Lathe motor starter or soft starter?

Light Hammer · Oct 27, 2015

After what amounted to a month long battle with my power co, I was informed that the three phase install would cost me around $25k so...

I ordered an American Rotary AD-30 RPC. Here's my question: My lathe (Monarch 60/61 series w/15hp motor) is equipped with what appears to be the factory installed starting system. Being that it's a 15 hp motor, will I need some form of "soft start" or will the existing components work. I ask because one of the letters I received from my power co indicated that all motors above 5 hp would need to have a "starting current reduction system" which, at least to me, means some form of soft starter.

TDegenhart · Oct 27, 2015

The problems I see are that being 3 phase, 15 Hp is too big for a single phase to 3 phase VFD. That leaves a RPC type of convertor as the primary source of 3 phase. After that, you could go with a soft starter or a reduced inrush starter such as an autotransformer. Dollar for Dollar, the soft start is probably cheaper. You didn't say what the factory starter is, but I would guess that it is just a across the line, not reduced inrush.

Tom

rons · Oct 27, 2015

Your issue is the soft start of the 30 HP motor in the AD-30.

Light Hammer · Oct 27, 2015

The electrician described the current starting arrangement as "slamming in when tripped" so I'm assuming that isn't very 'soft' lol

Whatever it is, it is as I purchased it and as it was wired and running on true three phase before. I ran the lathe before I purchased it and there wasn't any light dimming or bogging of the motor when switched on.

The AD-30 comes with a soft starting arrangement or so I'm told by the salesman.

TDegenhart · Oct 27, 2015

Is the problem with the starting of the RPC or the lathe after the RPC is running?

Tom

Edit: I purchased a 15 horse AD unit a couple of months ago. The person at the factory site told me that the difference between a "standard" RPC and the AD is that the AD has solid state circuity to add or subtract capacitance to maintain the generated leg at +- 5% of the line voltage. The other two legs pass through from the main supply and the load motor. There is no soft start according to the man I talked to. Best ye go back to American Rotary and find out what you do have.

Tom

Light Hammer · Oct 27, 2015

Let me clarify; I haven't received or wired the RPC yet. I am asking about a soft starter because of what I read (from the power co) which indicates that a soft start is required on motors over 5 hp. I'm asking to see if I need to purchase and install a different starter or run what I have. The current arrangement can be seen in this photo:

JST · Oct 27, 2015

The most typical is a reduced voltage start, which is fairly simple on single phase. Reduce to 50% to 60% of mains voltage, get the motor rolling over, then cut to mains voltage.

Can be done with resistors, a transformer, etc.

rons · Oct 27, 2015

Light Hammer said:
I'm asking to see if I need to purchase and install a different starter or run what I have. The current arrangement can be seen in this photo:

View attachment 152720

That metal has sure seen some paint over the years.

Before you do anything have you thought about just using a VFD. You would get
a soft start feature with a variable time period.

A soft start circuit for a three phase motor in simple terms looks like this:

The back to back SCR's or TRIACS are fired with a gradual reduction in phase
angle until the motor comes to speed.

Getting to your issue:
A soft starter is placed directly in front of the motor. Your future RPC has that already.
You would have to do that for the 15Hp. I'm wondering how critical that would be because you
will be running your 15Hp motor from a 30Hp generator motor. Maybe that is a question for
your power company person.

Are your existing controls stock? The two green contact blocks look like a forward/reverse
arrangement with a overload block directly below. The box on the lower right looks like a
transformer. I think you should trace out the wiring and post a picture. Make it two pages
if needed so people can read it. Let's understand what you have in there and where the wiring
can be modified. I think you can fit a soft start into what you have.

Summary:
- consider a VFD and cancel your RPC order.
- ask power company how critical a second soft start circuit is needed with a RPC generator already employing soft start.
- draw out circuit of existing controls and post.

wheels17 · Oct 27, 2015

Could you use a direct coupled pony motor to get the RPC motor to speed, and contactor it on once it us up to speed? Would this keep the POCO happy? Then use a VFD or soft starts as mentioned by TDegenhart to soft start the lathe? Lots of equipment. Or would the VFD have trouble with the RPC?

TDegenhart · Oct 27, 2015

That's an across the line reverser with the one on right is most likely for the coolant pump. Replace the reverser with a VFD but leave the coolant pump starter.

Tom

JST · Oct 28, 2015

I interpreted this as a question relating to the RPC, but at 15hp the motor of the lathe is also an issue. Same deal really, reduced voltage. The scr system works, but so does a simple resistor system.

abarnsley · Oct 28, 2015

Polyspeede makes 220 v single phase input ONLY.... VFD's to 50 hp.

