wire a soft start

Nothing you mentioned there would make it difficult to connect a VFD. Start and stop pushbuttons simply get disconnected from the existing connections and wired to the VFD. Need more detail on the rotary reversing switch, but it should be able to be wired to the VFD as well.

Can you post a link to a schematic so we can give you a rewiring diagram?

A schematic of your RPC would be helpful. Although a RPC is not a "soft start" per se; an appropriately sized RPC and load combination will start fast enough so that the breaker doesn't have time enough to trip. The idler motor, capacitors, and load motor comprise an electrical network in which total current circulates. A proper RPC will have an idler motor at least 1-1/2 times the HP rating of the load motor and sufficient capacitance during the starting interval to bring the load motor up to speed in about 1/2 second. Generally speaking, if wiring and breaker are correctly sized, the surge starting current of the RPC and load combination will be over before the breaker has had time to trip. A 3 HP idler motor and 5 HP load should be wired and breaker-sized for at least 25 amps. Suggest you speak with a local electrician to maintain code.

Bob (never did get my sack of apples) Swinney

I looked at a VFD. The way the control panel is wired, it would be hard to install. There is a rotary switch that flips two leads for reversing rotation ahead of a motor contactor. Estop logic can drop this contactor. There's start/stop pushbuttons on the operator panel for motor operation. All logic uses 110 VAC.

Iggy suggested a soft start. see this example:

Karl

Sorry but my post re. subject was incorrect: The idler motor HP should be at least 1-1/2 times that of the load motor. That implies an idler motor of at least 7.5 HP and combined HP of 12.5. Again, suggest check with a local electrician to determine single-phase wiring and breaker.

Bob Sw>

Nothing you mentioned there would make it difficult to connect a VFD. Start and stop pushbuttons simply get disconnected from the existing connections and wired to the VFD. Need more detail on the rotary reversing switch, but it should be able to be wired to the VFD as well.

Can you post a link to a schematic so we can give you a rewiring diagram?

least 1-1/2 times that

combined HP of 12.5.

I also noted in a previous thread to try starting the vertical spindle first to let that add another 5hp worth of idler to the converter before trying to start the apparently high inertia horizontle spindle.

A proper VFD will entirely eliminate the problems he's having, can eliminate the noisy RPC and if done properly a single VFD can operate either of the spindles.

least 1-1/2 times that

combined HP of 12.5.

By the way... How is your VFD working for you? Did you add a switch or you just use the panel buttons?

at least 1-1/2 times that

combined HP of 12.5.

I've been a bit busy since getting back from my dive trip, so the VFD is still sitting on the end of the Bridgeport table hooked up for testing. I did power it up again and play stepping around various frequencies and it still works nicely. I may get to work on it this weekend, before I have to run out of town on a work trip to Chicago for a few days.

start" per se; an

the breaker doesn't

comprise an electrical

motor at least 1-1/2

starting interval to

wiring and breaker are

will be over before the

wired and breaker-sized

My question is about a soft start. Do you or Pete know anything about them?

Adding more idlers didn't work. i can start other 5 hp motors, i.e. the main spindle. This one has a ton of gears to bring up to speed - too much start load.

I can handle wiring a VFD. I don't want to tear into the existing control. In particualar, I don't like the idea of mixing 24 Volt VFD logic on the same switches that have 110 power for everything else. There's already two lbs. of junk in a one lb. box. on the control panel. Not room for separate stuff.

Karl

Karl sez: "My question is about a soft start. Do you or Pete know anything about them?"

Sorry Karl, I know nothing about VFD's except that I strongly suspect you can't get one of the size you require for 100 bucks.

That being said, and reading further, you say: "Adding more idlers didn't work. I can start other 5 hp motors, i.e. the main spindle. This one has a ton of gears to bring up to speed - too much start load."

