Variable Frequency Drive or VFD is the system that controls the speed of a rotating device such as motor by applying varying frequency AC voltage. It is a kind of adjustable speed drive widely used in domestic and industrial applications such as pumps, ventilation systems, elevators, machine tool drives etc. The VFD is also called as Inverter drive or variable speed drive. By controlling the output AC frequency, it is possible to drive the device at different speed based on the requirements. It is essentially an energy saving system.
An Inverter system is meant for generating AC voltage from DC using an oscillator circuit and transformer. It generates high voltage Alternating Current from low voltage Direct Current. Oscillator circuit gives pulses of a specific frequency while the transformer produces the alternating current. The transformer induces an electric current in the coil and the changing magnetic field causes AC to flow in the other coil. The coil through which the current passes act like a magnet and produces a magnetic field. When the direction of current changes, polarity of the magnetic field also changes. If the number of turns in the secondary coil of transformer is twice of the primary coil, then the secondary voltage will be twice that of the primary voltage. So by increasing the number of turns in the secondary coil, it is possible to generate high voltage from a low voltage.
If DC is applied to the primary coil of the transformer, it will not induce current in the secondary since there is no change in magnetic field. If the direction of the Direct current in the primary is changing alternately, it is possible to get a square wave output at the secondary. This is the basic principle of the working of Inverter.
In short, the Inverter system converts low voltage DC to high voltage AC with a frequency close to the mains frequency (50Hz / 60Hz).
How to generate Sine wave?
Efficiency of the inverter system depends on the output waveform since the electric appliances are designed to operate in the sine wave of AC from the power grid. Most of the country uses 50 Hz while some uses 60Hz cycles of AC. To generate, sine wave output, it is necessary to feed sine wave input current. For this, an oscillator circuit is used in the sine wave inverters. An Amplifying transistor is made to oscillate by feeding some amplified output to its input through the positive feedback. It is possible to control the positive feedback so as to produce 50Hz / 60Hz by using crystal controlled circuits. The current from the switching transistor will be low so that it must be amplified using power amplifier to give high current to the primary of the transformer.
VFD Power Inverter
It is the system that gives AC output with varying frequency to control the speed of the rotating device according to the needs. Single Phase Variable Frequency Inverters are more common since most devices are working in Single phase AC supply. It consists of a full wave bridge rectifier to convert 230 / 110 Volt AC to approximately 300 / 150 volt DC. The output DC from the bridge rectifier is smoothed by a high value smoothing capacitor to remove ripples of AC. This fixed voltage DC is then fed to the frequency generating circuit formed of IGBT (Isolated Gate Bipolar Transistor) transistors. This IGBT circuit receives the DC and converts it into AC with variable frequency to control the speed of the device.
The frequency change can be achieved using electronic circuits or Microcontroller. This circuit vary the frequency of voltage (PWM) applied to the gate drive of IGBT circuit. Thus AC voltage of varying frequency appears at the output. The Microcontroller can be programmed to change the frequency of the output according to the needs.
The VFD System
The Variable Frequency Device has three parts like an AC motor, a Controller and an Operating interface.
The AC motor used in VFD is generally a three phase induction motor even though single phase motor is used in some systems. Motors that are designed for fixed speed operation is generally used but some motor designs offer better performance in VFD than the standard design.
The Controller part is the solid electronic power converter circuit to convert AC to DC and then to quasi sine wave AC. The first part is the AC to DC converter section having a full wave rectifier bridge usually a three phase full wave bridge. This DC intermediate is then converted into quasi sine wave AC using the inverter switching circuit. Here IGBT transistors are used for inverting DC to AC. The inverter section converts DC to three channels of AC to drive the three phase motor. The Controller section gives improved power factor, less harmonic distortion and low sensitivity to input AC transients.
Volts / Hz controlling
The controller circuit regulates the frequency of the supplied AC to the motor through volts per hertz control method. AC motor requires to adjust the applied voltage when the frequency changes to give the specified torque. For example, if the motor is designed to work in 460 volts at 60Hz, then the AC applied to the motor must be reduced to half ( 230 volts) when the frequency changes to half (30Hz). This regulation is based on the Volts/ Hz. In the above case, the ratio is 460/60 = 7.67 V/Hz.
Other voltage controlling methods
Besides Volts/Hz controlling, more advanced methods like Direct Torque Control or DTC, Vector control and Pulse Width Modulation etc are also used to control the speed of motor. By controlling the voltage in the motor, the magnetic flux and torque can be controlled precisely. In PWM method, the inverter switches produce a quasi sine wave through a series of narrow pulses with Pseudo sinusoidal varying pulse durations.
This section allows the user to start/stop the motor and to adjust the speed. Other facilities include motor reversing, switching between manual and automatic speed control etc. The operating interface consists of a panel with display or indicators and meters for showing the speed of motor, applied voltage etc. Membrane key pad switches is generally provided for controlling the system.
How it works?
Let us see the difference between the operation of an ordinary motor and motor run by VFD. When the ordinary induction motor is switched on manually through the AC switch, initially it draws very large current around 5-6 times than the actual current required to run the motor. As the load connected to the motor accelerates, the torque drops and then rises consuming large current till the motor attains full speed. In short, initial current consumption is very high in induction motor to attain full speed.
But in VFD controlled motor, the VFD system first applies a low frequency around 2Hz and low voltage initially. This prevents the inrush current at power on as seen in ordinary motor. After the initial stage, the frequency and voltage increases at a controlled rate to accelerate the load at the expense of minimum current. This soft starting allows to develop more than 100% torque than the rated value of the motor. At the same time, the VFD takes only 50% current.
Stopping steps of VFD motor
Stopping the VFD controlled Motor is opposite to the sequence of switch on and starting. During stopping, the frequency and voltage applied to the motor is first reduced to a controlled state. Frequency drops gradually and finally becomes zero and the motors shut down.
VFD devices can go wrong since it uses sensitive electronic circuits. Many factors like, line spikes, transients, magnetic flux induced problems, bearing corrosion etc can affect the working of VFD controlled Motor. Let us go to some trouble shooting tips.
POWER ON checking of VFD can be done only by an experienced person competent to repair high voltage circuits. It is dangerous if the VFD is tested without adequate precautions. Very high voltage is present in the circuit which can give a lethal shock.
1. Unplug the VFD and wait for few minutes to discharge the current in the capacitors.
2. Remove dust and dirt accumulated on the system, especially on the Heat sinks.
3. Check whether all the connections are tight or not. Loose connection in the incoming and output wires is the major cause of VFD failure.
4. Check the input / output fuses and input power supply.
5. Check the voltage in all phases (if the VFD is three phase type). The variation in the voltage in all phases should be less than 5%.Unbalanced line voltages in the three phase is the most important cause of VFD failure.
6. Check the Output voltage and current. Spikes, Transients, power outage etc in the input AC causes many problems in VFD. So make sure that the Input AC is transient free and there is no voltage hike or drop.
7. Using a multimeter, check the diode bridge to see whether the diodes are intact or shorted /open. An intact diode will give 0.3-0.6 forward voltage drop.
8. Check the bus capacitors also using the multimeter. A shorted capacitor will show zero ohms.
9. Check the variable speed drive for forward and reverse bias for any shorting or open circuit.
10. Check the Motor also. If there is overheating during operation, the windings may short leading to motor failure.
Source of VFD troubleshooting guide www. gejones.com