Re: RPC run capacitors


"Ignoramus19023" <ignoramus19023@xxxxxxxxxxxxxxxxxxxx> wrote in message
news:vLTIe.36486$iE2.28874@xxxxxxxxxxxxxxxxxxxxxxxx
> On Fri, 05 Aug 2005 22:20:26 +0100, pentagrid@xxxxxxxxx
> <pentagrid@xxxxxxxxx> wrote:
>> On Fri, 05 Aug 2005 17:29:48 GMT, Ignoramus19023
>><ignoramus19023@xxxxxxxxxxxxxxxxxxxx> wrote:
>>
>>>On Fri, 05 Aug 2005 17:16:59 GMT, Martin Whybrow
>>><aholeintheground@xxxxxxxxxxxx> wrote:
>>>>
>>>> "Ignoramus19023" <ignoramus19023@xxxxxxxxxxxxxxxxxxxx> wrote in message
>>>> news:_DMIe.36305$ZZ5.19736@xxxxxxxxxxxxxxxxxxxxxxxx
>>>>> I am considering adding some run capacitors to my self starting RPC. I
>>>>> am reading Jim Hanrahan's article at
>>>>>
>>>>> http://www.metalwebnews.com/howto/ph-conv/ph-conv.html
>>>>>
>>>>> and I am confused by something. I understand how self starting
>>>>> RPCwould start with one cap between one leg 1 and leg 3 (the generated
>>>>> one). That's how mine is wired. Jim makes a point that it works, but
>>>>> makes unbalanced voltage.
>>>>>
>>>>> But why would it start is capacitors are connected between 1-3 AND
>>>>> 2-3, like in this picture:
>>>>>
>>>>> http://www.metalwebnews.com/howto/ph-conv/fig1.html
>>>>>
>>>>> I cannot see how it would create assymmetric fields needed to spin up
>>>>> the motor. Is that because capacitances across legs 1-3 are greater
>>>>> than capacitance between leg 2-3?
>>>>>
>>>>> I could try to use run caps at run time and start caps at start
>>>>> time. In fact, I won a time delay relay for $9 on ebay yesterday, so I
>>>>> could set the RPC to start on start cap (both caps between leg 1-3)
>>>>> and then reconnect the same caps to become run caps, one between 1-3
>>>>> and another between 2-3. Same TDR could, then, turn out output current
>>>>> aftet time delay, allowing the RPC to spin up and switch to the run
>>>>> mode.
>>>>>
>>>>> As you can see, I am quite confused, but am willing to experiment. I
>>>>> have 4 unused Furnas 75 A contactors that I can wire, with the time
>>>>> delay relay, to do just about anything.
>>>>>
>>>>> Idler: 10 HP
>>>>>
>>>>> Capacitors: 92 mF each, 535 VAC rated, oil filled. I have 5 total, and
>>>>> use 2 for the starting leg, so three are unused.
>>>>>
>>>>> i
>>>>>
>>>> You are correct saying that it's because the capacitance between 1 + 3
>>>> is
>>>> higher than between 2 + 3.
>>>> My RPC, 10HP 440V 50Hz motor, has 40uF between 2 + 3, 60uF between 1 +
>>>> 3 and
>>>> 500uF switched by a start circuit between 1 + 3. The 500uF is a bit too
>>>> much, it starts very quickly (around 1/3 second) so I could probably
>>>> drop
>>>> that to around 200uF.
>>>
>>>Thank you Martin.
>>>
>>>I have five 92 mF caps total.
>>>
>>>What I have now is 2 capacitors connected between leg 1 and 3.
>>>
>>>It seems that about the only thing that I need, then, wire it as follows:
>>>
>>>1. 1 cap (92 mF) between legs 1 and 2, permanently connected.
>>>
>>>2. 1 cap (92 mF) between 1 and 3, permanently connected.
>>>
>>>3. 2-3 more caps (180-270 mF) between legs 1 and 3, connected
>>>at start time and switched off by the time delay relay (TDR),
>>>say 4 seconds after startup.
>>>
>>>4. (optional) add another contactor on output side that would only
>>>switch output on when the TDR actuates and switches off the start
>>>caps.
>>>
>>>Theat would get me an RPC that is:
>>>
>>>- properly balanced across the range of output HP
>>>- starts quickly
>>>- only turns on when balanced power is output and full rotational
>>>speed is achieved, that is, when good quality 3 phase power is available.
>>>
>>>Does it make sense?
>>>
>>>i
>>
>> With five equal value oil filled capacitors your options are fairly
>> limited but you can still finish up wth a perfectly good converter.
>>
>> One thing to remember is that capacitors of this type have extremely
>> low internal series resistance (milliohms)- that's why you get a
>> sizable bang if you short circuit a charged one!
