Thread: Need help sizing a buck/boost transformer

Can any one help me size a buck/boost transformer? I'm having problems with unequal 3 phase currents on my Phasemaster RPC. I checked my incoming line voltage from the utility company, it's 248 volts. I got the utility guy to come out, he remeasured the voltage and agreed that it is high but said that they can't do anything about it without causing problems for others, so I'm stuck. I called Kay Industries (Phasemaster) they say I need to get the voltage down to get their RPC to deliver equal amperage on all three lines. They suggested a buck/boost transformer and recommended a size, but I forgot what size they recommended. I could wait until Monday and call again but I thought someone here might be able help.

A few of the specifics:
RPC - Phasemaster MA-1-AX

With a 3.6KW saw running:
L1 - 7.4 A
L2 10.3 A
L3 11.6 A
L1-L2 245.5V
L1-L3 238.2V
L2-L3 251V
Single Phase incoming:
246V
20.5A - A phase
21A - B phase

With no load:
L1 - 4.5A
L2 - 4.4A
L3 - 0A
L1-L2 247.1V
L1-L3 269V
L2-L3 279.5V

Not sure if any of this means any thing but these are the numbers that the utility guy and I took this afternoon.

20A load with 10V buck is 200W Find a 300VA transformer is possible with a few tappings. say 0, 6, 12 V . Run the buck transformer primary from the load side which will reduce the current flowing in the buck transformer by 5%

That phase converter is too fussy about input voltage, 250V should be within normal supply range for a 240V supply.

Would there be anything wrong in using a 1KVA buck boost transformer? I think that is what the dude at Phasemaster recommended but at the time I thought the power company was going to bring the voltage down so I didn't take a note of his recommendation. The RPC will start a 5 hp motor but can run up to 10hp, I'm thinking that is why Phasemaster may have recommended a 1KVA transformer. Does a 1KVA make any sense?

Buck/boost transformers are sized primarily by current capacity. Since they're in series with the load they must carry 100% of the load current.

Then you determine the voltage change required. Once you know those two values, the KVA rating = volts * current / 1000.

Your load current appears to be 21 amps, so I would suggest a transformer rated 25 or 30 amps at 28 volts. This would drop your incoming voltage down to 220.

You don't say down to what voltage, whether it's 220 or 240.

A 30 amp 28 volt transformer would be rated 30 * 28 / 1000 = 0.84 KVA.

- Leigh

Thanks Leigh.
The Phasemaster manual only states that "If the input voltage is to high (exceeds 240V) the result could be excessive manufactured phase voltage accompanied by load current imbalance". This is my exact problem. They don't make any reference to low voltage causing problems.
So I need to get under 240 volts. But I would think voltage under 220 wouldn't be too good either.
So it looks like I need to size the transformer pretty accurately or I could wind up with too low of a voltage, especially if I use the same RPC for a smaller motor also. I plan to run several motors (1.5 to 5 hp) off the same RPC.
I think I'll call Phasemaster on Monday and get their recommendation before I buy something.

many buck/boost units are "4 way" wired, so that the primary and secondary are each doubled.

In that way the buck/boost can be adjusted by either parallelling or putting in series, the secondary. In parallel, current capability is double what it is in series.

Common winding types have secondary volts of 12/24, 24/48, or 16/32, depending on if parallel or series.

JST
Are the different voltages selectable by a switch or do you have to move wires to get the different voltages?

UA and CE approved electrical apparatus has to tolerate the line voltage +/- 10%. 10 Volts is well within 10% of 240.

Judging from the values you posted I'd say your saw was getting pretty good feed from your phase converter. If it starts well and develops full power (subjectively, the best you cvan hope for in an RPC is about 90% of rated HP before one leg goes over FLA.)

I'd say if your machinery works OK, your RPC aint broke so don't fix it.

Forrest, that's the problem my machinery doesn't work so well. I had to adjust the thermal overload of the saw to keep it from tripping. And now I'm adding another 1.5 hp motor and the heaters are tripping after about 1/2 hr of running. So I figure I should get this figured out so the heaters stop tripping and then turn the saw overload back down.

1) It is possibly more typical for the overcurrent to trip from LOW voltage, when same power pulls more current.

If, however, there is an imbalance, that can change the current draw also, posibly leading to the same problem.

2) The transformers just have wire leads, typically, so you either use a split nut, lug, or wire nut (I don't care for that at higher currents).

Yeah the imbalanced current on the saw is 11.6, 10.3, 7.4 I was able to adjust the thermal overload and get it to run ok but I have always worried that I might cause some damage. The current imbalance on the 1.5 hp motor I'm adding is 6.1, 5.9 and 2.5 amps, this trips the heaters. According to Phasemaster the high single phase voltage will cause this 3 phase imbalance. I ran into this problem when I put my Moore #1 jig borer in, but I switched that to a VFD and got variable speed and reversing to boot! The 1.5 hp motor is in a shaper that I'm adding, it has a Reeves drive so I don't need variable speed and reversing would just be plain silly for a shaper so I'd rather not go the VFD route this time. Besides I figure a transformer should be cheaper than a VFD.
Thanks for the help here, but I think I'll wait to order something until I talk to Phasemaster again on Monday. Phasemaster seems pretty reputable and should get me going.

Out of curiosity any idea why their RPC would go wacko if the single phase input voltage is over 240V?

Well, if you want to max the ratings on your saw motor you better go to a Phase Perfect. As I said earlier the best RPC will yield no better than 90% of motor nameplate HP. In the final analysis even a purpose built factory RPC is no better than a bandaid if maximum motor performance is paramount.

