RPC Question

for what it's worth, I have good luck starting the 6 hp spindle motor on my mill with a 600 uf starting cap, which is disconnected via a potential relay once it's up to speed - the relay coil is wired from the unenergized leg to neutral. it works very reliavbly. But if I want to start the 2 HP motor that moves the table, I am better off having the spindle running first.

Bill

to contact me, do not reply to this message, instead correct this address and use it

will iam_ b_ No ble at msn daught com

A Hanrahan self-starter usually takes 30-35 uF/idler motor HP to self start.

A conventional ROC usually takes 100 uF/idler motor HP to start within two seconds, which is about the limit for an electrolytic starting cap.

Since a Hanrahan converter uses run caps only, it doesn't matter if it takes longer to start as these cannot overheat and explode, unlike electrolytics, and you can thereby get away with less capacitance, which improves matters, as less capacitance is probably better, from a B-phase voltage control perspective.

Yes, indeed so.

This is but one disadvantage of the self-starter.

With separate starting, you can employ whatever (electrolytic) capacitance it takes to get the idler up to 90 percent of synchronous speed in 2 seconds or under, and, separately, whatever (oil-filled) capacitance it takes to have the idler produce a B-phase (what you are calling line 3) which is within ± 10 percent (± 5 percent is usually possible, too) of the single-phase line voltage.

Ideally, you want the fully loaded B-phase voltage to be ±10 percent (±5 percent is better, especially for "technical" loads, such as electronic equipment) provided the unloaded B-phase voltage is also within ±10 percent (same comment for ±5 percent).

If you can get the unloaded B-phase within +10 percent (same comment for +5 percent) and the fully loaded B-phase within -10 percent (same comment for - 5 percent), then you have an excellent RPC, well-matched to load.

If you cannot get ±10 percent (same comment for ±5 percent), then your idler is probably too small.

Don't forget the case of unidirectional vs. reversible loads.

If the load is predominately unidirectional, then you probably want more A-B (Line 1 to B-phase) capacitance than C-B (Line 2 to B-phase) capacitance. About 60/40 percent.

If the load is reversible, such as a machine which runs forward for some operations, using the conventional toolpost, and also runs reverse for other operations, using the rear toolpost, then you probably want the A-B and C-B capacitance to be the same.

Choices, choices, choices.

you can wire 220 with or without a neutral. To save you some time, I use Leviton 2711 plugs (and matching receptacle) - this is a 4 wire, 2 hot, ground, neutral arrangement. There is another 4 wire plug that has 3 hots, don't use that one. These are available from a nice seller on e-bay for about half of retail, by the way.

Having neutral at the machine is advantageous for me because then I can have an outlet there for a lamp or other accessory that wants 110 and not have extension cords on the floor to trip on.

If you would rather not have neutral, you can use a circuit with a second relay. this link provides a sensible and clear approach

if you would like the schematic for the two relay approach, contact me off line and I'll scan it and send it to you. You can also make a solid state affair with a triac instead of a relay and a op amp/zener/etc circuit to activate the relay - I figured that since I wanted a neutral anyway, I'd use the approach with the single relay. I had built the two relay one first (and it worked also, but the one relay one seems to work a little better). The second relay, if you use it, should pull in around 70 to 80 V and drop out below 30 to 50 V

- you can use a DC relay with a diode and cap, or an AC relay.

Bill

to contact me, do not reply to this message, instead correct this address and use it

will iam_ b_ No ble at msn daught com

It's been a few years since I've posted, but I have been lurking for most of it. I am resuscitating a B&S 2A universal mill to which I will mount a Bridgeport head. What I would like to do is use the 5HP mill motor as the idler in a RPC for the Bridgeport head, while still using it to power the table feeds and pump, and horizontal arbor when required. Yes, I could use a separate idler motor, but don't see the point as I will never need anywhere near 5hp on a mill. It has 32 spindle speeds and 32 feed speeds so I really don't need a VFD. (The bridgeport head is a VS off some kind of hydraulic driven mill)

I managed to start the mill motor using Hanrahan's self starting model, but I needed about 12x 50 uf run capacitors to do it. According to Hanrahan's article, it should only take 120-150 uf to start. I had to remove the chain drive to get the motor to come up to speed. With the chain on I could only get it up around 200 rpm (ran out of capacitors). The mills electrical system contains all the essentials for Hanrahans RPC. I just added the capacitors on the output of the contactor between line 1 and the non-fed line 3.

Hanrahans RPC with the separate starting circuit using start caps calls for around 50-100 uf per idler hp, yet the self starting version calls for

Wayne Sippola Moose Jaw, Canada

----- Original Message ----- From: "Ignoramus20351" Newsgroups: rec.crafts.metalworking Sent: Friday, January 06, 2006 9:21 PM Subject: Re: RPC Question

Very nice looking RPC!

Okay, so perhaps this particular motor I'm using needs a lot of capacitance to start. Seems odd to me that they would vary much. The motor is a GE. probably a late 1940's. Very large for 5hp.

