Potential relay?

Hi,
I'm new here but I thought this would be a good site to learn some more about Phase converters.

I do industrial wiring, controls, and automation for a living. However I never needed to mess with phase converters because 3phase was always available.

Now as a home project I would like to set one up.
I have a few questions to start:
A potential relay, is that the trade name for it? Where can I buy them?

Also is there a rule of thumb for sizing the starting caps and the running caps?

I have seen schematics that show them built without potential relays, however to start them it was set up in such a way a person either had to hold the start button to full speed in order to close the contact to the starting cap, or a single shot relay was implemented to time out when full speed was reached. Obviously both these methods have downsides. Does the potential relay take care of thees issues?
Does anyone have a complete schematic of a system I could aquire such as by email.

Thanks,

" A potential relay, is that the trade name for it? Where can I buy them?"

Grainger.

Grainger's order number 4E655 is really Steveco's part number 90-66, which is what just about everyone uses for their 220-240 volt RPCs.

The stated pickup voltage is 208-239 volts; the stated dropout voltage is 130 volts.

Some folks who have substituted a supposedly equivalent G.E. potential relay have reported problems with their RPCs.

Steveco is now a unit of White-Rodgers, which, in turn, is a division of Emerson Electric.


"Also is there a rule of thumb for sizing the starting caps and the running caps?"

There are several rules of thumb.

I'll state mine: "The 1-2-3-30 rule" ... very easy to remember.

1 microfarad per quarter HP for the power factor correction capacitor (must be a run type); 2 microfarads per quarter HP for the Phase C to Phase B capacitor (must be a run type); 3 microfarads per quarter HP for the Phase A to Phase B capacitor (must be a run type); and 30 microfarads per quarter HP for the start capacitor (may be an electrolytic capacitor).

Remember, the three-phase Phase A and Phase C correspond to the single-phase Line 1 and Line 2, respectively, and the three-phase Phase B is, by convention, the "manufactured" phase.

Also, any running capacitors must be rated at least 1.56 times the RMS line voltage (meaning about 370 volts, minimum, for a 220-240 volt RPC) and any starting capacitors must be rated at least the same as the RMS line voltage (meaning 250 volts, minimum, for a 220-240 volt RPC).


"I have seen schematics that show them built without potential relays, however to start them it was set up in such a way a person either had to hold the start button to full speed in order to close the contact to the starting cap, or a single shot relay was implemented to time out when full speed was reached."

This is a self-starting RPC, and generally requires 30-35 microfarads per HP to self-start.

Unfortunately, the "regulation" of such and RPM is not as good as is a "balanced" RPC.

With a self-starting RPC, there is no need to hold the start button. In fact, there is no start button, per se, just a disconnect.


"Obviously both these methods have downsides. Does the potential relay take care of thees issues?"

In general, yes.


[This message has been edited by peterh5322 (edited 08-27-2003).]

Auto, have you considered a VFD for your application? Many are rated to run from single phase and even threephase models will work from three phase if de-rated.

Thank You,
Does this potential relay have a set of N.C. contacts that are used to drop out the start cap when the required 239 volts are reached?

I'm assuming the relay coil is connected to T1 and T2 so that the contacts open when the required voltage is achieved?

Thanks for your help!

Forrest Addy,
Actually I did not consider a VFD. I'm partly doing this for an experiment and partly doing it because I know of several people who could use one and I know I can have it put to good use.

peterh5322,

"the power factor correction capacitor",

That is a new one for me. where does that one fit in and what is it's purpose?

Is there a schematic or ladder logic available for this ?

"Does this potential relay have a set of N.C. contacts that are used to drop out the start cap when the required 239 volts are reached?"

There is indeed a set of N.C. contacts. These contacts connect the start capacitor from Phase A (Line 1) to Phase B (the "manufactured" phase) during starting, after which time the start capacitor is automatically withdrawn from the circuit.

The potential-sensitive coil of the potential relay is connected from Phase C (Line 2) to Phase B, whereas the start capacitor is connected from Phase A (Line 1) to Phase B.

There is an internal connection within the potential relay's housing.


"I'm assuming the relay coil is connected to T1 and T2 so that the contacts open when the required voltage is achieved?"

T1 and T2 are motor winding designations, not necessarily phase designations.

(T1 -> T2 -> T3 is the same as T2 -> T3 -> T1, but not the same as T1 -> T3 -> T2).

The purpose of the N.C. contact within the potential relay is to maintain a path (via the start capacitor) between a line and its corresponding motor terminal *and* that motor terminal which represents the manufactured phase, until the motor comes up to about 90 percent of synchronous speed.

This path is maintained as a consequence of the "induction" inherent in an RPC of this type, between the line-powered phases, Phase A and Phase C, and the non-line-powered phase, Phase B.

For this reason, an RPC of this specific type should really be called an "Induction Phase Converter", and it is generally referred to by this term in the early electrical engineering literature.

Indeed, there are other kinds of rotary phase converters besides our now familiar Induction Phase Converter, and these RPCs are *not* induction phase converters.


[This message has been edited by peterh5322 (edited 08-27-2003).]

" [ The power factor correction capacitor ] is a new one for me. where does that one fit in and what is it's purpose?"

An RPC generally has a poor power factor, particularly if it is of the "self starting" type, where the P.F. can be as low as 0.2; or if the RPC is sized too big, or is operated too lightly loaded.

The P.F. of an RPC usually increases (improves) as the load is increased. But this increase due to load can be dramatically improved upon by the installation of a capacitor for the specific purpose of power factor correction.

This capacitor is installed directly across the powered terminals of an RPC, namely, directly across Line 1 (Phase A) and Line 2 (Phase C), on the load side of the RPC's contactor, if one is used, or the RPC's disconnect.