VFD question

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JonwoN

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Single phase 120/240 power on building. Co workers want to use a VFD to serve two 3 phase motor loads that are separately controlled. Is that possible with a vfd ?
Thank you for any inpuT!
 
No problem, simply derate by 33%.

Or, if you want to hack into the VFD circuitry, upgrade the rectifier diodes feeding the dc link and you don't even have to derate. May need to add some dc link capacitance too.

You do need 2 (two) vfds if you want to control the motors separately.
 
If I understand correctly
1 vfd sized 1.73 x (sum of motor hp or next larger size)
run at 60 Hz
motors connected in parallel to the vfd
each motor has a contactor/starter to start/stop it
interlock vfd to start when either motor is started

good question
a mod named jaref is the vfd guru
I'm curious too
 
JonwoN said:
Single phase 120/240 power on building. Co workers want to use a VFD to serve two 3 phase motor loads that are separately controlled. Is that possible with a vfd ?
Thank you for any inpuT!
Click to expand...

In general it is possible, especially for smaller Motors. Like many things there are certain limitations but it's not an uncommon thing to do. It might be almost is cost effective to get two small vfd rather than trying to run contactors off the output of 1 vfd.
 
We only have single phase at our shop and set up drives there before installing them. We have a small three phase motor for temporary load and a small transformer to get 480 for those drives.

I think the problem will be the second motor jumping in and out. Both together and your good to go.
 
There are 2 questions here.

1) Can a VFD be used to power a 3 phase motor from a single phase supply? The answer is in general yes, as long as things are sized appropriately.

2) Can a VFD be used to power multiple _separately controlled_ motors? This answer is in general _no_. In particular you probably don't want to have any sort of contactors sitting between the VFD and the motors, and you don't want to have the VFD running at fixed frequency and then start the motors 'across the line' on the VFD output. A VFD type device could be designed to function in this fashion, but a standard VFD won't be happy.

You will be much better off with two small VFDs, one for each motor, acting to convert single phase to three phase and then control the motors.

-Jon
 
winnie said:
There are 2 questions here.

1) Can a VFD be used to power a 3 phase motor from a single phase supply? The answer is in general yes, as long as things are sized appropriately.

2) Can a VFD be used to power multiple _separately controlled_ motors? This answer is in general _no_. In particular you probably don't want to have any sort of contactors sitting between the VFD and the motors, and you don't want to have the VFD running at fixed frequency and then start the motors 'across the line' on the VFD output. A VFD type device could be designed to function in this fashion, but a standard VFD won't be happy.

You will be much better off with two small VFDs, one for each motor, acting to convert single phase to three phase and then control the motors.

-Jon
Click to expand...
I agree 100%, especially in the issue of never tuning on a contactor feeding another motor if a drive is already running. You will damage the transistors.

An additional comment on sizing though.
At 230V, many VFDs are already designed to allow a single phase input without de-rating up to and including 3HP. Some mfrs offer two versions, one for 3 phase input, one for single phase, but some don't bother, they just offer one version useable either way. Check that out first.

But if it is over 3HP or any HP at 480V, you must at least DOUBLE the current rating of the drive compared to the motor FLC. Yes, the 1.732x sizing factor applies to the current rating and will cover the diode capacity of the rectifier, but you also need additional DC bus capacity to smooth out the additional DC bus ripple that results from rectifying single phase. Without it, you will eventually damage the transistors because they can misfire from the ripple. Using only a 1.732x sizing factor does not provide enough capacitance, 2x is the minimum. Then on many newer smaller drives it is even more, a 65% de-rate (2.85x sizing factor) because the capacitors can over heat. The alternative is to use the 2x sizing, but de-rate the ambient temp to 25C (77F), which is difficult to achieve. Bottom line, RTFM or ask the manufacturer of the drive.
 
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Thank you everyone for the input, based on the comments we decided to get two vfd's one for each motor. sizing is being done by my supervisor and the vfd manufacturer.
 
