Rotary Phase Converter issue

I have a 240V 30hp rated Desco phase converter with a 30hp Baldor Motor. I have been using it for probably 10 years to run the 15hp motor in My Monarch lathe and have not had any issues. When i installed it I checked voltages and they were like 244, 246,244 and I thought that was great. I recently got a mill that supposedly has a 20hp motor in it. The mill has hydraulic pumps so it doesn't start the motor with no load like on my lathe where the motor is declutched. The mill starts and runs, but upon startup it gives the idler motor fits, it starts buzzing and jumping around in the floor for about 5 seconds until the mill comes up to speed then it is fine. When it is running and I check the voltages they are still well balanced. I tried starting the lathe first to add its 15hp motor into the mix but it didn't seem to help any. Do I need to add more caps for starting? Would that help? Can I add to many starting caps? I need to get someone to help me so I can see the voltages on the phases at startup as of now I can't hit the start button and be close enough to see the voltages on the meter as it is on the other side of the mill from the start button.
Thanks
Brian

Brian, you were not clear if the voltages for the RPC output were at load or not. RPCs can not maintain those voltages throughout the load range. Usually they end up with higher voltages at idle because they drop fast when loaded. Off the cuff, I would say A-C/B-C should be 250v so they do not dip too low when loaded.

That being said, you have a bigger converter to help with that issue. It IS normal for a converter to wig a bit with load but dancing all over the floor for several seconds seems odd. I recommend voltage testing while loading to find out what is happening. You may be able to add significant changes with a little cap tuning to get start performance in line.

With any converter I ever built, they growl when loaded but are ok just sitting on the floor. Voltage sag should be checked though. That is a BIG issue regarding generated 3ph. Vdrop causes failures and nuisance trips.

I checked voltages today with the following results.
At idle my voltages are L1-L2 244V, L1-L3 240V, L2-L3 236V.
Under load with my 15hp Lathe they are L1-L2 241V, L1-L3 233V, L2-L3 225V
Under load with the 20hp Mill I am L1-L2 237V, L1-L3 225V, L2-L3 220V
On startup I am dropping to around 150V on the generated leg.
Any tips on what I can do?

Add caps! You did not indicate which leg was generated but I suspected a nasty voltage drop. Adding caps will probably help. A rough ballpark is 10uf/hp but will NOT be the same for both legs. Try to get about 5-8V higher than your incoming power at idle and that should help. Inrush which is 5-10x FLA will certainly cause a nasty voltage drop.

Viper L1 and L2 are my incoming 240V feeds and L3 is the generated leg. Should I add my caps from L1-L3 mostly since L1 has my highest voltage and L3 is the lowest? Currently none of my lines are higher than incoming voltage, would adding a bunch more caps allow me to get my voltages above line for no load conditions?

Yes, cap tuning will allow you to push voltage above incoming voltage. Target would be 240, 248, 248. I usually tune to about 240, 250, 250. If you have equipment that will be sensitive to the higher voltage and is say 208-230V, a buck/boost transformer will get everything back in check.

just start by adding about 100uf per side and start measuring. As you tune one side, the other is affected and as i say, they won't be the same on each side. I use HVAC caps that have a dual inegral cap. Typically like a 35 and 5. This makes it easy to fine tune. I would order up a pair of 50 and 100uf caps to get the bulk of the capacitance, then get some used or new HVAC caps to bring it home...

I had two since taken to the scrap yard hydraulic driven CNC lathes. The motors were only 15 HP each, but they pulled 300 amps until they were up to speed because they were directly connected to the hydraulic pump. I am assuming it is taking a huge inrush current to start that hydraulic pump motor, caps may help, but you might need a bigger converter.

If the load is purely a hydraulic pump motor and no sensitive electronics, I would certainly tune for that and probably run about 240, 255, 255V to better protect against inrush. It WILL make a difference in start performance but as noted above, super tough loads can require a bigger converter.

I do agree with viper about the caps,make sure that your adding run caps to L1-L3 and L2-L3 as your converter might only have run caps between L1-L3. Another thing that you could try to do is, turn on the lathe first and dont apply a load to it ( this would add to the phase conversion ) , then try startup the mill.

