Phase converter strange vibration
 

"Ignoramus19393" wrote in message
.. .
I built another self starting phase converter kit, with a 7.5 HP
idler.

That's because I have a 5 HP 3 phase compressor that I want to sell,
and I want to offer a phase converter with it so that it appeals to
garage mechanic types.

Almost everything is great. The compressor runs beautifully with the
7.5 HP idler. Voltages between legs are 240, 250, and 256 volts, which
I consider well balanced.

Here's the problem. The idler motor vibrates without load. As soon as
I turn on the 5 HP load, vibration stops. I am not sure why that is.

If I remove incoming 240V by turning the contactor off, vibration
stops instantly as well as the rotors spins down.

Any thoughts as to why that could be happening?

i

I

I'd be curious to know if the vibration is there when the RUN capacitor is
eliminated.
If you insist that balancing the RPC output voltages is important, ignor
this capacitor removal suggestion.

Jerry

"Ignoramus19393" wrote in message
...
On Fri, 19 Aug 2005 02:19:30 GMT, Jerry Martes
wrote:

"Ignoramus19393" wrote in message
.. .
I built another self starting phase converter kit, with a 7.5 HP
idler.

That's because I have a 5 HP 3 phase compressor that I want to sell,
and I want to offer a phase converter with it so that it appeals to
garage mechanic types.

Almost everything is great. The compressor runs beautifully with the
7.5 HP idler. Voltages between legs are 240, 250, and 256 volts, which
I consider well balanced.

Here's the problem. The idler motor vibrates without load. As soon as
I turn on the 5 HP load, vibration stops. I am not sure why that is.

If I remove incoming 240V by turning the contactor off, vibration
stops instantly as well as the rotors spins down.

Any thoughts as to why that could be happening?

i

I

I'd be curious to know if the vibration is there when the RUN capacitor
is
eliminated.
If you insist that balancing the RPC output voltages is important,
ignor
this capacitor removal suggestion.

Jerry, I have only one run capacitor between legs 1-3. That's what
allows the RPC to start. There is, possibly, too much capacitance. I
will experiment...

i

I

My point is -- There is very little to ever be gained from leaving the
start caoacitor IN the circuit, especially if the ultimate use of the RPC is
to power a compressor with an unloader.
Bump the capacitor into the circuit only long enough to get the idler
spinning a little then disconnect it. It doesnt add to the power
delivering capacity of the RPC.
Correct me if I'm wrong or if I misunderstand.

Jerry

On Fri, 19 Aug 2005 03:34:34 GMT, Ignoramus19393
wrote:

Jerry, you were 100% right. After I disconnected starting caps, the
loud buzzing noise was replaced with pleasant humming.

I will use a start pushbutton and your solid state relays to switch
caps off. (caps will be on only while the pushbutton is engaged).

i

Damn he's good.

Well done Jerry!

Gunner

"Pax Americana is a philosophy. Hardly an empire.
Making sure other people play nice and dont kill each other (and us)
off in job lots is hardly empire building, particularly when you give
them self determination under "play nice" rules.

Think of it as having your older brother knock the **** out of you
for torturing the cat." Gunner

"Ignoramus19393" wrote in message
.. .
I built another self starting phase converter kit, with a 7.5 HP
idler.

That's because I have a 5 HP 3 phase compressor that I want to sell,
and I want to offer a phase converter with it so that it appeals to
garage mechanic types.

Almost everything is great. The compressor runs beautifully with the
7.5 HP idler. Voltages between legs are 240, 250, and 256 volts, which
I consider well balanced.

Here's the problem. The idler motor vibrates without load. As soon as
I turn on the 5 HP load, vibration stops. I am not sure why that is.

If I remove incoming 240V by turning the contactor off, vibration
stops instantly as well as the rotors spins down.

Any thoughts as to why that could be happening?

i

I would suggest the 7.5 HP idler motor was originally designed for very high
starting torque; maybe for an air compressor. NEMA designates the design of
squirrel-cage induction motors in several classes according to their
electrical characteristics. The winding configuration of the rotor
(generally) is varied from class to class in order to modify starting
torque, running current, slip, efficiency and etc. In all liklihood, the
design of your motor was for high starting torque - such as class "C".
Leaving the start cap permanently in place (self starting RPC) could cause
excessive noise in a motor designed for high starting torque.

