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tracar

Plastic
Joined
Mar 16, 2018
adding capacitors between L1 and L2

i have retrofit a 24x60 inch cnc and have it working great, i can only get single phase power here So needed to build a 7.5HP rotary phase converter, as a static converter will not run the VFD controllers built into the cnc. im at the point where i have added capacitors and balanced ( my input voltage is 245vac... when i start my RPC the draw amps spike to almost 100A for a split second. ) after tuning balancing the rpc , L1 and L2 are matched in voltage 242vac the ghost leg is 251vac when running,

so with no caps between L1 and L2 the startup amp is near 100 and L1 and L2 input at idol measure 8.7a

at this point i add a cap 40umf between L1 and L2, the startup draw in amps goes down to 80a for a split second, used to be 100ish

so when i add another 40umf for a total of 80umf the L1 and L2 input measures 2.5A at idol with no change at the rpc output



it seems to be running more efficiently with lower amperage at the input source
is this safe to keep adding caps between L1 and L2 ?
is this somthing i should do ?
 
I'm am not exactly going to answer your question, but will offer my opinion.

I personally strongly dislike phase converters. I don't know what VFD you have, but I would be looking at making your existing VFDs work on single phase.

Most 3 phase input VFDs can run on single phase. The only ones that can't are the ones that have single phase protection (if this protection can't be turned off), and many look at DC bus ripple to trigger phase loss (easily fixed).

A VFD has an AC to DC converter (diodes), a DC bus consisting of capacitors tied to the DC converter and then a DC to AC inverter (IGBT transistors). The DC to AC inverter portion is exactly the same between single and 3 phase drives.

There are basically 3 things to consider when running a 3 phase drive on single phase assuming that single phase protection doesn't get you. Without doing anything, you can run a 3 phase drive on single phase but you should derate it 50%.

The first consideration is are the diodes large enough to handle single phase current. The diodes will see approximately 1.73 times the 3 phase current. If the diodes are too small put in larger diodes.

Second is the wiring large enough to handle the 1.73 times 3 phase input current. If not, replace the wiring.

Third is DC bus ripple. You solve this by adding capacitors - you can start by doubling the capacitance.

With that said, 7 1/2 hp is borderline where you want to run a single phase drive. Typically it would be limited to around 5 hp.

A better solution is to build your own AC to DC converter and feed the VFD as a DC input - many drives have the capability of accepting a DC input instead of AC input. It is very easy to build a AC to DC converter using diodes, capacitor, resistor and a contactor (relay). One AC to DC converter can feed multiple drives.

Seems daunting, but it really isn't. In my opinion, much better solution than dealing with phase converters.
 
markz528 said:
I'm am not exactly going to answer your question, but will offer my opinion.

I personally strongly dislike phase converters. I don't know what VFD you have, but I would be looking at making your existing VFDs work on single phase.

Most 3 phase input VFDs can run on single phase. The only ones that can't are the ones that have single phase protection (if this protection can't be turned off), and many look at DC bus ripple to trigger phase loss (easily fixed).

A VFD has an AC to DC converter (diodes), a DC bus consisting of capacitors tied to the DC converter and then a DC to AC inverter (IGBT transistors). The DC to AC inverter portion is exactly the same between single and 3 phase drives.

There are basically 3 things to consider when running a 3 phase drive on single phase assuming that single phase protection doesn't get you. Without doing anything, you can run a 3 phase drive on single phase but you should derate it 50%.

The first consideration is are the diodes large enough to handle single phase current. The diodes will see approximately 1.73 times the 3 phase current. If the diodes are too small put in larger diodes.

Second is the wiring large enough to handle the 1.73 times 3 phase input current. If not, replace the wiring.

Third is DC bus ripple. You solve this by adding capacitors - you can start by doubling the capacitance.

With that said, 7 1/2 hp is borderline where you want to run a single phase drive. Typically it would be limited to around 5 hp.

A better solution is to build your own AC to DC converter and feed the VFD as a DC input - many drives have the capability of accepting a DC input instead of AC input. It is very easy to build a AC to DC converter using diodes, capacitor, resistor and a contactor (relay). One AC to DC converter can feed multiple drives.

Seems daunting, but it really isn't. In my opinion, much better solution than dealing with phase converters.
Click to expand...



i do agree with you with the above and the use of RPC's and i have looked into alternate power input requirements since the day i had to repair a burned out ibgt due to a varistor failing on a surge, 100$ later and a new ibgt and new varistor and an hour of soldering...the vfd was up and running again, i did talk to the manufacturer of the vfd, colombo usa, and apparently they made a few versions of this 5HP vfd, and the main difference was , yes, the rectifier board within. and a couple of resistors, so modding it to take a DC is still an option, as the rest of the components in the cnc (stepepr drivers) are 3 phase, but Can take single also :) so at this point its all about cost,

the cnc uses 2 5HP spindles. so finding diodes to handle this might take some time....
 
Many folks getting into CNC think they need to get 3 phase power just because the nameplate on the machine says 3 phase. Many small CNCs can be run on single phase with little modification.

I've been running a Mori Seiki TV30 for over 4 years on 230V single phase and a friend has been running his Makino RMC55 on single phase for about the same amount of time. On the Mori, the only change needed was I added a small inverter to run the coolant pump. That was the only line powered 3 phase motor motor on the machine and the inverter was far less expensive than buying a single phase coolant pump.

My other 3 phase machines(engine lathe, bridgeport clone, and bandsaw) have all had inverters added to them. Then you get variable speed in any gear/belt range, soft start and reversing (great for power tapping).

If I had to run artificial 3 phase, I would bite the bullet and get a Phase Perfect.
 
