I have a 230 volt, single phase, 5 hp motor on an air compressor. Not long
ago the capacitor starting switches failed, and I was unable to find new
ones. I did however find a couple of used ones to get going again
temporarly.

What I would like to do is replace the centrifical/mechanical switches
with a timed relay, but I'm not sure how to size the relay. The motor
draws 23 full load amps, but I'm just swiching the capacitor start
circuit. What's minumum rating I should consider when I look for a relay?

"Maxwell" <luv2fly99@cox.net> wrote in message
news:s9Nuh.4275$uK1.857@newsfe23.lga...

I have a 230 volt, single phase, 5 hp motor on an air compressor. Not
long ago the capacitor starting switches failed, and I was unable to find
new ones. I did however find a couple of used ones to get going again
temporarly.

What I would like to do is replace the centrifical/mechanical switches
with a timed relay, but I'm not sure how to size the relay. The motor
draws 23 full load amps, but I'm just swiching the capacitor start
circuit. What's minumum rating I should consider when I look for a
relay?

Use a contactor - maybe 50 Amps? You can parallel up the contacts for more
capacity if you use a smaller model. Relays aren't a good choice here.

One older method was to measure the current drawn by the motor and switch
out the start winding when it dropped enough, however that is tricky to
do. The timer should be OK - about 5 seconds so a 1 - 10 second adjustable
timer should work.

That's a honking big motor for single phase!

It is a big motor indeed, but I never considered it would take that much
to switch off the starting cap. If it's going to be that expensive, I'd
probably be better off buying a new three phase motor. But thanks for the
help Homer.

"Maxwell" <luv2fly99@cox.net> wrote in message
news:FIPuh.54090$oA1.2503@newsfe19.lga...

It is a big motor indeed, but I never considered it would take that much
to switch off the starting cap. If it's going to be that expensive, I'd
probably be better off buying a new three phase motor. But thanks for the
help Homer.

I'm assuming it works off a pressure switch?

A contactor is the sort of thing with little resale value. Look around
junk yards and the like - you'll probably find one for $1. You'll pay more
for the timer!

Well now, it does have a 5 hp contactor that works off a pressure switch
now. On at 150 psi, off at 175 psi. That contactor makes connection of the
two 120v legs that supply power. But it also has a spring loaded switch
that switches out the starting capacitors at about 1400 rpm or so, it's a
17XX rpm motor. The 5 hp contactor and power leads to the motor are
something different.

The relay I want to ad would switch on the starting capacitors immediately
when the power is switched on by the existing 5 hp contactor, and break
that connection about 1.5 seconds later. So although I know the main
contactor has to be rated for switching a 5 hp single phase motor, that
normally draws 23 amps ( nd has a lock rotor rating of much much more), I
don't know how much amperage is being supplied through, and has to be
broken, by the capacitor circuit. Now that I think about it, it's only
about a 14 or 16 gage wire, so it would have to be a lot less aperage than
the power legs though the 5 hp contactor. The contactor closes a 30 amp
circuit wired with #10 copper.

"Maxwell" <luv2fly99@cox.net> wrote in message
news:JwTuh.18131$8U4.9385@newsfe20.lga...

Well now, it does have a 5 hp contactor that works off a pressure switch
now. On at 150 psi, off at 175 psi. That contactor makes connection of the
two 120v legs that supply power. But it also has a spring loaded switch
that switches out the starting capacitors at about 1400 rpm or so, it's a
17XX rpm motor. The 5 hp contactor and power leads to the motor are
something different.

The relay I want to ad would switch on the starting capacitors immediately
when the power is switched on by the existing 5 hp contactor, and break
that connection about 1.5 seconds later. So although I know the main
contactor has to be rated for switching a 5 hp single phase motor, that
normally draws 23 amps ( nd has a lock rotor rating of much much more), I
don't know how much amperage is being supplied through, and has to be
broken, by the capacitor circuit. Now that I think about it, it's only
about a 14 or 16 gage wire, so it would have to be a lot less aperage than
the power legs though the 5 hp contactor. The contactor closes a 30 amp
circuit wired with #10 copper.