Someone noted that line voltage a bit higher than nominal is an advantage and he's right. So long as the iron isn't saturating the motor will draw proportionately less current at the higher voltage. Also look at the motor service factor. If its 1.0 you have to size the breaker heater to the motor's nominal HP. If the service factor is 1.15 (not uncommon in industrial equipment) you can up-size the heaters proportionately ro reduce nuisance trips.

Residentail breakers subjected to extended service protecting an induction motor starting surges are often culprits where nuisance trips become frequent. If the single phase sevice is from a residential rated panel and there resides the troublesome breaker, change it out for a motor rated breaker if one can be found that's compatible.

I suggest if max HP is important for your saw you might think about re-motoring your saw to the next larger nominal size - or larger.

Personally, I'd install a load-meter in line of sight at the infeed station, connect it to register L3 Amps, and redline it at FLA. Then I would monitor the load-meter as I fed work into the saw. Save a $hitload of money. 4 kW Phase Perfect electornic phase converters are expensive. A larger single phase motor would be a better buy.

Hm. Wait! Is this a Felder, Laguna, or equivalent combination woodworking machine with the saw, planer, joiner and shaper each with a motor all built into one whopper of a machine? That's a special deal. Let me know.

Thanks for the info Forrest. Yes it is a Euro combo machine, not a Feldor but a Mini Max. I bought the Phasemaster RPC from Mini Max when I bought the saw. I'm not concerned about getting max hp out of the saw, but I had to adjust the thermal overload to keep it from tripping even at idle. After I adjusted the thermal overload the saw actually works fine, even with the phase imbalance, but I worry that I could cause some damage to the saw motor.
The real problem is I just added a metal shaper (1.5 hp) and the heaters trip at idle. The motor is rated at 4.8A FLA. At idle I measure 6.1, 5.9, and 2.5 due to the phase imbalance. I assume the 6 amp draw is tripping the heaters, so if I can correct the phase imbalance I should be back down to the FLA rating on the shaper and also be able to readjust saw the thermal overload back down. Phasemaster said that I need to get the single phase line voltage under 240 to correct the phase imbalance.

I'll look at the service factor of the shaper motor, maybe I can just up size the heaters to get the shaper working, although this wouldn't get the thermal overload on the saw back down to the factory set point.

Thanks for the help.

1) how are you measuring the currents at the load meter?

2) what are the phase to phase voltages measured, at the load machine?

3) my guess is that if you reduce the incoming line voltage to the
converter you won't see much change in the current readings if measured
the way I think they are being measured.

Originally Posted by jim rozen

1) how are you measuring the currents at the load meter?

We (utility guy and I) used a Fluke amprobe clipped around the three leads going to the saw and shaper motors while they were running and also while they were not running. We also used the amprobe clipped around the single shase lines coming into the RPC while the motors were running and not running.

2) what are the phase to phase voltages measured, at the load machine?
While the saw was running the voltages were L1-L2 245.5; L1-L3 238.2; L2-L3 251
With out the saw running the voltages were L1-L2 247.1; L1-L3 269; L2-L3 279.5
These were actualy measured in the RPC control cabinet just as the wires exit to the machine.

3) my guess is that if you reduce the incoming line voltage to the
converter you won't see much change in the current readings if measured
the way I think they are being measured.

What are you thinking Jim?

Well the dude a Phasemaster had a different story today. He thought that adding a buck transformer would be a waste of money. He felt the imbalance wouldn't cause the motor to heat up even with the over current draw on two legs and I should just up size the heaters and not worry about exceeding the FLA rating on the two legs. He rattled off some motor theory that went over the top of my head. He said if I wanted I could add a 1KVA buck boost transformer to even out the phase balance but better phase balance in itself probably wouldn't solve my problem.
So I'm kind of back to square one, how do I get the current draw on the shaper back down to less than the 4.8 FLA rating? Or should I not worry about it and just up size the heaters until they don't trip out?
I ran the shaper for about 1/2 hour taking some light cuts, pulled 6.9, 6.6, and 3.5 amps but tripped the heaters after 1/2 hr.
I'm thinking that I'll up size the heaters and see what happens.

Sorry, this is an old thread but my problems haven't gone away. I did up size the heaters on the shaper, and left the saw thermal overload turned up to the max, but now I've added a small horizontal mill and a power hack saw to the rpc and both these heaters trip. I suppose I could just up size the heaters in those machines also but something sure don't seem right here. I measured the input line voltage to the rpc tonight at 251 volts. I seriously thinking I'll buy a buck boost transformer and see if that will help. For me it seems like a shot in the dark but I think I need to do something to get better balance before I burn up a motor.

Originally Posted by Leigh B

Sorry, this is an old thread but my problems haven't gone away. I did up size the heaters on the shaper, and left the saw thermal overload turned up to the max, but now I've added a small horizontal mill and a power hack saw to the rpc and both these heaters trip. I suppose I could just up size the heaters in those machines also but something sure don't seem right here. I measured the input line voltage to the rpc tonight at 251 volts. I seriously thinking I'll buy a buck boost transformer and see if that will help. For me it seems like a shot in the dark but I think I need to do something to get better balance before I burn up a motor.

Old thread it may be. but it helped me decide the day for RPC's was long gone.

Bottom line in your case is that you are trying to use one medium-weight RPC that was speced for a specific load to (by now) run all manner of OTHER items. Nothing wrong with that in concept so long as they are not an overload ... but the many folks who have posted on PM as doing it all painlessly seem to have done one or both of two things you have not done:

A) Use a markedly oversized RPC so as to benefit from inertia - both mechanical and electrical. Simply less sensitive to load change that way.

B) Run some of the OTHER machines not even in-use, so as to have their unloaded motors provide MORE 'inertia' and smoothing in the system.

Your problem may go away most cheaply if you can simply switch-on the motor in another machine or two as extra 'idlers'.

Cheap as dirt to give it a try...

Have a care not to create a safety hazard, of course.

Bill