Looking for some help....Motor woes

[OldWood]

I did a re- motoring of my TS some time back when the smoke got out of the old one. The previous motor was a) underpowered and b) not sealed so every so often saw dust got across the centrifugal switch and the starter winding wasn't switched on - the motor just sat there and hummed. It was easy enough to clear but 25 years of doing that was just too much for the smoke !!

Anyway - I got a 2.2kw single phase motor off an old pressure cleaner. Twin capacitor and electronic switching. The start up load characteristics of the TS was clearly different from the power washer and one of the capacitors blew. I replaced the capacitor and had to repair the electronics - two triacs involved. I ran the motor off load and found that when the start-up cap switched off, the motor ran rough, so I removed the second cap and it was fine. But I did have to increase the start up time as the system inertia was such that the capacitor was being switched out too early and the motor didn't have enough speed to lock in.

My suggestion is that this is the same problem - the start up time is too short for the load you are driving.

Rob

 

[Digit]

For my two pennorth Bob I have seen such arrangements, if I understand the set up, that is.
With the arrangement that I am familiar with the relay coil is in series, (I think, it was a long time ago), with the motor windings and thus subject to the current/voltage changes due to the start up load. If the motor slows due to load the relay will fire up again.
It was much loved by the designers of a particular range of tumble dryers, now defunct, thankfully.

Roy.

 

[9fingers]

Hi Roy,

Yes I have seen that sort of arrangement in domestic stuff especially fridges
They still use a current controlled relay and a klixon overload protector today.

What I meant to say was that I had not seen relay or electronically switched starting on general purpose motors.

Having now studied the ABB catalogues I see that they are now making such motors - presumably to increase the reliability of capacitor start motors by getting rid of the infernal centrifugal switch.

Bob

 

[Digit]

infernal centrifugal switch. :lol:

Gone but not forgotten! :lol:
Which raises a question with me. My extractor motor came to a halt recently, I got it sorted, but I was led well up the garden path.
On rundown there is a loud click!
Starter switch says the brain, so take the end off the flipping motor, and guess what? No switch.
So what the heck the click is I haven't a clue!

Roy.

 

[9fingers]

Maybe the switch is at the other end??
Is there a rubbing noise after the click?
Or a magnetic switch based on the stator field?

I have two motors (chinese) where the centrifugal switch is on the outside of the motor bearing on a stub of the shaft. Nice and easy to service and no risk of bu88ering the switch when removing the end bearing cap.

Bob

 

[Digit]

No rubbing, no magnetic switch, and if there is a centrifugal switch on the opposite end I want the first poke at the designer, as the entire motor would have to be dismounted to get at it. That in turn would mean dismantling the blower casing as well!
That would make me very unhappy!

Roy.

 

[9fingers]

No rubbing, no magnetic switch, and if there is a centrifugal switch on the opposite end I want the first poke at the designer, as the entire motor would have to be dismounted to get at it. That in turn would mean dismantling the blower casing as well!
That would make me very unhappy!

Roy.

Well you are just going to have to take it apart to find out how it works!

Enquiring minds need to know!!

Bob

 

[Digit]

Sod that! I will if I need to but otherwise I'll remain mystified!
I had a mystery enough with that bit of turning. As I told you the parallelism was very good, so when I swapped it end for end I was surprised to find that it didn't run true.
After much head scratching I finally found that the dead centre in the headstock had been made with the centre off centre!
I was not amused!

Roy.

 

[GraemeD]

Thanks for all the feedback and thoughts, guys. It sounds to me like there are two current schools of thought....

1). a dud run cap, which I could prove pretty easily by getting a replacement and changing the one fitted. (Thanks for the offer of help with this Bob....I'll let you know how I get on).

2). the fact that the motor is not getting a long enough "kick" at the start for it to reach synchronous speed, and then struggling under load.

Option 1 was my initial thought, but can someone assure me that a dud cap would result in loss of torque under load only? Since the motor runs sweetly off load, with or without the start cap (I tried disconnecting the start cap in case it was being held in circuit permanently....my initial fear of a stuck centrifugal switch, since discounted as it's electronic).

Option 2 does also seem to cover what I'm seeing, as it is great off load....and impossible to stop by hand, so not exactly struggling to spin! The ABB catalogue says that the start cap is held in circuit until the point when the motor has reached its "break-down torque" (any idea what this is?)....capped at 2 seconds. If the load means that this is not long enough, then maybe I would get the effect I am seeing. There is no way to extend the 2 second timeout, so the only option here is to reduce the load....or use a more powerful motor. Do you think loosening the belt a bit would do this? Even if it slipped a bit....as an experiment to prove a point?

Thanks again for all the input.....I'm sure we're on to something now!

Cheers
Graeme

 

[Digit]

Break down torque is the greatest torque that that motor will produce, as the motor speeds up that torque decreases.
Two seconds is actually quite a long time. The cutter block can not be accelerated faster than the motor's own rotor, that is always the final factor in spin up time.
In a motor of your size that will be very quick, so unless the cutter block is either very heavy or tight for some reason two seconds should be plenty.

Roy.

