3 phase power and rotary phase converters - Page 2

3 phase power and rotary phase converters · 01-11-2009, 12:06 PM

Say I have a 10 Horsepower motor on a planer and a rotary phase converter. The single phase power to the rotary phase converter is rated at and cabled to handle 100 amps.

Everytime you try to start the motor, it somewhat comes up to speed and then it trips the 100 amp breaker. If you take the belts off the motor starts fine. The planer head spins freely. If you put the belts on to the head it trips the breaker.

Checking the wiring it was a single phase leg, manufactured leg, and a single phase leg.

Switching it around so that the single phase legs come first and the manufactured leg comes last makes it work properly.

So I assume the single phase legs have to come first, but why? If there were true 3 phase power from the power company I don't think this would be an issue would it?

Do the single phase legs have to come first and the manufactured leg to come last in order for the motor to reach full power? Why?

DaveD · 01-12-2009, 11:32 AM

Travis, something is drastically wrong here. With my limited experience (one 10HP converter feeding a 5HP lathe) the motors should start almost instantaneously. Fast enough that you can't count to 2 before they are up to speed. I would think it (planer motor) is getting very hot if it takes a minute to get up to speed and it possibly is overloading the RPC also.

Here is my sequence for 10hp idler and 5hp lathe motor:
Power to the RPC control panel
Hit start button on control panel
10HP idler spins up to running rpm (1800) almost instantly (less than 1 second?)
voltage between 2 original phases is 243V
between wild leg and one original phase 255V
between wild leg and other original phase 259v

Turn on lathe at 920RPM via low voltage controls and contactor in lathe
Lathe comes instantly up to speed. Its a gear head lathe. 920 rpm is middle of the lathes range.

Voltages change by less than 5 volts 245/259/259
lathe is drawing 6.3/9.3/8.8 amps. manufactured leg is small one.
No cutting load on lathe.
At 2000 RPM and no cutting load the amps are about 1 amp higher on each leg.

Inrush current is very high (70+A?, don't remember)when starting the RPC but 50a fuses work just fine because the inrush doesn't last but a second or two. I'm feeding the RPC control panel with #6 copper.

Lathe motor nameplate says 14.2A at 220V, 1735 RPM, 4 pole

I can't easily measure amps under load because I have too many things to watch at the same time and am new to using a metal lathe so don't want to look away from running it.

My lathe has a 8" chuck and I would guess it has a comparable 50% load compared to your 10HP planer. So you need a 20HP idler and all the associated wiring size to run it. The info I have read says a 15HP idler would be marginal.

I think the cord to my lathe is #10 stranded wire (can't remember)

Are you sure its (idler motor itself) at least a 15HP idler? You said its rated at 20HP but...but...
Does the planer have a separate motor for the feed? if so can you run it without running the head motor?
How does it act? If it spins up ok then what happens if you then start the head motor?
If you have separate feed and head motors are you trying to start both at the same time?

I believe you can cascade idler motors and/or 3 phase machines to get more capacity. Can you add in another 5HP or so of idler motor?

If it isn't wired incorrectly (including a phase missing at the planar) then I would guess the RPC is undersized no matter what KAY says in their literature.

One more thought...I'll bet the reason the breaker is tripping is is because the inrush current is lasting a very long time with the planar motor taking up to a minute to get up to speed. A bigger breaker will not solve the problem (other than help burn out the planar motor).

DaveD · 01-12-2009, 11:38 AM

Scott...I agree with you. If you are running everything with contactors (AND momentary switches/interlocks pulling in the contactors) it wouldn't restart.

The problem would arise (from www readings) if someone uses a, always energized, ordinary toggle switch (flipped to on) to pull in the contactor.

DaveD · 01-12-2009, 11:43 AM

Here is a website with miles and miles of good info on RPCs. Good designs (on their 'sticky') too.I spent a lot of time there understanding RPCs.

http://www.practicalmachinist.com/vb...splay.php?f=11

and a good detailed article on tuning a RPC

http://home.att.net/~waterfront-wood...econverter.htm

cpowell · 01-12-2009, 09:18 PM

Originally Posted by DaveD

Travis, something is drastically wrong here. With my limited experience (one 10HP converter feeding a 5HP lathe) the motors should start almost instantaneously. Fast enough that you can't count to 2 before they are up to speed. I would think it (planer motor) is getting very hot if it takes a minute to get up to speed and it possibly is overloading the RPC also.