JonwoN said:
Thank you everyone for the input, based on the comments we decided to get two vfd's one for each motor. sizing is being done by my supervisor and the vfd manufacturer.
Click to expand...
If I may ask what you are powering with these motors?

If they are general purpose motors and don't need speed regulation or soft starting the least expensive route would just be to replace the three phase motors with single phase motors in most instances.
 
kwired said:
If I may ask what you are powering with these motors?

If they are general purpose motors and don't need speed regulation or soft starting the least expensive route would just be to replace the three phase motors with single phase motors in most instances.
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20 hp hydraulic pump , 6.6 hp cut off saw. which i have been told are both 230v 3 phase ( i did not personally look at nameplate otherwise i would have service factor , hertz , code letter and everything on the nameplate basically ). The situation is the phase converter is too loud, and the company wants a quieter means of generating 3 phase for said equipment. This is merely bid work at this time once they see the price we are sure they will elect to move the phase converter outside. Any input is highly appreciated!@#$ :D
 
petersonra said:
In general it is possible, especially for smaller Motors. Like many things there are certain limitations but it's not an uncommon thing to do. It might be almost is cost effective to get two small vfd rather than trying to run contactors off the output of 1 vfd.
Click to expand...
It's what I would do.
But what do I know............
 
JonwoN said:
20 hp hydraulic pump , 6.6 hp cut off saw. which i have been told are both 230v 3 phase ( i did not personally look at nameplate otherwise i would have service factor , hertz , code letter and everything on the nameplate basically ). The situation is the phase converter is too loud, and the company wants a quieter means of generating 3 phase for said equipment. This is merely bid work at this time once they see the price we are sure they will elect to move the phase converter outside. Any input is highly appreciated!@#$ :D
Click to expand...
20 HP single phase motors are not so easy to come by and phase converter or VFD would be the more common application here. Keep in mind for most off the shelf VFD's this means you need at least a 40 HP drive to run it with single phase input.

I find it kind of funny they think the phase converter is too loud when you probably can't even hear it over the driven loads when they are running.
 
Well then here's a new wrinkle. 27HP is a large draw on a single phase 240V service. I'm guessing these motors alone will draw close to 100A, so if you have a 100A meter, common for 120/240V services, you will not be able to feed it. If you have a 200A meter and there isn't much else going on it could work, but make sure you run this by the utility first.
 
Jraef said:
Well then here's a new wrinkle. 27HP is a large draw on a single phase 240V service. I'm guessing these motors alone will draw close to 100A, so if you have a 100A meter, common for 120/240V services, you will not be able to feed it. If you have a 200A meter and there isn't much else going on it could work, but make sure you run this by the utility first.
Click to expand...

That and the code required increase in wire size etc for a 40 hp drive regardless of the service entrance size.
 
ptonsparky said:
That and the code required increase in wire size etc for a 40 hp drive regardless of the service entrance size.
Click to expand...

Great point. 125% of the DRIVE rated input current.

Then in looking for typical VFD input currents for 40HP 240V rated drives, I saw some that say the source must be no less than 66kVA. A typical 240V 200A service is. 50kVA transformer, so that may add yet another wrinkle (unless you have a 320A meter).
 
Jraef said:
Well then here's a new wrinkle. 27HP is a large draw on a single phase 240V service. I'm guessing these motors alone will draw close to 100A, so if you have a 100A meter, common for 120/240V services, you will not be able to feed it. If you have a 200A meter and there isn't much else going on it could work, but make sure you run this by the utility first.
Click to expand...

Out here in the boonies it is more common to see larger single phase services then it may be in cities.

I just converted a 2000 amp single phase service at a school building in a small town to 208/120 three phase because we added major HVAC loads to the facility. That 2000 amp service however was only served by a 100 kVA utility transformer and peak demand before changes only reached around 83 kVA.
 
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