 

[OldWood]

Roy,
I was surprised when I ran into this problem with my TS refurbishment as I would have thought a 3hp motor would have run it up pretty fast - it was a 0.75 hp previously. I can't remember now - it will be a log book somewhere - what timing I did have to re-set it to but it was quite a bit longer than I expected. There is also the question of how much tolerance there is on the 2 seconds - it could quite easily be down to 1.5 for instance.

Graeme's is an older machine and the combination of the pulleys, belt and cutter block will all represent a reasonable amount of inertia. I worked with my cabinet maker brother for a while and his elderly P/T took a markedly long time to wind up particularly first thing on a winter's morning, so based on my experience I do favour the lack of delay time as the cause.

As to a solution ? I was an electronic engineer so I just worked out the timer circuit and adjusted the appropriate component.

This however is typical of what you want if you are prepared to rewire the starter capacitor to an external adjustable delay timer

http://cpc.farnell.com/jsp/level5/modul ... 274858.xml

Omron do similar devices.

Rob

 

[Digit]

Point taken Rob, but in this instant the poster states that it is a 6 inch planer, from that I assumed a straight two pulley single belt drive, two or three knife block.
Assuming that is correct I would expect two seconds to be sufficient, if the timing is not reasonable accurate then that indeed could be the problem.

Roy.

 

[GraemeD]

Roy's right in that this is a simple two pulley arrangement, with a two cutter block. It is quite heavy compared to the one in my Elektra Beckum P/T, but doesn't feel too onerous when you spin it up by hand. Hard to be sure though.

If we're thinking this is the issue, I'm wondering whether I could put in a manual switch to connect in the start cap, and hold it in for a bit longer and see if it does the job. If it does, something like Rob's timed relays would be worth investing in as a more permanent solution.

I'm actually a practising electronics engineer myself, though I work inside chips, so all this power stuff is well outside my expertise! However, it wouldn't worry me at all to add my own start cap circuit, separately timed. The inbuilt one is all potted in resin, so there won't be any way to reuse that unfortunately.

Graeme

 

[OldWood]

Graeme - as I went off to go to bed immediately after posting that idea about the timer relay, your suggestion of course came to mind. Going back to the P/T that my brother had, that had to be hand switched over, so if you can locate the appropriate wires, then I would suggest a manual switch test with a 'on-and', 'two-and', etc count.

Yes, I can well imagine power electrics when you are dealing with microwatts and millivolts can be slightly daunting, but a wire is a wire, and a current is a current. At least you know what is happening and have that as a confidence starting point.

Fortunately the timer I worked on was potted in rubber which could be dug out, plus it was discrete wire ended components which were easy to mess around with.

Keep us posted - if the timing does turn out to be the problem this could be a reference thread on the matter as more motors appear with electronic switching.

Rob

 

[GraemeD]

Great minds and all that! I'll try and have a play later today, if I can find a suitably rated switch....the starting currents are high after all. It should however start to tell us if we're looking in the right direction for this problem, so I'm keen to have a go.

As an engineer, I'm more than happy with the concepts here, and also putting things together if I know it's the right way to do things.....just that my microwatts don't bite back if you get something slightly wrong!

Graeme

 

[Digit]

I had to make all sorts of adjustments like that Graeme, I started an electrical engineering course being well clued up valve based circuitry, as we progressed through the course I had to re think from voltage amplification to current amplification.
Keeps the brain active.
By the time I was well into ICs and PC boards I ended up developing motors in the 100s of horsepower range. I lived in Birmingham's library for while! :lol:

Roy.

 

[9fingers]

Maybe we need an electrical/electronic, past and present engineers section on the forum?

Who do you work for Graeme? until I retired last year I was often a specifier/user/mananging the design of ASICs

Bob

 

[GraemeD]

Seems like there are a few of us on here, so that may be a nice idea! If you were involved in ASICs, then you'll understand what I do. I run my own small consulting business with a staff of four (www.heliontech.com), offering ready-made building blocks to enable chip designers to drop in certain functionality quickly and at low risk. We specialise in encryption, so we get to work with all kinds of interesting people and companies around the world, as you can imagine! All my work these days is on a PC....no soldering irons involved these days in what I do! Hence my spare time woodworking (with a fair bit of ww machine rebuilding) keeps me grounded.....proper engineering!!

Graeme

 

[9fingers]

Seems like there are a few of us on here, so that may be a nice idea! If you were involved in ASICs, then you'll understand what I do. I run my own small consulting business with a staff of four (www.heliontech.com), offering ready-made building blocks to enable chip designers to drop in certain functionality quickly and at low risk. We specialise in encryption, so we get to work with all kinds of interesting people and companies around the world, as you can imagine! All my work these days is on a PC....no soldering irons involved these days in what I do! Hence my spare time woodworking (with a fair bit of ww machine rebuilding) keeps me grounded.....proper engineering!!

Graeme

Yes we used to use a similar approach to reduce risk on flea power mixed signal/analogue asics but the plethora of asic vendor processes and their use of different tools always seemed to leave plenty of risk and pooh traps for us to find!
Latterly I was in the role of being responsible for managing the work but not allowed to get involved to the degree I could help prevent the problems!
Early retirement and making sawdust seemed a no-brainer when the opportunity arose and I can keep my mind active by technical contributions on this forum and a few others.

Bob

 

[Digit]

Maybe we need an electrical/electronic, past and present engineers section on the forum?

No we don't! I already spend too much time on here walking down memory lane!

Roy.