Is there a simple way of reducing startup speed of an AC induction motor?

[bigal_scorpio]

Hi to all, especially Eric, Nigel, Mike and too many others to mention.

After a long time away from the workbench through ill health I am finally back to normal, almost.

I have become interested in woodwork again after my son recently expressed an interest in moving into it and as I had some years experience in the field, though a long time ago I decided to dig out my old tools and give him some help and hopefully tuition.

All was going well until I realised I had at some stage cannibalised my table saw and used the motor for something I can't even remember.

I have a brand new 2500w induction motor, single capacitor version that is ideal for the job both size and power wise, I remember the saw was always a little underpowered and you can never have too much power on a table saw so I thought it would be ideal. It came from a damaged return leaf shredder and runs really well and at the right speed for the pulley drive.

The only problem I have is that the starting jolt of the motor is way too powerful and kicks quite a bit which does no good to the drive train.

I need a simple method of getting the motor to start just a little less fiercely if possible but I have no experience with induction motors. I am not even certain what type it is? Capacitor run or start? So is there a foolproof way of telling and a simple way of achieving this?

Thanks for reading my lengthy tome guys.

Al

 

[crutschow]

Here's a thread on a similar problem that may give you some ideas.

A capacitor start motor will typically have a cylindrical capacitor mounted on the outside of the motor. A capacitor run motor may have one or two capacitors.

 

[alec_t]

Perhaps use an inrush current limiter such as
**broken link removed**
Not sure if this would provide a long enough ramp-up time.

 

[bigal_scorpio]

Here's a thread on a similar problem that may give you some ideas.

A capacitor start motor will typically have a cylindrical capacitor mounted on the outside of the motor. A capacitor run motor may have one or two capacitors.

Hi Carl, I read that thread before I posted but sadly didn't understand it at all. As for the motor, it has just a single 450v 45uF capacitor, which I understand (I think) is within the range for either run or start?

Al

 

[bigal_scorpio]

Perhaps use an inrush current limiter such as
**broken link removed**
Not sure if this would provide a long enough ramp-up time.

Hi Alec,

They look ideal if only I could work out if they do one that would suit the 2500W the motor is rated at. Do you understand the criteria for choosing the values or indeed what actual values are available? I seem to get the idea that there are only a few values and they are paralleled or seriesed up to get the required result? Am I getting the gist?

If they do one of the right value I would assume that setting the inrush to the actual normal run current of the motor would possibly slow down the start speed to some degree but would I need to take into account that the motor would always start with no load?

They also seem to be a little easy to damage from the literature, soldering, dropping, cold, damp and even touching them is frowned on. I wonder how they are in reality and how they would fair mounted in the somewhat hostile environment of my tablesaw case?

Very, very interesting but also baffling document wise!

Al

 

[cowboybob]

If the cap on the motor is just a starter cap (as opposed to a run cap), you might just try a lower value cap. Same voltage rating, just 2/3 (possibly less) or so of the 45uF on it now.

A lower value cap won't shift the phase as much (at start-up) and this will have the effect of a less "vigorous" start-up torque (which was needed for the blower - considerably greater initial inertia to overcome).

Whether a start or run cap, this sort of slight, briefly powered modificaton shouldn't hurt anything.

 

[dr pepper]

You can get soft start boxes made for the purpose, might be a little expensive though.
Using a different value capacitor will slow startup however current in the run windings will still be very high, prolonging start up lengthens this maybe to the point of an overheat.

 

[alec_t]

Do you understand the criteria for choosing the values or indeed what actual values are available?

If you know the motor winding resistance, then dividing the peak mains voltage by that resistance will give you the motor winding inrush current. There's also capacitor charging current to add on.
I don't know the full range of values available for the surge limiters. Try various manufacturers data. I didn't realise they could be damaged so easily.