VFD questions

[homebuilt burner]

I was a mechanic for many years. Now part of my job involves VFDs. I do not work on them but around them and could use some knowledge. Please correct me if I am wrong. I realize I don’t know enough and I do not touch.

They vary the frequency to speed up and slow down an electric motor.??

Do they always need 3 phase input voltage?

Do they always output 3 phase voltage?

Can the output frequency be measured?

Do they hold residual volts/amps? Like a condenser or capacitor.

Does the output voltage change and can it be measured?

 

[txvwnut]

I was a mechanic for many years. Now part of my job involves VFDs. I do not work on them but around them and could use some knowledge. Please correct me if I am wrong. I realize I don’t know enough and I do not touch.

They vary the frequency to speed up and slow down an electric motor.??
Yes

Do they always need 3 phase input voltage?
No

Do they always output 3 phase voltage?
I believe so

Can the output frequency be measured?
Yes

Do they hold residual volts/amps? Like a condenser or capacitor.
No, at least mine doesn’t

Does the output voltage change and can it be measured?
Output will be the same as input and yes

See answers above in the quoted post

 

[MattT]

They vary the frequency to speed up and slow down an electric motor.??

They vary voltage and frequency mostly on a linear scale. So to run a 230v 60Hz motor at half speed the VFD will output 115v at 30Hz.

Do they always need 3 phase input voltage?

Can be single or three phase input.

Do they always output 3 phase voltage?

Mostly yes. Some can output DC to stop the motor but it's rare anyone uses that feature.

Can the output frequency be measured?

Yes with the right meter. The output is a square wave high frequency which equates to the requested sine wave frequency.

Do they hold residual volts/amps? Like a condenser or capacitor.

Yes. How they work is they convert the incoming AC power and convert it to DC. This DC power is stored in a capacitor bank inside the VFD and then switched back into AC to drive the motor. The capacitor bank will stay charged for some time after input power is removed.

Does the output voltage change and can it be measured?

Yes. As above you need the right test gear to measure it accurately.

 

[marinusdees]

In my limited experience, if you feed a vfd (freq drive) single phase, it spits out 3 phase. I'm sure the boo birds have other opinions. Evidently some freq drives don't, I have not encountered one, but my experience is limited. Admittedly so..

 

[Gokart]

I was a mechanic for many years. Now part of my job involves VFDs. I do not work on them but around them and could use some knowledge. Please correct me if I am wrong. I realize I don’t know enough and I do not touch.

They vary the frequency to speed up and slow down an electric motor.??

Do they always need 3 phase input voltage?

It depends on the rating of the VFD. Some models are rated for single phase input (typically 1-5 hp), or there are applications specific guidelines you can follow for single phase use. For instance I have a 60 HP vfd (schneider) that I am feeding with 240V single phase to fed a 30 HP 240 Three Phase Motor. In this instance I needed to add a line reactor.

Do they always output 3 phase voltage?

Yes

Can the output frequency be measured?

Yes, your meter just needs to be capable of measuring HZ.

Do they hold residual volts/amps? Like a condenser or capacitor.

The do hold voltage, as there are capacitors in the VFD's. I would always check for voltage present, on the DC buss before you work on them. You can see this in action when you power off a bigger drive, as it will take some time for the screen to power down. This is the capacitors bleeding off.


Does the output voltage change and can it be measured?

VFD's typically output in a V/F mode, so on a 480V system w/ a motor running at 30 HZ (half speed), you would expect to see 240V across the phases (480/ (60/30))

 

[homebuilt burner]

So they vary the frequency and the voltage?

 

[Matt Matt]

They vary the frequency to speed up and slow down an electric motor.??yes with volts

Do they always need 3 phase input voltage? no

Do they always output 3 phase voltage?no

Can the output frequency be measured?yes

Do they hold residual volts/amps? Like a condenser or capacitor.yes

Does the output voltage change and can it be measured?yes

So they vary the frequency and the voltage?yes to a max. There are 120 SP models that out put 240 3 Phase.

There are 3 Phase in=>3 phase out.
There are single phase in => 3 phase out.
Some have brake resistors and some are without.

 

[American Locomotive]

So they vary the frequency and the voltage?

VFDs generate a voltage that's proportional to the output frequency. So if you have a 480v motor and run it at 60Hz, the drive will output 480v. If you run the motor at 30Hz, the drive will output 240v.

There are other control schemes that could allow the drive to output different voltages at a given frequency, but you don't really need to sweat the details.

 

[TRWham]

A couple of practical things you might encounter:

VFDs are hard on motors because they can generate voltage spikes and the output is not a nice, smooth sine wave. There are inverter duty motors that are built to handle higher voltage and generally have heavier windings and insulation.

Motor leads over 150' can lead to even uglier waveforms at the motor, so pay attention to layout. You can add reactors to tame that. This was never an issue for me, but I imagine some plants would need the drives to be remote from the motors.

We applied many Yaskawa drives in refrigeration and they were very supportive with training and education, so I imagine you might be able to attend some brief classes to get some background.

 

[exmaxima1]

Do they always need 3 phase input voltage?

Most all VFD's can accept both single phase and 3-phase input power. If you only have single phase power available, you only connect 2 wires (vs 3 wires for 3-phase power). But for single phase power you you will need 1.73 times more current than for 3-phase. And that generally means you need to de-rate the VFD that same amount, so a 3hp motor would need a 5hp VFD (or simply double the VFD size for running a 3-phase VFD on single phase power).

 

[acmikee]

use VFD rated motors if you want it to last also install grounding brushes on the motors shafts.