My mistake - the main question I had and I phrased it wrong. The Self Starting version in Hanrahans article uses much less capacitance than the one using the separate start circuit. I don't understand why. I will probably go this route, as that much capacitance is more easily done with start caps. I expect I will need around 800 uf to start with the drive chain on the motor.

I suspect I will go this route. Have to go and buy some start caps. I've been collecting run caps over the years knowing I would eventually build a phase converter, but don't have much for start caps. You said the potential relay is wired from the unenergized leg to neutral. Does 3 phase wiring have a neutral? I was expecting to use the

I don't think this is it. Hanrahans article gives time to start with the self starting method from 1 sec using 150 uf to 2.6 seconds using 120 uf of run caps (5hp idler). So if we go with a 1 sec start time using 150 uf of run caps, why 250-500 uf of start caps to start the same motor. Maybe it just starts faster yet. My problem is I need approx 600 uf of run caps using the self starting method, with no load on the idler. I need to start it with a minimal load so will need something more than this to use the mill motor as the RPC idler. As the self starting method's line 3 voltage goes up with the capacitance, I expect I will have to go with the separate start caps and use my run caps to balance the RPC. Wayne

The code gives you a time to 48 volts or less (48 volts being the limit voltage for low-voltage circuits), which means that each value of capacitance would require a separate calculation and perhaps a separate value.

For practical purposes, a 1.5 kilohm 5 watt wire wound resistor is fine.

Caps which are permanently shunted by the idler or load motor are going to be bled down, anyway, so you really only have to provide bleeders across caps which are switched, which would certainly include the electrolytic starting caps.

Most caps have at least two places per terminal for the connection of

For my own convenience and safety, I usually place a 1.5 kilohm 5 watt wire wound resistor on each capacitor bank: A-B, C-B, A-C (if used for power factor correction), and A-PR-B (PR = Potential Relay, for starting, assuming an intermittent, potential relay starter is employed).

This way, when I am moving around caps during "tuning" each bank will have its own resistor.

An especially careful person might solder a 1.5 kilohm 2 watt carbon composition resistor across each individual capacitor.

If this was done at the base of the "Fast-on", a terminal position wouldn't be lost.

Thanks, I had a surplus contactor that only had one pole left, so using a second start switch, I use this contactor to engage the capacitors. Push both start switches until the mill comes up to speed and let go. Seems to work alright. I have it starting now under the minimal load of the drive chain. Just have to find a box to mount it all nicely in and add on the receptacle for the vertical head. Also need to get the main contactor to stop buzzing. From a thread on RCM a couple of weeks ago, I expect the shade pole thingy is broken. Learn lots of stuff here! Wayne

I checked the B phase voltage with the 600 uf of run caps still in the circuit. About 320 volts! I've redone it to use the oil filled caps in a separate start circuit. (yeah, I could use electrolytic caps, but I didn't have any handy) I am using a separate push button which engages a separate contactor. How much resistance do I need to add to bleed down the caps after they are disconected?? Wayne

The self-starter uses oil-filled caps, which have no time limit, and it does not matter if the motor takes a tenth of a second or ten seconds (or ten hours) to start up.

With electrolytic caps, there is a time limit (and a number of starts limit, too), thereby forcing you to use enough capacitance s the startup occurs in less than two seconds (better yet, less than one second).

The issue is the high equivalent series resistance (which causes high self-heating) in the electrolytics.

The oil-filled caps don't have this property, or this restriction.

My first RPC was a self starting RPC described by Hanrahan. It had some disadvantages.

1. It vibrated needlessly and was too noisy (60 Hz noise). That was, as I later learned, due to imbalance in the motor due to having caps on one leg only. I cannot imagine this vibration is good for the idler.
2. Due to same imbalance, the voltage in leg L2 was excessively high (like 264V, IIRC, for the 245V legs).

I would not recommend to make a RPC this way, honestly, it is just too bad. It is not too expensive to buy another contactor or a solid state relay(s) to change the phase converter from a start configuration to a run configuration.

I thusly redid my phase converter completely, with several objectives in mind.

1. Higher power
2. A neater enclosure
3. Better phase balance
4. (Important for me) as low starting current as I could have

My own approach was to go with what I had, which was two motors -- 10 HP and 7.5 HP, several oil capacitors, and an enclosure from a Ferrups UPS.

The answer was that I had a two stage phase converter. The first stage was the same old 10 HP motor started with same old caps between L1-L3. Essentially the same self starting phase converter I had prior. However, when I pushed another switch button, after the first stage speeds up, a second stage would be activated. In the second stage, there was a second 7.5 HP motor, and caps from L1-L2 (another side for balancing) and Leg1-Leg 2 (for power factor correction).

The advantages is that all goals 1-4 were realized. I did not have to start a single big motor at once (two smaller motors started at different times), everything is neatly balanced by caps, and power factor is corrected.

Correction of PF alone accounted for reduction of idle current from

The nicest thing for me, though, is that the RPC is now very quiet, even at serious loads (welder), and does not vibrate, so that I can keep other tools and stuff on top of it while it is operating.

Now the loudest part of this RPC is two case fans that are there for removing warm air.

(I made some minor changes since I took the photos, maybe I will take more soon)

I highly recommend avoiding unbalanced caps.