Types of Starters Used for Starting DC Motors | Electrical Engineering

The torque of a dc motor is directly proportional to the product of flux and armature current and is quite independent of speed, i.e., T ∝ ɸI_a. Hence in order to have high starting torque for a given armature current, the flux must be increased to the maximum value possible.

In case of a dc shunt motor, the flux ɸ remains constant as field is connected directly across the constant voltage supply mains and the armature current is controlled by connecting a starting resistance in series with the armature. The torque, which is directly proportional to armature current (as shown in Fig. 1.58) is limited by the maximum starting current- full-load current will develop full-load torque, twice full- load current will provide twice full-load torque, and so on. As the motor speeds up the starting resistance is cut out.

In case of a dc shunt motor reduced-voltage starting method cannot be used because a reduction in supply voltage would reduce the flux proportionately and hence the starting torque.

In the case of a dc series motor as the field winding is connected in series with the armature therefore, the current in the series field winding and the armature is the same. Since up to saturation point the flux is directly proportional to the current flowing through the field and after saturation point the flux is independent of current and remains almost constant, the torque, therefore, varies as the square of the armature current up to saturation point and varies directly as the armature current beyond saturation point. The torque- armature current characteristic is shown in Fig. 1.58.

In case of series wound motor since torque is independent of the applied voltage and the speed, hence reduction of voltage applied across the series motor is a very suitable method for its starting, which is achieved by connecting a resistance in series with the motor.

When the series motors operate in pairs, series-parallel method of voltage control can be employed.

Starters for Shunt and Compound Wound DC Motors:

Starters are used to limit the starting current to a safer value and obtain the required starting torque.

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 time 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.

The starter for a shunt motor (three-point starter) is shown in Fig. 1.59. At start all resistances are in series with the armature, the holding coil (through the starter arm A) is across the line. As the speed builds up and the back emf increases the arm A is moved to the right. This decreases the armature resistance and the armature has greater emf.

At normal speed when the back emf is large enough to limit the armature current, the holding coil and starter arm A make contact. Both the field and armature are directly across the line. Current through the holding coil keeps the arm in this position. If for any reason the line voltage should fail, the magnetic field of the holding coil ceases and under the action of the spring the arm goes back to its off position. This will keep the line voltage open. Because of this action the starter protects the motor.

Restarting after a line voltage failure can only be done by using the protective series resistance in the armature circuit. The starter is also provided with an overload release coil to protect the motor against the flow of excessive current due to overload. This coil is connected in series with motor and so carries full-load current.

When the motor is overloaded it draws heavy current, which also flows through the coil and magnetises it to such an extent, that it pulls its armature upwards and so short circuit the no-voltage release coil, as shown in Fig. 1.59. The no-volt release coil being short circuited demagnetises and releases the starter arm which goes back to the off position and the motor is automatically disconnected from the supply mains.

In a three-point starter, discussed above, 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 the holding coil current is no longer able to create sufficient electromagnetic pull to overcome the spring tension, the starter arm pulls back to the off position.

Hence three-point starter is 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 motor is shown in Fig. 1.60. In this arrangement ‘no-volt release coil’ circuit is independent of shunt field circuit and, therefore, it will not be affected by the change of the current in the shunt field circuit.

DC Series Motor Starter:

The series motor starter serves the same purpose as the three- and four-point starters employed with shunt and compound wound motors. However, series motor starter has different internal and external connections.

A series motor starter with holding coil and overload release is illustrated in Fig. 1.61. Holding coil protects the motor against “no volt” and “no load” whereas overload release protects the motor against the flow of excessive current.

For switching off the dc series motor, the line switch should always be opened rather than throwing back the starting arm, as in case of dc shunt and compound motors. If it is done, then heavy sparking is caused at the stud 1, because here the field circuit is broken and the entire energy stored in the magnetic field is dissipated in the form of heavy spark.

