Starting of Electric Drives:
The most important processes associated with controlled electric drives are (i) starting, (ii) speed control (iii) braking, (iv) reversal of direction of rotation. All these are transient processes that occur in a drive system. Starting of Electric Drives involves a change in its state from rest to a steady-state speed of rotation. The process of starting is the most important phenomenon in the entire operation of the drive.
The Starting of Electric Drives by switching on the supply. There are, however, three possibilities which have to be considered in the starting of electric drives :
- Interference with the supply in the form of an excessive voltage drop which is more than that can be tolerated by other equipment or other consumers connected to the same power supply circuit.
- The starting currents will add to the motor heating by an amount that depends upon their rms values and the frequency of starting. The excessive currents may cause damage to the motor itself. In a dc motor the limitation may be good commutation rather than heating, as dc machine have a certain maximum limit for the current dictated by the commutation process.
- Damage to the connected load through too rapid acceleration. This may also be taken to include unacceptable discomfort to passengers in lifts and trains.
In some cases none of the above is applicable and full-voltage or direct-on-line (DOL) starting is permissible, but in many cases one or more have to be guarded against, usually at the expense of additional equipment.
Necessity of Starting Equipment:
If a motor is switched directly on to the supply it will, except in the small sizes, draw a current much above the permissible limit. The main purpose of the starting equipment is, therefore, to limit the starting current to a safe value without, at the same time, reducing the starting torque to a value less than that required. This limitation of current may be accomplished by reducing the voltage applied to the motor by means of a resistance in series with the motor or by sonic other suitable means as briefed below.
1. Direct-on-Line Starting or Full-Voltage Starting:
This involves the application of full line voltage to the motor terminals. Whether or not this method of starting is used, depend, upon the following factors :
(i) Size and design of the motor (ii) kind of application (iii) location of the motor in the distribution system and (iv) capacity of the power supply circuit and the rules governing such installations as established by power supply companies.
As a general rule, dc motors up to 2 kW and squirrel-cage induction motors as well as certain small synchronous motors up to 4 or 5 kW are usually started by this method.
2. Reduced-Voltage Starting:
The starting of a dc motor is usually accomplished by inserting a suitable external resistance in the armature circuit and as the motor speeds up the starting resistance is cut out in steps.
Reduced voltage for starting of 3-phase induction motors is achieved by (i) stator resistance starting (ii) stator reactor starting (iii) star-delta starting and (iv) auto-transformer starting. The above methods are applicable equally to the synchronous motors. With reduced voltage starting the transition to full voltage may be done either before or after synchronisation although the former method is usually preferred.
Slip-ring induction motors, though can be started by using the starting methods that are used in case of squirrel-cage induction motors, are usually started with full-line voltage across the stator terminals and by introducing variable resistance in each phase of the rotor circuit. The external resistance introduced in each phase of the rotor circuit not only reduces the current at the starting instant but increases the starting torque also. As the motor accelerates, the external resistance is cut out in steps so that the available electromagnetic torque remains maximum during the accelerating period. Ultimately when the machine attains the normal speed, the rotor winding is short circuited automatically.
3. Starting By Means of Smooth Variation of Voltage or Frequency:
With ac motor-dc generator sets, dc motors can be started by smooth variation of applied voltage and with variable frequency sources both induction and synchronous motors can be started by smooth variation of supply frequency, simultaneously varying proportionally the voltage applied to the motors.
4. Starting of AC Commutator Motors:
AC commutator motors may he started by applying a reduced voltage or by shifting the brushes. Both of these methods are also employed for speed control, and in most cases starting can be satisfactorily affected by adjusting the control equipment to the low-speed position and then switching on full-line voltage. In some cases it may be necessary to provide in addition special starting tappings on an auto-transformer or a series resistance.
Special Features of Starting Equipment (Starters): There are a number of features common to starters of motor of different types. Firstly, it is desirable to provide some protection against the flow of excessive currents for a prolonged time duration, even though currents of the same magnitude are permissible for a short period during the initial acceleration. Such ‘overcurrent’ protection can be had by providing means to return the starter to the ‘off’ position or by disconnecting the supply by other means. The operation of the device may be achieved electromagnetically by energization of solenoid connected in series with the motor, or thermally by a bimetallic strip heated by the motor current. In the former case a “dashpot” is required to prevent operation during acceleration period; in the latter case, the inherent delay caused by the thermal capacity of the device may be sufficient.
Secondly, it would be pointless to provide starting equipment which remained in the “full-on” position in the event of failure of power supply, since the restoration of supply would cause the very result for the prevention of which the starting equipment was provided. Furthermore, unexpected restarting of the motor and connected load could endanger equipment and personnel. A “no-volt release” is, therefore, provided in all starters, to return them to the “off’ position in the event of failure of supply. This applies even to D-O-L starters for the second reason mentioned above.
A third common feature is a matter of definition. Most starters operate in a number of discrete steps and not by continuous smooth variation. It is necessary to define the number of steps, n, as the number of accelerating positions, including “full-on”. Since the starter has an “off’ position, there are (n + 1) positions in all.