Amperage calculation before a VFD single phase to 3-phase

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Kevin Williams

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Katy Texas
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Electrical Contractor
I am really in a pickle here.
The Contractor signed off on 208 3-phase pump 7.5 Hp and that we had 208 single phase coming in.
I have 120/240 single phase coming in. This is 650 ft away mind you.
The pump people refused to change the pump. So, I must step the voltage down to 208 Volts.
the pump has an FLA of 27 at 3-phase what will my single-phase FLA be before VFD.
 
The first step is that the VFD will need to be oversized (de-rated) to operate from a single-phase supply. Common rule of thumb is to double the rating of the drive. So you probably will have a 15 hp VFD. Next step is to confirm with the VFD supplier what the single-phase input current will be at 208 V single-phase. They should tell you what the input current requirement are. It will likely be more than the 47 A due to inefficiency in the rectifier section and drive losses. You need to make sure that whoever is sizing the VFD knows it will be getting a single-phase input and that the VFD supplier is OK with this. I wouldn't size the feeder or transformer until you get data from the VFD people. This is getting to the upper range of applications I've seen with single-phase input. Also it would be best to put your transformer at the drive to take advantage of the 240 V down the 650 ft. This seems like not a great solution, overall.
 
Yes, I usually have to double the size of the drive when it's used in a single phase in, three phase out situation. Largest I have done is a 15 hp 240v 3phase submersible motor and pump where only single phase was available. 10 hp is as big as they make a single phase sub motor for water wells. Used a 30 hp Yaskawa drive. Had to install a new panel and service for the pump house. What distance from your pump will the VFD be? You may want to install a dv/dt filter. I prefer the TCI V1K.
 
Wait: you thought you had 208V single phase, and were provided with a 208V three phase motor and VFD that takes 208V single phase input, but now have to deal with the fact that you really have 240V?

You probably don't need to step the single phase voltage down to 208V. Just supply the 240V to the VFD. Very likely the VFD is intended for use over a range of voltages.

Remember that part of adjusting frequency is being able to adjust voltage to match, so the VFD will be able to supply 208V 60Hz from a 240V supply if it is rated for 240V input.

The motor is quite likely a 208-230V motor anyway, and will be happier with the higher voltage.

The VFD will compensate for motor power factor, so there isn't a simple correlation between motor 3 phase rated current and the single phase load current.

As others noted the VFD will need to be oversized to deal with the single phase input.

I expect the drive you use will have a 50-60A rated input, but that the actual running current would be around 30A if the pump gets fully loaded.

Jon
 
For good measure, double that 27A to say 60A and look for a VFD that can deliver that amperage -->about 15HP. VFD manufacturers have instructions on how to choose supply requirements for their units, IMO.
 
winnie said:
Wait: you thought you had 208V single phase, and were provided with a 208V three phase motor and VFD that takes 208V single phase input, but now have to deal with the fact that you really have 240V?

You probably don't need to step the single phase voltage down to 208V. Just supply the 240V to the VFD. Very likely the VFD is intended for use over a range of voltages.

Remember that part of adjusting frequency is being able to adjust voltage to match, so the VFD will be able to supply 208V 60Hz from a 240V supply if it is rated for 240V input.

The motor is quite likely a 208-230V motor anyway, and will be happier with the higher voltage.

The VFD will compensate for motor power factor, so there isn't a simple correlation between motor 3 phase rated current and the single phase load current.

As others noted the VFD will need to be oversized to deal with the single phase input.

I expect the drive you use will have a 50-60A rated input, but that the actual running current would be around 30A if the pump gets fully loaded.

Jon
Click to expand...
Thank You, so get out there and play with it. I've never set one up this will be my first.
 
Dzboyce said:
Yes, I usually have to double the size of the drive when it's used in a single phase in, three phase out situation. Largest I have done is a 15 hp 240v 3phase submersible motor and pump where only single phase was available. 10 hp is as big as they make a single phase sub motor for water wells. Used a 30 hp Yaskawa drive. Had to install a new panel and service for the pump house. What distance from your pump will the VFD be? You may want to install a dv/dt filter. I prefer the TCI V1K.
Click to expand...
What they are saying is the VFD is rated at 78 amps it's built in to a control panel.
 
winnie said:
You probably don't need to step the single phase voltage down to 208V. Just supply the 240V to the VFD. Very likely the VFD is intended for use over a range of voltages.
Click to expand...

I totally agree w/ Jon here. And also agree you should eyeball the actual VFD specs. It would be very unusual (imo) to find a 208 VAC only unit, but absolutely check.

By using 240 instead of 208 VAC Mains, you are creating a slightly higher DC bus voltage. And when the VFD starts to pulse width modulate that bus to create skinny and fat pulses that mimic a sine wave the slightly higher DC bus value will have the VFD make overall slightly skinnier pulses. All that math is taken care of inside the the VFD to ensure the "area under the curves" is the same.
 
winnie said:
You probably don't need to step the single phase voltage down to 208V. Just supply the 240V to the VFD. Very likely the VFD is intended for use over a range of voltages.
Click to expand...
This^^^.
You program the motor voltage into the VFD anyway, so it can be ANYTHING up to and including the input voltage. No need to step it down ahead of the VFD, you just tell the VFD that it is a 208V or 200V motor (if it truly is) and the VFD will automatically limit the output to 208 or 200V.

Doubling the size of the VFD to the motor nameplate FLA is the MINIMUM requirement, but in reality you MUST consult with the VFD mfr and get them to respond IN WRITING that this is their approved de-rate method. Some smaller VFD designs require a 65% de-rate (motor FLA/.35), because of how the DC bus capacitors are sized and configured. Basically IF the VFD has what's called a "DC Bus Choke", it's a safe assumption that a 50% de-rate is going to be permissible. But many small Asian sourced drives do not use a DC choke, they just use more capacitors (because caps are cheaper for them), so the capacitors will over heat if not given a further de-rate. Bottom line, go by what they PRINT, not by what some salesman tells you, because if the VFD burns up, you will be left holding the bag.
 
What JRAEF said is true. Capacitors should have voltage ratings that are higher for them to survive. You already have a current requirement which should be 1.732 times the load amps (three-phase to single-phase) plus the factor 1.414 to account for the peak voltage that the capacitor will be subjected to (if used in the VFD extensively)--> that's about 2.5 times (40%). Employing chokes on the DC bus could temper those bad things that go with cheap designs but we can't be sure if those cheap designs have entered our markets nowadays! Still, you are the boss, choose wisely.
 
When installing a starter for a single phase motor such as vfd do you terminate the neural to starter line terminal as you would ungrounded phase conductors? Or is only black wire single single phase connected to line and neutral unbroken back to main panel?
 
I mean unbroken except for control solid state purpose

My company I work for is cheap and uses old school contact 3 phase starters

And old pneumatic time delay relays for motors Etc
 
Jpflex said:
When installing a starter for a single phase motor such as vfd do you terminate the neural to starter line terminal as you would ungrounded phase conductors? Or is only black wire single single phase connected to line and neutral unbroken back to main panel?
Click to expand...
If you are using a mag starter for a 120V motor, you will typically put the Neutral through the contactor and OL relay, but it is not 100% necessary. You can ALSO run the neutral straight to the motor because if you switch only the Hot line, the motor is still controlled by the starter. The only thing you cannot do is to switch the Neutral without also switching the Hot.

As a separate issue: IF you are using a VFD that converts the 120V into 230V 3 phase for a 3 phase motor, then you MUST run the Neutral to the VFD, because otherwise you do not have a complete 120V source feeding it.
 
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