Well I figured it out some but only by trial and error now I realise what needed to happen. I now realise that the phase to phase with the lowest voltage is where the extra caps need to be added. I 1st started adding 80uf to L1-L3 and 60uf to L2-L3. That raised L1-L3 alot and L2-L3 a little. I then realised that the low voltage needed the extra caps for balance so I added a 150uf to L2-L3 with no extra caps in L1-L2 and ended up with the folowin results.
Idle L1-L2 242V, L1-L3 250V, L2-L3 250V 3.3%peak imbalance
Lathe No load L1-L2 241V, L1-L3 240V, L2-L3 243V 1.3% peak imbalance
Lathe loaded L1-L2 240V, L1-L3 240V, L2-L3 240V no imbalance
Mill No Load L1-L2 237V, L1-L3 226V, L2-L3 230V 4.8%peak imbalance
Mill Under Power L1-L2 235V, L1-L3 225V, L2-L3 222V 5.8% peak imbalance

Since the addition of the caps didn't seem to bring the L3 leg up much under the highest load case running the mill I am thinking this is probably as good as it is going to get without having a bigger idler motor. It did seem to reduce the vibrating of the idler motor on startup but it is still hating life as it take about 7 seconds for the mill to come up to speed and everything to go back to being happy after the button is pushed. Once it is running everything seems happy.
I saw some of the HVAC multi power caps and thought about getting them but wasn't for sure how they should be wired in since the leads are marked Fan, C, and Her. I might would try one of those to tweak things with if I knew how to hook it up right. Any ideas?

herm = cap run for compressor, fan = cap run for fan, C = common shared between both herm and fan. Super simple. basically just two caps in one with one side common so just adding a jumper on the cap will bring in more capacitance.

Sounds like you are pretty close on your tuning. If your systems will tolerate, I might look to push idle imbalance a little higher so things stay balanced better under load which is where it is most important. Or reducing voltage sag anyway. Would be interested to see how many amps you are asking of your converter for that 7 second start. I have run fast spindle CNC machines in them and push the piss out of them without issue. The idlers growl and wiggle a bit but that is pretty normal.

Hi Brian,

Does your Desco have any round plastic motor starting caps, or they all metal can run caps? Many static units have only motor starting caps, and if that is all in it what is it's rating XXX Uf and XXX Volts and X # of caps?

What phase L1 or L2 are they hooked to? Is the Baldor motor 1200 , 1800 or 3600 rpm?
The 3600 (3450 actual) RPM motors don't provide a solid "wild leg". the 1800(1725) RPM units are better. And in my opinion the 1200 (1140) RPM have the most guts. The 900 RPM and lower are nearly worthless as a phase converter.

What size is the transformer feeding your shop? 10, 25 or 50 KVA They normally have large numbers on the front under the secondary terminals. You should have a 25 or 50 KVA out there (30 HP is 22 KW). If you have a 10 it's time to get the power company to upgrade it. The electric heat class of residential service gets you a larger transformer out there.

If L1-L2 is dropping 7 Volts with some load on the mill. It is time to get the clamp on ammeter going to see what your running amps are. Make sure your wire size is a minimum of 4 sizes larger than needed for your running amps. Also check voltage L1-L2 at your main service with the phase converter off and with load on the mill.

From the posts you have done, it looks like you may have several problems at the same time. So the questions will help to identify those areas that have problems.
Possible sources are:
1. Small power company service, good source of big voltage drops during peak loads.
2. Running the "pony motor" single phased, Helper caps on the mill only may be needed.
3. Overloading of existing wiring, large momentary currents need larger wires to reduce voltage drop.
4. Evaluate the "pony motors" ability to handle large overloads.

I run a Denison press with a 5HP motor on the hydraulic pump and a big heavy 3 HP 1.15 s.f. 1200 RPM "pony motor". At high tonnage The 5 HP is run to a stall (locked rotor) condition momentairly. The "pony motor" jumps all over during the stall. The control is homemade.

Bill

"Also check voltage L1-L2 at your main service with the phase converter off and with load on the mill."

I am wondering why you are telling him to single phase his mill?