>> The same thing happens when you connect a charged capacitor to an
>> uncharged one - VERY high peak currents flow as the charge voltage
>> equalises on the two capacitors.
>
> An excellent point.
>
>> The capacitors are not greatly bothered by this treatment but it's
>> very unkind to the switch contacts or relay contacts used to parallel
>> connect two capacitors if there is a substantial voltage difference at
>> the instant of connection.
>
> Yep, makes full sense.
>
> Now, based on common sense, unlike CONNECTING capacitors,
> DISCONNECTING them should not produce any big sparks.
>
> I have a plan for a start circuit with a separate contactor. The
> sequence of events is as follows.
>
> 1. I turh the ON switch.
> 2. The contactor that connects starting caps engages.
> 3. When it engages, it actuates the SECOND contactor that supplies
> primary 240V voltage to the electric motor.
> 4. Everything starts spinning.
> 5. After a few seconds, a time delay relay that I won on ebay today
> (NO and NC), will open a pair of contacts and DISCONNECT the starting
> caps.
>
> I think that I know how to wire this properly so that all of this
> happens, and yet everything shuts down when I turn the on/off switch
> to off. I have a mental picture. Basically both contactors would have
> a neutral wired directly to neutral, but the 110V line for signal
> would go through the switch, then to 3rd poles of second contactor,
> and to the 3rd pole of the first contactor but through a time delay
> relay. then from the switched side of both contactors to the hot
> switch terminal of the second contactor.
>
> 6. When the NO contacts on the time delay relay close, it would
> actuate the third contactor that would allow power to be output.
>
> Sorry if I am speaking in strange language, I am a computer
> programmer. It should work, logically.
>
> read on...
>
>> Brute force oversizing of the switching contacts can give reasonable
>> contact life but it's much neater to avoid the problem by using
>> separate start and run capacitors that are never parallel connected.
>> This is easily done with your capacitor collection.
>>
>> Use three parallel connected as your start capacitor. Only in circuit
>> for the few seconds needed for the idler to run up to speed.
>
> Right.
>
>> You now have a bit unbalanced but perfectly usable converter system.
>
> right. it's much nicer to have something working and add stuff to it,
> than build a "dream system" without testing to only realize that
> something went amiss. That's how I do my programming too.
>
>> Ideally the load motor should not be switched in until the idler is up
>> to speed. However, if the load motor is initially running light, this
>> start capacitance is probably enough for a simultaneous idler and load
>> start.
>>
>> Your START switching should be a changeover contact which EITHER
>> connects the start capacitor OR the run capacitor across L1 and L2.
>>
>> For the run capacitor, try one capacitor or two capacitors series
>> connected to halve the effective value - whichever gives the best
>> voltage balance with full load on the load motor.
>
> Looks like one run cap per side should be good, based on Jim
> Hanrahan's writings. I'll see.
>
>> Do not place a capacitor across L2 L3. With the values you have
>> available this would do more harm than good.
>
> Hm, why is that so?
>
> Anyway, my main question is: if I do all this (as I outlined in steps
> 1-6), will I get a Mercedes Benz of phase converters? Or am I wasting
> my time for load up to and under 5 HP?
>
> i

I

It would be rude for me to agree that you'd be wasting your time by
sophisticating this RPC. I can tell you that nothing you are likely to
*ever* use in your house will be able to operate any better because of
*anything you do beyond just spinning up that 10 HP idler.

Jerry


.

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