Post By Jraef
single phase power into a VFD rated for 3-phase?
I have a static phase converter, which I bought when I got my first three-phase machine, a Bridgeport mill.
I am adding a VFD to another machine. I think the VFD (brand is GE) is rated for 3HP, and is supposed to have 3 phase power input.
At first, I just connected the VFD in the same way that I connected the Bridgeport mill, in parallel with the lines coming from the static phase converter. The VFD worked OK, except for when I would turn another machine on or off. Then it would give an error message, from the input power surge.
As a test, I just disconnected the leg that is generated by the static pahse converter, to see if the VFD would work with single phase input. I was pleasantly surprised that it does seem to work now. Furthermore, I can simplify wiring by not using the 4-wire twistlock connectors, and keep the simple (#12 AWG 3-conductor wire with 220VAC single phase input).
My question is, should I have any worries about damaging the VFD and/or motor by doing this? I have read a couple of posts about derating a VFD that is run on single phase power, but I don't quite understand how that works. Is there a general rule of thumb for this sort of thing? The manual that came with the VFD did not mention anything about wiring it for single phase power.
Can a person use single phase power for any VFD that is labelled for 3-phase power input?
Thanks in advance for your knowledge,
Here's how it works inside...
Here's how a VFD works on the INSIDE...
AC power comes in at the input terminals (single or three phase... doesn't matter).
This is rectified into DC... commonly referred to as LINK VOLTAGE.
Link voltage, being DC, is then sent to three switching-amplifiers (one for each output phase) to be three sine- or modified-sine outputs, operating at a frequency determined by desired MOTOR speed.
The de-rate issue revolves around the fact that in a three-phase input situation, you have THREE bridge rectifiers... all piling their DC power onto the link-voltage 'rails'.
If you run a three-phase inverter from a single-phase source, the input rectifier might be carrying a very substantial current... because all of it's equivalent power is being drawn through smaller recifiers... not to mention a few 'other' things.
A VFD consists of a rectifier/filter section which converts the incoming AC to DC. This high-voltage DC is then converted back to AC with additional controls which provide the speed control. The problem with single-phase AC power is at the input side of the VFD, the part that converts incoming power to DC.
When you rectify a three-phase power source, the output voltage from the rectifier never drops below 50% of the peak voltage. This makes it real easy to filter and provide pure DC to the rest of the equipment.
However, when you rectify single-phase power, the output voltage of the rectifier drops to zero 120 times a second. The task of filtering this to provide pure DC is much more difficult.
The standard rule-of-thumb is to derate the VFD by 50% when operating from a single-phase source. Thus, if you want to run a 5 HP motor, you need a VFD rated for 10 HP.
However, I would not recommend operation of any VFD on single-phase power unless such usage is proper according to the manufacturer. If not discussed in the owner's manual, you can always contact the maker's technical support department and ask for assistance.
The "rule" is basic 1 phase / 3 phase math. Power in a 1 phase circuit is V * A * pf. Power in a 3 phase circuit is V * A * pf * 1.732 (sq. root of 3). So the unique situation of a VFD in this case is that the MOTOR is using the power at that 1.732 value, yet the SUPPLY is not, so the power drawn by the VFD from the supply is 1.732 x the power used by the motor. Test it out using 1HP.
1HP = 746W
Amps for 746W in a 230V 1 phase system is 746 / 230 * .8pf = 4.05A
Amps for 746W in 230V 3 phase (the motor) is 746 / 230 * .8pf * 1.732 = 2.34A
4.05 / 2.34 = 1.732 ... hey!
So a 3 phase 1HP VFD probably has components rated for 2.34A, but when connected to a 1 phase source, the input will draw 4.05A from the supply. If the diodes used in the rectifier section are not capable of taking 4.05A through them, they fry. The ripple issue is also important, thats why we round up to 2.0 (50% derate) instead of 1.732 just to allow for extra capacitors that will come with the larger size.
Many small VFDs however are using components on the rectifier side and the DC link that are so cheap that they can afford to oversize them without much cost. So up to 3HP (typically), they don't need derating. But those are usually the ones that SAY they can have 1 phase or 3 phase input. If they don't expressly say it, then they may not have the oversized components and you run a risk of frying the rectifier. In addition if you don't have enough capacitance to smooth out the extra ripple in the DC link, you can end up damaging the transistors on the output side. The net effect is the same to you; the magic smoke is released and it is never worth trying to shove it back in.
You MAY be able to add your own rectifier and caps.
