quick and easy upgrade for single speed scroll saws

[Walney Col]

I've just bought one of these PWM controllers and wired it up to my trimmer router and a couple of motors (one from a grinder and one from a bandsaw) and it suddenly struck me that there's no obvious reason why you couldn't use one to add variable speed to an el-cheapo single speed scroll saw.

Only £6.99 on ebay and simplicity itself to wire up (two wires in two wires out) and seemingly capable of handling 2KW.

http://www.ebay.co.uk/itm/201094837...l?_sacat=0&_from=R40&_nkw=201094837973&_rdc=1

I have no involvement whatsoever with that seller other than having just bought one for my own use and I'm sure PWM controllers are available elsewhere if you care to look.

If anyone wants a demo give me a shout and I'll show you how I did it.

Col.

 

[powertools]

With having zero knowledge of electrics I would be very interested in knowing more about this.

 

[CHJ]

PWM controllers are Ok for reducing the power to incandescent lamps or universal motors, they are basically what is in your pistol grip trigger speed controller on your drill or jigsaw.

They work by switching off part of the power curve and reducing the energy available to the device and subsequently in the case of universal motors limit the power available to carry out useful work, although the more sophisticated devices have feedback sensing circuits to try and compensate for this loss of torque.

If you use them to control an AC synchronous motor the only way they effect a speed reduction is to supply insufficient power to maintain magnetic lock between the rotor and stator so that due to the increased magnetic slip is slows down.
This is very inefficient and power output of the motor takes a nosedive, in some instances due to the lack of back EMF, (spinning motor acting as a generator*) to counteract the input current the current consumption can rise dramatically and the motor overheat with little effective power output and reduced natural cooling because its integral cooling fan is moving a lower volume of air.

*This is why a synchronous motor has a high starting current, there is nothing to stop it presenting something approaching a short circuit to the power source until it starts self generating.



Those with electrical/electronic expertise will probabley cring at the above simplistic comments but I think a simplified descrition is all that is needed for someone without electrical motor knowledge.

 

[powertools]

Thanks Chas for the time spent in posting your reply.
I still don't understand these things and have no idea what type of motor is fitted on both my saws so I think I will just carry on as I have been with the single speed.

 

[CHJ]

Thanks Chas for the time spent in posting your reply.
I still don't understand these things and have no idea what type of motor is fitted on both my saws so I think I will just carry on as I have been with the single speed.

If the motor has Brushes it will be a "Universal" motor, it does not care if it is fed with mains Alternating Current power or Direct current and will slow down with little detriment to the motor but will have less torque (power) at slower speeds, may get over warm if design relies on built in cooling fan.