Starters for DC Shunt and Compound Wound Motors

Starters for DC Shunt and Compound Wound Motors Points : Starters for DC Shunt and Compound Wound Motors, Three-Point Starter, Operation, No-Volt Release, Over-Load Release Coil, Four-Point Starter, Precaution, Controller for Series Motors, Drum Controllers, Semi-automatic starters, Magnetic Blow-outs, Automatic Starters, Motor starters are generally manufactured in convenient sizes and styles for use as auxiliaries with dc shunt and compound motors. Their primary function is to limit the current in the armature circuit during the starting or accelerating period. They are always rated on the basis of output power and voltage of the motors with which they are to be employed. The simplest type of starter consists of an additional resistance (a rheostat) inserted in series with the armature alone (not with the motor as a whole), as illustrated in.

This starter is, however, modified to include a few protective devices, such as no-volt release, over- load release etc. There are two standard types of motor starters for dc shunt and compound motors, the three-point type and the four-point type starters. The four- point starter is employed when wide range of speed by shunt field control is required. When no (or little) speed control is required, either type of starter may be used. In this context it is• pertinent to note that while starting dc shunt and compound motors, it is advantageous to keep the field excitation at its maximum value.

A large field current, therefore, a higher value of flux will result in a low operating speed and in higher motor torque for a particular value of starting current because motor torque is proportional to the product of flux per pole and armature current. Thus for a given load torque, the motor will accelerate quickly and take less lime to reach the lower operating speed from the starting instant. This will result in less heating of the armature during starting. Thus the rheostat, in series with the shunt field winding, should be at zero resistance position at the time of starting of the dc shunt and compound motors.

Three-Point Starter The three point starter with its electrical connections and protective devices (no-volt release and over-load release) is illustrated in. It consists of a series starting resistance divided into several sections and connected to brass studs, brass are by which the connection to shunt field is made, no-volt release and overload release. Since only three terminals (L, F and A) are available from the starter, it is called a three-point starter. The last stud of the starting resistance is connected to terminal A to which one terminal of the armature is connected. The +ve supply line is connected to the line terminal L through main switch. From line terminal L, supply is connected to the starter arm through over-load release. A spiral spring S is placed over the layer to bring the starter arm to the OFF position in case of failure of supply. A soft iron keeper is attached to the starter arm which is pulled by the holding coil or no-volt release under normal running condition. The far end of the brass arc is connected to the terminal F through the no-volt release and to the terminal F one terminal of the field is connected. The -ve supply line is connected directly to the remaining ends of armature and field winding of the dc shunt motor. Operation When the motor is at rest, the starter arm is in the OFF position due to action of strong spiral spring S. for starting the motor the dc supply is switched on by closing the main switch keeping starter arm in OFF position. The starter arm (or handle) is then turned clockwise to the first stud and brass arc (or strip). As soon as it comes in contact with first stud, whole of the starting resistance R is inserted in series with the armature, the field winding is directly connected across the supply through the brass are and the holding coil is also energized. As the starter arm is turned further the starting resistance is cut out of the armature circuit in steps and finally entire starting resistance R is cut out of armature circuit. With the starter arm reaches the ON position, it is held against the action of spiral spring S by the force of attraction between holding coil magnet and soft iron keeper attached to the starter arm. The starter arm should not be held for an unduly long time in an intermediate position as it is likely to burn out the starting resistor. No-Volt Release When the starter arm reaches “ON” position, the resistance is completely cut off and motor starts running at normal speed. If the supply gets interrupted or disconnected, the starting arm will remain in the same position i.e. “ON” position and when the supply is switched on or gets restored, in back emf will be acting in circuit, the armature being directly across the supply mains, and resistance of armature being low, the motor will draw excessive current and will get damaged. Hence for the protection of the motx, some device must be provided, so that starter arm may reach the “OFF” position automatically as soon as the supply is cut off or disconnected or fails and for this purpose “no-volt” release coil is provided.
No-volt release coil consists of an electro-magnet connected in series with shunt field which holds the arm in the “ON” position. Now when the supply fails or gets disconnected the electromagnet demagnetizes and so releases the starting arm A, which goes back to “OFF” position due to the spring attached to it and gets disconnected from the supply main.
The other important advantage of connecting the no-volt release in series with the shunt field winding is that it prevents the motor from running away owing to an open shunt field because open-circuited shunt field will demagnetize the electro-magnet and release the starter arm A and, thus the starter arm will go back to its OFF position and the supply will be disconnected. Over-Load Release Coil This coil is provided for the protection of the motor against the flow of excessive current due to over -load. This coil is connected in series with motor so caries full-load current. When the motor is overloaded, it draws heavy current, which also flows through this coil and magnetizes it to such an extent, that it pulls its armature upwards and so short-circuits the no-volt release coil. The no-volt release coil, being short-circuited, demagnetizes and releases the starting arm, which goes back to “OFF” position with the action of spring attached to it and the motor is automatically disconnected from the supply mains. Thus the motor is disconnected from the supply and is protected against over-loading. The starters for motors upto 15 kW are provided with over-load release to disconnect the motor form the supply mains in the event of an over-load. Larger motors are provided with separate automatic circuit breakers. Four-Point Starter In three point starter no-volt release coil is connected in series with the shunt field and the field rheostat and, therefore, the current flowing through the field is the same current that flows through the holding coil. Thus if sufficient resistance is cut in by the field rheostat so that holding coil current is no longer able to create sufficient electro-magnetic pull to overcome the spring tension, the starter arm will fall back to the “OFF” position. It is this undesirable feature of the three point starter that makes it unsuitable for use with speed-controlled motors and that has resulted in the wide spread application of four point starters.

