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Voltage/Transformers/Phase Converters?

300sniper

Gun Plumber
Joined
May 20, 2020
Member Number
695
Messages
412
Loc
Greenwood, CA
I've been running a 3ph 220v CNC turning center trouble free off a 40hp RPC and my single phase service for years, which is always right around 250v (248v-252v every time I've checked). I added an additional 220v 3ph motor to this machine and it only lasted a couple days of intermittent use. This motor is nearly identical to another motor that has been, and still is trouble free in this machine. The motor dealer said it was probably caused by too high of voltage, and they typically fail at even 230v. I do know I am beyond the 10% voltage tolerance, but everything else has been working fine for years.

So now I am thinking about getting a transformer to step down the voltage. This is where my questions are. I can find a 30kva, 240-208 3ph transformer pretty easily. Will a 240-208 transformer work to lower a 250v service to around 220v, or at least close enough to put me in the machine's range?
 
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Probably not - transformers are built around a voltage ratio. That one is likely expecting 460V at the input or a 2:1 ratio. Unless it's spec'd to have 250V as the input I'd look elsewhere.

Some kind of voltage regulator or power conditioner is probably more in line with what you're looking for (variable input voltage -> fixed/constant output voltage)
 
Probably not - transformers are built around a voltage ratio. That one is likely expecting 460V at the input or a 2:1 ratio. Unless it's spec'd to have 250V as the input I'd look elsewhere.

Some kind of voltage regulator or power conditioner is probably more in line with what you're looking for (variable input voltage -> fixed/constant output voltage)

It’s rated 240 to 208 or 208 to 240.
 
You're looking for a voltage regulator.


There are xfrmr's with multi-tap that you can tap up/down in 5-10v increments, but they'll all be designed to be stepping up or down from another common voltage.


The failure was probably caused by the RPC. I hate those things and the ghost issues they can cause throughout the system. They're magically creating a phase through inductance which can do crazy shit as the load varies. I always recommend having the power company setup an open delta or go to a solid state solution like a VFD.


If you're already looking at transformers, look at how to setup an open delta system and a real 3ph service.


What does the voltage drop to from neutral to each phase, and phase to phase (A-B, A-C, B-C) when the machine is loaded?
 
I've been running a 3ph 220v CNC turning center trouble free off a 40hp RPC and my single phase service, which is always right around 250v (248v-252v every time I've checked). I added an additional 220v 3ph motor to this machine and it only lasted a couple days of intermittent use. This motor is nearly identical to another motor that has been, and still is trouble free in this machine. The motor dealer said it was probably caused by too high of voltage, and they typically fail at even 230v. I do know I am beyond the 10% voltage tolerance, but everything else has been working fine.

So now I am thinking about getting a transformer to step down the voltage. This is where my questions are. I can find a 30kva, 240-208 3ph transformer pretty easily. Will a 240-208 transformer work to lower a 250v service to around 220v, or at least close enough to put me in the machine's range?

YES! If you install this transformer on the three phase side, feed it 250V on the 240V side you'll get 217V on the output. ASSuming the transformer's windings will take the 250V input... about 20% more than it's rated for. Simple math, no voodoo involved. Best bet is to see what the hi-pot is on the transformer, and as long as it's considerably more than 250V it should be fine.
 
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YES! If you install this transformer on the three phase side, feed it 250V on the 240V side you'll get 217V on the output. ASSuming the transformer's windings will take the 250V input... about 20% more than it's rated for. Simple math, no voodoo involved. Best bet is to see what the hi-pot is on the transformer, and as long as it's considerably more than 250V it should be fine.

Considering I have a 240v service and 252v is still technically in that spec, I think the transformer should be fine. I’ll verify.
 
Have you tried dialing it back with the run capacitors in the phase converter?
 
You're looking for a voltage regulator.


There are xfrmr's with multi-tap that you can tap up/down in 5-10v increments, but they'll all be designed to be stepping up or down from another common voltage.


