VFD Output

When the drive signal to a VFD is varied, does the output voltage vary or does the frequency of the output vary?

The simple answer is in the name...Variable Frequency Drive. Others will answer this better that I can in detail. Yes the input varies or can vary the frequency...one way to view this is set the Inverter to display HZ ..assuming it has a display/opertator interface.

A VFD uses an AC input to create a DC output in a sawtooth waveform that can be variable, technically depending on setup of drive all 3 of the main components, frequency, voltage and current may vary. the main component is frequency.

One formula for speed in rpm is:
(120xfrequency)/number of poles....ie if a motor has 4 poles and is running at 45hz then 120x45=5400 diveded by 4 = 1350

Both. In general, VFDs maintain a constant volts/Hz ratio.

Here's a link...

Try this link. It provides a good explanation but brief.

http://www.motordoc.org/downloads/drives.pdf

Below the nominal output frequency, 60 Hz in the US, most VFDs vary both frequency and voltage. At 50% speed, for example, a nominal 60 Hz 460 Volt VFD will provide 30 Hz at 230 Volts.

Above the nominal frequency most VFDs will maintain a constant voltage. In general the output voltage cannot exceed the input voltage.

Tom gave a great explanation, I would add that many drives give the option to change the V/H ratio although such mods are rarely needed.-

A VFD uses an AC input to create a DC output in a sawtooth waveform that can be variable

Click to expand...

VFD uses an AC input to create a DC voltage is stage is an intermediate one in the VFD this dc voltage is usally called"DC link" or "DC bus" this dc voltage is then chopped using power module called IGBT module(Insulated gate bipolar transistor) to produce AC output with a specific frequancy (speed) according to the speed reference applied to the VFD.
Due to the C/C's of the the AC induction motor there is a ratio V/f(Voltage to frequancy)which must be kept constant .So the VFD change the output voltage with frequancy to maintain this ratio.
As tom said this valid up to the base frequancy of the motor (name plate freqancy) 60Hz in USA and 50Hz in EGYPT.above this frequancy we can only increase frequancy(speed) because we reach the motor nominal voltage .this has a disadvantage because the torque will start to decrease (V/F will not be constant in this range)

A VFD uses an AC input to create a DC output in a sawtooth waveform that can be variable

Click to expand...

VFD uses an AC input to create a constant DC voltage this stage is an intermediate one in the VFD this dc voltage is usally called"DC link" or "DC bus" this dc voltage is then chopped using power module called IGBT module(Insulated gate bipolar transistor) to produce AC output with a specific frequancy (speed) according to the speed reference applied to the VFD.
Due to the C/C's of the the AC induction motor there is a ratio V/f(Voltage to frequancy)which must be kept constant .So the VFD change the output voltage with frequancy to maintain this ratio.
As tom said this valid up to the base frequancy of the motor (name plate freqancy) 60Hz in USA and 50Hz in EGYPT.above this frequancy we can only increase frequancy(speed) because we reach the motor nominal voltage .this has a disadvantage because the torque will start to decrease (V/F will not be constant in this range)

I thought most modern drives are pulse with modulated where the output voltage peak to peak remains constant. The resulting current simulates a sine wave where the frequency and amplitude varies. This plays havoc on digital voltmeters when troubleshooting.

You are correct, samnegg. Since the DC bus is at a reasonably constant voltage (it varies as the input AC varies only), the output transistors can only switch this DC on and off.

The AC current wave is recreated by varying the width of the output pulses and then depending on the inductance of the motor to integrate these PWM pulses back to something resembling a sine wave. In practice, the current wave usually is recreated quite well but the voltage still hardly resembles AC at all. And, yes, the poor waveform causes considerable voltmeter and ammeter error unless the meter is designed specifically for measuring AC with large amounts of higher frequency noise mixed into the fundemental wave.

The rate at which the output transistors chop the DC bus to produce the pulses is commonly referred to as the "carrier frequency" and, these days, can vary from around 1KHz to about 20KHz. In my view, anything over about 8KHz is excessive and unneccessary from a drive-motor system standpoint.

Interestingly, if you were to put only resistive loads, say incandescent lamps, on the drive output, you never would see any AC. The output pulses would remain due to no inductance being present.

Also, there are high performance AC drives out there such as the ABB ACS600 and ACS800 that do not have any carrier frequency. The output pulses are generated based on a complex motor model and formulas which release pulses based on torque and speed demand and motor rotor position, updating the calculations every few microseconds or so. This tends to make the output pulses appear random in frequency rather than evenly spaced in time.

Some manufacturers make devices called "sine filters" which are intended to be placed in the motor leads. These devices do all the pulse integration to recreate the sine wave and spare the motor from this task. Normally they are not used due to cost but, especially where there are long motor lead lengths or the motor insulation is substandard, they can be helpful.

