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Rotary Converter, or VFD???

slufkin

Aluminum
Joined
Jun 19, 2008
Location
Upstate NY
I am soon to take possesion of a nice little Bridgeport, and I am in the market for a small lathe.
I am still up in the air as to how I want to handle my three phase conversion.

I have an old 5hp 3PH motor and a small 3PH Panel Box laying around. I was originally thinking of buying a static converter to start the 5hp motor to make it into a rotary converter, feeding the panel, then run outlets from there.

However the more I read from you guys, the more I think I might want to just put VFD's on each machine and plug them into a 220VAC outlet.

Then there is always the old static converter on each machine, but I always hated them on a mill because of the pause you have to have to reverse the motor when power tapping.
 
If you wish to instant reverse the mill motor you will need to use the RPC. It is the only one of the three that will allow you to do this.

It is much better to build the RPC circuit than to buy a static phase converter as you can do it better and for half the cost.

For 5 hp idler you will need a Steveco potential relay model 90-66 and about 500 mfds of electroyltic start caps. Steveco is about $25 and the caps will be about $25 so you can have your start circuit for $50 and this is what you get in a static converter however the potential relay will not be a Steveco nor will be of the same quality.

A 5 hp RPC is one of the very easiest to build and when equipped with capacitors on both generated phases, you will have a fine performing RPC. I run a 2 hp Bridgeport, a 3 hp Colchester lathe and a 1 hp Harig surface grinder from my 5 hp. I can run any two at the same time.

I like VFDs and use a couple of them. The big advantage is low noise and convience plus you have variable speed. I am thinking about putting one on my lathe but then I think about how much work I will have to do and I just start the RPC and carry on.
 
"Steveco is about $25 and the caps will be about $25 so you can have your start circuit for $50 and this is what you get in a static converter however the potential relay will not be a Steveco nor will be of the same quality."

This is an interesting, and important point.

The SPC boxes usually found out there are often incredibly poorly built, and the so-called potential relay is often just a conventional relay with a series resistor which makes it behave as if it was a potential relay. The relay's contacts may or may not be substantially built.

OTOH, the Steveco is a true potential relay, one which has a very high impedance, and very sensitive coil, and it needs no series resistor. Besides which, the Steveco is adjustable both as to drop-out and as to drop-in voltage, although the 90-66 model comes preset and sealed for 240 volt operations. Also, the Steveco is specifically designed for the rigors of commercial HVAC equipment. It is an industrial grade product in every sense of the term.

If your RPC is 5 HP and under, the 90-66 is all you need (besides caps, of course).

If your RPC is over 5 HP, then the 90-66 can drive a definite purpose contactor which, in turn, drives the large starting caps required for over 5 HP RPCs.

(The Steveco is limited to about 550 µF at 240 volts).

I believe Steveco's OEM catalog (White Rogers Corp) shows a user-adjustable model. The 90-66 is factory calibrated and then is sealed. The user-adjustable model is unsealed.
 
"If you wish to instant reverse the mill motor you will need to use the RPC."

It is possible to plug-reverse using an SPC, but it requires an auxiliary relay.

You would wire the auxiliary relay so it always passes-through the A and C phases as L1 and L2.

Then, the drum switch, which ordinarily reverses A/L1 and C/L2 (B being L3) is used to exchange the starting circuit with respect to A and C, while the running circuit remains the same.

The A-B and C-B caps would be the same value, about 2.5 µF per quarter HP (10 µF per HP).

In forward operation, the starting cap (about 110 µF per HP) would be placed across A-B, and the coil would read the voltage across C-B.

In reverse operation the, the starting cap would be placed across C-B, and the coil would read the voltage across A-B.

To effect a plug-reverse, you would first move the drum switch to the OFF position, wait a moment until you heard the potential relay drop-in, and then immediately move the drum switch to the opposite position.

The starting cap will be engaged and will firstly be employed to decelerate the motor towards a stop, and will secondly, and immediately be employed to accelerate the motor in the reverse direction, thereby accomplishing a plug-reverse.
 
"If you wish to instant reverse the mill motor you will need to use the RPC."

It is possible to plug-reverse using an SPC, but it requires an auxiliary relay.

You would wire the auxiliary relay so it always passes-through the A and C phases as L1 and L2.

Then, the drum switch, which ordinarily reverses A/L1 and C/L2 (B being L3) is used to exchange the starting circuit with respect to A and C, while the running circuit remains the same.

The A-B and C-B caps would be the same value, about 2.5 µF per quarter HP (10 µF per HP).

In forward operation, the starting cap (about 110 µF per HP) would be placed across A-B, and the coil would read the voltage across C-B.

In reverse operation the, the starting cap would be placed across C-B, and the coil would read the voltage across A-B.

To effect a plug-reverse, you would first move the drum switch to the OFF position, wait a moment until you heard the potential relay drop-in, and then immediately move the drum switch to the opposite position.

The starting cap will be engaged and will firstly be employed to decelerate the motor towards a stop, and will secondly, and immediately be employed to accelerate the motor in the reverse direction, thereby accomplishing a plug-reverse.

Wouldn't it be easier to just program the VFD to do a reverse? :D Dave
 
Actually, it's easier to use a tapping head!

I'd just forget about the plug reverse, and use a tapping head. They can be picked up CHEAP, and ever since I got one, I LOVE it!! Anyways, plug reversing is kinda hard on the machine. You can set the accel/decel time on a VFD low, and add a braking resistor to improve the time it takes to reverse. Allthough you might have duty cycle problems. I dunno, I've never tried it. With a VFD there will be a slight pause still between forward and reverse. I dunno how fast exactly you can get a mill to reverse with a VFD. I'd just get a tapping head, and forget about that. :cheers:
 
Plug reversing is in no way hard on motors.

This having been said, unless I were running a VMC with true repeat tapping
capability, a tapping head like a procunier is way, way better than trying to
tap via plug reversing.

Jim
 
I didn't mean the motor so much as the MACHINE.

I was thinking more along the lines of the spindle splines, and the other things in the mill that have to reverse instantly. Allthough I'm SURE making the motor INSTANTLY make that stuff turn the other way isn't exactly good for it. The surge current HAS to be more starting from turning the opposite direction than stopped! I suppose that's not hard on the motor at all though. :skep:
 








 
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