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

On Aug 26, 7:21*pm, T i m wrote:
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

On Wed, 26 Aug 2009 12:34:52 -0700 (PDT), NT
wrote:

On Aug 26, 7:21*pm, T i m wrote:
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


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?

T i m wrote:
On Wed, 26 Aug 2009 12:34:52 -0700 (PDT), NT
wrote:

On Aug 26, 7:21 pm, T i m wrote:
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


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 :
I am tinkering with the idea of building a test jig and a basic bench
grinder could make the ideal platform.

It sounds as if what you need is a variable speed inverter, that way
you would have full speed control.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk

In article ,
T i m writes:
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?

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.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

On Wed, 26 Aug 2009 21:58:49 +0100, Bob Minchin
wrote:

I am tinkering with the idea of building a test jig and a basic bench
grinder could make the ideal platform.


Why bother with soft start? No grinder I know has or needs it.

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

On Wed, 26 Aug 2009 22:01:24 +0100, Harry Bloomfield
wrote:

T i m explained :
I am tinkering with the idea of building a test jig and a basic bench
grinder could make the ideal platform.

It sounds as if what you need is a variable speed inverter, that way
you would have full speed control.

Whilst that does sound nice it also sounds expensive? ;-(

T i m

In article ,
T i m wrote:
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?

You don't generally need a soft start on an induction motor as the torque
is very poor below operating speed.

--
*Why do psychics have to ask you for your name? *

Dave Plowman London SW
To e-mail, change noise into sound.

On Wed, 26 Aug 2009 21:21:32 +0000 (UTC),
(Andrew Gabriel) wrote:


Am I right in thinking you can make induction motors 'soft start'
(reduced starting torque) by the inclusion of a series resistor?

Not really enough detail about what you're trying to achieve.

Well as I said it's just a idea at the moment g. 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.

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.

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).

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.

Ok, then that may be easier (if needed at all from what you say etc).

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.

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.

On Wed, 26 Aug 2009 22:56:28 +0100, "Dave Plowman (News)"
wrote:

In article ,
T i m wrote:
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?

You don't generally need a soft start on an induction motor as the torque
is very poor below operating speed.


Ok, I may well be ok then but I just thought I'd ask before I bought
the grinder. ;-)

Cheers, T i m

T i m wrote:
On Wed, 26 Aug 2009 22:56:28 +0100, "Dave Plowman (News)"
wrote:

In article ,
T i m wrote:
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?
You don't generally need a soft start on an induction motor as the torque
is very poor below operating speed.


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

On Wed, 26 Aug 2009 23:12:19 +0100, Bob Minchin
wrote:


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.

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 :
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.

Ok, then that may be easier (if needed at all from what you say etc).

However a bench grinder motor is not intended for capacitor start.


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.

Ah, ok, understood.

So not reaching synchronous speed means lots of heat generated inside
the motor, so not a good idea at all.


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).

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?

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. ;-)

3000RPM is around the bottom end of the rev range of a 2T engine, so
why can't you use a bench grinder?

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk

On Wed, 26 Aug 2009 23:18:41 +0100, Harry Bloomfield
wrote:

T i m has brought this to us :
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.

Ok, then that may be easier (if needed at all from what you say etc).

However a bench grinder motor is not intended for capacitor start.

What do they use then (never taken note or had need to look inside one
that I remember).


snip

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).

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.

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?

You can/could get in-line speed controllers for these.

Auto washing machine motors have various windings which do allow quite
low speed running.

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?

Could you perhaps use a pillar drill which has various gearing?

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. ;-(

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. ;-)

3000RPM is around the bottom end of the rev range of a 2T engine, so
why can't you use a bench grinder?

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

In article ,
T i m writes:
On Wed, 26 Aug 2009 21:58:49 +0100, Bob Minchin
wrote:

I am tinkering with the idea of building a test jig and a basic bench
grinder could make the ideal platform.


Why bother with soft start? No grinder I know has or needs it.

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

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.

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.

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).

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

On 26 Aug, 19:21, T i m wrote:

I am tinkering with the idea of building a test jig and a basic bench
grinder could make the ideal platform.

Don't think I've ever seen a bench grinder with deliberate soft start.

Am I right in thinking you can make induction motors 'soft start'
(reduced starting torque) by the inclusion of a series resistor?

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.

On Thu, 27 Aug 2009 07:46:24 +0000 (UTC),
(Andrew Gabriel) wrote:


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

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.

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. ;-)

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.

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.

Ok.

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).

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 g) 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

In message
,
Andy Dingley writes
On 26 Aug, 19:21, T i m wrote:

I am tinkering with the idea of building a test jig and a basic bench
grinder could make the ideal platform.

Don't think I've ever seen a bench grinder with deliberate soft start.

Am I right in thinking you can make induction motors 'soft start'
(reduced starting torque) by the inclusion of a series resistor?

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.

The Electrical Apparatus Company was still making those when I was an
apprentice:-)

regards

--
Tim Lamb

On Thu, 27 Aug 2009 20:15:23 +0100, Tim Lamb
wrote:

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.

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