The failure was probably caused by the RPC. I hate those things and the ghost issues they can cause throughout the system. They're magically creating a phase through inductance which can do crazy shit as the load varies. I always recommend having the power company setup an open delta or go to a solid state solution like a VFD.


If you're already looking at transformers, look at how to setup an open delta system and a real 3ph service.


What does the voltage drop to from neutral to each phase, and phase to phase (A-B, A-C, B-C) when the machine is loaded?

The machine doesn’t use a neutral. Getting “real” 3ph here is not even remotely an option. I spent over $10k with the utility just to upgrade to 400a from 200a. There’s nothing wrong with a RPC in this application. Putting a VFD in front of a CNC machine doesn’t make much sense to me. I’m pretty sure the voltage issue in will still be the voltage issue out.
 
Have you tried dialing it back with the run capacitors in the phase converter?

That’s something I don’t understand. I’m pretty sure when I hooked it up years ago, the line in was bonded to the line out on the RPC leading me to believe the voltage in will the the same as out on the non manufactured legs.
 
I think the dealer is feeding you a line of BS. They fail even at 230V? Either the quality is complete crap, or they're full of shit. Most motors I have seen are nameplated for 230V, so +10% gives you 252VAC - you're at upper end, but within spec. Even outside of spec, I can't image it failing that quickly unless you're way above spec - like running a 230V motor on 460..... I could see reduced life from the voltage you're at, but it should still last many years. Same thing with the RFC - probably some weird harmonics and voltage balance issues, but motors are tough. No way any reputable vendor would engineer a motor to be that sensitive.

I suspect you're more likely to damage motors from too low of voltage than too high trying to throw a transformer on it. The electronics in the CNC should be all rights be far more sensitive than an electric motor, and if you haven't had problems with that system or any of the other motors I'd place good money that the basic system is working fine, and the problem was specific to that motor.
 
The machine doesn’t use a neutral. Getting “real” 3ph here is not even remotely an option. I spent over $10k with the utility just to upgrade to 400a from 200a. There’s nothing wrong with a RPC in this application. Putting a VFD in front of a CNC machine doesn’t make much sense to me. I’m pretty sure the voltage issue in will still be the voltage issue out.

They should have set you up back then.

An open delta creates 3ph from single phase using transformers. I'm well aware the motor doesn't have a neutral, I've only done a few thousand. I've seen plenty of issues with RPC's, but they usually only show up when I put the power quality analyzer on it while the machine is running cycles. Motors are hard to kill, so they mask issues.

What's your measured running voltage on the legs along with the current?
 
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I'd try a mains isolation transformer
should knock the high leg down a bit if it's one with all one core across all six windings
 
Oh right, are you using a meter that automatically calculates RMS voltage?
Some meters will only read peak to peak, so your 220v ends up reading something like 290 or some shit iirc
 
I don't think your RPC is the problem. That seems like a motor problem to me. It's out enough for me to bitch to the power company that's supposed to be supplying it. I'm making an assumption that you have some sort of PLC running your mill. I lose PLC power supplies and contactors when I have shitty voltage, not motors.
 
I don't think your RPC is the problem. That seems like a motor problem to me. It's out enough for me to bitch to the power company that's supposed to be supplying it. I'm making an assumption that you have some sort of PLC running your mill. I lose PLC power supplies and contactors when I have shitty voltage, not motors.

I have no doubt the RPC is NOT the problem. I think my voltage is too high. I do have my doubts about the new motor failing so quickly due to it, since I’ve a couple nearly identical motors running on this machine trouble free for 5+ years, but I have had a power supply fail ($4k+ refurbished), but I know plenty of others that have had these fail even within the 220v tolerance. I think that was probably just a timing thing.

Either way, I should probably try to get the voltage within the machine’s spec.
 
I don't think your RPC is the problem. That seems like a motor problem to me. It's out enough for me to bitch to the power company that's supposed to be supplying it. I'm making an assumption that you have some sort of PLC running your mill. I lose PLC power supplies and contactors when I have shitty voltage, not motors.