One more thought...I'll bet the reason the breaker is tripping is is because the inrush current is lasting a very long time with the planar motor taking up to a minute to get up to speed. A bigger breaker will not solve the problem (other than help burn out the planar motor).

These thoughts seem most plausible to me too. Something is bad wrong. Please keep in mind that the 10 HP planer motor ONLY needs 10 HP under a tremendous load. Motors only supply the power needed to meet the load (plus any efficiency losses). FWIW the planer motor should have its own overload protection - heaters/other.

I would venture a 2HP motor would spin that planer head up to speed very quickly under "no load" conditions. The 2 HP motor just wouldn't be able to take a WIDE swath of wood like the 10 HP can. I would make darn certain that the wiring in the motor wiring box is solid. Open the box, open the wire nuts and make sure the connections are good.

Does the planer motor only drive the cutterhead or does it drive infeed/outfeed rollers too? Just wondering if you could have a mechanical problem.

The moment of inertia of a planer head is not as large as something like a huge fan impeller.

Measure voltages and currents NO Load and then LOADED. I would rule out wiring/connections, then RPC, then motor. Not saying it can't be high amps but I don't believe you should be seeing high amps. You aren't dealing with a 200 foot feeder run are you?

Chuck

cpowell · 01-12-2009, 10:02 PM

Originally Posted by Travis Porter

For this one, he has a 220V switch set up to turn power on and off to the converter. On mine (which is a Kay), I have a rotary on/off switch built into it. I have looked into it, and it does have capacitors, some buss type fuses, and a couple of other things. I haven't looked into it in great detail before, but after reading your post and reading up on the internet some it is starting to make some sense.

Correct me if I am wrong, as all of this is my opinion after what I have been reading. All advice and comments are appreciated, as I am always wanting to learn more.

A rotary phase converter is a 3 phase motor (preferably a wye type not a delta type) that has capacitors for starting and capacitors for running. The start capacitors are normally activated with a time delay relay that drops them out after start up. The run capacitors should be the electrolytic type as they will hold up better.

Playing with this guys stuff and reading through documentation I am beginning to believe their is either an issue with the size of wiring somewhere, the run capacitors, or the 3 phase motor the convertor is built upon. If the motor on the planer starts up without the planer head on it, and it worked before he bought it (saw it crank up) it tells me the converter he bought is acting up. Since it is a 3 phase motor as well, it is probably the capacitors (if the wiring to the planer is good).

It seems like there is really very little to go wrong with a 3 phase motor. It either works or it doesn't.

One thing that bothers me about all electrical stuff is this. You get told and you read you need to run this big wire to handle the load to a device. You open the covers and get into it and there you see 16 gauge, 18 gauge etc. Why do you have to run such big solid wire to a motor and then everything from the connection point is tiny?

Your thinking is not off base. I would rule out wiring/loose connections - especially at the motor connections, then would suspect capacitors, then RPC. 3 phase motor at planer would be last thought.

Small wires inside manufacturers' panels are normal and acceptable.
Resistance = resistivity * length/area (cross sectional)..if wire length is relatively short then resistance is negligible. While manufacturers may do things differently we must follow NEC guidelines in all of our wiring practices. The mfgs are being safe, they are just working with very short lengths of copper.

Does the Griz have a 3 pole contactor? I have seen bad contacts on new contactors.

If you understand vectors then I could go into as much detail as you wish to learn about 3 phase/single phase power, power factor, capacitors and so forth over lunch after the MWTCA meeting. Bring a few bar napkins.

I'll even buy you lunch if you bring me that problematic planer you've been wrestling for far too long.

Chuck

scsmith42 · 01-12-2009, 10:34 PM

Originally Posted by cpowell

Please keep in mind that the 10 HP planer motor ONLY needs 10 HP under a tremendous load.
Chuck

As Gene explained to me, an electric motor experiences it's greatest current draw at the point where the motor stops turning.

This occurs in two circumstances - tremendous load on the motor, as Chuck mentioned above, and upon startup when the motor is already stopped turning when the power is applied.

Travis Porter · 01-13-2009, 12:30 AM

The feed rollers are on a separate motor and start and run with no issue. With the belts off the planer head spins nice and easily and the motor spins easily. The two belts are big and stiff and make the thing tougher to spin, but it isn't like you can't do it.

I didn't get a chance to talk to him today, but I think the phase converter is messed up or the wire size is somehow off. I am curious to see what he has found and done.