To the Op:
Can you re-wire the hydraulic pumps on the mill with 3 phase contactors and switches so that they are switchable and only start them after the spindle motor is running at speed?Instead of them starting at the same time as the spindle motor?
Having the pumps start seperatly would likely take a large start load off of the rotary converter IMO.
Also how are the hydraulic pumps plumbed?Do they deadhead when not in use or circulate the fluid untill called for?

A deadheaded pump takes a lot of power and heats everything significantly.

"A deadheaded pump takes a lot of power and heats everything significantly."

Not unless it is a positive displacement pump. The typical centrifugal pump hp requirements and amp draw decreases with decreasing flow. You can burn up your pump or boil the water in it, but the motor will be coasting.

If the mill has 3600rpm motors, it will give the converter fits getting started. Less torque to pull up to speed and takes them longer to pull up. My new 3hp 3600rpm Van Norman makes my 5hp converter sound like it is having a coniption fit when you hit the start button, while my old 1100rpm 5hp on the L&S barely bumps the converter on startup.

The pump is driven off the opposite end of the motor shaft that drives the spindle so no way to shut it off for startup. The spindle is declutched though. I can't get tothe motor to read the data plate on the mill motor so i honestly don't know its true size or specs as to rpm. All I know is it is labeled 220V on the outside. My Phase converter is fed from a 2AWG wire and that is as big as the service to my shop has coming in so I don't see any point in going bigger. The wire does not get warm while running so I think its size is fine. My phase converter is off a 100amp breaker and I have never tripped it even on startup so I think my loads are ok. The mill is on a 60amp breaker and it hasn't kicked out on me either, once it is running everything seems fine and it functions happy so I think I will just have to igore the dimming lights and buzzing converter for the stratups. Oh and the desco converter has plastic start caps and a timer in it as well as the oil filled caps. I think the same transformer has been haging on the pole out here since the 1940's I have been here 25 yeears and I can't recall it ever being replaced. I don't think anything could be read off of it, but I have a 300amp service to my house which also feeds my shop so It must be of decent size.

Sounds like the same deal I am experiencing with the 3600rpm motor and no clutch to the spindle or feed. A 3600rpm motor has half the torque of a 1750 of the same hp rating. Given equal loads to get spun up, the 3600 is going to have to grunt harder and longer to get up on the torque curve to where it can then easily maintain the load. I have a 1750rpm 3hp head on my bridge mill that zings up effortlessly even at top speed (2500rpm spindle speed) with the converter barely buzzing. My 3hp 1100rpm radial drill doesn't hit it very hard. Same for the 5hp 1100rpm lathe motor that is clutched (an that's a 5hp motor on a converter with a 5hp idler!). This new 3hp 3600rpm motor mill makes it honk for a couple of seconds until it gets spun up fully, then all is back to normal. I think it's just the way it's gonna be.

I have a Desco 30Hp RPC. It is about 6 years old. I was told when I bought it that the largest motor you can start with it is half the rated hp which in my case would be a 15hp motor.

You trying to start a 20hp motor is probably why it is not working if the same holds true for your converter.

I have an Okuma LB-15 cnc lathe. It is a 15hp machine and when it starts up the draw on my RPC is noticeable. Also when ramping up to speed and braking to stop the chuck it will knock out power to my other cnc lathe and my VMC sometimes......a HUGE pain in the ass. Especially on a short cycle time.

Later,
Russ

Hello,

I`m new on this page.
I recently purchase a second hand Temco XR21 30hp phase converter to power up a 10hp panel saw, a 10hp Ingersoll 2 stage air compressor and a 3 phase Miller welder Dimension 652. The panel saw starts up easy, but the air compressor and the welder won`t start up.
When we start up the compressor, the voltage drops significantly. All of the machines are 380v. We use a 30kva transformer to transform the 220v 3 phase to 380v 3 phase. What could be a possible problem?

Another issue is that the 1 phase with the neutral voltage is 96.6v (L1 + N) , 105.4v (L2+N), 107v (L3+N)
The 220v readings are: L1+L3= 224.7v, L2+L3= 231.8v, L1+L2= 220.6v
The 380v readings are: L1+L3= 374.5v, L2+L3= 397.8v, L1+L2= 385.5v

The startup for a 5HP, 34amp, 220v, 1phase is also going difficult.
Does anyone know what could be the possible issue?
Thank you all in advance for any advise.