A four point starter with its internal wiring connected to a long shunt compound wound motor. It is obvious that when the arm touches stud no. 1, line current divides into three parts:
1. One part passes through starting resistance, armature and series field.
2. Second part passes through the shunt field winding
3. The third part through no-volt release coil and protective resistance.

Since in this arrangement, “no-volt release coil” circuit is independent of shunt field circuit, it will not be affected by the change of the current in the shunt field circuit. It means that the electromagnetic pull exerted by. the holding coil will always by sufficient and will prevent the spiral spring from restoring the arm to the “OFF” position, no matter how the field rheostat is adjusted. The possibility of accidentally opening the field circuit is quite remote; hence there is greater acceptance of the four-point starter over the three-point starter.

A motor is started with a four-terminal starter in the same way as with a three-terminal starter. Any desired speed, above normal, of the motor can be obtained by adjustment of the field rheostat in series with the shunt field. It is necessary to ensure, before starting a dc motor that the field circuit is closed, the rheostat in series with the shunt field winding is at zero resistance position and starting resistance in series with the armature circuit is at maximum value.

For stopping the motor, the line switch should always be opened rather than throwing back the starting arm. In shunt motors, the line switch can be opened without ay appreciable arc, since the motor develops a back emf nearly equal to applied line voltage and the net voltage across the switch contact is small. The electro-magnet energy stored in the field does not appear at the switch but is discharged gradually through the armature. On the other hand if the starting arm is thrown back the field circuit is broken at the last contact button. Owing to the inductive nature of the field, this will cause a hot are and burn the contact. Precaution While stopping the motor, all the resistance in the field rheostat is cutout, so that motor speed falls to its normal value; then the line switch is opened. This procedure ensures that, the next time the motor is started, it will be with a strong field and resultant strong starting torque. Controller for Series Motors A controller is a device for regulating the operation of the machine to which it is connected. A d.c. motor controller performs the basic functions of starting, speed control, reversing, stopping and providing some measure of protection for the motor that it governs. Common type of controllers are the face plate, the drum and the magnetic controller.
A starter is a controller whose main function is to start and accelerate a motor. Controllers are used with motors for lifts, cranes, electric trams, trains and for any intermittent loads. Drum Controllers A drum controller consists essentially of a cylinder, insulated from its central shaft on which are mounted a series of copper segments. As the cylinder Is rotated by a handle attached to the shaft, the various contact segments come in contact with the contact fingers. The contact fingers are held in place by adjusting screws and by springs strong enough to ensure a good electrical connection with the contact segments. By designing the controller with the different sequences for making and breaking of the contacts as the drum is rotated, a different switching operation may be attained. When a shunt or a compound motor is operated by a handle of a drum controller, the connections established at the ‘start position’ are such that the field circuit is connected across the line and also the armature circuit is completed through a starting resistance. As the drum is moved to next steps, the starting resistance is gradually cut out until the armature is connected directly across the line. It may also be possible, after the starting resistance is cut out to achieve speed control with the help of additional contacts inserting resistances into the field circuit. Reversing may be achieved by rotating the drum in the opposite direction from the “neutral” or “off’ position. In fact, when this is done, the armature connections and the direction of rotation are reversed. In certain types of drum controllers, the central shaft has a series of projecting cams which move the movable contacts against some stationary contacts in a present order.

The drum controller has a much more robust construction than a face plate starter which is generally used for starting a motor which is intended to run for some time. The controller is employed for starting, speed control, stopping arid reversing of the motor during the operation time. For this reason, the contact segments are made easily replaceable due to the heavy wear in service, and the resistance elements are given a more liberal temperature rating. Master Controllers Master controllers are employed for high voltage or heavy currents. A master controller carries only relatively low voltages and currents and actuates relays or contactors, which perform the actual operation of opening or closing the circuit. Semi-automatic starters Any control which is set in action by the closing of a switch or pressing of a button or operating a master controller is semi-automatic, i.e., whenever the rate of. closing the switch or contactor is independent of the operator and only depends on the change of voltage or current of the motor. Magnetic Blow-outs In order to prevent the contacts of controller from being damaged by any are that may be formed when the contact are opened, magnetic blow-outs are often used to put off the arc.
A magnetic blow-out is an electromagnet so constructed that its field is set up across the arc. When the contacts open, the arc is moved at right angles to the field in the same way as when a current carrying conductor is acted upon. The field and the contacts are so arranged that the arc is drawn away from the contacts, breaking it in a short time. To make the field strength proportional to the current being interrupted, the blowout coil is connected in series with the circuit being opened. The inner surfaces of the magnet’s pole faces are lined with insulation, so that the arc cannot jump to them. In some bearers of high interrupting capacity, the arc is blown into a restricted channel called an arc shute. The arc come into intimate contact with the walls of the arc shute, where it is cooled and is quickly de-ionized. Automatic Starters Often starters of the automatic type are used. Automatic starters, have many advantages over the hand- operated starters. For example, they cut out the starting resistor at a definite present rate and prevent blowing off of the fuses and the opening of the circuit breakers which might be caused due to high accelerations. In many equipments, where a motor is used intermittently, it may be started or stopped by merely pushing a start or stop push button. Instead of a push button switch, the motor may controlled by a float switch, a pressure switch or any other type of automatically operated switch. Automatic starting, stopping and reversing of d.c. motor is commonly achieved by means of magnetically operated switches called magnetic contractors. Controllers using magnetic contactors to control the operation of a motor to which they are connected, are called as magnetic controllers.