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Definition of static frequency Converter
Static frequency converter is a device which alters the frequency of input signal according to the input set point. As the name specifies, it consists of solid-state switching devices which are either on or off according to the input control signal.
Introduction to static frequency converter
In mechanical converters the frequency of operation is low because of friction and mechanical losses. In order to obtain high frequency switching and fine frequency control, the solid state switching devices are used in frequency controlling.
General operation of solid state devices
In the on condition the solid state device approximates to an ideal closed switch with zero voltage drop across it with a current that is determined by the external circuit. In off condition, the device is approximated as ideal open switching with high impedance which blocks the current flow in the circuit. This solid device can be used in frequency converters if it is powered on/off by a low power control signal without any delay in changing states. How ever in practical devices some losses will exert which can be minimized by proper choosing of the devices.
The development of a thyristor with its availability in high power applications have given new boost to static methods of adjustable frequency AC generation. The thyristor can be triggered in to on condition by external signal on gate electrode, but the thyristor cannot be switched off by an external signal. Turnoff or commutation of thyristor can be achieved by the interruption of anode current of thyristor.
unlike the thyristor, the devices like power transistor, Field effect transistor, gate turn off thyristor can be both turned on and turned off by the control signal to their control terminals (base and gate). This principle is also called forced commutation.
The thyristors are extensively used in very high power applications in the range of 40000 horse power to adjust the applied frequency and voltage.
Types of static Frequency converters
The are three basic types of Static converters as follows:
- DC link static converter
- Cyclone converter
- High Frequency Ac link converter
All the frequency converters fall under one of these categories. All the three types of power converters offers reverse rotation of motors by changing the turn on sequence of the power devices with switching of the power supply.
Advantages of static Frequency converters
1) The main advantage of static frequency converter is that it is independent of supply voltage and load variations. The output frequency is entirely depend on the oscillator which is to control the on/off of static devices. So it is completely independent of fluctuations in the ac supply voltage and frequency. In other case it also independent of load variations. In induction motor there will be slight speed change because of rotor torque but this will be constant through out the speed range. In synchronous motor the motor speed is solely depends on reference oscillator.
2) Static frequency converters have low installation costs, because they don’t require heavy machinery and careful alignment.
3) Static converters occupies less space and offers low noise compared to mechanical converters with heavy machinery.
4) The operating cost of static devices is low due to its high efficiency and the movable parts which deteriorates the life of the components.
5) It offers a wide variety of voltage and frequency control with a great control due to its independence of supply voltage variations.
6) The output volts/hertz can be adjusted to suit the adjustable speed motors and a large starting torque can be provided if it is needed.