VFD's - An Explanation Please

goldstar

goldstar

Senior Member
Location
New Jersey
Occupation
Electrical Contractor
I've never had an occasion to use or have wired a VFD. If someone could explain it's function and/or explain what applications it can be used in I would appreciate if you would post it here.

I recently wired a hood exhaust system for a small pizza shop and did several other small electrical jobs within the store. The owner was sold a 3-phase, 240V mixer by the company who installed the hood exhaust system. There is no 3-phase service not only in the store but in the entire small strip mall. This company, said they will install a VFD inside the mixer to convert it to single phase. I'm not sure how that will work. In addition they tell me that they will need to convert the existing 20A, 240V circuit to 30 amps and can't I just switch the breaker out :(. I already know the answer to that one but I am clueless about the VFD. Any help would be appreciated. Thanks in advance.
 
VFD stands for Variable Frequency Drive. A 3-phase motor's speed is controlled not by the voltage sent to the motor, but by the frequency.

Examples:
60Hz = nameplate speed (1750RPM for a 4-pole motor).
30Hz = 1/2 nameplate speed.

In your case, the vendor is using a VFD to convert from single phase to 3-phase power. This works because a VFD takes in AC from the line, rectifies it into high-voltage DC, and then chops that DC up back into a synthesized AC waveform at whatever frequency is programmed. In your case, it'll probably be set to 60Hz, and will be used simply to convert single phase to 3-phase power.

You need the larger circuit because that power has to come from somewhere. For a given amperage, a 3-phase supply can deliver 1.73 times as much power as an equivalent single phase supply.


SceneryDriver
 
Beat me to it. VFDs can be used to create 3phase power from a single phase supply as well as controlling the speed of an AC motor by varying the frequency delivered to the motor. This is a gross simplification of what is happening but I think it answers your question.
 
Thanks. I'm having difficulty posting a photo of the nameplate. But, here's some info on the nameplate :

Volts - 230
Hz - 60
RPM - 1725
HP - 2
Amps - 5
Ph - 3
 
The simplest explanation I can think of:

Imagine an audio amplifier with the input signal supplied by a variable-frequency audio signal generator. At 60 Hz, it would resemble the utility supply, but can be adjusted for other frequencies.

Of course, as a 3ph source, a VFD has three "amplifiers" and output terminals, with the normal 120-degrees of separation maintained among the three outputs as the output frequency is varied.
 
I would call a local VFD sales company and ask them exactly what will be large enough to put out a three phase from your single phase power. VFD'S have incoming ( line side ) go to full wave rectifiers that have a capacitor to smooth out the power. Large drives have a capacitor bank. The DC is feed to IGBT'S that can switch power up to 5 Hertz. I do not recommend purchasing drives from Amazon. Like a box of chocolate you never know what you are going to get. My old cheap boss purchased a cheating Chinese drive from Graingers. He spent over six hours and numerous phone calls to try to get it to operate properly.He returned it for a made in USA drive. Had it running in less then 30 minutes. I had a 60 HP 480 volt return fan feed from a VFD continue running after it blew the B phase fuse. Drive control transformers usually are feed by the A & C phase. This fan was only running at about 60% of full speed. A 480 volt three phase drive operates on a DC Buss voltage or 650 to 666 volts DC. With the one fuse blown the DC Buss voltage was much lower. If this is your first drive install I would recommend that you have the local drive wholesaler perform the start up. Might only cost a few hundred dollars but money well spent in the drive knowledge you will learn. We always had a company perform drive start ups because it extended the free parts & warranty out to three years at least on Danfoss drives. In a emergency I would replace a drive and punch in the motor specs & set some parameters like ramp up & down speed and min & max speeds and get them running but have the drive tech go over it on his next visit. These drive tech are great. We had a problem with 8 or 10 drives on a roof that would trip out once or twice a year what they called earth fault. Would megger the motors and they always passed. Finally asked the drive tech about this problem. Here early models of this drive had a ribbon cable defect and he replaced them for free.Great drive company supplied replacement ribbon cables no charge. Never would have found this problem in my own.
 
goldstar said:
.. This company, said they will install a VFD inside the mixer to convert it to single phase.
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Along with the excellent lessons for how VFD's work, there's another interesting aspect.

When equipment is suggested by trusted opinion without bids, markup over 10 times is not unusual.

So a combination 3-phase mixer + VFD + 10 times equipment markup, was suggested for 1-phase pizza shop.

With average ~$7 profit per pizza, ~$1 for the dough, ~30 pizzas sold per day, how long before the mixer pays for itself?

Since owner didn't bother to check for 1-phase mixer from restaurant supplier, or for pre-owned mixer from equipment auctions, a trusted contractor was able to suggest the equipment source.
 
For a 2hp motor someone (you) could change the motor from 3 phase to a single phase motor. If you do this have the mixer company supply the motor. It is possible depending on the load that they would have to bump up to a 3 hp motor on single phase.

Or go along with the vFD. The VFD makes things a little more complicated, but it will work fine.

Either way there is no free lunch.

