Magnetic Starter?
 

I received some very helpful information from the members of this group. i
am extremely thankful for it. If I may ask another question. What is the
advantage of a magnetic starter on a 5hp single phase compressor. A couple
of individuals suggested I scrap the magnetic starter that came with my
compressor and wire it without it.

Thanks in advance for the help.

Joe...

"JB" wrote in message
...
| I received some very helpful information from the members of this group.
i
| am extremely thankful for it. If I may ask another question. What is the
| advantage of a magnetic starter on a 5hp single phase compressor. A
couple
| of individuals suggested I scrap the magnetic starter that came with my
| compressor and wire it without it.
|
| Thanks in advance for the help.
|
| Joe...

If they want to scrap it then are they willing to guarantee the
protection you just threw away?

The overload part of the starter allows the high inrush currents that
happen when the motor first starts up, but will open up when extended high
currents are going to toast something. The contacts are made for switching
this kind of load and will last a very long time and not weld themselves
shut eventually, unlike most switches you could use.
You might look and see if a manual starter might work for you. No
control wiring required.

On Thu, 21 Apr 2005 19:34:13 -0400, "JB"
wrote:

I received some very helpful information from the members of this group. i
am extremely thankful for it. If I may ask another question. What is the
advantage of a magnetic starter on a 5hp single phase compressor. A couple
of individuals suggested I scrap the magnetic starter that came with my
compressor and wire it without it.

Thanks in advance for the help.

Joe...

Joe,
The way a magnetic starter works is this: It is a relay that uses a
low current to close the contacts which handle the large current of
the device being powered. They also, usually, have a feature where
they only need momentary power applied to stay on. Once the relay
contacts close they also supply power to the relay coil along with
powering the device. This is good because if power is lost for some
reason the relay contacts open and the device will not turn itself
back on once power is re-established. Your compressor may be wired so
that the pressure switch supplies power to the magnetic starter. If
so, then the starter probably does not power its own coil so that when
max pressure is reached the pressure switch shuts off power to the
coil and the compressor motor turns off. In any case, if the magnetic
starter is functioning properly why remove it? You can be sure that if
it's original equipment it wasn't put there just for fun.
ERS

I agree with Eric. I am an electrician and know all about motor starters.
They perform a few basic but very useful functions. On a motor of this size
I would highly advise keeping it in the circuit and functioning properly. In
the long run it could easily save you the cost of a new motor.
Larry
"Eric R Snow" wrote in message
...
On Thu, 21 Apr 2005 19:34:13 -0400, "JB"
wrote:

I received some very helpful information from the members of this group.
i
am extremely thankful for it. If I may ask another question. What is the
advantage of a magnetic starter on a 5hp single phase compressor. A
couple
of individuals suggested I scrap the magnetic starter that came with my
compressor and wire it without it.

Thanks in advance for the help.

Joe...

Joe,
The way a magnetic starter works is this: It is a relay that uses a
low current to close the contacts which handle the large current of
the device being powered. They also, usually, have a feature where
they only need momentary power applied to stay on. Once the relay
contacts close they also supply power to the relay coil along with
powering the device. This is good because if power is lost for some
reason the relay contacts open and the device will not turn itself
back on once power is re-established. Your compressor may be wired so
that the pressure switch supplies power to the magnetic starter. If
so, then the starter probably does not power its own coil so that when
max pressure is reached the pressure switch shuts off power to the
coil and the compressor motor turns off. In any case, if the magnetic
starter is functioning properly why remove it? You can be sure that if
it's original equipment it wasn't put there just for fun.
ERS

It may have an "overload" connected to it as well so that if the motor
draws too much current for any reason, it cuts it out before it cooks the
motor windings

Tom

"Larry Naumann" wrote in message
...
I agree with Eric. I am an electrician and know all about motor starters.
They perform a few basic but very useful functions. On a motor of this
size
I would highly advise keeping it in the circuit and functioning properly.
In
the long run it could easily save you the cost of a new motor.
Larry
"Eric R Snow" wrote in message
...
On Thu, 21 Apr 2005 19:34:13 -0400, "JB"
wrote:

I received some very helpful information from the members of this group.
i
am extremely thankful for it. If I may ask another question. What is
the
advantage of a magnetic starter on a 5hp single phase compressor. A
couple
of individuals suggested I scrap the magnetic starter that came with my
compressor and wire it without it.

Thanks in advance for the help.

Joe...

Joe,
The way a magnetic starter works is this: It is a relay that uses a
low current to close the contacts which handle the large current of
the device being powered. They also, usually, have a feature where
they only need momentary power applied to stay on. Once the relay
contacts close they also supply power to the relay coil along with
powering the device. This is good because if power is lost for some
reason the relay contacts open and the device will not turn itself
back on once power is re-established. Your compressor may be wired so
that the pressure switch supplies power to the magnetic starter. If
so, then the starter probably does not power its own coil so that when
max pressure is reached the pressure switch shuts off power to the
coil and the compressor motor turns off. In any case, if the magnetic
starter is functioning properly why remove it? You can be sure that if
it's original equipment it wasn't put there just for fun.
ERS

JB

You really dont want to accept the advice of the "couple of individuals"
who suggested you discard the magnetic starter.
If the magnetic starter has included in it, overloload switches that match
your 5 HP 3 phase motor, you have an excellant switch for your motor. It
surprizes me that you have a manual switch to replace the magnetic starter.

If you choose to keep the magnetic starter and wish to actuate it with
push bottons, I will give you a couple used, high quality Allan Bradley
pushbuttons. I'd even throw in a couple indicator lights.

You can get all the help you need for woring that magnetic starter from
qualified RCM guys.
Ugly's Electrical Referances booklett (Home Depot) has all the information
you'd need for wiring the magnetic starter.

