How to slow start a router brushed motor, 120 VAC

[AlainB]

Hi,

I want to use a 1/2 HP router with the CNC machine that I am building. It is an old Black & Decker router. My problem is that the torque at start up is too great, ...a hell of a kick if I may say so.

Would there be a way to soften this a little bit, getting full speed maybe in one second or more instead of now wich is almost instantanious.

Many thanks!

Alain

 

[Speakerguy]

Yes, lemme go lookup a cheap easy way. BRB.

ETA: Here is a page on soft-start circuits for audio power amplifiers (to limit inrush current in huge toroids). Maybe you can adapt something? The simplest is a series resistor for startup and then a relay to bypass it for normal operation.

**broken link removed**

Of course, how much would a soft start router cost? My woodworker's router (Dewalt 621) has a very nice ~1 second soft start.

 

[Mickster]

Alain,

in what way is the router start-up torque causing problems?

Are you noticing sideways movement within the Z axis as the router starts?

If so, it's likely that any movement during start-up will also be apparent when taking a cut, in even the softest material, due to a lack of mechanical rigidity. Chatter will probably be observed on the walls of a pocket or profile cut, along with a poor top finish on a surfacing operation.

What material are you using to mount the router to the Z axis, what material is used for the Z axis mounting carriage, what material/diameter is used for the Z axis rails and how are the rails attached to the Y axis?...along with...how is the router mounted - bottom plate only, or bottom plate & body clamp - how much backlash exists within each axis... etc. are all questions that point to the machine's rigidity.

A simple machine constructed from MDF/Ply is barely capable of producing precise results in aluminium, as is an aluminium machine when trying to cut steel. However, we build the simpler machine to make progressively-better parts for a better machine, to tolerances which are hard to reproduce in quantity by hand.

How do I know? I have a 48" X 24" X 6" MDF & Aluminium DIY CNC machine that has a little runout in the Y axis leadscrew, which is currently poised to re-make it's own inaccurate hand made parts to correct the runout, then make parts for the more accurate machine.

Anyway, I'm rambling somewhat...

Aside from machine rigidity, correct feeds and speeds need to be observed for the material being cut and the cutter being used. A two-flute cutter requires a different horizontal feed-rate to a three or four-flute cutter... in a similar way to that which prescribes that a HSS cutter requires a different feedrate to a TCT or STC cutter. Chip-load-per-tooth is the important factor.

..Back to your original quandry.

Start-up torque shouldn't really be a problem in a DIY machine...you normally start the spindle, run the G-code, then stop the spindle at the end of the cutting operation. Unless you are using a spindle controller card to process multiple M3/M4 & M5 commands, stopping & starting between Z moves.

 

[AlainB]

Thanks for the link speakerguy. I get the picture. I will try using bulb lamps as ballast.

Alain

 

[Sceadwian]

Lamps will waste a bit of power but if that doesn't bother you it should work nicely. Check a local industrial supply store and see what they have available for motor 'run' capacitors. We use them on a couple very simple drive setups on barrels at work that prevent the large loads on them from blowing the motor when it's starting. A quick conversation with someone should find you a capacitor rated for needs.

 

[eblc1388]

With rating less than 400W it will probably be a series wound universal motor.

How many wires does the router motor have?

 

[AlainB]

It is an ordinary north american router: 120 vac, 5 amp. with brushes. 2 wires plus ground. 3/4 hp, not 1/2 hp as I have stated before and turn at 20 000 rpm.

I went to the flea market yesterday and got this condenser. I beleive it is a motor run capacitor. 24 mfd, 400 VAC with a bleed resistor. It is oil filled.

Could I use it for my purpose and if so, how to wire it?

EDITED:

I tried it in serie and it is not working. Just a small humm from the motor. I did not wired in parallel. I suppose that the bleeding resistor would blow up immediately, if not worst.

 

[Hero999]

It's a universial motor so no start/run capacitor is required.

The capacitor in your picture has all sorts of applications from starting motors to power factor correction.

A simple way to limit the start up torque is to connect a resistor in series with it then disconnect it after a couple of seconds. If you connect, a 47Ω 100W power resistor in series with it then by pass it with a timed relay it should do the trick.