Old time electric-motor speed control
Jón Fairbairn
Thanks to Liz Baker (ta!) I'm now the proud owner of a
rather old but working Kenwood Chef (A701A). However, it has
a bit of a hot electricity smell¹, and makes an intermittent
sparking noise and at the same time the motor stutters a
bit.
So I took it apart -- there's some wiring that's become
fatigued so will need replacing, but the big question that
I'm hoping someone here can answer is: how (on earth) does
the motor speed control work? At least, someone might be
able to point me at someone who can answer it.
Here's a description of the innards:
The mains comes in through an unexceptional filter circuit
(two chokes and what I take to be a twin capacitor in a can)
and switch. It goes straight from there to two wires coming
out of the motor. Two more wires come back from the motor
and are connected to the ends of a parallel R-C circuit
(450R, can't read the capacitor). In parallel with the RC
circuit is a choke(?) in series with a peculiar device.
The peculiar device looks something like a bare switch with
flat contacts. Behind each of the contacts is a circular
copper(? bronze?) disc about 1cm in diameter. I don't have a
camera else I'd post a picture.
The speed control is a cam (does that make it on topic?)
that tilts the whole capacitor/switch &c assembly away from
the end of the motor spindle. On the motor spindle is a
centrifugal governor, like the ones on steam engines except
that it's made of bronze springs instead of brass links. As
the speed of the motor increases the governor pushes the
"peculiar device". Since turning the control tips the
assembly away from the governor, the higher you set the
control the faster the motor can go before activating the
device. I could almost believe that the device is a switch
that simply cuts the inductor out of the circuit when the
speed gets too high, except that's just too crude for
words. It isn't that, is it? (Please someone say no!). If it
is, why doesn't it spark like crazy the whole time instead
of once every couple of seconds at most?
[1] I think that's from a lunking great 450Ohm resistor -- I
don't know what temperature it reaches, but shortly after
power off it's hot enough to smoke when touched (ouch!)
--
Jón Fairbairn Jon.Fa...@cl.cam.ac.uk
http://www.chaos.org.uk/~jf/Stuff-I-dont-want.html (updated 2004-03-03)
Theo Markettos
>
> Thanks to Liz Baker (ta!) I'm now the proud owner of a
> rather old but working Kenwood Chef (A701A). However, it has
> sparking noise and at the same time the motor stutters a
> bit.
Ah, I have the same model. I've not got around to taking it apart yet (the
mains cable looks a bit suspect and in need of earthing) but it does make
the hot electrical smell. Seems to work fine though, with no sparking.
> The speed control is a cam (does that make it on topic?)
> that tilts the whole capacitor/switch &c assembly away from
> the end of the motor spindle. On the motor spindle is a
> centrifugal governor, like the ones on steam engines except
> that it's made of bronze springs instead of brass links. As
> the speed of the motor increases the governor pushes the
> "peculiar device". Since turning the control tips the
> assembly away from the governor, the higher you set the
> control the faster the motor can go before activating the
> device. I could almost believe that the device is a switch
> that simply cuts the inductor out of the circuit when the
> speed gets too high, except that's just too crude for
> words. It isn't that, is it? (Please someone say no!). If it
> is, why doesn't it spark like crazy the whole time instead
> of once every couple of seconds at most?
My synchronous motor theory is very rusty (it was never very bright in the
first place), but might the R/C circuit be attached to a damping coil on the
motor? Are you sure that the 'choke' is really that - what resistance does
it measure? If it were a resistor, say, cutting it in and out might affect
the loading of the damping coil, causing more power to be dumped in the
resistor. Though I'd guess that would need a very hefty resistor to take
the power. Are you sure your sparking isn't just dead brushes?
How about sci.electronics.design?
Theo
(probably talking out of an unconventional orifice)
Elizabeth Baker
"Jón Fairbairn" <jon.fa...@cl.cam.ac.uk> wrote in message
news:wfn00tw...@calligramme.cl.cam.ac.uk...
> Thanks to Liz Baker (ta!) I'm now the proud owner of a
> rather old but working Kenwood Chef (A701A). However, it has
> a bit of a hot electricity smell¹, and makes an intermittent
> sparking noise and at the same time the motor stutters a
> bit.
Oops! Good job I wasn't selling it.
> So I took it apart -- there's some wiring that's become
> fatigued so will need replacing, but the big question that
> I'm hoping someone here can answer is: how (on earth) does
> the motor speed control work? At least, someone might be
> able to point me at someone who can answer it.
ISTR Paul Bird saying he used to mend them for a living.
http://groups.google.co.uk/groups?q=%22kenwood+chef%22+repair&start=10&hl=en&lr=&ie=UTF-8&c2coff=1&selm=SqJ5c.7%241p6.0%40newsfe1-win&rnum=12
Maybe he can help.