15 hp is not too large.. wire VFD direct to motor, power company happy.. Might step up to a 20 hp VFD if lathe has a spindle clutch..

VFD is less weight for shipping than a heavy rotary..

And of course will provide a very nice soft start.. At less than the cost, of a dedicated soft start only unit..

Small rotary for pumps and such.. Or just install single phase pump..

hitandmiss · Oct 28, 2015

Light Hammer said:
After what amounted to a month long battle with my power co, I was informed that the three phase install would cost me around $25k so...

I ordered an American Rotary AD-30 RPC. Here's my question: My lathe (Monarch 60/61 series w/15hp motor) is equipped with what appears to be the factory installed starting system. Being that it's a 15 hp motor, will I need some form of "soft start" or will the existing components work. I ask because one of the letters I received from my power co indicated that all motors above 5 hp would need to have a "starting current reduction system" which, at least to me, means some form of soft starter.

So the question now is how big is the transformer on the pole.

It should be 25KVA or larger if you are the only service attached and a 50 KVA if there are more than just you.

You need to know the inrush on your RPC. The system I worked with many years ago, the inrush was over 400Amps and 1/2 Second duration.

JST · Oct 29, 2015

The inrush is limited by the impedance of the pole transformer.

But, the motor may be labeled with a code letter. If it is, that will give the kVA per HP of locked rotor current. When the motor is started, the initial surge is essentially that of a locked rotor situation, since the motor is not turning. You use the kVA and the mains voltage to get the current.

The rating will be for the motor as a three phase motor, and will be less for single phase input, possibly not in an entirely predictable way. But that will give a starting point.

If you assume the pole transformer has about 5% impedance, then you can figure out about what the actual surge current will be, from the impedances of motor and transformer.

It may not be worth while to go through that. If you have a specific goal for the starting current with soft start, you can choose parts to give that goal current, if not already provided.

But probably, you can make sure that the voltage to the motor on start is 57% of normal line, and I doubt anyone will be able to argue with that. It's equal to the wye-delta start, which is an accepted method, and satisfies the requirement to "have a soft start" on it.

A VFD obviously can do whatever you want as far as motor starting, but may not be justified simply for a soft start.

Light Hammer · Oct 31, 2015

The pole transformer is a 25kva with my shop and one home attached to it. The power co engineer stated that they would upgrade the transformer size however he wanted me to run my setup and see if there were any problems before exploring that option.

The RPC came on Thursday. It was well packed and appears to be in good order. The sales rep at American Rotary stated that there is no "soft starter" however the idler/generator has been given a "soft start rating" and that "because of design of the generator motor, starting the 30 hp generator is equal to starting a normal 10 hp motor." On this, we'll see.

On the transformer subject, the engineer stated that they "performed a load test which indicated that the average draw on the transformer was 12.4 kva. I take it this means I'm not currently using the transformer to it's capacity. Hopefully it's big enough and I won't have to go down the upgrade road.

JST · Oct 31, 2015

If the requirement for soft start is just to satisfy powerco, wait and see if it all works.

More might be necessary for cases in which the motor drags down the voltage when started, blinking lights, and maybe shutting down computers etc.

The RPC may be a "double rotor cage" type which inherently has lower start current. That may be what they mean.

Light Hammer · Nov 8, 2015

I wired up the RPC today. Pulling four 4 ga conductors through 40 feet of conduit was absolutely no fun. Anyway, after a double check of the layout I took a leap of faith and hit the switch. The RPC idler fired right up with very minimal light dimming. I took a second leap of faith and started the lathe motor. Just as the RPC idler motor, the lathe motor fired right up(Keep in mind that the belts aren't installed so it's only starting the motor shaft). Much to my surprise, I did something correctly!

... or so it would seem...

Now, out of curiosity, I used a Fluke clamp on meter to check the amp draw of the lathe motor. What I found was: T1 - 17.5a, T2 - 16.4a, T3 - 9.5a.

Is it common to see different amp draws across the three feed lines or do I have a problem?

JST · Nov 8, 2015

Reasonably common at light load and consequent bad power factor. They should even out as the load gets heavier. If not, the "balance" capacitors may want some adjustment. RPCs are made for a range of motor powers, so they may not be perfect for yours.

Light Hammer · Nov 9, 2015

JST said:
Reasonably common at light load and consequent bad power factor. They should even out as the load gets heavier. If not, the "balance" capacitors may want some adjustment. RPCs are made for a range of motor powers, so they may not be perfect for yours.

Got it, thanks! I'll check the lines again after I get the belts back on and see if things even out a little more.

Lathe motor starter or soft starter?

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