However, there is one significant difference. In case of a dc series motor, the flux does not remain constant but varies with the current because the line current, field current and armature current are the same. Thus the back emf at any given speed varies as the current varies between the upper and lower limits. The practical result of this is that a series motor starter has a smaller number of steps than that required for a starter of a dc shunt motor of the same rating with the same current limits. This is because an increase in current causes increase in back emf and thus the current rise tends to be self-limiting. As a result of the fewer steps, the resistance of each “section of the starter resistor is greater than for the shunt motor starter.

Since flux varies during the starting operation and its relation to the field current is non-linear, the determination of the number of steps is rather complicated.

Drum Type Series Motor Speed Controller:

DC series motors are often employed on cranes, elevators, tramcars and other applications, where the motor is under the direct control of an operator. In these applications, frequent starting, variations of speed, stopping and reversing may be necessary. A manually operated controller, more rugged than a starting rheostat, called a drum controller, is employed.

The controller is in the form of rotating drum having segments which makes contact with the fixed points. Speed controller of this type is shown in Fig. 1.62 in which K is an arc braking coil- EM is a braking electromagnet; FLS is a switch for rotation in one direction and RLS is a switch for rotation in reverse direction. The controller has six positions for forward and six positions for reverse rotation of the motor. The working positions of the controller are shown by vertical dotted lines. The electromagnet EM, is connected in parallel with the motor and releases the motor at starting. When the motor is disconnected it is braked mechanically.

Now when the knife blade switch is closed and the controller is set in forward position 1, the connections are along vertical line one. In this position segments make contacts with fixed points 6 and 7, 8 and 9. In this position current flows from +ve bus-bar through armature winding of the motor, arc braking coil K, all the starting resistances 1-6, fixed points 6 and 7 through controller segments, series field winding, the forward limit switch FLS, fixed points 9 and 8 through controller segments and returns back to the negative bus-bar.

In the second forward position, the segments of the controller come in contact with fixed points 5 and 6 thereby bringing some of the starting resistance out of circuit, so speed of the series motor increases. In the subsequent forwarding positions 3rd, 4th and 5th, additional steps of the starting resistances are brought out of the circuit and finally in 6th position all the starting resistances are short circuited and motor attains maximum speed.

In all the six forward positions the direction of current in armature as well as in series field winding is same (from right to left), as shown in Fig. 1.62.

But in first reverse position the current flows from + ve bus-bar through armature winding, arc braking coil K, all the starting resistances 1-6, fixed points 6 and 10 through controller segments, the reverse limit switch RLS, the series field winding, fixed points 7 and 8 through controller segments and returns back to the – ve bus-bar. Hence it is obvious that in reversing positions direction of flow of current in armature winding remains unchanged, while reverses through the series field winding, thereby reversing the direction of rotation of motor.

Automatic Starters:

Push-button types of automatic starters are quite common in use in industry. Even an inexperienced operator, with the help of such starters, called the auto-starters, can start and stop the motor without any difficulty.

The operation of such starters depends upon, either the time delay or the counter emf developed across the armature terminals.

Counter-EMF Starter:

This type of starter depends upon the buildup of the back or counter emf to operate the contactors (a heavy duty relay designed to open or close an electrical power circuit), which in turn causes the starting resistors to be shorted. The connections of a counter-emf starter are shown in Fig. 1.63.

When the motor is switched on, the counter emf developed by the motor is zero and the voltage across contactor coil A or B is insufficient to energize the relays. Consequently the contactors A and B are normally open and, therefore, starting resistances R₁ and R₂ are in series with the armature. As the motor picks up speed and builds up a counter emf the voltage across coils A and B increases until the operating voltage of coil A is reached. Contactor A is then energized, closing the normally open contact A, shorting out the starting resistor R₁. The motor continues to pick up speed building up the counter emf until the operating voltage of coil B is reached. Contactor B is energized, closing normally open contactor B, which in turn shorts out the starting resistor R₂ and places the armature directly across the supply mains.

The drawback of counter-emf automatic starter is that if the motor fails to start, the counter emf remains zero, the voltage sensitive relays cannot operate and so the starting resistances may burn out. Such occurrences can, however, be avoided by using definite time-limit starters.