Jerry


"JB" wrote in message
...
I received some very helpful information from the members of this group. i
am extremely thankful for it. If I may ask another question. What is the
advantage of a magnetic starter on a 5hp single phase compressor. A couple
of individuals suggested I scrap the magnetic starter that came with my
compressor and wire it without it.

Thanks in advance for the help.

Joe...

On Thu, 21 Apr 2005 19:34:13 -0400, "JB"
wrote:

I received some very helpful information from the members of this group. i
am extremely thankful for it. If I may ask another question. What is the
advantage of a magnetic starter on a 5hp single phase compressor. A couple
of individuals suggested I scrap the magnetic starter that came with my
compressor and wire it without it.

Keep the mag starter in the circuit. For openers, it has overload
heaters that will sense a problem like a locked rotor far faster than
the circuit breaker at the panel - usually fast enough to save the
motor. This is doubly critical on a 3-phase unit, because the circuit
breaker doesn't care that one phase is dead - the motor sure does!

(If you want double insurance on 3-Phase you can hook a simple $20
electronic phase monitor up after the starter contactor, which will
detect a dropped phase, rotation reversal, or big voltage imbalance
and open the contactor even before the overload heaters do, in well
under a second. Cheap insurance.)

When you get above 1 Horsepower, the switching contacts in some
pressure switch models are rated for and /can/ handle the motor load
directly, but they /shouldn't/. They slowly burn themselves up and
fail open, and occasionally they weld closed and the compressor fails
to shut down. With a 5-horse it happens much faster.

And having a mag starter makes it extraordinarily simple to put an
"Off" switch on the compressor so it doesn't run at night, or when
your shop is not in use. You can use any simple switch (or even a
timer!) because it's a pilot-duty device switching 50 milliamps, not
the full 50 amps.

I have a 4-horse compressor without a starter, and I'm going to ADD
a mag starter to it as soon as I find one for the right price. (Which
is "Free to a good home." I run into them at work occasionally.)
Does that tell you something?

-- Bruce --

--
Bruce L. Bergman, Woodland Hills (Los Angeles) CA - Desktop
Electrician for Westend Electric - CA726700
5737 Kanan Rd. #359, Agoura CA 91301 (818) 889-9545
Spamtrapped address: Remove the python and the invalid, and use a net.

....
(If you want double insurance on 3-Phase you can hook a simple $20
electronic phase monitor up after the starter contactor, which will
detect a dropped phase, rotation reversal, or big voltage imbalance
and open the contactor even before the overload heaters do, in well
under a second. Cheap insurance.)
....

Here's something I hadn't heard of. Are they readily available from a place
like Grainger or McMasterCarr?

Would it be a good idea to wire one of these into the Estop circuit on large
machines? If this device drops, open a NC contact in front of MCR? (Master
Control Relay)

I had a weird one where my generated leg dropped out after my hydraulic pump
started on my huge CNC lathe. (pump comes in on Estop reset) The pump kept
going on two legs (as all three phase motors will) until I tried to bring in
my 20 hp spindle. It popped the main circuit breaker in the machine. No big
deal, but this means the control lost power also.

Karl

Those who suggest starting a 5 HP motor without a mag starter are
working from a vast background of ignorance. The starting currents are
too high for pressure switch contacts. They will quickly burn and pit.
The running current protection is there to protect the motor. Have you
priced a 5 HP single phase motor lately?
'Nuff said.
Bugs

Magnetic starters are a good idea on any heavy equipment, because you
can design any kind of control logic into the system that you want.
Ladder logic is used to work up time delayed starts & stops, machine
sequencing, limit switches, etc, etc. etc.
Bugs

On Fri, 22 Apr 2005 12:02:27 GMT, "Karl Townsend"
remove .NOT to reply wrote:

...
(If you want double insurance on 3-Phase you can hook a simple $20
electronic phase monitor up after the starter contactor, which will
detect a dropped phase, rotation reversal, or big voltage imbalance
and open the contactor even before the overload heaters do, in well
under a second. Cheap insurance.)
...

Here's something I hadn't heard of. Are they readily available from a place
like Grainger or McMasterCarr?

Grainger carries the Square-D starters with Motor Logic (tm)
electronic overload, which has a phase monitor built in. (Cat 395 PP
369) This will catch any possible failure mode I can think of.

ICM makes separate wire in phase monitors.

Would it be a good idea to wire one of these into the Estop circuit on large
machines? If this device drops, open a NC contact in front of MCR? (Master
Control Relay)

I had a weird one where my generated leg dropped out after my hydraulic pump
started on my huge CNC lathe. (pump comes in on Estop reset) The pump kept
going on two legs (as all three phase motors will) until I tried to bring in
my 20 hp spindle. It popped the main circuit breaker in the machine. No big
deal, but this means the control lost power also.

If you're talking a manufactured leg from a phase converter, you are
probably better off with a plain old starter with three heaters. Or a
phase monitor that can be adjusted to rather wide tolerances of "OK".

The voltage on the manufactured leg is probably going to vary enough
to drive an electronic monitor nuts - it's expecting to see nice clean
and rock steady utility-fed 3-phase that doesn't vary more than a few
volts, at worst a motor generator plant...

And yes, you could tap into the control loop of any motor starter
and have it drop out the main control power to the equipment - hell,
you can do about anything with relays. Give me 10,000 relays (and a
few other critical components), and I can build you a complete
telephone exchange switch. One that will take a whole lot of daily
maintenance to keep running, but they worked for 75 years...

-- Bruce --
--
Bruce L. Bergman, Woodland Hills (Los Angeles) CA - Desktop
Electrician for Westend Electric - CA726700
5737 Kanan Rd. #359, Agoura CA 91301 (818) 889-9545
Spamtrapped address: Remove the python and the invalid, and use a net.