You will have to upsize the branch circuit either way
 
I wired a restaurant and the hood fan contractor did exact same thing! no one checked the electrical service and just assssumed it was 3 phase. A vfd appeared one day. no explanation.
 
garbo said:
I would call a local VFD sales company and ask them exactly what will be large enough to put out a three phase from your single phase power. VFD'S have incoming ( line side ) go to full wave rectifiers that have a capacitor to smooth out the power. Large drives have a capacitor bank. The DC is feed to IGBT'S that can switch power up to 5 Hertz. I do not recommend purchasing drives from Amazon. Like a box of chocolate you never know what you are going to get. My old cheap boss purchased a cheating Chinese drive from Graingers. He spent over six hours and numerous phone calls to try to get it to operate properly.He returned it for a made in USA drive. Had it running in less then 30 minutes. I had a 60 HP 480 volt return fan feed from a VFD continue running after it blew the B phase fuse. Drive control transformers usually are feed by the A & C phase. This fan was only running at about 60% of full speed. A 480 volt three phase drive operates on a DC Buss voltage or 650 to 666 volts DC. With the one fuse blown the DC Buss voltage was much lower. If this is your first drive install I would recommend that you have the local drive wholesaler perform the start up. Might only cost a few hundred dollars but money well spent in the drive knowledge you will learn. We always had a company perform drive start ups because it extended the free parts & warranty out to three years at least on Danfoss drives. In a emergency I would replace a drive and punch in the motor specs & set some parameters like ramp up & down speed and min & max speeds and get them running but have the drive tech go over it on his next visit. These drive tech are great. We had a problem with 8 or 10 drives on a roof that would trip out once or twice a year what they called earth fault. Would megger the motors and they always passed. Finally asked the drive tech about this problem. Here early models of this drive had a ribbon cable defect and he replaced them for free.Great drive company supplied replacement ribbon cables no charge. Never would have found this problem in my own.
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I'd do none of this. not his problem. wire the drive YOU say will work. checkbook in hand? ok. blows up. oh, well. don't get involved but wiring the thing
 
goldstar said:
In addition they tell me that they will need to convert the existing 20A, 240V circuit to 30 amps and can't I just switch the breaker out :(.
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Along the lines of what Golddigger just posted, I looked at a VFD that will accept 240V single phase input and run a 2HP 230V 3 phase motor. The VFD INPUT current rating is 18A from the 240V single phase source, so per 430.122 you must size the CONDUCTORS at 125% of that value (22.5A), and as per their UL listing of the VFD, the MINIMUM fuse or circuit breaker to feed it is 25A. So #10 conductors, 30A breaker is appropriate (25A would be OK too, but it doesn't change the conductor size). 20A no good. If everything is wired in conduit and you are using THHN, with nothing else in the conduit and the run is short, you MIGHT be able to use the #12.
 
Jraef said:
If everything is wired in conduit and you are using THHN, with nothing else in the conduit and the run is short, you MIGHT be able to use the #12.
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#12cu @ 75C per T310.16 maxes out at 25A, or 20A x 1.25%, per 430.122(A) VSD loads.

After factoring single to poly phase conversion, a 2 HP mixer may be too large for the #12 circuit.

If the fire marshal has seen field modified equipment, or unlisted VSD assemblies, burn down restaurants before, then suddenly any fire caused by gas leaks, vandals, or homeless encamped on the roof, becomes an electrical fire, attributed to the unlisted VSD assembly.
 
Be aware the addition of the VFD may likely cause GFCI tripping. Might be in long term better to get a non VFD option as the VFD would get nuisance tripping issues.
GFCI is required in 210.8(B)(2) for the kitchen area of non dwelling units.

(B) Other Than Dwelling Units.
All 125-volt through 250-volt receptacles supplied by single-phase branch circuits rated 150 volts or less to ground, 50 amperes or less, and all receptacles supplied by three-phase branch circuits rated 150 volts or less to ground, 100 amperes or less, installed in the following locations shall be provided with GFCI protection:
 
Fred B said:
Be aware the addition of the VFD may likely cause GFCI tripping. Might be in long term better to get a non VFD option as the VFD would get nuisance tripping issues.
GFCI is required in 210.8(B)(2) for the kitchen area of non dwelling units.

(B) Other Than Dwelling Units.
All 125-volt through 250-volt receptacles supplied by single-phase branch circuits rated 150 volts or less to ground, 50 amperes or less, and all receptacles supplied by three-phase branch circuits rated 150 volts or less to ground, 100 amperes or less, installed in the following locations shall be provided with GFCI protection:
Click to expand...
I know that Danfoss drives has have a built in what they call "Earth Fault " on all of thier hundreds of drives I have PM'ed in last 30 years. Often wondered if this would qualify as ground fault protection. Most of trip out on the earth fault were false. Would make sure motor was at a complete stop and use a,1,000 volt megger on the 480 volt motors and they usually had a meg reading over 10 mega ohms to ground. Looking thru the screen that held the last 10 reasons drive faulted often would come across that for some reason a drive would trip out once or twice a year on false earth faults. Did have earth fault shut downs on drives where the motor was grounded and no fuses blew. Not an expert but appears that NEC 210. 8 (B) (2) does not pertain to a hard wired drive that feeds non residential exhaust hood and motor is hard wired. When in doubt ask the AHJ.
 
garbo said:
..1,000 volt megger.. once or twice a year on false earth faults. ..appears that NEC 210. 8 (B) (2) does not pertain to a hard wired drive.
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If you accept your own office in my pizza joint, will pay all-you-can-eat, to keep the VFD circuit radio noise shielded & isolated from other GFCI devices, in kitchen & fuse box.

All I ask, is when GFCI's or earth faults force VFD shutdown during production, you kneed & toss the pizza dough in the show window.
 
garbo said:
I know that Danfoss drives has have a built in what they call "Earth Fault " on all of thier hundreds of drives I have PM'ed in last 30 years. Often wondered if this would qualify as ground fault protection.
Click to expand...
No. The NEC specifically uses the term “GFCI”, which is Class A protection at 5mA. The GF protection built into the VFDs is intended to protect only the motor, not the circuit or people, it is never Class A.
 
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