Let us see the basic behavior of an 3 phase induction motor in short,
An induction motor is an energy conversion device, converting electrical energy into mechanical energy and some heat energy. For an induction motor, torque is proportional to the motor terminal voltage squared. (V²). So, for example, half the voltage produces a quarter of the torque.
The induction motor has two basic functions to fulfil :
1. To accelerate itself and its load to full speed.
2. To maintain the load at full speed efficiently and effectively over the full range of loading
The load starting torque is usually near zero, an induction motor will develop far too much torque when connected directly to the supply. So, At the instant of start-up, there is an un-necessary heavy power surge on both the electrical supply and the mechanical drive components The unwanted/excess torque generated is as shown by the shaded areain the figure below
The mechanical effects
The sudden mechanical impact at start up on the load, followed by the rapid acceleration to full speed causes excessive wear on :-
>> Belts and pulleys
>> Gears and chains
>> Couplings and bearings and :-
>> Cavitations in pumps etc........
The electrical effects
The electrical impact of large starting current can be severe enough to cause:
>> Large voltage dips and damage to computers & other electronic equipment on the line.
>> Supply voltage drops in case of weak supply and thus affect the entire operation or community.
>> Pitting and burning of switching contacts due to high currents which are many times the motor full-load current.
>> Increased line losses.
>> Risk of exceeding sanctioned maximum demand.
As utility power systems continue to be run at maximum capacity, the effect of starting motors across the line can put stress on the factory’s power distribution system. The lights go dim, process control systems can fail or trip out or you may be restricted as to when and how often you are allowedto start the motor. And this all just because of the heavy current drawn by the motor during starting.
There are several factors to be considered while selecting the starting equipment for any electric motor driven load. These include:
1. The source of power and the effects, the motor starting currents will have on the source and the stability of the system voltage.
2. The starting and breakdown torque characteristics of the motor (motor speed torque characteristics)
3. The motor starting characteristics (torque) that correspond to the motor best suited to the load characteristics at full load and speed.
4. The starting characteristics of motor torque, load torque, accelerating load torque (load inertia) the motor acceleration and running torque of the driven load.
5. The available short circuit capacity of the distribution system.
6. Process considerations: shock, vibration, mechanical hammer, the control and maintenance of different starting methods.
The initial inrush currents, locked rotor currents and the resulting torque values produced are the factors that determine whether the motor can be applied directly across the line, or whether the current has to be reduced to get the required performance to match the load requirements and/or utility line voltage flicker or voltage dip specifications.
There are three basic methods for starting fixed speed 3 Ø AC motor,
1) Full Voltage Starting [ DOL]
2) Reduced Voltage Starting [ SOFT STATER]
3) Part Winding Starting [AUTOTRANSFORMER]
Soft Starter falls under the 2nd method. Where the motor terminal voltage is gradually increased with either electronically or by using HFSR method, A care is always taken that the motor torque will be always greater than that of load requirement,
Now, in short, to avoid all the ill effect mentioned above, the right device which should be used for motor starting is called as Soft Starter
The Result is :-
>> In a simple, reliable and cost effective piece of equipment, all the problems of electro-mechanical starting have been banished to the past!
>> Optimising continually monitors the motor to improve the part-load efficiency by reducing the degree of over-fluxing of the stator.
>> The part load pF of the motor is improved.
>> This produces a significant reduction in kVAr and kVA, as well as a useful reduction in kW.
>> Electrical losses in the stator and feed cables are reduced.
>> Motor performance is unimpaired.
>> There is virtually no fixed-speed application of an induction motor which cannot benefit from the fitting of a Jayashree soft-starter.
>> In particular, pumps and pumping systems benefit from the soft-stop feature and the reduction in cavitations effects.