Buck Boost Transformer, Before or After RPC?

300sniper · Sep 2, 2020

I've got a 3ph 220v name plate machine after a RPC. I am on the high side of the 240v, always 248v-252v every time I've measured it. I see minimal/no voltage loss after the RPC. It's honestly been running trouble free this way for years, but added an additional motor for a higher pressure coolant pump, and the motor only lasted a couple days of intermediate use. My hydraulic pump on this machine uses an identical spec motor and has been running fine for ever. Regardless if this was a quality issue or voltage issue, I should probably get closer to the nameplate voltage.

Now I am looking at buck boost transformers. I am trying to figure out if it is better to buck the voltage before or after the RPC. Wiring (and mounting in my situation) would be easier before the RPC, but maybe it is better to buck it after for some reason?

I've dealt with standard step up/down transformers but the buck boost transformers are new for me.

Edit to add: the RPC nameplate is 230v, which I don't think makes any difference.

DavidScott · Sep 2, 2020

Before is easier and cheaper, what I did, but will reduce the amperage out of the phase converter the same percentage drop as the voltage.

thermite · Sep 2, 2020

DavidScott said:
Before is easier and cheaper, what I did, but will reduce the amperage out of the phase converter the same percentage drop as the voltage.

ISTR most of those seen on PM were either a single higher KVA in one leg of the input single-phase feed, ELSE two somewhat lower KVA, one each of the "pass through" legs, AND NOT the "generated" leg of the 3-Phase load side?

Even so, a reduction in enrgy applies, either side. But that WAS the goal, so..!

drcoelho · Sep 2, 2020

Given the minimal change in voltage you are looking for, most likely not much difference in price of transformer either before or after.

jim rozen · Sep 2, 2020

drcoelho said:
Given the minimal change in voltage you are looking for, most likely not much difference in price of transformer either before or after.

He'll need one before the idler, as opposed to two, after the idler.

JST · Sep 2, 2020

So, how is the balance of the RPC output (generated leg vs the straight-through legs)?

Was the coolant pump working harder than the hydraulic pump?

These things affect heating which is the main factor in killing motors.

A heavily loaded motor operating at more than a few percent imbalance may be overloaded.

Sometimes open flow is a heavier load than a throttled flow. Coolant is often a fairly open flow, aside from pipe friction, although "higher pressure coolant" suggests a heavy load and a positive displacement pump..

Any of these things, or some others, can affect how a motor behaves, and how long it operates. And there are such things as "defects" as well.

So....... in what way did the motor fail?

thermite · Sep 2, 2020

jim rozen said:
He'll need one before the idler, as opposed to two, after the idler.

A decent coolant pump wouldn't hurt, either!

Prolly cheaper, too. No need to over-think it, the older one having lasted as long as it has.

Strostkovy · Sep 2, 2020

Whatever transformer you can get cheapest.

When checking for phase balance and derating motors accordingly, you need to measure current, not just voltage. Depending on the motor, a small imbalance in voltage can be a significant imbalance in current. The heat generated in motor windings is proportional to current, so you will need to either adjust your balancing or decrease your load until the current through each leg is below the FLA rating of the motor.

Some voodoo occurs with phase angle being off from 120 degrees due to capacitors used to boost one leg. This can in theory cause imbalance, rpm oscillation, and high inductive heating from cyclic slip, but that's got to be a lot of phase angle error to do that.

300sniper · Sep 3, 2020

JST said:
So, how is the balance of the RPC output (generated leg vs the straight-through legs)?

Was the coolant pump working harder than the hydraulic pump?

These things affect heating which is the main factor in killing motors.

A heavily loaded motor operating at more than a few percent imbalance may be overloaded.

Sometimes open flow is a heavier load than a throttled flow. Coolant is often a fairly open flow, aside from pipe friction, although "higher pressure coolant" suggests a heavy load and a positive displacement pump..

Any of these things, or some others, can affect how a motor behaves, and how long it operates. And there are such things as "defects" as well.

