Hi,
I'm in the process of restoring an old unisaw (3 HP left titl with low
voltage control). I finally got all the mechanicals cleaned and tuned up
today, re-mounted the motor, and fired it up.
What I'm seeing is that when I press the start button, the motor runs, but
stops as soon as I release the button. I thought maybe it was because there
wasn't any load on the motor, so I put the belts back on and still the same
problem.
Right now, I'm at a loss on how to narrow down the problem. I'm no
electrical engineer, but I can do basic stuff with an Ohm/Volt meter if told
what to do.
Thanks,
Winthrop
My previous response should be clarified. The latching circuit has to be
through a low voltage contact relay or through a third set of contacts if
your relay has three contactors. Otherwise you will need a latching switch
on the low voltage circuit. DO NOT use the motor power (120/240) contacts to
latch the relay.
Frank
"W CHAN" <[email protected]> wrote in message
news:DP%[email protected]...
> Hi,
>
> I'm in the process of restoring an old unisaw (3 HP left titl with low
> voltage control). I finally got all the mechanicals cleaned and tuned up
> today, re-mounted the motor, and fired it up.
>
> What I'm seeing is that when I press the start button, the motor runs, but
> stops as soon as I release the button. I thought maybe it was because
there
> wasn't any load on the motor, so I put the belts back on and still the
same
> problem.
>
> Right now, I'm at a loss on how to narrow down the problem. I'm no
> electrical engineer, but I can do basic stuff with an Ohm/Volt meter if
told
> what to do.
>
> Thanks,
> Winthrop
>
>
So you've a magnetic hold-on switch that isn't holding.
Start with contact cleaning, hope that works. Else, switch.
"W CHAN" <[email protected]> wrote in message
news:DP%[email protected]...
> Hi,
>
> I'm in the process of restoring an old unisaw (3 HP left titl with low
> voltage control). I finally got all the mechanicals cleaned and tuned up
> today, re-mounted the motor, and fired it up.
>
> What I'm seeing is that when I press the start button, the motor runs, but
> stops as soon as I release the button. I thought maybe it was because
there
> wasn't any load on the motor, so I put the belts back on and still the
same
> problem.
>
> Right now, I'm at a loss on how to narrow down the problem. I'm no
> electrical engineer, but I can do basic stuff with an Ohm/Volt meter if
told
> what to do.
>
> Thanks,
> Winthrop
>
>
"W CHAN"
> I'm in the process of restoring an old unisaw (3 HP left titl with low
> voltage control). I finally got all the mechanicals cleaned and tuned up
> today, re-mounted the motor, and fired it up.
> What I'm seeing is that when I press the start button, the motor runs, but
> stops as soon as I release the button. I thought maybe it was because
there
> wasn't any load on the motor, so I put the belts back on and still the
same
> problem.
>
> Right now, I'm at a loss on how to narrow down the problem. I'm no
> electrical engineer, but I can do basic stuff with an Ohm/Volt meter if
told
> what to do.
First things first.
Under absolutely NO circumstances whatever, do you ever install a latching
relay on a table saw for the most obvious of reasons.
You will have lost the most basic safety feature, under voltage protection.
Since you have low voltage control, there is a 3 pole motor starter complete
with an auxiliary normally open low voltage control contact.
Can't diagram the circuit with ASCII so a verbal description will have to
suffice.
This control contact is wired in parallel with the normally open "start"
push button.
This is known as the basic "holding" circuit.
During normal operation, closing the "start" push button energizes the motor
starter coil causing the 3 power poles to close, energizing the motor.
Also the auxiliary contact closes which establishes a parallel circuit
around the "start" push button, allowing you to release it and still keep
the motor starter energized.
In the event you lose power or you push the "stop" button, the motor starter
is de-energized, will open and the saw will stop.
Thus the term "under voltage protection".
HTH
--
Lew
S/A: Challenge, The Bullet Proof Boat, (Under Construction in the Southland)
Visit: <http://home.earthlink.net/~lewhodgett> for Pictures
Here is a schematic of a basic relay and a latching relay. You need the
latching relay schematic.
http://www.homewiringandmore.com/howdoesitwork/relays/relaybasic.html
Frank
"W CHAN" <[email protected]> wrote in message
news:DP%[email protected]...