Hope this answers some questions.

Dick "Interestingly, if you were to put only resistive loads, say incandescent lamps, on the drive output, you never would see any AC. The output pulses would remain due to no inductance being present."

Do you mean that if I was to connect 3 bulbs (which I have done in the past) I would no see a strobe effect. Please explain for I did this a few times to test some com. and analog cards with Inverters and they do flash with frequency.

For instance, at 1 Hz... they go 1 Hz.

hi
You are right piere I try it many times to connect a three pulbs connected delta or star to the VFD output and it goes as i told the frequancy to be.
Thx

I was referring to the carrier frequency pulses which would be at 1KHz to 20KHz, depending on the drive. You would not see these pulses in the light output due to the heating/cooling time delay of the filaments.

You just might see the strobe effect of reducing the fundemental output frequency below 60Hz. I've never done this but, it seems to me that, at low frequencies, the voltage would be too low to light the bulb at all. At 1Hz, I doubt that the bulb would even glow since it would be seeing such narrow pulses that the equivalent voltage would be only around 15-20VAC.

To repeat, my point about the incandescant lamps in the earlier post is that inverters depend on the inductance of the motor to recreate the AC current waveform. This affects the motor insulation and motor temperature in ways that must be considered when choosing components for a drive/motor system. Other than a curiousity, I know of no useful purpose for incandescant lamps wired across an inverter output.

hello !
attached is a picture of how the VFD works.
You can get more info for free here
http://www.vacon.fi/support/nxdocuments.html

(all manuals)

Just an FYI for those that dont know or have forgotten

120vac is equivalent to 120vdc with resistive type loads...ie incandescent lamps. 120v is the "working" or "rms" voltage of an ac sine wave with an amplitude of approximately 170 volts. Many will know this and understand it. This 120vac is the equivalent of 120vdc as far as resistive loads are concerned.

I rarely if ever state these things correctly even though I do understand them.

Resistive loads do not create an EMF or a current that opposes the incoming as inductive loads do.

DC voltage, when "pure" has no frequency..seen on a scope it will be a flat line at the voltage level.

IN REALITY though creating DC voltage anytime AC is the source there is a thing called ripple. An example of ripple is when you use a single diode. A single diode "chops" the AC sine wave in half..ie allows electron flow in one direction, which is the way DC works. To obtain "pure" DC we use capacitors, clamping diodes and/or zener diodes.

We get to connecting resistive loads to an AC drive inverter. As far as I am concerned the inverter produces a DC output...some call it a BUS. This output is converted to a sine wave type output that is modulated in many cases...therefore it "acts" like an AC source of power to an inductive load...ie a motor. The motor itself is part of the "reason" the inverter acts like an AC source...ie the inductive load creates an emf that opposes the incoming power. With AC inverters the motor also create other factors that have to be considered.

Overall though an AC inverters output is DC voltage with a controlled frequency variation.

Hook an incandescent lamp...plain old light bulb (maybe several tied together to accept the voltage levels) and the inverter @ 60HZ will light the lights. At lower frequencies you will see a "pulse" from the lights.

WHY? Because the "working" or in ac terms "rms" voltage isnt there. The pulsing causes the DC voltage level to goto ZERO....and STAY there longer...ie the light will be off longer the lower the frequency. Since we arent dealing with a "true" sine wave the off time may be longer than the on time or vice versa.

Inverters are an awesome design of ingenuity, if you ever had to deal with some of the older model DC systems...like MaxLine etc you would understand why these things are awesome. Many applications that use to REQUIRE a DC motor and controller can now easily be replaced with an AC inverter and 3 phase motor.

I originally answered this question with a "it depends" kind of answer and that frequency was the main variable. Technically it is. Many have said it maintains a constant volt/hertz relationship. I guess it does...again tho this is a "it depends" thing. Yes its constant BUT the constant relationship may vary...depending. The current relationship may vary...ever seen a zero speed constant torque application? That implies a voltage and current but no frequency so its stationary but has the "power" to "hold" a load.

Maybe some of this is skewed somewhat because of my way of thinking. My learning says if you have current you must have voltage...lack of frequency tho could remove speed from the factor...this is a simplistic view because other factors MUST apply because its overall an AC ckt...just using a controlled DC.

Personally I appreciate DickDV spending his time giving us the knowledge he has gathered over the years pertaining to AC drives. He offers some very informative posts. Thank you Dick.

I also wonder why you would connect lights or resistive load to an AC inverter? Yes the lights will pulse depending on frequency...that more or less shows the output is DC.

I keep a light bulb and pig tail (a 2 wire socket for a bulb) around as troubleshooting tool...use 240v bulbs and it works wonders. I obtained this habit from working with gensets that had to be paralled.

These are my thoughts and opinions, I look forward to hearing others.