For sure you don't want the contacts to weld together however it may be that
the starting current through the cap start circuit is quite reasonable once
the motor gets up to speed.

So any half way reasonable contactor should
do the job OK - inc. a pre owned one!

On Sun, 28 Jan 2007 02:17:42 GMT, "Homer J Simpson"
nobody@nowhere.com> wrote:


"Maxwell" <luv2fly99@cox.net> wrote in message
news:JwTuh.18131$8U4.9385@newsfe20.lga...

Well now, it does have a 5 hp contactor that works off a pressure
switch now. On at 150 psi, off at 175 psi. That contactor makes
connection of the two 120v legs that supply power. But it also has a
spring loaded switch that switches out the starting capacitors at
about 1400 rpm or so, it's a 17XX rpm motor. The 5 hp contactor and
power leads to the motor are something different.

The relay I want to ad would switch on the starting capacitors
immediately when the power is switched on by the existing 5 hp
contactor, and break that connection about 1.5 seconds later. So
although I know the main contactor has to be rated for switching a 5
hp single phase motor, that normally draws 23 amps ( nd has a lock
rotor rating of much much more), I don't know how much amperage is
being supplied through, and has to be broken, by the capacitor
circuit. Now that I think about it, it's only about a 14 or 16 gage
wire, so it would have to be a lot less aperage than the power legs
though the 5 hp contactor. The contactor closes a 30 amp circuit
wired with #10 copper.

On Sun, 28 Jan 2007 02:17:42 GMT, "Homer J Simpson"
nobody@nowhere.com> wrote:


"Maxwell" <luv2fly99@cox.net> wrote in message
news:JwTuh.18131$8U4.9385@newsfe20.lga...

Well now, it does have a 5 hp contactor that works off a pressure switch
now. On at 150 psi, off at 175 psi. That contactor makes connection of
the
two 120v legs that supply power. But it also has a spring loaded switch
that switches out the starting capacitors at about 1400 rpm or so, it's
a
17XX rpm motor. The 5 hp contactor and power leads to the motor are
something different.

The relay I want to ad would switch on the starting capacitors
immediately
when the power is switched on by the existing 5 hp contactor, and break
that connection about 1.5 seconds later. So although I know the main
contactor has to be rated for switching a 5 hp single phase motor, that
normally draws 23 amps ( nd has a lock rotor rating of much much more),
I
don't know how much amperage is being supplied through, and has to be
broken, by the capacitor circuit. Now that I think about it, it's only
about a 14 or 16 gage wire, so it would have to be a lot less aperage
than
the power legs though the 5 hp contactor. The contactor closes a 30 amp
circuit wired with #10 copper.

For sure you don't want the contacts to weld together however it may be
that
the starting current through the cap start circuit is quite reasonable
once
the motor gets up to speed.

---
"Quite reasonable"??? It goes to zero, you fucking moron, once the
motor gets up to a certain speed and the centrifugal switch opens,
disconnecting the caps from the starting winding. So you don't know
anything about motors, either? What a surprise.
---

So any half way reasonable contactor should
do the job OK - inc. a pre owned one!

---
"Half way reasonable"? What the hell is that goddam gobbledygook
supposed to mean?

Plus, the OP doesn't need a contactor, he needs a relay that
connects the capacitors to the starting winding when power is
connected to the motor via the 5HP contactor he's using now. Then
1.5 seconds or so later, he wants the relay to drop out and
disconnect the caps from the starting winding. He's stated that
very clearly, I think, so either you can't read worth a shit or your
reading comprehension is very, very poor. The turn-off delay
function is easily achievable, so the only thing that's up in the
air is how much current the relay contacts have to be able to
handle.

To the OP:

If you can measure the current in the starting winding and post what
you find then any number of us (except Homer) can help you pick a
suitable relay.

