VFD

VFD's are built using electronics, they work real good and the output voltage uses feedback control. Don't expect to hit the knowledge lottery here, you must put in some effort to learn. GA?

Your question looks like explanation notice to subordinates.

"Sorry sir,...i will submit the explanation very soon."

VFD or variable frequency drive... not variable voltage drive. The voltage is usually constant, I've not come across one that has a variable voltage.

How it works.... simple version: AC incoming is rectified to DC, then inverted to AC thru the IGBT's which in turn gives you the Hz output at a predetermined voltage.

Google it.. then if you have questions please come back

When the frequency is reduced in a vfd the VOLTAGE will be reduced proportionally. This is done usually by the V/F limiter.Otherwise the core flux density will increase resulting in excessive core losses.

Yes I know that.... the question was how is the voltage controlled, the voltage is not its the Frequency that is raised or lowered depending on the speed required, the voltage change is direct result of the Hz change, however as you say there are limits to be set and I did not want to try and give the OP a master class in VFD's without him doing some work. It sounds to me that this is a college class project and while it could be looked at as "research" asking CR4 is an easy was of getting research done

I agree with you britch, but I think it is OK to say that vfd can be used to control mechanical systems by adjusting the frequency and voltage supply to typically a 3 ph electric motor. As the rotor responds to the rotating magnetic field created work is done. If the frequency gets too low or the rotor does not comply with the magnetic forces, the magnetic circuit could begin to saturate creating losses and higher than necessary current in the motor windings. Most vfd's reduce the drive voltage below a given frequency such as line frequency. Often vfd's are used with some kind of feedback, but they can run open loop also.

Regards,

Luther M

It sounds like by controlling the frequency of an oscillator the resultant output voltage will follow! Similar to a simple light dimmer application, by varying the oscillator frequency, one can control the firing angle(s) of thyristor(s), such as triac or SCR, which will determine the amount of time this thyristor(s) will be triggered conducting (ON state). This basic AC control concept has a lot of different applications, both in single phase and polyphase AC controls. When incorporated with a feedback loop, it can also be applied affectively in magnetic amplifier applications such as ferro-resonant type of voltage stabilizers. Above statements were from an old personal experiences.

very quickly, light dimmers change the voltage, not the frequency, while VSD, by inverting the DC bus voltage gives you the Hz you require, the voltage of the sine wave will follow. But if you are going into detail, then you MUST talk about total Harmonic distortion (THD), short circuit ratios, motor output and a whole mountain of other things that the OP has not even heard of.

However if the VSD maintains a Volts to Hz ratio the the HP output of the motor changes, as well as the motor RPM increasing or decreasing.

With modern VSD/VFD in the application I use them, we then feed the VSD output into a Step-up transformer to give 2000 volts plus for the ESP's, then we also have to take into consideration the transformer ratio too! plus the additional harmonics...

Light dimmers , motor speed controls, etc are all pretty much the same. The main difference is the type and output power handling capacity required for any given application. This will also determine what type of electronic component that will be driven by the VFO to much that particular application. The key is the variable frequency oscillator that any end user will or can adjust depending on the desired effect. The variations in the oscillator frequency will in turn can be converted into voltage, current, etc. necessary for any given application. A typical application example, The VFO frequency variations can also be translated into an equivalent varying small DC voltage that can be used to effectively saturate the core of a transformer, (most linear portion of the hysteresis curve) which in effect can control its magnetic characteristics via the alteration of the magnetization curve.

In reply

There is no way that dimmers and speed controllers are the same...see below

"Dimmers are devices used to vary the brightness of a light. By decreasing or increasing the RMS voltage and hence the mean power to the lamp it is possible to vary the intensity of the light output. Although variable-voltage devices are used for various purposes, the term dimmer is generally reserved for those intended to control resistive incandescent, halogen and more recently compact fluorescent (CFL) lighting".

More specialized pulse-width modulation equipment is needed to dim fluorescent, mercury vapor,solid state and other arc lighting.

There are not many application for a VFD (not VSD) in lighting control, only in the theater because of the heat

"Modern dimmers are built from silicon-controlled rectifiers (SCR) instead of potentiometers or variable resistors because they have higher efficiency. A variable resistor would dissipate power by heat (efficiency as low as 0.5). By switching on and off, theoretically a silicon-controlled rectifier dimmer does not heat up (efficiency close to 1.0)".

I quote from; see link

http://en.wikipedia.org/wiki/Dimmer

Dimmer: decreases or increases the RMS voltage and hence the mean power to the lamp it is possible to vary the intensity of the light output.

VSD or VFD: converts AC to DC then back to AC using IGBT's to produce the required Hz and voltage to control the RPM of an AC asynchronous motor, usually 3 phase.

Of course! since they differ only in names due to differing final applications! The circuit design approach remained the same since you'll need a VFO as the INITIAL stage in your control mechanism. And depending on the chosen type of output requirement, it may be a proportionally varying frequency, voltage, or current, as well as the amount of power capacity required. The Driven or final stage component, (IGBTs, IGFETs, thryristors, etc.) will also be determined based on the type output required! This basic and general approach in circuit designs is applicable to both AC or DC, analog or digital applications. As simple as a light dimmer, to UPS' or to a more complex power regulation applications, The VFD in question is only one of those different applications requiring an initial VFO. I just don't limit myself on different terminologies since ultimately they will be determined by their final usage or application and labeling when marketed!

