Help! unisaw won't stay on

[W CHAN]

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

[George]

So you've a magnetic hold-on switch that isn't holding.

Start with contact cleaning, hope that works. Else, switch.

[Mike Faithfull]

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.

[Frank K.]

[Frank K.]

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

[Lew Hodgett]

"W CHAN"

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

[WoodChuck]

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

protection.

[Lew Hodgett]

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.

[WoodChuck]

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]

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.

[Oughtsix]

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 c>"W CHAN"

[Lew Hodgett]

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.