Hi all,
I am tinkering with the idea of building a test jig and a basic bench grinder could make the ideal platform.
Am I right in thinking you can make induction motors 'soft start' (reduced starting torque) by the inclusion of a series resistor?
Cheers, T i m
more or less any motor can be soft started that way
NT
Indeed, but I believe an induction motor is more difficult to speed control (my ideal solution), hence the soft-start as an acceptable compromise.
So I use a series resistor to limit the starting torque but I assume it would also limit the final torque. Then I assume I could either switch out the starter resistance (or it could be automatically switched out after a time delay etc [1]) to give me the full power of the motor if it's unable to reach full revs with less_than_full torque?
Cheers, T i m
[1] I imagine they make starter relays that may do this already?
Why bother with soft start? No grinder I know has or needs it.
Bob
T i m explained :
It sounds as if what you need is a variable speed inverter, that way you would have full speed control.
Not really enough detail about what you're trying to achieve.
Induction motors don't generally need soft start until you get up to the sort of power where you'd use a 3-phase motor, and then you do the soft start using a star-delta switch which starts the motor on 240V per winding, and switches to 415V per winding when the rotor has got up some speed. Generally, this is only done to limit the inrush current with a stationary rotor, and not because you want the motor to run up slowly -- induction motors tend to do that anyway because they naturally have poor starting torque.
For an induction motor which uses a start capacitor on a single phase supply, reducing the value of the start capacitor will reduce the starting torque.
For any type of induction motor, a series resistor will reduce the starting and running torque, but this isn't normally done because...
An induction motor must normally be allowed to reach its sync operating speed (minus the designed slip for many types). If you don't allow an induction motor to reach the right speed, it will generally overheat (unless it has been designed with such a bizzare operating mode in mind). For this reason, you generally don't want anything that limits the starting or running torque, because this risks having the motor fail to reach its design speed.
Wellyousee, it's not going to be used as a grinder. ;-)
All I actually need is a reasonably stable shaft spinning at up to around 3000 rpm and able to apply a little torque while it does so. I initially considered using a Picador shaft / bearings but then I would also need a motor and coupling.
And whilst some grinders do have variable speed control they are more expensive than I can afford for this experiment / project.
T i m
Whilst that does sound nice it also sounds expensive? ;-(
T i m
You don't generally need a soft start on an induction motor as the torque is very poor below operating speed.
Well as I said it's just a idea at the moment . BIL is into Lambretta scooters and decided to 'update' his latest restoration project with the electronic ignition system they tend to be made with these days (well, in India anyway). Due to the nature of the beast the parts are typically bits used on other makes and models (I think the regulator is from a Vespa and the CDI unit as used on some Ducatis etc) and whilst this isn't unusual in the motor game they generally do at least come fitted and working from the factory and fully (and generally correctly) documented.
It would seem that the paperwork he was supplied with on this 'kit' seems to be at odds with my understanding of the use of the charge, exciter and trigger coils but in spite of a re-wire it still doesn't work (as in not spark, let alone actually 'running'). He's already replaced the CDI unit but again nothing (but he could have killed them both). Sooo, I was thinking of a simple jig that could allow the mounting of the stator and flywheel (by the use of a 'universal' stator back plate and replaceable 'noses' for the flywheel / grinder spindle) and a simple mounting for the CDI unit, spark plug, regulator, dummy battery (or real one), lights and kill switch etc.
Ok. I was just thinking how fast my bench grinders spin up (admittedly from memory) and the potential of the shaft spinning within the flywheel if not fitted with a keyway or tightened onto a proper taper as it would on the crankshaft. I was hoping to be able to just 'nip it up' with a nyloc nut and a suitable self centring taper [1] on the grounds that might be more flexible (allowing a greater range of flywheels etc).
Ok, then that may be easier (if needed at all from what you say etc).
Ah, ok, understood.
FWIW I was thinking that even a hand cranked bench grinder would be ok for this project as initially it's a go / no-go test. However, if the cheapo bench grinder idea worked then it could be further used to stress / heat / cold test, especially the CDI unit (with the aid of the hot air gun etc). He does have a few scooters that might also get upgraded this way (also allows for 12V lighting) hence the 'universal' approach. To be fair the company he bought the kit from have offered to look at it all for him if he was to get the scooter and parts up there but I sense he would rather sort it himself if at all possible ... hence my test jig idea. ;-)
Cheers, T i m
[1] Or unique collar / spacers turned to accept different flywheels.
Ok, I may well be ok then but I just thought I'd ask before I bought the grinder. ;-)
Cheers, T i m
Why not use a universal motor with feedback?
You can rob all the bits out of an old washing machine.
Make a note of all the wiring - they are all different.
Bob
Hmm, ok that could be another option (thanks). I might have a good motor from a washing machine (the controller went to a good home on here) but I was concerned about the time this would all take and all the other jobs I'm supposed to be doing. ;-(
And if this doesn't work I can always use another bench grinder. ;-)
T i m
T i m has brought this to us :
However a bench grinder motor is not intended for capacitor start.
So not reaching synchronous speed means lots of heat generated inside the motor, so not a good idea at all.
Now we know what you are trying to do....