> Hi,
>
> I'm in the process of restoring an old unisaw (3 HP left titl with low
> voltage control). I finally got all the mechanicals cleaned and tuned up
> today, re-mounted the motor, and fired it up.
>
> What I'm seeing is that when I press the start button, the motor runs, but
> stops as soon as I release the button. I thought maybe it was because
there
> wasn't any load on the motor, so I put the belts back on and still the
same
> problem.
>
> Right now, I'm at a loss on how to narrow down the problem. I'm no
> electrical engineer, but I can do basic stuff with an Ohm/Volt meter if
told
> what to do.
>
> Thanks,
> Winthrop
>
>
It's a good bet that the saw does not have an under voltage relay. Just a
magnetic contactor, 1 NO start button and a NC stop button.
It should be relatively east to troubleshoot.
BTW, I am presently drawing up a control system that includes motor
controls and a PLC.
WoodChuck
"Lew Hodgett" <[email protected]> wrote in message
news:[email protected]...
>
> "WoodChuck" writes:
> > You were right on in your description of the motor control circuit, also
> > known as 3-wire control, and it works just as you described. However,
you
> > are completely wrong on the "under voltage protection." Undervoltage
> > protection uses a separate relay that senses a drop in the supply
voltage
> of
> > 3% to 5% below the pickup voltage. This protects the motor from "brown
> > out." The standard 3-wire control circuit automatically "unlatches" on
> loss
> > of power and thus prevents an unwanted restart once power is restored.
> >
> > WoodChuck
>
> Perhaps my use of the term "under voltage protection" was too broad in
this
> era of "brown outs".
>
> The standard motor starter does not, as you point out, provide "brown out
> protection" where the voltage reduction causing the "brown out" is
something
> less than 15%-20% of nominal line voltage, the normal drop out voltage.
>
> Have forgotten exactly what NEMA specifies as the drop out voltage for a
> motor starter, but the above is close.
>
> Perhaps a more accurate description would be "brown out protection" which,
> as you point out, requires an additional device in addition to the
standard
> motor starter, and covers the 0%-20% voltage drop area.
>
> In any event, doubt this saw would have "brown out" protection.
>
>
> --
> Lew
>
> S/A: Challenge, The Bullet Proof Boat, (Under Construction in the
Southland)
> Visit: <http://home.earthlink.net/~lewhodgett> for Pictures
>
>
"W CHAN" <[email protected]> wrote in message
news:DP%[email protected]...
> Hi,
>
> I'm in the process of restoring an old unisaw (3 HP left titl with low
> voltage control). I finally got all the mechanicals cleaned and tuned up
> today, re-mounted the motor, and fired it up.
>
> What I'm seeing is that when I press the start button, the motor runs, but
> stops as soon as I release the button. I thought maybe it was because
there
> wasn't any load on the motor, so I put the belts back on and still the
same
> problem.
>
> Right now, I'm at a loss on how to narrow down the problem. I'm no
> electrical engineer, but I can do basic stuff with an Ohm/Volt meter if
told
> what to do.
It sounds like the no-voltage-release (NVR) is operating in the control box
when it shouldn't (is this what you mean by "low voltage control"?). Check
with your voltmeter that power is being fed from the control box to the
motor and disappears immediately you let go of the 'start' switch. If
that's the case, then you most likely need a new relay or possibly a new
'off' switch in the control box. Yes, sounds strange, but the NVR works by
having a relay 'hold itself on' by applying power to its energising coil
through an auxiliary pair of contacts. The 'start' switch momentarily
shorts out the auxiliary contacts to make the relay pull in and when it does
so, the contacts close and remain closed after you release the start button.
If the incoming power drops, the relay releases and won't pull in when power
is restored until you push the start button again, thus preventing the saw
from starting unexpectedly. The 'stop' switch is simply a normally-closed
pair of contacts in the circuit feeding the energising coil of the relay so
that when you push the button you open the circuit to the coil causing the
relay to drop out, removing power to the load. You can check the operation
of the contacts on this switch easily enough with the resistance measurement
function of your test meter - having, of course, first removed all power to
the unit. (Don't just switch off, disconnect the saw from the mains power
supply.)