So....... in what way did the motor fail?

The motor just didn’t start one day. It was extremely hot to the touch. I checked continuity between phases, and there wasn’t continuity between the phases that should have and was where there shouldn’t have been. This was from an internet search, so I can’t remember what the combo was off the top of my head.

As far as balance after the RPC goes, today at idle, at the pump relays, it was 249-249-250 between phases. It’s been a long time since I checked it while running, but when I did, the numbers were nearly identical.

The coolant pump shouldn’t have been working harder than the hydraulic pump. They are both designed specifically for the mated pump. Pretty much a pump/motor combo.

JST · Sep 3, 2020

"There are such things as defects".

Other than that, the pump/motor pair may be well designed, but the key is what goes on in the application. A positive displacement pump (high pressure) and a restricted flow is a heavy load, where a centrifugal pump and a restricted flow is not.

And a positive displacement pump with open flow is a light load, where a centrifugal pump with open flow might even be an overload.

300sniper · Sep 3, 2020

So I’m thinking if I buck it down after the RPC, that will (I think) in theory make my RPC have a bit more capacity. Maybe 6a, probably negligible.

And has anyone heard of Larson Electric? From my google searching, they seem to be the only ones I can find that can do 240V to 220v. Then again, buck/boost transformers are new to me and I may not be searching correctly.

thermite · Sep 3, 2020

300sniper said:
So I’m thinking if I buck it down after the RPC, that will (I think) in theory make my RPC have a bit more capacity. Maybe 6a, probably negligible.

And has anyone heard of Larson Electric? From my google searching, they seem to be the only ones I can find that can do 240V to 220v. Then again, buck/boost transformers are new to me and I may not be searching correctly.

Buck//Boost is generally just a manner of connecting what is otherwise a full-isolation low-voltage transformer.

For example, 120/240 VAC connectible primary, then 6/12, 12/24, 18/36, 24/48 VAC connectible secondary.

So "everybody" makes them. You "probably" want an 18/36 but could use the generally more easily found 12/24.

One does have to do a minor bit of math to select the Ampacity // VA / (fractional) KVA, but they are all around us, Big Box to eBay as GE, Acme, Jefferson, Hammond, other major-maker or minor-maker brands as "dry type" and generally "outdoor" rated. Nearly all are safely NEMA-cased, internally potted-up solidly and are highly weather-resistant.

Some are sold for low-voltage grounds and garden lighting, for example.

Full load, full isolation is a different animal. There, it is about "adjusting taps". Some have one option @ roughly ten-percent. Others designed for "global" markets or even just the Japanese domestic market (more than one standard co-exist) may have "many, many" taps.

Hammond (Canadian firm) and "not-only" have good educational information online. Most of the makers have that.

JST · Sep 3, 2020

As I recall, the reason for all this is a failure of a motor. Other motors on the same power are fine, even a machine actually rated at 220V, but one added motor failed.

To my simple engineering mind, you have no clue at the moment that the problem was due to high voltage, which does not seem to be high out of spec. The high limit is 264VAC for 240V. Same as 132V for 120V.

Again, to my simple mind, it would be good to have some assurance that the problem is due to voltage before spending time and money on a voltage "fix".

So your motor failed. Does it smell overheated? If not, then the probability is that it is NOT an overvoltage (or UNDER voltage) failure. And, high voltage is a HELP to many motors, reduces current for same power, and leads to LESS overheating, so long as you do not "saturate" the iron, which high voltage can do.

You may want to check more closely just to avoid going off on this road, and then having the same failure occur again because you did not find the problem.

Ther also may be NO problem, and you had a defective motor. Then all this concern for a buck transformer is worthless effort. You should try to determine if there is a real problem, to determine what the problem really is, and only THEN can a solution be found.

You might post a pic of the motor data plate, and maybe give more details. Answers about transformers may NOT be helping fix your problem, assuming there really IS a problem.

jim rozen · Sep 4, 2020

thermite said:
Hammond (Canadian firm) and "not-only" have good educational information online. Most of the makers have that.

