I'll be picking up my Bridgeport 2 HP 2J in a couple of weeks, so it's time to start working on a rotary phase converter. I know that my ideal size is 3 HP for the converter. There's a possibility of getting two 2 HP motors incredibly cheap. Can you gang them together, or is there a disadvantage to doing that?
I"m early in my search, so I don't know if there are any reasonably priced 3 HP motors out there in our rural area at the present time.
RWL
It is definitly possible since I do the same thing, with 10 and 7.5 HP idlers. Been doing this for perhaps 4 years by now. Gives you numerous advantages.
i
You can, and they need not be mechanically connected.
As Ig stated, there are numerous advantages, which hopefully he'll elaborate on.
I see the advantages as lower start up cost, lower running cost, greater flexibility (run as many as you need), easier start up (won't brown out the neighborhood), AND, I suspect, overall better power regulation, and, all things being equal, quieter operation. Also less heat buildup.
I have an arsenal of about 6 idlers. :)
In fact, I see no advantages to one big idler over numerous small ones, except one: simpler wiring. Others may disagree, however.
Best to separate start up caps from run caps, and even better to have a bank of switchable caps for best load/voltage matching.
With a bank of switchable caps (to both L1-L3 and L2-L3, (L1-L2=line)), you can use the caps you need to start up, then switch those out, switch in the run caps predetermined for the idlers chosen, *and* the anticipated load. Stage the idlers in, one by one, as needed. Only need start up caps for the first idler started.
Get 3 cheap digital multimeters from HF (also 3 of their dert cheap clamp-on multimeters) to let you observe voltage and current in all three legs simultaneously. Really helps in the tuning process.
Another poss. superior alternative for your BP is a VFD, esp. if the head is belt, and not variable speed. This way, you won't need an rpc, at least not for this machine.
I will list them, and I know that you mentioned quite a few.
My phase converter is one of relatively few homemade things that I am actually very happy with and that does not give me any trouble.
i
I also have a dual motor unit. 10 and 15 in my case. I'll just emphasize that you need to delay the starting of motor 2 by several seconds. Spreads start load.
Karl
That's what I do also. I have a starting switch and a button. I flip the switch, which starts the first motor. At this point there is no power on the output of phase converter, and the first motor is noisy, because it is unbalanced electrically.
When I hear that it started, I push a button. That closes the contactor that starts the second motor, and also connects balancing caps for the other leg, and enables output.
At that point the RPC is running and producing balaned 3 phase power, and quiet.
I have a phase convertor that runs my downdraft grinding table. The Phase convertor is a 7.5hp the table is 3hp. On occasion I shut the phase convertor off without shutting the table off. I also once in a while, start the phase convertor with the switch on the grinding table on. Does this practice hurt anything? Steve
Could hurt the person who is changing the saw blade at the moment. :)
Otherwise, if that stuff starts quickly, I do not think so.
I can't tell a difference in startup time either way. There are no exposed moving parts. Basically a vacuum cleaner inside a table with grates on top. Here is a link to a newer model.
Others have given you good advice on rotary phase converters, but I'll give you my take. Having run Bridgeports on rotary converters, static converters and VFDs, a rotary would be my last choice. A VFD is much more convenient, even with a 2J variable speed head, and a static converter is more than adequate. RCM dogma says that you can't plug reverse a BP with a static converter, but I did it for many years, as does a friend of mine with 3 mills in a commercial shop.
Of everything, VFDs are the best option for a Bridgeport. Despite having a phase converter, I run mine on a VFD and could not be any happier.
What is "plug reverse"?
The reversing issue on a static converter:
On static conversion, the motor can be reversed via the BP switch, as long as the leads being interchanged are L1 and L2 -- or so I think. AND, of course, the starting caps placed between L1-L3 or L2-L3.
The problem with static converters is that they seem to strain the motor after startup, unless they are switched out, and balanced run capacitors are switched in.
But if this is done, then I agree, static conversion, at least for one motor, may be preferable -- altho, you will have to de-rate that motor.
Another 3 ph motor elsewhere on the grid would help "re-rate" the motor, but then you have the curious scenario where the statically converted motor is acting like an rpc!
So I would say that if you only have one motor, and you don't mind de-rating it, static conversion would be preferable to an rpc.
But otherwise (>1 motor, no de-rating), an rpc would proly make more sense. I'm thinking of statically converting a compressor motor.
AND, interestingly enough, I have a residential central air unit that I just discovered has a 3-ph compressor with... you guessed it.... static conversion!!
Which is possibly why the compressor is so goddamm noisy, cuz of the inherently imbalanced start capacitors, that are never switched out, of course. Will be a chore to wire this up properly....
I've had two VFD's for some time, but haven't hooked them up yet -- thinking of making a kalamazoo 9x13 hydraulic the first guinea pig -- but then I could only put the blade motor on the VFD.
I'm still in rpc-mode. :) VFDs, tho, are theoretically Da Bomb.
Well, it does "violate" motor recommendations, but if the larger motor is not started under load, it doesn't seem to hurt much. Small motors can start larger ones, and I do it myself.
It just goes to show that 3-ph motors form a kind of 3-ph grid, and once one is started, any other 3-ph motor can be started. Which is quite apropos of the OP, ito of ganging motors.
Plugging is reversing or decelerating the motor by rapidly switching two motor leads. E.g., reversing a Bridgeport by switching directly from forward to reverse without pausing at off.
It's been quite a while since I've had a BP connected to a static converter, but that sounds right.
I'm not aware of any undue strain on the motor without run caps, as long as, as you say, you don't push the motor too hard. And if you're driving a Bridgeport's motor to capacity, the machine is probably dancing across the floor.
I can "tune" an rpc almost by ear -- the combination of caps across the various legs that results in the quietest motor operation corresponds with the most uniform voltage across all three legs!!
I assume this noise is some kind of electromechanical imbalance in the motor -- in the stator winding(s), the rotor, etc.
Needless to say, stock commercial rpc's with those big-assed start caps that are never switched out make a helluva racket, AND yield a 270 V third leg.
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