After that long (!) explanation, the fault may also just possibly be inside
the motor where centrifugal switch contacts fail to release a 'start'
connection, (often involving an external capacitor) and make a 'run'
connection, or indeed, where a motor winding has failed, but leaving a
'start' winding intact. Exploring and repairing this area is, I would
suggest, probably beyond someone who "can do basic stuff".
Remember, mains voltages can be lethal, so unless you are sure you can make
the suggested measurements in complete safety, employ the services of a
professional. Good luck.
"WoodChuck" writes:
> BTW, I am presently drawing up a control system that includes motor
controls and a PLC.
Won't admit how long I been designing, building, and selling discrete
machine tool automation systems, but have made a couple of $'s along the
way.
SFWIW, probably the most complex automation system you will ever find is the
one used to control a line up of progressive stamping presses, usually about
5-6, along with the associated material transfer "gandy dancers" that are
used as an integrated system to stamp out things like automobile hoods.
In addition to the individual PLC control for each press and "gandy dancer",
it also requires a lot of real time communication and information exchange
to make it work.
Good luck on your project.
--
Lew
S/A: Challenge, The Bullet Proof Boat, (Under Construction in the Southland)
Visit: <http://home.earthlink.net/~lewhodgett> for Pictures
"WoodChuck" writes:
> You were right on in your description of the motor control circuit, also
> known as 3-wire control, and it works just as you described. However, you
> are completely wrong on the "under voltage protection." Undervoltage
> protection uses a separate relay that senses a drop in the supply voltage
of
> 3% to 5% below the pickup voltage. This protects the motor from "brown
> out." The standard 3-wire control circuit automatically "unlatches" on
loss
> of power and thus prevents an unwanted restart once power is restored.
>
> WoodChuck
Perhaps my use of the term "under voltage protection" was too broad in this
era of "brown outs".
The standard motor starter does not, as you point out, provide "brown out
protection" where the voltage reduction causing the "brown out" is something
less than 15%-20% of nominal line voltage, the normal drop out voltage.
Have forgotten exactly what NEMA specifies as the drop out voltage for a
motor starter, but the above is close.
Perhaps a more accurate description would be "brown out protection" which,
as you point out, requires an additional device in addition to the standard
motor starter, and covers the 0%-20% voltage drop area.
In any event, doubt this saw would have "brown out" protection.
--
Lew
S/A: Challenge, The Bullet Proof Boat, (Under Construction in the Southland)
Visit: <http://home.earthlink.net/~lewhodgett> for Pictures
Lew,
If you review the diagram frank posted I think you will find that it
is exactly what you are describing without the extra two poles of the
relay (contactor) drawn in. The posted circuit is what I have seen on
most quality power tools. On cheaper tools I have seen the designers
use a button to physically close the contacts and a normally closed
push button to disengage the contactor coil.
Chan,
Are you sure it is low voltage control? Is there a transformer some
where in the mix? What is the voltage across the coil on the
contactor? If it is line voltage (220v) then you are just missing the
normally closed push button that keeps the contactor engaged until the
circuit is broken by opening (pressing) this push button. (labeled "G"
in the above listed link)
On Tue, 12 Aug 2003 20:15:01 GMT, "Lew Hodgett"
<[email protected]> wrote:
>"W CHAN"
>
>> I'm in the process of restoring an old unisaw (3 HP left titl with low
>> voltage control). I finally got all the mechanicals cleaned and tuned up
>> today, re-mounted the motor, and fired it up.
>
>> What I'm seeing is that when I press the start button, the motor runs, but
>> stops as soon as I release the button. I thought maybe it was because
>there
>> wasn't any load on the motor, so I put the belts back on and still the
>same
>> problem.
>>
>> Right now, I'm at a loss on how to narrow down the problem. I'm no
>> electrical engineer, but I can do basic stuff with an Ohm/Volt meter if
>told
>> what to do.
>
>First things first.
>
>Under absolutely NO circumstances whatever, do you ever install a latching
>relay on a table saw for the most obvious of reasons.
>
>You will have lost the most basic safety feature, under voltage protection.
>
>Since you have low voltage control, there is a 3 pole motor starter complete
>with an auxiliary normally open low voltage control contact.
>
>Can't diagram the circuit with ASCII so a verbal description will have to
>suffice.