Vancbiker said:
Many folks getting into CNC think they need to get 3 phase power just because the nameplate on the machine says 3 phase. Many small CNCs can be run on single phase with little modification.

I've been running a Mori Seiki TV30 for over 4 years on 230V single phase and a friend has been running his Makino RMC55 on single phase for about the same amount of time. On the Mori, the only change needed was I added a small inverter to run the coolant pump. That was the only line powered 3 phase motor motor on the machine and the inverter was far less expensive than buying a single phase coolant pump.

My other 3 phase machines(engine lathe, bridgeport clone, and bandsaw) have all had inverters added to them. Then you get variable speed in any gear/belt range, soft start and reversing (great for power tapping).
Click to expand...

Totally agree!
 
so would it not be safer and easier to find an isolation transfoermer 240vac in -> 240vac out, then to the rectifier block, then add varisters and caps to smooth the voltage which should be around 340vdc. which is just what the voltage reading was at the VFD bus B+ and B-

or is isolation needed ?
 
tracar said:
....or is isolation needed ?
Click to expand...

Don't think you would need it. Typical spindle drives do not have an isolation transformer in the input power retification stage. Some larger spindle drives (40+HP) have a reactor on the input side. I'm guessing that is used to minimize noise fed back into the line power.
 
i finally got the answer from the manufacturer:

***************

There are two ways to connect DC voltage to our drives.

**In either of these cases, you will lose the UL approval agency ratings and the drive’s warranty, should damage come as a result of connection loss or failure from your external DC supply. These drives are rated for their V-rms levels on the input, but there's no UL testing that covers DC voltage input.



1) If you go in through the B+/B- terminals, there is NO INRUSH PROTECTION. So, unless you can guarantee there is no huge inrush or you can add external limiters or bring the DC supply voltage up slowly, there could be problems with inrush currents. Also, the B+ and B- terminals on our SM Vector drives generally are designed for dynamic brake circuits, and generally have smaller copper areas for current carrying. Again, the biggest caveat is inrush current. Also you cannot connect the DC bus terminals between drives for same reason.



2) One solution that circumvents the inrush problems is to put the DC voltage right into the input terminals of the drive (pick any 2 of L1, L2, and L3). This will give you inrush protection.

It will heat up a single pair of diodes instead of spreading out the heat across the full-wave bridge. It shouldn't be an issue, as the diodes are rated to handle the necessary current, but there's no extensive testing in this configuration. The DC level you need to supply at the input terminals would have to be Vin (AC) times the square root of 2 equals the DC voltage equivalent required. So then in order to run a 230V motor, assuming we use the minimum AC input voltage spec at -15%, you would need a minimum of 230V*(sq root of 2)*0.85 = 277V, so the 320Vdc you mentioned is fine. If the output load is high or fluctuates a lot the drive's input voltage may drop and may nuisance trip. If you can put DC into the input terminals of the drive, there is less likelihood of tripping.



Again, you will lose UL rating and warranty for drives if there’s any issue with wiring or surges from the DC supply.
*******************************



i think the winning solution is 240ac to L1 and L2.. with soft starting spindles
 
tracar said:
i finally got the answer from the manufacturer:

***************

There are two ways to connect DC voltage to our drives.

**In either of these cases, you will lose the UL approval agency ratings and the drive’s warranty, should damage come as a result of connection loss or failure from your external DC supply. These drives are rated for their V-rms levels on the input, but there's no UL testing that covers DC voltage input.



1) If you go in through the B+/B- terminals, there is NO INRUSH PROTECTION. So, unless you can guarantee there is no huge inrush or you can add external limiters or bring the DC supply voltage up slowly, there could be problems with inrush currents. Also, the B+ and B- terminals on our SM Vector drives generally are designed for dynamic brake circuits, and generally have smaller copper areas for current carrying. Again, the biggest caveat is inrush current. Also you cannot connect the DC bus terminals between drives for same reason.



2) One solution that circumvents the inrush problems is to put the DC voltage right into the input terminals of the drive (pick any 2 of L1, L2, and L3). This will give you inrush protection.

It will heat up a single pair of diodes instead of spreading out the heat across the full-wave bridge. It shouldn't be an issue, as the diodes are rated to handle the necessary current, but there's no extensive testing in this configuration. The DC level you need to supply at the input terminals would have to be Vin (AC) times the square root of 2 equals the DC voltage equivalent required. So then in order to run a 230V motor, assuming we use the minimum AC input voltage spec at -15%, you would need a minimum of 230V*(sq root of 2)*0.85 = 277V, so the 320Vdc you mentioned is fine. If the output load is high or fluctuates a lot the drive's input voltage may drop and may nuisance trip. If you can put DC into the input terminals of the drive, there is less likelihood of tripping.



Again, you will lose UL rating and warranty for drives if there’s any issue with wiring or surges from the DC supply.
*******************************



i think the winning solution is 240ac to L1 and L2.. with soft starting spindles
Click to expand...

First, don't think you are worried about UL - are you? On a side note, I actually sit on the UL power conversion committee that sets this standard - as an end user representative.

In my suggestion the inrush would be regulated by the capacitor charging resistor - the resistor would be shorted out after a couple of seconds.

Sounds like the B terminals won't work for you because you use the braking resistor and the internal wiring is smaller. I have never heard of putting DC into the AC line but that does make sense. They say that it can handle the current, so that does seem like a very good solution.

The only question in my mind is how big the capacitors need to be on the DC supply you use (I can help figure that out - PM me if you would like to discuss). Also you don't really need an isolation transformer but they don't hurt.
 
Just curious - where do they take the internal control power from? The DC bus? Need to make sure its not from L1 and L2 unless it is capable of taking the DC in. I would get that clarification from them.
 
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