Liz
Who normally gives away quality junk
Jón Fairbairn
> Jón Fairbairn <jon.fa...@cl.cam.ac.uk> wrote:
> >
> > Thanks to Liz Baker (ta!) I'm now the proud owner of a
> > rather old but working Kenwood Chef (A701A). However, it has
> > a bit of a hot electricity smell?, and makes an intermittent
> > sparking noise and at the same time the motor stutters a
> > bit.
>
> Ah, I have the same model. I've not got around to taking
> it apart yet (the mains cable looks a bit suspect and in
> need of earthing)
The rating plate claims that it's "double insulated", so
doesn't need earthing. I don't know whether to trust that,
but the construction looks OK: the motor is insulated from
the chassis. The mains cable on mine also looks a bit
suspect, but I reckon on replacing it with more twin.
> My synchronous motor theory is very rusty (it was never
> very bright in the first place), but might the R/C circuit
> be attached to a damping coil on the motor? Are you sure
> that the 'choke' is really that
fairly
> - what resistance does it measure?
with my ancient cheapo DMM, zero, assuming that the
"peculiar device" really is a switch. It's shrink wrapped,
but looks like fairly thick wire wound round a core. It
looks quite similar to the chokes on the input leads.
> If it were a resistor, say, cutting it in and out might
> affect the loading of the damping coil, causing more power
> to be dumped in the resistor. Though I'd guess that would
> need a very hefty resistor to take the power.
It's too small for that, though the big resistor that
doesn't get switched in and out dissipates a lot.
It's in the circuit when the motor is running too slow,
which means that the two wires from the motor are connected
via it (the RC circuit won't amount to much compared to the
choke). So my current guess is that there is a separate set
of "starter" windings on the motor that are fully on when
it's too slow, and damped when it's too fast.
> Are you sure your sparking isn't just dead brushes?
I can see it coming from the "peculiar device", so
yes. Besides, the one brush I inspected was in surprisingly
good nick.
> How about sci.electronics.design?
It's not very electronic, more electromechanical would they
mind? Maybe we should start alt.support.kenwood-a701a :-)?
Jón Fairbairn
> "Jón Fairbairn" <jon.fa...@cl.cam.ac.uk> wrote in message
> news:wfn00tw...@calligramme.cl.cam.ac.uk...
> > Thanks to Liz Baker (ta!) I'm now the proud owner of a
> > rather old but working Kenwood Chef (A701A). However, it has
> > a bit of a hot electricity smellš, and makes an intermittent
> > sparking noise and at the same time the motor stutters a
> > bit.
>
> Oops! Good job I wasn't selling it.
I don't think it's a real problem. If I can get someone to
confirm that the "peculiar device" really is just a switch,
I can clean the contacts and that should do the trick.
>
> ISTR Paul Bird saying he used to mend them for a living.
>
> Maybe he can help.
Ah, thanks. I'll send him an email if he doesn't pipe up in
this thread.
> Who normally gives away quality junk
Oh, this is definitely tqt! I'm rather impressed that
kenwood service still have some (admittedly not many) spare
parts for a model this old.
Paul Bird
in later life.
In early 1978 I was privileged to attend the (then) Havant Kenwood factory
for a dry course (including said Chef, and cookers) and a wet course
(dishwashers and washing machines), courtesy of Thorn Domestic Appliances of
Empire Way, Wembley. Now built over by the way and unrecognizable but
that's' not why you called.
So far as I recall, and I've still got an old Chef somewhere, the motor
speed is governed by a centrifugal switch. The motor increases in speed
when power is applied and the centrifugal switch has increasing pressure
applied to it as a result until the two contacts part, at which point the
power is removed from the motor and it slows down again, the process
continues as described resulting in the aforesaid erratic motion.
The cf (for brevity) is adjustable by means of two slotted screws,
accessible through the curved plastic slotted casing, with a long thin, flat
(as opposed to Philips or Pozidriv), screwdriver, by inverting the whole
machine on a soft surface (to avoid damage to the top casing. At this point
flour will fall out, and crumbs, and whatever else you mixed but didn't get
to use in the previous twenty years.
Adjust both screws, in the same direction (ISTR anticlockwise to slow it
down and cw to speed it up), BTW this with the power OFF, repeat OFF.
Turn it up the right way, apply the power and put one hand gently in the way
of the rotating mixer holder (the bit that goes around on the front). Give
the machine sometime to settle down into its "new" speed. Watch a clock
with a second hand. This is always good for passing the time if you are
bored but in this case you are looking for about one rotation a second, or
60 rpm.