 

[Dingleburry]

Mostly yes. Some can output DC to stop the motor but it's rare anyone uses that feature.

Can the output frequency be measured?

Yes with the right meter. The output is a square wave high frequency which equates to the requested sine wave frequency.

Yes. As above you need the right test gear to measure it accurately.

The "right test equipment" you need is a DMM with low pass filter. Fluke 87v has it.
Heres a post of me showing the differences.

https://www.garagejournal.com/forum/showpost.php?p=7471213&postcount=33

VFDs can also do a DC injection when at 0 hz to say, hold a fan from spinning from the wind, of if its cold out to keep the motor warm etc, just gives a constant DC voltage when not in operation.

So they vary the frequency and the voltage?

Its called volts per hertz. A motors resistance is based on its frequency. So lower frequency, lower resistance, you need to lower the volts or the amps will go through the roof.

There are other control schemes that could allow the drive to output different voltages at a given frequency, but you don't really need to sweat the details.

If you wanna reduce the max frequency output, you cant just put 40hz, cause itll still use the maximum volts even though only at 40hz causing the drive or motor to overload. Cause now its at 100% hz (40) and 100% volts. Easiest way is to adjust your input % from the potentiometer and leave max freq at 60.

 

[homebuilt burner]

Thank you for all of the information.

 

[Dingleburry]

Below 60hz you get full torque, but reduced HP, above 60hz you get full HP but reduced torque.
Therotically.

 

[MattT]

If you wanna reduce the max frequency output, you cant just put 40hz, cause itll still use the maximum volts even though only at 40hz causing the drive or motor to overload. Cause now its at 100% hz (40) and 100% volts. Easiest way is to adjust your input % from the potentiometer and leave max freq at 60.

Depends on the drive. A lot of them will have base frequency and voltage settings which control the V/Hz ratio. On those max frequency only limits the frequency output. Gotta read and understand the manual before playing with drive parameters.

Below 60hz you get full torque, but reduced HP, above 60hz you get full HP but reduced torque.
Therotically.

You get full torque until the drive maxes out on voltage which is roughly equal to input voltage with a three phase input. Haven't played with single phase input enough to know for sure how much output voltage it's capable of.

So what you say is true for most installations but when you you have 480v input and wire the motor for 230v you get full torque up to around 120Hz and HP over motor nameplate above 60Hz

 

[bjcouche]

Most SMALL drives (VFD's) can run on single phase input, like under 20HP or so. What you'll find is from 20HP to 10,000+Hp, these larger drives can not operate on single phase for two main reasons. Firstly some have more complex input rectifier configurations utilizing SCR's, or power factor correction schemes and detect the lost phase. Another reason is that the larger drives often have 3 phase motors internally to power fans.
A drive takes the input AC voltage, converts it to DC, then converts it back to AC again. an example would be, 480VAC, 60Hz -> 675VDC -> 0-480VAC, 0-60Hz.

Some drives you can skip the AC input altogether and just supply it with 675Vdc, if you just happen to have that high voltage DC lying around.... For 240V drives, just divide by 2.

Brian

 

[6768rogues]

We had to replace a number of VFDs at about the 5 to 7 year mark. They don't last forever.

 

[Stuart in MN]

We had to replace a number of VFDs at about the 5 to 7 year mark. They don't last forever.

VFDs, even the little cheap ones, have gotten a lot better over the years. Sure, they can and will break down, but the ones you buy today are most likely going to last a lot longer than ones bought even five years ago. [they keep getting smaller and smaller, too...I put in 100hp VFDs 25 years ago that were the size of a large refrigerator, today a 100hp VFD is a little box that hangs on the wall.]

 

[Augus7us]

The life of the VFD also depends on the environment. I use NEMA 4X enclosures on mine because there is a lot of metal dust in my shop.

This thread was confusing me but I think that is because we are talking smaller 1-5HP VFDs and as someone else mentioned the larger industrial 20HP+ VFDs. I've only ever used the smaller ones in my hobby shop so I've never seen any that can input 480v 3ph. One the smaller stuff you typically need a transformer with your VFD to step up to 480v.

When I purchased and installed my first VFD four years ago, I can say they are pretty simple to setup and tune. Also, if you buy from a reputable dealer, you typically get good support.

Thought I would add my limited experience with them.

 

[bjcouche]

As someone who has worked with and repaired Industrial VFD's of all sizes in industry, I can ASSURE you that they are NOT getting more reliable as newer models come out. The prices are becoming cheaper and cheaper due to competition and buyers demanding lower and lower prices. Manufacturers are responding with lower and lower quality products, which do not last as long as the previous generation products. The lower prices should benefit the home shop garage journal crowd and the reduced lifespan shouldn't bee too much of an issue due to the use not being 24 hours per day operation.
For those who want to make their VFD's last longer, when you are not using them, turn the power to them off, as in have a disconnect switch on the input power. Keeping them powered up 24/7 will shorten their lifespan because the capacitors are still charged and being stressed.
Brian

 

[MattT]

This thread was confusing me but I think that is because we are talking smaller 1-5HP VFDs and as someone else mentioned the larger industrial 20HP+ VFDs. I've only ever used the smaller ones in my hobby shop so I've never seen any that can input 480v 3ph. One the smaller stuff you typically need a transformer with your VFD to step up to 480v.

480v drives are available all the way down to fractional HP sizes.

Not exactly sure what you mean by "One the smaller stuff you typically need a transformer with your VFD to step up to 480v".? Stepping 240v single phase up to 480v to feed a 480v drive does work but I don't think that's what you're referring to.