the windingsconnected in delta.

4 At the time of Starting when the statorwindings are start connected, each stator phase gets voltage , where is the line voltage. Since the torque developed3rd Class / AC MachinesDr. Inaam Ibrahim38by an Induction motor is proportional to the square of theapplied voltage, star- delta Starting reduced the Starting torque toone third that obtainable by direct delta Starting . K2M Main Contactor K3M Delta Contactor K1M Star Contactor F1 Thermal Overload RelayFig.(2) Induction Motor with Star Delta Starter3rd Class / AC MachinesDr. Inaam Ibrahim39 Auto Transformer StarterThe operation principle of auto transformer Method issimilar to the star delta starter Method . The startingcurrent is limited by (using a three phase autotransformer) reduce the initial stator applied auto transformer starter is more expensive, morecomplicated in operation and bulkier in construction whencompared with the star delta starter Method .

5 But an autotransformer starter is suitable for both star and deltaconnected Motors , and the Starting current and torque canbe adjusted to a desired value by taking the correcttapping from the auto transformer. When the star deltamethod is considered, voltage can be adjusted only byfactor (3) shows the connection of a 3phase inductionmotor with auto transformer Class / AC MachinesDr. Inaam Ibrahim40 Fig.(3) shows with auto transformer Impedance StarterThismethod allows external resistance to be connected tothe rotor through slip rings andbrushes. Initially, the rotorresistance is set to maximum and is then gradually decreasedas the motor speed increases, until it becomes Class / AC MachinesDr. Inaam Ibrahim41 The rotor impedance Starting mechanism is usually verybulky and expensive when compared with other methods. Italso has very high maintenance costs. Also, a considerableamount of heat is generated through the resistors when currentruns through them.

6 The Starting frequency is also limited inthis Method . However, the rotor impedance Method allows themotor to be started while on (4) shows theconnection of a 3phase Induction motor with rotor (4) Shows the rotor Class / AC MachinesDr. Inaam Ibrahim42 Example (9):It is desired to install a 3-phase cage Induction motor restrictingthe maximum line current drawn from a 400 V 3-phase supplyto 120 A. if the Starting current is 6 times full load current, whatis the maximum permissible full load kVA of the motor is directly connected to the is connected through an auto-transformer with atapping of 60% is designed for used with star-delta startingMaximum line current, Starting current Since the maximum line current drawn from the supply is 120A Maximum permissible rating of the motor 3rd Class / AC MachinesDr. Inaam Starting Maximum permissible rating of the motor Starting Maximum permissible kVA rating of the motor 3rd Class / AC MachinesDr.

7 Inaam Ibrahim4411. SPEED CONTROL OF Induction MOTORSThe speed of an Induction motor is given asN = 120f/p (1-S).So obviously the speed of an Induction motor can be controlledby varying any of three factors namely supply frequency f,number of pole P or slip main methods employed for speed control of inductionmotors are as voltage frequency resistance energy basic principles of these methods are described belowPole changingThe number of stator poles can be change by Multiple stator windings Method of consequent poles Pole amplitude modulation (PWM)3rd Class / AC MachinesDr. Inaam Ibrahim45 The methods of speed control by pole changing aresuitable for cage Motors only because the cage rotorautomatically develops number of poles equal to thepoles of stator Multiple stator windingsIn this Method the stator is provided with two separatewindings which are wound for two different polenumbers.

8 One winding is energized at a time. Supposethat a motor has two windings for 6 and 4 poles. For 50Hz supply the synchronous speed will be 1000 and 1500rpm respectively. If the full load slip is 5% in each case,the operating speeds will be 950 rpm and 1425 rpmrespectively. This Method is less efficient and morecostly, and therefore, used only when Class / AC MachinesDr. Inaam of consequent polesIn this Method a single stator winding is divided into fewcoil groups. The terminals of all these groups are brought number of poles can be changed with only simple changesin coil connections. In practice, the stator winding is dividedonly in two coil groups. The number of poles can be changedin the ratio of 2 (1) shows one phase of a stator winding consisting of 4coils divided into two groups a b and c d. Group a bconsists of odd numbered coils(1,3) and connected in c d has even numbered coils (2, 4) connected in terminals a,b,c,d are taken out as (1) Stator phase connections for 4 polesfig.

9 (1-b)fig. (1-c)3rd Class / AC MachinesDr. Inaam Ibrahim47 The coils can be made to carry current in given directionsby connecting coil groups either in series or parallel shown infig. (1-b) and fig.(1-c) this connection, there will be a total of 4 poles givinga synchronous speed of 1500 rpm for 50 Hz system. If thecurrent through the coils of group a b is reversed ( ), thenall coils will produce north (N) order to complete the magnetic path, the flux of the polegroups must pass through the spaces between the groups, thusinducing magnetic poles of opposite polarity (S poles) in theinter pole (2) Stator phase connections for 8 polesfig. (2-b) series connectionfig. (2-c) parallel connection3rd Class / AC MachinesDr. Inaam Voltage ControlThe torque developed by an Induction motor is proportionalto the square of the applied variation of speedtorque curves with respect to the applied voltage is shown infig.

10 (3). These curves show that the slip at maximum torque smremains same, while the value of stall torque comes down withdecrease in applied , we also note that the Starting torque is also lower atlower voltages. Thus, even if a given voltage level is sufficientfor achieving the running torque, the machine may not Method of trying to control the speed is best suited forloads that require very little Starting torque, but their torquerequirement may increase with speed T .Fig. (3): Torque -speedcurves for various terminal voltages3rd Class / AC MachinesDr. Inaam Ibrahim49 Supply Frequency ControlThe synchronous speed of an Induction motor is given by The synchronous speed and, therefore, the speed of motorcan be controlled by varying the supply emf induced in the stator of an Induction motor is givenby Therefore, if the supply frequency is change, E1will alsochange to maintain the same air gap flux.