static phase converter

Nope. A start cap between L1 and L2 or L3 and L2, *and* a relay to switch it out as soon as the RPM gets high enough to be self-sustaining. The start cap can't tolerate continuous operation. :-)

And as soon as it reaches the critical RPM, the third phase is disconnected from *everything*. :-)

How many horsepower is the load motor? That is one of the several reasons for not liking static converters. Each one is made with a different size capacitor which is good only for a fairly narrow range of horsepowers. Look them up and note how many models over the horsepower range.

Better to make a rotary converter for this task. (Which is a static converter for starting the idler motor and then getting out of the way, and tuning caps to balance the phases.)

Good Luck, DoN.

Static phase conversion works with a 3 phase motor because the L1 to L2 capacitor feeding the motor magnetising inductance tries to generate approximately 60 deg phase shift (supply to L1 and L3). This results in an open vee 3 phase system. 60 deg is the vee included angle at L3. See Electric Motors 2nd edition for a fuller explanation.

It only works because the inherent iron airgap in an induction motor results in comparatively low value of magnetising inductance. The magnetising VA is now a significant or large fraction of the full load current (Low Power Factor). This enables the phase shift to be fairly constant over a usable load range.

With suitable choice of capacitor this inductive component also makes it possible for the phantom phase voltage to be close to the correct voltage.

Because standard 3 phase transformers have no inherent air gap the magnetising VA is a small fraction of the full load current. This means that the primary impedance appears predominantly resistive and directly proportional to load.. The best that a capacitor can now do is provide some phase shift that is critically dependent on load and necessarily lower in voltage than the L1-L2 voltage.

It's a bit better but a very long way from the desired balanced

3 phase. Adding also an L2-L3 capacitor can only make things worse - the two capacitors now act as a voltage divider so, for the same phase shift, the output voltage is even lower.

Since only a a limited improvement is possible it's well worth trying PrecisionmachinisT recommendation of L1-L2 without any capacitors.

If this isn't good enough then the simple solution is to use a 3 phase motor as a rotary converter as suggested by Lloyd Sponenburgh and Don Nichols.

Jim

A Kill-A-Watt shows this clearly for a single-phase 120V motor, such as on a washing machine where the load varies substantially during the cycles.

Since the KAW is cheaper and easier to understand than a scope and current probe, do you know a way to balance the currents in a 240V 3 phase rotary converter with them?

jsw

to those who say "this won't work" - I've been running my mill (6hp cutting motor, 2 hp traverse motor) on a simple single capacitor "static converter" for about a decade with no problems at all. Please explain what about this constitutes "not working"

In the original poster's case, there is no motor.

In other words, what you describe above is a "rotary transformer", and what Karl proposes is not.

That you are also happen to be drawing mechanical power from the shaft of your rotary transformer is not relevant to Karl's situation.

SNIP

This repeat of an earlier post might help :-

A converter of this type is basically a capacitor/inductor phase shift system which produces an open vee 3 phase system. This phase shifter is a series resonant circuit and when it is set up to give the 60 deg phase shift it is working a long way below its natural resonantfrequency. 60 deg is of course the correct phase angle between the two legs of an open vee system.

The motor(s) is the inductor in the system and unfortunately the apparent inductance of the motor changes with rotor speed. For any particular rotor speed greater than about

90% of synchronous speed (the lower limit varies a bit with motor type) it is possible to choose a capacitor combination which produces a pretty close approximation to balanced 3 phase at the motor terminals.

For near the full load rated speed of the motor, large run capacitance is needed with most or all of it as a single capacitor feeding the phantom phase from supply live. At light load the speed of the rotor rises and if the capacitor value is chosen to achieve the right phase angle the phantom phase voltage will be excessive. This could be corrected by feeding the capacitor from a lower voltage single phase source but this would mean feeding it from an autotransformer across the supply.

It is much simpler (and of course everybody does this) to use two capacitors arranged as a voltage divider to simultaneously achieve the correct phase angle and phase voltage.

The effective capacitance of the two capacitors connected in series across the supply is the sum of the capacitances because the source impedance of the supply is zero and this effectively parallels the two capacitors.

Because the they also act as a voltage divider, this sum capacitance is effectively fed from a voltage of supply voltage times C1/(C1+C2) where C1 is the top capacitor and C2 is connected phantom phase to neutral.

Because it looks nicely symmetrical there seems to be a tendency to believe that C1 and C2 should be equal and any inequality in their value must result from some strange second order effect. This is NOT true. There is nothing magic about equal C1 and C2. It simply results in a capacitor of value C1+C2 fed from half the supply voltage. At this low effective supply voltage it is only possible to get close to balanced operation at no load or light loads which enable the rotor to operate close to synchronous speed.

As the load increases with consequent slowing of the rotor speed the total capacitance needs to increase with both more in C1 and less in C2. By the time full load is reached the optimum value for C2 is usually zero.