You might want to look at this other thread. You could take a 35a bridge rectifier, and a capacitor, and wire it to the VFD if it has terminals for the DC bus on the outside like my hitachi does. So long as you don't go too crazy with a capacitor, the surge current won't kill the bridge. You'll want to add another cap the same size as the one in the VFD, just so the ripple won't be an issue. You can always open up the VFD to wire it in if you know what you're doing, but BEWARE, there are DANGEROUS LETHAL voltages present in there. If you don't know anything about electronics, DON'T DO IT!! Check out this thread for more info.
How to avoid de-rating 3phase only VFDs?
Thank you for the helpful information.
I appreciate the knowledge that all of you have shared.
The VFD is rated for 3 HP, so I gather, from the rule of thumb, that it will be sufficient to run the 1-HP motor on my bandsaw.
If I need to run a bigger motor, I suppose one option is to make a rotary phase converter using a spare 5HP 3-phase motor.
I'm still working through a similar situation, except I need to operate a 10 HP 3-phase motor.
I looking at the PC1-100 converter that is rated for a 10 HP output(33 amps) and requires 70+ amps of single phase input current.
I sent the following message to them, and recieved the response below:
----- Original Message -----
Sent: Wednesday, March 05, 2008 2:39 PM
Subject: POLYSPEDE MODEL PC1-100 Application
I have a 1965 vintage Leblond Tool & Diemaker lathe with a 10 HP motor and a factory variable speed option. It was recently moved to a location where 3 phase power is not available, so I'm in the market for a 220 V single phase to 3 phase converter. I believe that the PC1-100 specifications match up with the requirements of this application, but I would like your confirmation that the PC1-100 will be up to the task.
As long as it has a AC induction motor then you can use the PC1-100. You will have to use the PC1-100 for starting and stopping + direction control. NO contactors on the output of it.
My follow on question is why I can't use the break and direction control built into the lathe?
Covered here in detail many times, but the short short version is that opening a contactor on the output of a VFD while under load will damage the transistors. The VFD can do both functions on its own.
Originally Posted by johnly
Is a motor driven RFC immune to this condition?
Regarding the original brake and direction control, I think that is the main disadvantage of using a VFD.
The VFD has settings for ramp up and ramp down rates. I try to set them to be the fastest possible without causing a VFD overload fault.
There are ways of adding braking resistors to a VFD, which will allow for quicker stops. But I am not experienced with this sort of thing.
If you use a rotary phase converter (is that what you mean by "RFC?"), then you can use the original lathe contact switches and mechanical brake. The rotary phase converter essentially is just a big 3-phase motor running on one phase. The other two legs of the 3-phase power are generated by the energy that is stored in the magnetic field of the motor as it idles. The disadvantage of using a rotary phase converter is that you don't get to enjoy the variable speed control of a VFD.
The ideal device would be a variable frequency source of 3-phase electrical power. Without such a device, a VFD still is quite useful.
Making rotary phase converters is not that complicated or expensive, if you use old surplus 3-phase motors as idlers. To run a 10HP lathe, I guess you should have about 15HP of 3-phase motors. It doesn't have to be just 1 motor. You could, for example, use three 5HP motors, if those happen to be available.
Depending on how you use your lathe, you might be able to get by with a smaller rotary phase converter, as long as the motor is powerful enough to get the lathe motor started. I had trouble staring my 3HP Clausing Colchester lathe on a static phase converter. But I think that was only because the lathe had a magnetic starter which wouldn't activate using a static phase converter. I was able to start it just by running a 1.5 HP motor in parallel on the static phase converter circuit (that was my Bridgeport mill). I now have a dedicated 3HP idler motor sitting within the lathe cabinet, which serves that purpose. When I want to turn on the lathe, I first turn on the idler motor, which is on a simple 3-connector toggle switch.
In case anyone is interested, I have put this VFD on eBay, as my very first listing as a seller.
The item number is 120336004280.
What do you think VFD stands for???
VFD stands for Variable Frequency Drive. Which means a VFD cranks out variable frequency 3 phase AC. So, Jon_Spear, a VFD DOES exactly what you say that WITHOUT, a VFD is still useful. A VFD can do BOTH, convert single to 3 phase, AND crank out variable frequency 3 phase AC. So, you guys BOTH need to do some research on the subject.
Most VFDs 3hp and under are rated for single OR 3 phase input, but not ALL.
Rotary Phase Converter is referred to an an RPC.
VFDs can do LIMITED braking without a braking resistor. A braking resistor allows faster braking.
VFDs over 3hp rated for single pahse are expensive, a VFD rated for 3 phase that will take single phase is MORE expensive, because you have to de-rate it by 50%.
Check it out, all kinds of useful info on the net!
Thanks for the heads up ... it's on my watch list
I have put this VFD on eBay