The plc and control components are running off single phase something (some control transformer) Power those on an independent circuit. Don’t power them through the 3 phase circuit.
 
Have you checked frequency?

I would say voltage regulator, but I'm not as familiar with RPCs and how that would react. Does it create a load and increase voltage or load at the RPC motor?
 
The plc and control components are running off single phase something (some control transformer) Power those on an independent circuit. Don’t power them through the 3 phase circuit.

OK. Ours are on a transformer running off our 3 PH. I'm probably over my head at generating on site, ours is 3 ph 13.2 coming in from the line at work.
 
To drop voltage a small amount we used to use Buck and Boost transformers. A typical situation for these was when an old 240 volt delta service was replaced by a 208 volt wye. However , the OP is looking at over voltage that normally isn't an issue. But the Buck and Boost will drop it.....Disclaimer, I never messed with phase convertors....
 
Maybe some don’t, I don’t know. Either way, a VFD probably isn’t a good idea for a CNC machine with multiple transformers, motors, controls, etc.

VFD is the worst idea for a VMC unless it's setup in a way that you could replace the existing drives with VFDs that would work with the existing control. Probably not going to work very well....

I have a cnc knee mill that uses a VFD on the spindle and then single phase servo drives for movement....but this is a home converted setup running on linux. I have a 4ths axis that I bought for it but have been struggling on how to get it to "talk" to my controller.
 
VFD is the worst idea for a VMC unless it's setup in a way that you could replace the existing drives with VFDs that would work with the existing control. Probably not going to work very well....

I have a cnc knee mill that uses a VFD on the spindle and then single phase servo drives for movement....but this is a home converted setup running on linux. I have a 4ths axis that I bought for it but have been struggling on how to get it to "talk" to my controller.

Yeah, a VFD shouldn’t have any switching after it. It’s a motor control, not a machine control.


Now I’m thinking the buck/boost is probably the best bet. I’ve got to decide to buck the voltage before the RPC or after. Before would be extremely simple, both wiring and mounting. After is a bit more complicated wiring and mounting two transformers, but in theory, should allow a bit more capacity from my RPC, although pretty negligible. Cost is about a wash from Larson Electric, which I haven’t heard of.
 
I don't think your RPC is the problem. That seems like a motor problem to me. It's out enough for me to bitch to the power company that's supposed to be supplying it. I'm making an assumption that you have some sort of PLC running your mill. I lose PLC power supplies and contactors when I have shitty voltage, not motors.

Correct. Get an inverter rated motor and run it. He got a lemon motor and someone is not owning up to that fact.
 
What application is the 3 horse motor used for? Is it a high load application such as a coolant pump? Supplying 250v to a 220v rated smaller motor under high load would definitely cause a high current draw. If you replace the motor, check the current draw and see if it was exceeding the nameplate rating. My guess is it does....
 
What application is the 3 horse motor used for? Is it a high load application such as a coolant pump? Supplying 250v to a 220v rated smaller motor under high load would definitely cause a high current draw. If you replace the motor, check the current draw and see if it was exceeding the nameplate rating. My guess is it does....

No, higher voltage to a motor drops the current draw..
 
What application is the 3 horse motor used for? Is it a high load application such as a coolant pump? Supplying 250v to a 220v rated smaller motor under high load would definitely cause a high current draw. If you replace the motor, check the current draw and see if it was exceeding the nameplate rating. My guess is it does....

It is for a positive displacement pump, so yes, high load. The pump and motor are made by the same overseas company and designed to be close coupled together. TMy external bypass is set just below what it is rated for and the internal bypass is set at what it is rated for.
 
No, higher voltage to a motor drops the current draw..

No sir, it does not work that way on a motor. When you are over the range of the nameplate voltage, the iron core will get saturated, the current draw will go up, and bad things will happen from there.
 
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