radial arm saw failing to start any sparks in here?

[8ace]

I have a dewalt radial arm saw that has a switch with a relay type thing built in to it. It then passes through a condenser ( I think this is also know as a start cap) the saw has been reluctant to spin up for a while you have to spin the blade to get it going then switch it on ( I have since read this is start cap failing. Now it won't start at all .

I'll check over weekend for cable continuity fuse already checked. Can a start cap or condenser completely fail to pass a current preventing starting or could it be the switch. It worked before winter set it but has been covered over in the workshop overwinter. Both parts are about £20 and £30 each so I don't really want to buy one to find out it is the other

Any help many thanks

 

[user 6228]

I have a dewalt radial arm saw that has a switch with a relay type thing built in to it. It then passes through a condenser ( I think this is also know as a start cap) the saw has been reluctant to spin up for a while you have to spin the blade to get it going then switch it on ( I have since read this is start cap failing. Now it won't start at all .

I'll check over weekend for cable continuity fuse already checked. Can a start cap or condenser completely fail to pass a current preventing starting or could it be the switch. It worked before winter set it but has been covered over in the workshop overwinter. Both parts are about £20 and £30 each so I don't really want to buy one to find out it is the other

Any help many thanks

Start cap that's what's gone. you can get them cheaper from Maplin or cpc farnell or RS

 

[8ace]

There's a safety feature on the saw that the switch won't lock on unless there is power to stop it getting left on when power at mains is off so it doesn't start on mains power on and the switch is not engaging like theres no power.

I'll take it apart again tonight and get the details of the cap and get one ordered then thanks

 

[oliver90owner]

Start capacitor will prevent it starting, run capacitor will keep it running properly after starting. You give no details, apart from the make, so not too helpful.

FYI these motors are of different types. Some are wound with the armature taking current to produce a turning force, others simply magnetise coils to induce the rotor to turn.

This latter type are called induction motors and there are two common types used for relatively small motors - ones which start easily and those which need to start under some load. The ones which start under some load often have two windings - high power (and short lived if run continuously!) and the normal runnning windings. Both windings start the motor and when up to a predetermined speed, where the normal running windings need no further assistance, the high power windings are disconnected, often by a 'centrifugal' switch.

Now, while your problem may be the start capacitor, you may have exacerbated the problem by using it in the way you have. The initial problem could also have been the switch which energises the start windings. It could be the start windings. It may now be the running side as well.

I would advise you check the windings, for burning, before spending money if cost is important. That is in addition to checking the capacitor(s).

 

[Gilberdyke John]

I have a dewalt radial arm saw that has a switch with a relay type thing built in to it. It then passes through a condenser ( I think this is also know as a start cap) the saw has been reluctant to spin up for a while you have to spin the blade to get it going then switch it on ( I have since read this is start cap failing. Now it won't start at all .

I'll check over weekend for cable continuity fuse already checked. Can a start cap or condenser completely fail to pass a current preventing starting or could it be the switch. It worked before winter set it but has been covered over in the workshop overwinter. Both parts are about £20 and £30 each so I don't really want to buy one to find out it is the other

Any help many thanks

A switch with a relay type thing - do you mean a motor starter with overload protection incorporated? Has it tripped?
Is the saw a hobby sized unit or a serious bit of kit?
Single phase 230 volt or three phase 400 volts? Your mention of a capacitor strongly suggests it's single phase but please confirm.
Rab has given you some good info in his reply and I think the capacitor in a small single phase motor is the most likely culprit. It will have some data printed on its casing, the most important being rated voltage, capacitance value and duty. Almost any electronic suppliers or motor rewind shop will be able to supply, probably far cheaper than the price you quoted.
If you have a fluorescent light fitting with a capacitor of similar value, it's possible to borrow it as a test replacement to check that a working capacitor starts the motor. Don't use this as a bodge solution!
It's up to you to take all required precautions to protect your self from electrical hazards during testing.
Getting the little men up your arm is unpleasant and could be fatal.

 

[8ace]

Its a single phase dw1751 with only 1 capacitor that I can see. I know the switch was replaced before I had it (old work saw) I'll remove motor cover and check windings it is single phase 240 always has been for the 20 years ive known it but the cap says 450v dbhsfqv 400v dbhsffpv 20uf 450v 50h2

 

[Gilberdyke John]

Its a single phase dw1751 with only 1 capacitor that I can see. I know the switch was replaced before I had it (old work saw) I'll remove motor cover and check windings it is single phase 240 always has been for the 20 years ive known it but the cap says 450v dbhsfqv 400v dbhsffpv 20uf 450v 50h2

While you are (safely) checking in the motor terminals make sure live and neutral reach the motor when the switches are made.

 

[oliver90owner]

The capacitors will need to be rated above the voltage it is exposed to. You may think that 230V is the voltage of the supply, but it is not. 230V could be as high as 254V and that is the root mean square average ( DC equivalent ) so the peak voltage of the sine wave AC waveform (which is zero volts 100 times each second!) may be as high as 360Volts - and that is if the supply voltage never transiently rises above that 254V AC supply. So 450Volts rating for the capacitor is not excessive as they will almost certainly fail if exposed to higher voltage than the maximum voltage spec.

Your electric motor is a design which, dependent on how the windings are configured, could either be used on a single or three phase supply.

If you have a 'no volts' trip circuit with thermal cut out, that is just another area of possible failure. It will have resistive coils in series with the motor windings which will hold on the mains supply by magnetising switch contacts against springs which, if the stronger force, will disconnect the supply (and the magnetic field which held the supply in the 'on' position). The heat dissipated by the motor, particularly at start up conditions, may trip the thermal overloads in the starter as added protection for the motor windings. Good, these safety features for avoidance of motor burn-out! They are there for good reasons but are set up, assuming normal operation.

 

[8ace]

Bought this cap http://m.ebay.pl/itm/261596699974?nav=SEARCH didn't start so i ordered the exact switch that is in there and bingo! Starts better than I ever remember. It is the 20uf cap I bought is that one suitable long term? The cap is slightly shorter than before. I think when they changed the switch and housing from the older non safety one, old orange type they used the old starter because it has never started up this quick before so could have been on its way out for a while maybe?

 

[oliver90owner]

Dimensions are irrelevant as long as it fits. Capacitance value is often rough and ready as tolerance could be +-10% or +-20%. Voltage tolerance is an important parameter and must be higher than the supply.

Thermal overloads are usually set by the fitting leccy such that the motor is tripped out before the windings are overheated by overloading.

 

[8ace]

Excellent thanks everyone for your help. I have another table saw but this is my go to saw and is so much easier to set up for hive construction, got another 3 on the go now for future expansion☺