These effects are very noticeable if you're using a single motor on a variable load up to near rated full load power and some compromise necessary. The saving grace is that industrial motors are surprisingly tolerant of reasonable overvoltage when operating at light loads so the trick is to size the capacitors for at or near full load and to accept some overvoltaqe at light loads. This increases the motor losses at light load but the total motor losses still remain well below the losses at rated full load so temperature rise is acceptable.

Summing up - if you need to cope with heavy loads on a static converter throw away the bottom capacitor and be sure to size C1 for operation near full load.

None of this helps with starting torque - this is inherently poor with the static converter arrangement however large the starting capacitor. This is because correct low speed phasing requires the capacitor to be fed fed from a voltage many times the supply voltage.

Jim

An accurate true power meter is a good way of setting up a 3 phase converter because minimum copper loss occurs when the three winding currents are equal. It's a pretty flat optimum but that's not important - if you're within a few watts of optimum who cares!

It doesn't tell you in which direction to change the capacitor value(s) so you have to experiment. It's best to use it is a method for setting up at or near full motor load. Results on light load or no load will be inaccurate and not carry through to heavier loads.

You may find my reply to Bill Noble helpful.

Jim

I saw several Kill-A-Watt on the closeout table at a local radio shack store for about $20.

The catch is that the KAW runs on 120V, not 240V, and thus needs to run off a 120V stepdown transformer and sense the 240V current on a different phase.

jsw

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Jim (and the rest of us) are talking about it not working feeding into a three phase *transformer* -- not a motor as you are doing.

Note that jim was *recommending* a rotary converter. The combination of the static converter and your mill's spindle motor makes a rotary converter for the smaller traverse motor. Do you ever use the traverse motor without the spindle motor running?

At Phase-a-Matic's web site, a downloadable brochure shows the HP ranges for different models.

Regular duty models

PAM-300 is for 1 to 3 HP PAM-600 is for 3 to 5 HP PAM-900 is for 4 to 8 HP

Heavy Duty Models

PAM-300HD is for 1 to 3 HP PAM-600HD is for 3 to 5 HP PAM-900HD is for 4 to 8 HP

Note that *none* of the models says it is adequate for starting (alone) both your 2HP traverse motor and your 6HP spindle motor.

They even say:

====================================================================== Do not add the HP of the power feed, coolant pump, etc. These rely on the generator effect of the main motor.

The only time you would add the HP of two or more motors together would be if they always started at exactly the same time. ======================================================================

It is interesting that the Phase-A-Matic web page actually includes a suggetsion at the bottom for using their static converter as part of a rotary converter. (Method No. 2)

The primary difference between the Regular and the Heavy Duty series (based on when to use the HD series) is that the Regular uses a plain start capacitor, and the HD uses instead a more expensive run capacitor instead -- to handle the longer connection period under the heavy starting conditions.

Enjoy, DoN.

I had missed that this was for a transformer - sorry. And yes, the traverse motor works fine with the spindle motor stopped, there is only one static converter and it seems to power both just fine.

Lots of 240V power meters easily available on UK Ebay.

Jim

I do have a digital storage oscilloscope and current probes so I don't need to hack my KAW. I was just wondering if anyone had seen it done. Clamp-on meters can give you the total steady-state current per leg, perhaps that's enough if you don't care about inrush or reactive current.

Lady Ada's KAW hack:

jsw

A clamp on current meter can give a reasonable indication but it is not as good as a true power meter because the reading also includes the irrelevant VA current. The results are biased towards minimum VA which is not necessarily the same as minimum true power.

A clamp on current meter has no particular advantage with this method as there is no requirement to swap the meter between phases.

All that is required is a single 240V true power meter to monitor the single phase 240V power input to the motor/capacitor network.

Jim

My target audience here is trying to select appropriate caps for a rotary converter without owning or knowing how to use expensive specialized measuring equipment. The particular advantages of clamp-on ammeters are low price and safety, they need no physical connection to live power.

jsw

You need to know the complete load profile, including the power factor of the leg using the capacitor to calculate the size needed. I am not familiar with that equipment but some experimentation, using phase angle meters may be in order to get it right.

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I should be careful what i ask for. I can see from these pages that caps should only go from L3 to L2. If I understood more, I could size the caps. Let's assume 20 amp load, how much caps to balance the L2 as close as possible?

Karl

A simple AC voltmeter would probably do the same thing.

When L1-2 = L2-3 = L3-1 are the same voltage you have 60 or 120 degree displacements depending how you want to look at it.

The currents may be different in the unit unless the 3 phase motor is totally isolated and run on it's own.

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My target audience here is trying to select appropriate caps for a rotary converter without owning or knowing how to use expensive specialized measuring equipment. The particular advantages of clamp-on ammeters are low price and safety, they need no physical connection to live power.

jsw

A welder can be a completely different beast as the phase supplies do not need to be balances when they all contribute their own amounts of current to the single output. This is a quite different than a 3 ph motor that requires a little more balanced supply.

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I will ask him. I sold him the welder, but gifted the caps.

i

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Hmm ... what is the rating of the static converter? They suggest that it will sort of work a little way outside the rating range.

Enjoy, DoN.