A motor as used in an electric drill is more suited to have its speed controlled and many have a built in speed control, but put a heavy load on one a very low speed and even they will over heat due to poor air flow. You can/could get in-line speed controllers for these.
Auto washing machine motors have various windings which do allow quite low speed running.
Could you perhaps use a pillar drill which has various gearing?
3000RPM is around the bottom end of the rev range of a 2T engine, so why can't you use a bench grinder?
What do they use then (never taken note or had need to look inside one that I remember).
Understood. I can't see it being a 'heavy load' though, other than overcoming the inertia at startup? I mean, not like running a big sanding disk or wire brush when you could possibly stall it etc?
I'm not sure how important the speed (control) thing is though. I mean, if it produces a decent spark from nearly zero rpm (kick starting) up to a couple of thousand that will prove all but the more exotic faults I should think?
Hmm, another good idea (thanks). I was just concerned that there was as little as possible movement in the shaft / bearings, don't want the flywheel touching the coils (or anything for that matter) when it's up to speed. ;-(
I think I can, I was just concerned about the crude (torsional) interface I might be able to make on a quick_ad_dirty solution.
He did say he might have a spare damaged crank I could use so if I used the flywheel half and was able to accurately and rigidly mount it on the grinder spindle (I can counter bore the crank half on the ML10) Then I could make use of the woodruff key and retaining nut (if only just 'nipped up').
If, (as seems to be the general consensus here so far) the starting torque of yer std bench grinder isn't very high anyway then I might not have a problem in any case. Also, ignoring the potential for any longer / reliability / temperature testing it needn't actually be on for very long in any case and would by definition go from 0 to max to
0 rpm with a single on - off cycle (enough for a reasonable go / no-go test).I think I was mainly looking for some fairly simple but rigid bearings and a bit of electric drive. If the stator would fit back over the body of the grinder casing that would be good as then I could keep the flywheel as inboard as possible. If not the flywheel could be mounted inside out and the stator mounted on the outside.
I will check to see if the chuck / spindle on my pillar drill is up to it as that would make a pretty good platform (and as you say with a range of speeds). I don't think I have enough room for the flywheel over the bed on the ML10 but again, worth a quick measure up (flywheel on a shaft or directly in the 3 or 4 jaw and stator mounted on a plate on the toolpost).
Cheers, T i m
3000 RPM (being 50Hz * 60secs) is the synchronous speed of a 2-pole induction motor on 50Hz. With slip, it will probably be rated around 2900 RPM for max torque.
I don't know what sort of motor bench grinders typically use, not having one myself.
Variable speed control for induction motors requires decreasing the frequency and the applied voltage by the same amount, but the torque available drops off quite significantly as speed is reduced. This tends to work for things like fans where little torque is required at lower speed, but not for anything which requires any additional starting torque. Indeed, some sort of check is required to make sure the motor overcomes its own bearing torque, such as starting at full power and then ramping down the the required speed, or having a feedback tacho or similar to ensure the rotor starts moving. This only gets you a small speed variation because of the rapid drop off in torque. If you want to go still slower, I think you have to drive the induction motor more like a pulsed stepper motor (which requires a multi-phase motor, but you are probably using that anyway if you're using variable frequency control).
Don't think I've ever seen a bench grinder with deliberate soft start.
Make that a _big_ series resistor, ie chunks of old fire element wire. You'll cook a little one. It also requires switchgear, either manual or automatic (centrifugal)
What you might find handy is to look at a 1900-ish handbook of Modern Electric Motors etc. (I've just acquired a copy of Rankin Kennedy). In this period, the usual starter was a manual swinging lever across an arc of copper studs with coils of resistance wire between them. As life was simpler then, books of the period describe things with the sort of shed-based technology we're likely to recreate with our bodgery.
A cheap source of rotating machines with variable speed (shift while spinnning) drives is a low-end woodturning lathe.
And that should be fine as it should allow me to test the system (stator, flywheel, CDI and regulator) at a typical 'in use' RPM. If I wanted to test it right up to 6k then I might need to think again. ;-)
Ok.
No, I think the natural inertia of the flywheel should allow it to spin down from whatever revs it can manage back to zero and I can observe the spark all the way down.
As I said, initially I'm happy for any revs that actually show the thing to be working at_all (so FWIW it could even be hand cranked) and anything outside that could be considered a bonus.
*If* my pillar drill could hold the flywheel rigidly enough to ensure it can't touch the stator coils and spin the flywheel at sufficient speed to generate a spark (so 10 rpm probably ) then that would certainly be good enough to offer my BIL some way of bench checking the stuff he has with the minimum of effort and cost.Cheers, T i m
The Electrical Apparatus Company was still making those when I was an apprentice:-)
regards
Indeed. ;-)
That's sounds exactly like the soft start throttle I made for my electric endurance racing motorbike. 6" square sheet of Tufnol with three 1/4" brass bolts with their heads turned down to a shallow dome and set in a 45 deg arc. Spring loaded 'wiper', actuated by the twist grip went from 'park' to the soft start position (battery via several strands of eureka wire would into a spring coil) to full speed. From standstill in first gear the 'resistor' would glow a nice dull red for a couple of seconds until I got under way and the current backed off. Once rolling it didn't need to do much other than absorb the current 'shock' of up-shifts. ;-)
T i m
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