What tends to happen over time is that the screws move (presumably outwards
from there mountings), so as to increase the speed of the motor, and symptom
reported by the customer (oh happy days driving around the East end of
London in 1978-80), is that "it throws the flour all over the place". The
cure was to turn the machine over, turn the two aforementioned screws
clockwise a bit, evenly, i.e. the same on each side, then back the right way
up and watch the clock again until down to 60rpm.
This would then result in a burning smell as the flour that had accumulated
on the large resistor, began to burn off. IIRC the resistor was wired
across the contacts to reduce sparking. No doubt there was a capactor there
as well since my electronics training doesn't bring back much about
resistors reducing arcing but I stand to be corrected.
Thanks for my 15secs of fame, all the above from memory twentyfour years
ago. HTH etc
Paul
Paul Bird
so here is the precis and errata from above.
The motor tends to speed up over time and needs adjusting back down to 60rpm
at the slowest speed control setting on the side, by screwing in the two
slotted screws clockwise, accessible through the plastic casing under the
motor, using a long thin flat ended screwdriver.
Try a quarter turn clockwise each then turn the machine back over and try it
on the slowest speed, put your hand on the mixing attachment holder and let
your fingers gently catch the protruding bit each time it comes around while
watching a second hand, you are looking for about 60rpm and the movement at
this minimum speed will be somewhat erratic. There will also be a burning
smell from the resistor but don't worry about this. Once the flour has
burnt off the smell will go.
If it still exceeds 60rpm on the slowest speed control setting then turn the
machine over and turn the two screws in a bit more, but evenly on each side
to keep the centrifugal switch bracket evenly balanced.
Continue until about 60rpm is achieved and ignore the burning smell.
Paul
p underscore bird at ntlworld dot com
if you want anymore info directly.
Paul Bird
news:wfn00tw...@calligramme.cl.cam.ac.uk...
<snip>
> So I took it apart -- there's some wiring that's become
> fatigued so will need replacing, but the big question that
> I'm hoping someone here can answer is: how (on earth) does
> the motor speed control work? At least, someone might be
> able to point me at someone who can answer it.
It's a crude centrifugal switch.
<snip>
> The peculiar device looks something like a bare switch with
> flat contacts. Behind each of the contacts is a circular
> copper(? bronze?) disc about 1cm in diameter. I don't have a
> camera else I'd post a picture.
Peculiar device = centrifugal switch?
<big snip>
> I could almost believe that the device is a switch
> that simply cuts the inductor out of the circuit when the
> speed gets too high, except that's just too crude for
> words. It isn't that, is it? (Please someone say no!). If it
> is, why doesn't it spark like crazy the whole time instead
> of once every couple of seconds at most?
Don't know why it doesn't spark like crazy, too long ago, had to give the
service manuals back in 1980 when I left, but no doubt soon some bright
spark (NPI) will be along with one dragged off the archives.
> [1] I think that's from a lunking great 450Ohm resistor -- I
> don't know what temperature it reaches, but shortly after
> power off it's hot enough to smoke when touched (ouch!)
Yes, it's hot enough to burn the flour that has accumulated from cake
mixture over the years, hence the burning smell.
Paul
p underscore bird at ntlworld dot com
Sorry I don't remember more, but the basic speed adjustment I've posted
elsewhere.
Paul Bird
23Mb apparently
http://www.kenwoodservice.co.uk/instructions/A701instructions.htm
Found this after getting curious for the service manual and did a Google
Paul
Jón Fairbairn
> The motor tends to speed up over time and needs adjusting back down to 60rpm
[...]
> using a long thin flat ended screwdriver.
unfortunately the only screwdriver I have that's thin enough
isn't really long enough...
> Try a quarter turn clockwise each then turn the machine
> back over and try it on the slowest speed, put your hand
> on the mixing attachment holder and let your fingers
> gently catch the protruding bit each time it comes around
> while watching a second hand, you are looking for about
> 60rpm
I've got it down to 75rpm after managing one quarter turn on
each screw, but I'm waiting for my fingers to recover before
trying again.
> and the movement at this minimum speed will be somewhat
> erratic.
When you say erratic, do you mean that it'll drift up and
down, or that it'll make jerky changes in speed (which is
what this one is doing)?
> Continue until about 60rpm is achieved and ignore the
> burning smell.
Many thanks. I'd put the motor cover back on before I read
your message, so I'm glad it's possible to make the
adjustments without taking it off again. Do you remember the
correct procedure for taking it off/putting it back on? The
choice seems to be between undoing a circlip that holds the
button for the head-lift mechanism in, or knocking out the
hinge pin. I did the latter because the circlip lacks the
two holes that my circlip pliers need to get purchase, but
the result is that putting it back together requires fitting
the hinge pin while trying to line the hole up against the
(considerable) force of the head-lift spring. I hadn't
realised that my lack of physical strength was going to make
it so difficult!