I think you are either

1. Getting your VFD's and VFO's mixed up

2. You don't know the difference between a VFD and a VFO

3. or are just trying way to hard to impress the forum by telling everyone about VFO's, that do not exist in a variable frequency drive.

I have looked for VFO's and the answers I got is as follows

"A variable frequency oscillator (VFO) in electronics is a oscillator whose frequency can be tuned (i.e. varied) over some range.[1] It is a necessary component in any tunable radio receiver or transmitter that works by the superheterodyne principle, and controls the frequency to which the apparatus is tuned."

could you be confused by a Voltage Controlled Oscillator?

"A voltage-controlled oscillator or VCO is an electronic oscillator designed to be controlled in oscillation frequency by a voltage input. The frequency of oscillation is varied by the applied DC voltage, while modulating signals may also be fed into the VCO to cause frequency modulation (FM) or phase modulation (PM); a VCO with digital pulse output may similarly have its repetition rate (FSK, PSK) or pulse width modulated (PWM)."

Applications of a VCO

VCOs are used in: * electronic jamming equipment * function generators, * the production of electronic music, to generate variable tones, * phase-locked loops, * frequency synthesizers used in communication equipment.

There is one application that is not mentioned, in EVERY VSD there is a crystal to give the snubber firing boards that control the IGBT's their reference to work from

Voltage-Controlled Crystal Oscillator as a Clock Generator: A clock generator is an oscillator that provides a timing signal to synchronize operations in digital circuits. VCXO clock generators are used in many areas such as digital TV, modems, transmitters and computers.

As a new member to the forum, you need to stay on tack with the OP question, you have managed to work me away from it, added to the OP's confusion and proved that you can confuse the issue with unrelated information that really has no real value to the OP. The best and only answer that explained to the OP in detail was from jmueller, after that nothing else required.

Quoted texts are from:

http://en.wikipedia.org/wiki/Variable-frequency_oscillator

and

http://en.wikipedia.org/wiki/Voltage-controlled_oscillator

I concur to all given circuitry details for a VFD! Earlier comments given are conceptual ideas on how instrumentation controls, its design mechanisms can be approached or looked at! Actual circuit similarities as well as differences will depend Only on the particular chosen application! It is very general and open ended! As these 2 cents of mine were taken from old designing experiences back in the mid 1960s, will put me way far from being either !! - I'm dead sure I'm neither.

I agree this is a pretty good explanation but I would like to add a few things:

For Part 1: The rectifier can be passive (diode bridge) or active SCR's. If passive there will likely be a pre-charge resistor and a bypass contactor. This is for start-up to protect the rectifiers when trying to charge the capacitors mentioned in #2. Once the caps are above some threshold voltage the bypass contactor engages removing the pre-sharge resistor from the current path. SCR's can just vary their firing angles to accomplish the same result.

Part 2: Dynamic braking is done via a "Chopper" which is usually a transistor of some type not a contactor.

The Output of the IGBT's is usually a "PWM" output. Some VFD's allow you to change the carrier frequency of the PWM. The higher the frequency the greater the switching losses on the IGBT's but the quieter the motor will operate which can be a big deal, particularly in HVAC applications.

In contrast to a light dimmer the VFD is different in that it has the "Front-End" to convert the incoming voltage to DC. It also runs at much higher carrier frequency. A Light Dimmers carrier frequency is line frequency where as a VFD can be anywhere from 2-12KHz or more.

Shawn

Shawn33,

Your additions are helpful. Depending on the application, slowing the driven load can be as simple as a controlled decrease in the output voltage and frequency while the driven load takes away the stored inertial energy in the motor and load. Or, if the stored inertial energy is more than can be taken away by the load in the time desired for stopping, then the VFD's design has a way to take back the energy. This results in adding to the voltage in the DC bus section. The VFD's have two design methods for keeping the charge on the DC bus from getting too high--either dump it to a resistor load bank outside of the drive (by SCR's as you noted), or (on line regenerative types of drive designs) put it back onto the "incoming" power line.

In my earlier post I noted that "most" drives have a preset (but user-adjustable) carrier frequency. It is heard as a singing from the motor's laminations. I have worked with a few that switch the IGBT's on and off without the use of a clocked carrier frequency. They were very quiet.

Regarding typical light dimmers, they have no DC bus and output a variable voltage at the same frequency as the incoming line frequency.

--JMM

VFD is variable frequency drive. This is used to vary speed of the motors by varying the frequency. However correspondingly the voltage will also get changed proportionaly. The referred motors are used for driving the centrifugal drives like fans/pumps. when these drives are operated on part load due to process requirement, the overall efficiency of the process is becoming low. However if we operate at the optimum speed without killing developed pressure by the drive, the eficiency is not affected. Further the starting current will also be reduced considerably (approximately 1.5 to 2 times) thereby reducing burdon on distribution system (normally directon line starting will draw current six times full load current). We understand the payback period for additional cost involved in providing VFD is 12 to 18 months.

R.Thiyagarajan