Frequency converter is used to convert the frequency of ac current, that is it converts a 50 or 60 hz ac current to ac current of any desired frequency. The device may also change the voltage if it is required.
Frequency converters are used in order to facilitate a accurate control of critical processes, such as: Cooling system (radiators, pumps), Fuel system (booster, heaters…), Ventilation ( engine hall ventilation). The frequency converter is also a energy saver and in many applications also noise limiter.
To change the frequency of ac current frequency converter goes through two stage conversion. First it converts ac to dc, then secondly dc to ac of desired frequency.
So frequency converter has two sets of working- first the rectifier circuit converts ac to dc and than inverter circuit with thyristors/ IGCTs / IGBTs converts dc to ac with desired frequency. The changing or shifting of frequency happens in the converter stage.
If voltage conversion is desired, a transformer will usually be included in either the ac input or output circuitry and this transformer may also provide galvanic isolation between the input and output ac circuits.
A battery may also be added to the dc circuitry to improve the converter’s ride-through of brief outages in the input power.
When use a frequency inverter in electric motor and an electricity generator, electric drives are way ahead of combustion engines, since, unfortunately, a car engine which sucks up exhaust fumes during braking and downhill rides and converts them into fuel and fresh air is still pending. The electric motor can deliver this, although during its first century of existence, its use has largely been hampered by two basic drawbacks:
An electric motor has no accelerator pedal.
A socket has no sort of “water tap”.
When an electric motor is running, it generates a voltage with a polarity opposite to the feeding voltage.
Therefore the current is excessively high at the first instance of switching on when the motor is not yet running. For big motors precautions have to be taken not to damage it or blow any fuses. As the motor speeds up, this induced voltage increases. In fact when exceeding the speed where the applied voltage and the mains voltage are equal the motor will generate a higher voltage than that found in the line. Current will flow the other way round, and the motor has inversed its function into that of a generator.
That is good, since it offers excellent energy efficiency advantages especially for cranes, elevators etc. which actually become power plants during downward motion.What is not so good is that the line always has approximately the same voltage, but with respect to other loads, e.g. lights, this has to be so. Hence, provisions have to be foreseen again if the motor speed shall be varied. In the old days, this used to be an onerous task. One had to use transformers with multiple taps, such as in locomotives, but which was a bulky and expensive solution, or limit the current with resistors, such as in trams, which was an inefficient solution.
And things become even more difficult when it comes to AC motors, be they single-phase or three-phase. The principle of an electric motor is always to create a rotary movement by attracting and repelling magnetic forces. In the strict terms of physics electric motors do not even exist, but all of these would need to be called magnetic motors from a purist point of view: An electric magnet attracts another – also electric, or permanent – magnet until it has come as close as can be. Then the polarity of the current in (one of the) electric magnet(s) is inversed, and the attracting force inverts into a repelling one. The motor’s mechanical design is made up so as to allow such motion only around in a circle because a rotating motion is desired. AC motors can be built simpler than DC motors because the periodic swaps of polarity occur anyway and do not have to be generated within the machine.
But it becomes obvious that the variation of rotational speed is difficult for DC motors, since it depends largely on the supplying voltage, which is approximately stable, and it is impossible for AC motors, whose speed coheres strictly with the frequency of the network, which is in technical terms totally stable.
Now either type of electric motor has to be designed in a way so that at the desired (rated) speed the voltage generated in the motor is about the same as the applied (rated) operating voltage. With DC motors the induced voltage has to be somewhat lower than that in the line. When loaded, the DC motor will lose a bit of speed, yielding a further drop of the induced voltage and hence a higher difference to the line voltage and a higher input current, matching the higher load. So it adapts (more or less) by its nature to varying load.