It seems you may be confusing idler motors with start capacitors. Your statement is illogical re. "Adding more idlers didn't work . . . . ." Are you sure you didn't mean "Adding more capacitance didn't work ? It seems you are proceeding wihout any regard, or understanding, of RPC design philosophy. You should go back an reread my article, "Induction Motors and Rotary Phase Converters" in the November / December ed. of The Home Shop Machinist.

Bob (forget the damn sack of apples, you're on your own) Swinney

Don't know much about the "soft start", it appears to pretty much be a "VFD lite" and you could do the same with a VFD configured to start when input power was applied.

You wouldn't ever be mixing 110VAC and 24VDC on the same switch the former 110VAC switches just become 24VDC logic level inputs to the VFD. Most of the 110VAC would be eliminated, or even all of it. What exactly is there anyway, I thought just controls for two 5hp spindles and an E-stop.

Make that November / December 2001 Ed. of Homeshop Machinist. The article also appears in Metal Web News along with a lot of others....

Bob Swinney

Bob Swinney

And a very fine article it was too. I remember reading it when it came out. But, IIRC, not a dang thing in it about soft start devices.

Karl

I think his point is that a properly setup RPC will start even the worst loads.

Possibly a dumb question, but have you actually got the thing running in that upper gear and confirmed that there isn't something binding?

Karl,

About as close as you can get to "soft start" with a conventional RPC is to properly design and balance one. A RPC, with sufficient start and run capacitance to bring the load motor up to speed within 1/2 second which is fed from a correctly sized breaker provides a form of soft start. Implicit in understanding this is the fact that a RPC is an electrical network consisting of idler motor, load motor(s), and capacitance. In such a network, single-phase current is fed through an appropriate circuit breaker to run three-phase motors. The RPC circulating current path is not through parallel motors as is commonly thought. Phase relationships vary widely throughout the network, but if properly designed, current and voltage will be in phase across the input to the network. Circuit breakers for motor service are designed to stand large surge currents so long as those currents are of short duration. Analysis of the currents flowing in a RPC is quite complex and touched on briefly toward the end of my article. Generally speaking, problems occur with RPC's because of design shortcuts. It is a common mistake for one to short-cut the amount of start and run capacitance used in a RPC.

Bob Swinney

And a very fine article it was too. I remember reading it when it came out. But, IIRC, not a dang thing in it about soft start devices.

Karl

I think my news server is acting up, but here goe again . . . .

Karl,

About as close as you can get to "soft start" with a conventional RPC is to properly design and balance one. A RPC, with sufficient start and run capacitance to bring the load motor up to speed within 1/2 second which is fed from a correctly sized breaker provides a form of soft start. Implicit in understanding this is the fact that a RPC is an electrical network consisting of idler motor, load motor(s), and capacitance. In such a network, single-phase current is fed through an appropriate circuit breaker to run three-phase motors. The RPC circulating current path is not through parallel motors as is commonly thought. Phase relationships vary widely throughout the network, but if properly designed, current and voltage will be in phase across the input to the network. Circuit breakers for motor service are designed to stand large surge currents so long as those currents are of short duration. Analysis of the currents flowing in a RPC is quite complex and touched on briefly toward the end of my article. Generally speaking, problems occur with RPC's because of design shortcuts. It is a common mistake for one to short-cut the amount of start and run capacitance used in a RPC.

Bob Swinney

And a very fine article it was too. I remember reading it when it came out. But, IIRC, not a dang thing in it about soft start devices.

Karl

That is no problem -- you feed the motor directly from the VFD, and use a selected three of the four terminals which flip the two leads to select forward or reverse command to the VFD (dropping to 5V logic commands instead of the 115 V ones which are currently in use.

Most VFDs have the ability to accept the start/stop pushbuttons (working at 5V) without problems.

Not necessary (nor desirable) with the VFD. I'm sure that you would feel more comfortable re-wiring the panel at 5V instead of 110 V.

There, I can't help you, since I've never worked with one of these. Perhaps go to the maker's site and download the manual if possible?

Good Luck, DoN.