>
>This control contact is wired in parallel with the normally open "start"
>push button.
>
>This is known as the basic "holding" circuit.
>
>During normal operation, closing the "start" push button energizes the motor
>starter coil causing the 3 power poles to close, energizing the motor.
>
>Also the auxiliary contact closes which establishes a parallel circuit
>around the "start" push button, allowing you to release it and still keep
>the motor starter energized.
>
>In the event you lose power or you push the "stop" button, the motor starter
>is de-energized, will open and the saw will stop.
>
>Thus the term "under voltage protection".
>
>HTH
You were right on in your description of the motor control circuit, also
known as 3-wire control, and it works just as you described. However, you
are completely wrong on the "under voltage protection." Undervoltage
protection uses a separate relay that senses a drop in the supply voltage of
3% to 5% below the pickup voltage. This protects the motor from "brown
out." The standard 3-wire control circuit automatically "unlatches" on loss
of power and thus prevents an unwanted restart once power is restored.
WoodChuck
"Lew Hodgett" <[email protected]> wrote in message
news:[email protected]...
> "W CHAN"
>
> > I'm in the process of restoring an old unisaw (3 HP left titl with low
> > voltage control). I finally got all the mechanicals cleaned and tuned up
> > today, re-mounted the motor, and fired it up.
>
> > What I'm seeing is that when I press the start button, the motor runs,
but
> > stops as soon as I release the button. I thought maybe it was because
> there
> > wasn't any load on the motor, so I put the belts back on and still the
> same
> > problem.
> >
> > Right now, I'm at a loss on how to narrow down the problem. I'm no
> > electrical engineer, but I can do basic stuff with an Ohm/Volt meter if
> told
> > what to do.
>
> First things first.
>
> Under absolutely NO circumstances whatever, do you ever install a latching
> relay on a table saw for the most obvious of reasons.
>
> You will have lost the most basic safety feature, under voltage
protection.
>
> Since you have low voltage control, there is a 3 pole motor starter
complete
> with an auxiliary normally open low voltage control contact.
>
> Can't diagram the circuit with ASCII so a verbal description will have to
> suffice.
>
> This control contact is wired in parallel with the normally open "start"
> push button.
>
> This is known as the basic "holding" circuit.
>
> During normal operation, closing the "start" push button energizes the
motor
> starter coil causing the 3 power poles to close, energizing the motor.
>
> Also the auxiliary contact closes which establishes a parallel circuit
> around the "start" push button, allowing you to release it and still keep
> the motor starter energized.
>
> In the event you lose power or you push the "stop" button, the motor
starter
> is de-energized, will open and the saw will stop.
>
> Thus the term "under voltage protection".
>
> HTH
>
>
> --
> Lew
>
> S/A: Challenge, The Bullet Proof Boat, (Under Construction in the
Southland)
> Visit: <http://home.earthlink.net/~lewhodgett> for Pictures
>
>
"Frank K." writes:
> The latching circuit I posted a link to is not a mechanical latch, but an
> electrical latch. Here is the link again.
> http://www.homewiringandmore.com/howdoesitwork/relays/relaybasic.html
>
> This circuit will stop the saw in case of a power outage. It will not
> restart until you restart it.
>
> Frank
Ah ha, got a look at the circuit.
It is a standard holding circuit meant to release when there is an
undervoltage condition and not automatically restart when power is restored,
just as you describe.
It is definitely the way to correctly wire a table saw as an example.
I have a bone to pick with the use of the word "Latching" as it applies to
electrical control devices and how it was used by the site you referenced.
The example you show is definitely not latching as I have used it.
I don't want to confuse things but sufice to say that a standard latching
relay is a mechanical device with an electrical coil being energized to
close the device and a totally separate electrical coil that must be
energized to open the contacts.
There are VERY FEW legimatite applications for a latching relay.
SFWIW, many large industrial companies prohibit them from being included in
equipment they purchase, without a written exemption for a specific
application, and some will not accept a latching relay under any
circumstances.
BTW, how did this thread come back to life? Thought it died at least a month
ago.
--
Lew
S/A: Challenge, The Bullet Proof Boat, (Under Construction in the Southland)
Visit: <http://home.earthlink.net/~lewhodgett> for Pictures