I'm not sure how accurate it is, but my meter was indicating about 28 amps
at startup, and 5 amps during run on the capacitor circuit, and about 120
amps at start up, and 23 amps during run on the line voltage.

I couldn't find a size on the wires in the capacitor circuit, but it
appears to be about a #14, it only measures .116" on the outside of the
insulation.

Why??? The capacitor is already connected at start up (or it would not
start up). It is disconnected by the spring loaded switch when it gets
up to speed...

"John Fields" <jfields@austininstruments.com> wrote in message
news:92epr2dco468v2q9fm11fsh9a77j0sm65h@4ax.com...
On Sun, 28 Jan 2007 02:17:42 GMT, "Homer J Simpson"
nobody@nowhere.com> wrote:


"Maxwell" <luv2fly99@cox.net> wrote in message
news:JwTuh.18131$8U4.9385@newsfe20.lga...

Well now, it does have a 5 hp contactor that works off a pressure switch
now. On at 150 psi, off at 175 psi. That contactor makes connection of
the
two 120v legs that supply power. But it also has a spring loaded switch
that switches out the starting capacitors at about 1400 rpm or so, it's
a
17XX rpm motor. The 5 hp contactor and power leads to the motor are
something different.

The relay I want to ad would switch on the starting capacitors
immediately
when the power is switched on by the existing 5 hp contactor, and break
that connection about 1.5 seconds later. So although I know the main
contactor has to be rated for switching a 5 hp single phase motor, that
normally draws 23 amps ( nd has a lock rotor rating of much much more),
I
don't know how much amperage is being supplied through, and has to be
broken, by the capacitor circuit. Now that I think about it, it's only
about a 14 or 16 gage wire, so it would have to be a lot less aperage
than
the power legs though the 5 hp contactor. The contactor closes a 30 amp
circuit wired with #10 copper.

For sure you don't want the contacts to weld together however it may be
that
the starting current through the cap start circuit is quite reasonable
once
the motor gets up to speed.

---
"Quite reasonable"??? It goes to zero, you fucking moron, once the
motor gets up to a certain speed and the centrifugal switch opens,
disconnecting the caps from the starting winding. So you don't know
anything about motors, either? What a surprise.
---

So any half way reasonable contactor should
do the job OK - inc. a pre owned one!

---
"Half way reasonable"? What the hell is that goddam gobbledygook
supposed to mean?

Plus, the OP doesn't need a contactor, he needs a relay that
connects the capacitors to the starting winding when power is
connected to the motor via the 5HP contactor he's using now. Then
1.5 seconds or so later, he wants the relay to drop out and
disconnect the caps from the starting winding. He's stated that
very clearly, I think, so either you can't read worth a shit or your
reading comprehension is very, very poor. The turn-off delay
function is easily achievable, so the only thing that's up in the
air is how much current the relay contacts have to be able to
handle.

To the OP:

If you can measure the current in the starting winding and post what
you find then any number of us (except Homer) can help you pick a
suitable relay.



I'm not sure how accurate it is, but my meter was indicating about 28 amps
at startup, and 5 amps during run on the capacitor circuit, and about 120
amps at start up, and 23 amps during run on the line voltage.

I couldn't find a size on the wires in the capacitor circuit, but it appears
to be about a #14, it only measures .116" on the outside of the insulation.

"me" <me@here.net> wrote in message
news:Xns98C666F6D6D4Fmeherenet@38.119.71.210...

Why??? The capacitor is already connected at start up (or it would not
start up). It is disconnected by the spring loaded switch when it gets
up to speed...

The switch is toast. The OP wants a work around.

"me" <me@here.net> wrote in message
news:Xns98C666F6D6D4Fmeherenet@38.119.71.210...


Why??? The capacitor is already connected at start up (or it would not
start up). It is disconnected by the spring loaded switch when it gets
up to speed...


The switch is toast. The OP wants a work around.


Yeah, but then you have Capactive phase motors that