Use of Thyristor in VFD

To get variable frequency out of a VFD, the output must be built up at a different frequency to incoming mains power. A diode cannot do this, but a thyristor is off until it is triggered on by a short pulse to its gate. A thyristor is also called a Silicon Controlled Rectifier - SCR. Note that it can only conduct with anode positive, hence "rectifier".

A thyristor will stay conducting until the anode current in it falls to zero. This is a complication for its use but even from a DC supply it was possible to use a second thyristor to apply a negative pulse to the anode forcing the anode current to zero.

Variants of the thyristor which can be turned off by a pulse to the gate are now available.

Changing of voltage/ frequency is done by IGBT of the inverter circuit. I am asking about thyristor which is used in converter circuit for converting AC to DC. In some VFD I seen simple diode bridge for this purpose but some are using thyrister.

A diode allows current to pass in one direction. An SCR only allow current to flow in one direction after a signal is put on the gate terminal. So an SCR performs 2 functions, rectification and control.

To control voltage, on every cycle the SCR must be turned on. To reduce voltage, the gate is triggered later, and to increase voltage it is triggered earlier, letting more current through. A large capacitor is used to filter the voltage pulses into a smooth average.

https://en.wikipedia.org/wiki/Silicon_controlled_rectifier

IGBTs are more modern devices. SCRs were invented in the 1950's.

https://en.wikipedia.org/wiki/Insulated-gate_bipolar_transistor

Yes but my question is why we need to use a control device in converter section?

Yes but my question is why we need to use a control device in converter section?

A VFD is a AC power supply that varies frequency in order to drive a motor at variable speed. A motor is an inductive device. Its impedance is proportional to frequency. At low frequencies, the VFD has to lower the voltage in order to limit the current delivered to the motor.

Up to the motor rated speed (i.e. frequency) the voltage increases linearly up to the motor voltage rating. The current will be constant at the motor rated current. Above the motor rated speed, the voltage is held constant at the motor rated voltage. As the frequency increases, the motor impedance increases, so the current (and torque which is proportional to current) will reduce.

https://controltrends.org/wp-content/uploads/2010/10/VFFundamentals.pdf

You may find this helpful...

https://en.wikipedia.org/wiki/Insulated-gate_bipolar_transistor#Difference_between_Thyristor_and_IGBT

You are not helping an answer by using "converter" for AC to DC - converter can also mean DC to AC. You could call the VFD a "frequency converter". As I wrote in comment #6, AC to DC is usually done by rectifier and DC to AC is done by an Inverter.

Your first reply made it clear you meant thyristors in the rectification process.

Varying the DC input voltage to the inverter will vary its AC output voltage proportionally. The first inverter I saw was brutally simple. Battery input was chopped into a square wave by two thyristors driving a push-pull transformer. The square wave transformer output was then filtered to leave the 50Hz sine wave [single phase].

I pointed out in comment #6 that the voltage variation part of the process [ably described by Richters comments] can be done by thyristors [Silicon Controlled Rectifiers - SCR] in the AC to DC "circuit block".

I think this answers your second question and your initial post.

In some cases, simple diodes are used for rectification, as you can see in the above image, but in others, transistors are used for rectification.

On the left, you have your 3 AC phases. The diodes conduct so each pair of phases charges the capacitor in turn resulting a DC voltage.

On the right, 6 igbt's are turned on and off (pulse width modulated) in the right sequence to synthesize sine waves between the motor phase pairs. The video below explains how this works, about the 12 minute mark.

Lalit_nawarda,

What type transistors are used for rectification?

As far as I know, SCR have reverse blocking capability same as forward, but the transistors have poor reverse blocking, including IGBT; ref IR Application Note AN983 [now Infineon] page 3.

https://www.infineon.com/dgdl/Infineon-IGBT_Characteristics-AN-v01_00-EN.pdf?fileId=5546d462533600a40153559f8d921224

Of course, one can put a diode in series with an IGBT, an extra 1 volt drop is negligible at several hundred volts.

Regards,

67model

The Thyristor is used to control the inrush current to the capacitor bank in the VFD so the DI/DT is not exceeded. Once the capacitors are charged the Thyristors are turned on fully to present near zero reduction of input voltage. In some of the larger drives, 150KW, this was achieved by an inline resistor which after the capacitors were initially charged the resistors were bypassed by a contactor.

The SCRs are fired to control the dc bus voltage to the frequency conversion components, what ever they may be. This provides stable inverter performance during both line and load variations, compensating for load I squared R changes and line voltage and impedance changes. The better quality inverters (variable frequency motor drives) have regulated front ends. This voltage is normally adjustable, to account for environmental variables where it might be advantageous, such as an unstable supply or load where the other components might suffer from over voltage, typically

In addition to changing the frequency of the AC going to the motor, the VFD also needs to change the voltage. VFDs (and soft-starters) need to reduce the voltage when the frequency is reduced.

In order to change the voltage, SCRs or thyristors can be gated ON at different points of the AC waveform. After filtering, the pulsating DC waveform at the bottom will produce a smaller voltage than the one at the top.

This ability to change the voltage independently of the frequency allows the drive to be programmed to give a voltage boost during starting (because a low voltage at low frequency might not be enough to get the motor to start turning). When the motor has gained speed, this boost can be removed and the normal voltage vs frequency ratio can be used.

regards,

Vulcan

P.S.

I included an illustration but editor keeps rejecting the post (parameter missing). I don't know if this post will show up. Keeping fingers crossed. (",)

Hello Vulcan.

I am also finding pictures will not go into post. I enter the file reference OK but pressing the insert button does nothing except cause the hand icon to flutter a few times.

Also, the list of posts has become unreliable - they are listed with dates from long ago.

I find that clicking on the "Create Date" header of the date column causes them to be sorted into proper date order.

Undefined: <...we...>.

...we...? For clarity of definition, should you not have replied to post #4 ??

SCR's aren't used in VFDs but they have been used in 3-ph motors to ramp a motor up in speed as a soft-start. They're used because they have a gate to trigger the SCR into conduction. As such, by delaying the point the SCR turns on you can effectively control how much of the phase conducts. As a result the motor doesn't present the full load to the line instantaneously.

Thyristors instead of diodes are used in the rectification side of some VFDs using a tecnique called phase controlled rectification (PCR) which is timed firing of SCRs at the descending part of the wave because it's an efficient way to provide lower than the peak rectified voltage as input to the switching side of the cirquit. This can under some conditions, like lower than full power increase the total efficiency of the system, since efficiency on PWM switching drops when difference between input and output voltage is higher and under these conditions VFD's vector control system again for efficiency reasons must lower the output voltage and compensate for the increased slip with a bit higher frequency. S.M.

Simply, because <...thyristors...> will make the circuit work properly and <...regular diodes...> won't.

Please feel free to try it, though please make sure all other CR4 readers stand well away before so doing.