Bob Swinney

Since you already have the capacitors, I would suggest you measure the
voltages that you get now with no starting caps. And then see what
reconnecting some of the caps you disconnected and remeasuring the
voltages.

Yep it isn't necessary. But I think you will find that some is good, a
lot is not. My experience is that with a little playing around, a
starting push button and a relay are not necessary. Your mileage may
vary.

Dan


Ignoramus19393 wrote:
Jerry, you were 100% right. After I disconnected starting caps, the
loud buzzing noise was replaced with pleasant humming.

I will use a start pushbutton and your solid state relays to switch
caps off. (caps will be on only while the pushbutton is engaged).

i

If you multiply the capacitance by the resistance, you get what is
known as the RC time constant. The capacitance is normally in ufd or
farads times 10^-6. So if you multiply by resistance in Megohms ( ohm
times 10^6 ) you get the time constant is seconds. In one time
constant, the voltage drops by 63%. So the rule of thumb is to wait
about three times constants so the voltage is only 5% of what it was.

For 100 mfd, try 10,000 ohms. (100 * 10^ -6 ) times ( 10,000 /
1,000,000 ) or
( 100 * 10^6 ) times (1/100 * 10^6 ) or one second. So waiting three
seconds for that size discharge resistor would be prudent.

With a 10 K resistor and 240 volts, the current thru the resistor would
be 24 milliamps. And the power in the resistor would be 5.7 watts.

So maybe a two watt 100 K resistor would be a better choice. It would
mean you ought to wait at least 30 seconds before assuming the
capacitors are safe.

Dan


Ignoramus677 wrote:


It was interesting to see how disconnecting the cap produced sparks.

What was even more interesting is that, after unplugging, when I
shorted my 3 parallel big behemoth vodka bottle sized capacitore, to
remove the charge, there was a huge spark and a sound like a .22 shot.

I think that I will add a discharge resistor, too. Any idea how many
ohms for every 100 mF is best?

If we were dealing with DC t=rc etc....
This is AC you might have any voltage when disconnected.
It discharges and charges in real time - 1/60hz is 16ms. So every 8ms it is grounded.
Every 8ms it is at a peak. Design for peak but live with some varying values.
Still do what you are doing, but remember it isn't DC.

Martin

Ignoramus677 wrote:

On 19 Aug 2005 09:09:45 -0700, wrote:

If you multiply the capacitance by the resistance, you get what is
known as the RC time constant. The capacitance is normally in ufd or
farads times 10^-6. So if you multiply by resistance in Megohms ( ohm
times 10^6 ) you get the time constant is seconds. In one time
constant, the voltage drops by 63%.


approximately to e^-1 of the original charge, right?


So the rule of thumb is to wait about three times constants so the
voltage is only 5% of what it was.

For 100 mfd, try 10,000 ohms. (100 * 10^ -6 ) times ( 10,000 /
1,000,000 ) or
( 100 * 10^6 ) times (1/100 * 10^6 ) or one second. So waiting three
seconds for that size discharge resistor would be prudent.

With a 10 K resistor and 240 volts, the current thru the resistor would
be 24 milliamps. And the power in the resistor would be 5.7 watts.



So maybe a two watt 100 K resistor would be a better choice. It would
mean you ought to wait at least 30 seconds before assuming the
capacitors are safe.


I like that better, actually. I can wait 30 seconds.

Thanks Dan.

i


Dan


Ignoramus677 wrote:


It was interesting to see how disconnecting the cap produced sparks.

What was even more interesting is that, after unplugging, when I
shorted my 3 parallel big behemoth vodka bottle sized capacitore, to
remove the charge, there was a huge spark and a sound like a .22 shot.

I think that I will add a discharge resistor, too. Any idea how many
ohms for every 100 mF is best?






--
Martin Eastburn
@ home at Lions' Lair with our computer lionslair at consolidated dot net
NRA LOH, NRA Life
NRA Second Amendment Task Force Charter Founder

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Exactly,
Dan

Ignoramus677 wrote:

approximately to e^-1 of the original charge, right?