McMaster Carr actually has an excellent tutorial on these, in their catalog.

johansen · Sep 4, 2020

250 or 260 volts didn't kill your motor.

my guess is you dropped a phase and that is why the motor didn't start and was hot.

i'm also guessing your motor has an internal thermal circuit breaker and that is why it didn't catch on fire but as you describe was hot to the touch. usually motors fail catastrophically when they don't start. some motors are small enough they can handle locked rotor current for a long time.. but anything above 1/2 hp usually can't.

SomeoneSomewhere · Sep 4, 2020

This is why the NZ electrical code, and I imagine many others, mandates a thermal overload relay or other closely-sized thermal protection for motors above 370W (1/2HP).

Redpie · Sep 4, 2020

Buck boost switching regulators are noisy and emit a lot of EMI. If the circuit includes the proper EMI filtering circuits then you should be fine. You can tell this by how many capacitors and common mode chokes are used in your switching mode power supply. If there is none, and your system includes a LCD LED screen, speakers, motors, a poorly designing PCB, etc... Then you will get a dirty signal to those devices. In your case you'll get motor whine. Part of the reason why you don't hear electric car motors is because the filtering components are big, expansive, and well engineered. So to answer part of your question it will depend how much noise you want in the circuit, and if you can put up with it. I would put it after as far as noise goes.

Current draw is more important then voltage when it comes to electric motors. Current is proportional to torque. In your case I would focus on trying to supply current to the device that needs it the most. Again I would put it after if the device does not include the proper feedback loops to keep a steady current supplied to the device.

Hopefully this helps. I'm an electrical engineer. I'm currently designing a system for a fully automated injection molding machine.

thermite · Sep 4, 2020

Redpie said:
I'm an electrical engineer.

VERY recently credentialed, it would seem?

One does wonder what anyone in Utah did lately that was so preverted as to deserve "buck/boost switching regulators" for an ignorant coolant pump when it is not YET clear than even the age-old passive - and EMI-free - lumped inductances are all that essential?
Meanwhile..

Sniper?

If all else works a treat, has good balance, has not BEEN a problem, and the coolant pump really IS problematic?

KISS method. Leave the rest of the rig TF alone.

Put in a SMALL transformer to serve the fragile pump.

ONLY.

mymachinery · Sep 4, 2020

300sniper said:
So I’m thinking if I buck it down after the RPC, that will (I think) in theory make my RPC have a bit more capacity. Maybe 6a, probably negligible.

And has anyone heard of Larson Electric? From my google searching, they seem to be the only ones I can find that can do 240V to 220v. Then again, buck/boost transformers are new to me and I may not be searching correctly.

Go to the acme electric web site, the ones that make transformers. you can download a file for selecting the correct transformer. you can even print the connection diagram. find used transformers on EBay!!!!!!

JST · Sep 4, 2020

Putting the transformer after tends to be more expensive, and often simply transfers the noise from the motor to the transformer. And it increases the series impedance feeding the motor.

But, the problem being solved here is unknown.... So picking a solution is way premature.

It is a classic case of deciding, in the absence of hard evidence, that "this has to be the problem" (i.e. totally guessing) and then picking a solution to alleviate the (guessed at) problem.

Generally very poor problem solving technique.

Redpie:

So am I. I have designed VFDs... and filters... the transformer is usually more expensive after than before, and generally is not a very good noise filter. You want a noise (sine) filter, you design one. They (filters, and transformers) add series impedance also, which lowers motor performance.

However, this is an RPC, which uses the third (unpowered) winding of a three phase induction motor that is operated on single phase, to supply power out at the back EMF voltage. Not much noise generation.

And, we still have no definite idea why ONE motor failed, and others are fine. That does suggest the voltage is not the issue, but gives little guidance as to what really IS the issue.

Running into the weeds about a potential problem that is not proved to be "THE" problem, is not ideal.

Buck Boost Transformer, Before or After RPC?

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