Thanks again,
Jón
Jón Fairbairn
> So far as I recall, and I've still got an old Chef somewhere, the motor
> speed is governed by a centrifugal switch. The motor increases in speed
> when power is applied and the centrifugal switch has increasing pressure
> applied to it as a result until the two contacts part, at which point the
> power is removed from the motor and it slows down again,
I think it just removes power from some windings rather than
the whole motor -- at least, when I stuck the end of a
ty-rap between the contacts the motor still ran, albeit with
rather less vigour than normal.
> Thanks for my 15secs of fame, all the above from memory
> twentyfour years ago. HTH etc
Pretty well remembered! Thanks.
Jón Fairbairn
Got that, thanks. It's the user manual, though -- useful all
the same.
Paul Bird
<snip>
> When you say erratic, do you mean that it'll drift up and
> down, or that it'll make jerky changes in speed (which is
> what this one is doing)?
The latter. But the average will be 60rpm.
> Do you remember the correct procedure for taking it off/putting it back
on?
No. That was a skill I possessed at the time but have long since forgotten.
Circlips. Don't get me started. They can travel some distance and
frequently do.
> The choice seems to be between undoing a circlip that holds the
> button for the head-lift mechanism in, or knocking out the
> hinge pin. I did the latter because the circlip lacks the
> two holes that my circlip pliers need to get purchase, but
> the result is that putting it back together requires fitting
> the hinge pin while trying to line the hole up against the
> (considerable) force of the head-lift spring. I hadn't
> realised that my lack of physical strength was going to make
> it so difficult!
Fun isn't it! I do remember that the hinge pin was one of our regular stock
items which implies that it wore out with great regularity probably owing to
the strength of the spring that caused you such trouble.
> Thanks again,
>
> Jón
You're welcome.
Paul Bird
> "Paul Bird" <pb...@ntlworld.com> writes:
>
> > If you have broadband try this:-
> >
> > 23Mb apparently
> >
> > http://www.kenwoodservice.co.uk/instructions/A701instructions.htm
> >
> > Found this after getting curious for the service manual
> > and did a Google
>
> Got that, thanks. It's the user manual, though -- useful all
> the same.
I know. If you find the service manual on the web do let me know. I know
for a fact that Electrolux in Coventry will have one because they are the
most recent owners of the company that was once T.D.A. Electrical Ltd, and I
am pretty certain that they microfiched all the manuals and now issue them
on a portable display to their service engineers. Whether or not they would
allow a non-employee to have a copy is an entirely different story but I
would certainly start with them if I were looking for one.
Paul
dalewh...@gmail.com
1. Does 60rpm refer to the WHISK rotation speed, or the speed at which the chrome 'arm' goes around? (the bit you push the whisk into)
2. When you say 'slowest setting', is that "at position 1 on the dial" or at the very slowest (i.e. immediately after leaving the OFF position) ?
Sorry if they sound like dumb questions, I just need some clarity :-)
Thanks, Dale
Paul Bird
> Several people have mentioned that "lowest speed is 60rpm" which is a good start. I have a A901E to set up, and I have two questions...
> 1. Does 60rpm refer to the WHISK rotation speed, or the speed at which the chrome 'arm' goes around? (the bit you push the whisk into)
> 2. When you say 'slowest setting', is that "at position 1 on the dial" or at the very slowest (i.e. immediately after leaving the OFF position) ?
> Sorry if they sound like dumb questions, I just need some clarity :-)
> Thanks, Dale
Kenwood factory in Havant in 1978.
https://www.ifixit.com/Guide/How+to+adjust+Kenwood+Chef+A701A+minimum+speed/28280
PB
Paul Bird
> 2. When you say 'slowest setting', is that "at position 1 on the dial" or at the very slowest (i.e. immediately after leaving the OFF position) ?
2. Very slowest, immediately after leaving the OFF position. i.e. turn
it on then rotate the knob anticlockwise until it hits the detent and stops.
Caveat: My instructions were for the A701 which was available new at the
time. I cannot comment on whether they are applicable to the A901E.
PB
dalewh...@gmail.com
Dale
husto...@gmail.com
Initially it works. The speed can be controlled and goes according to the speed knob turning. I was happy and though it works. BUT then...after a while(2-3mins later) the speed begins to creep up and stays at top speed. Once again the controller is out of control and original problem is back.
Strange enough, after I OFF and rested the motor (20mins)to cool down. It works again initially, but problem persist after 2-3mins and goes to top speed and stays. Wonder if it's the CHOKE resistor that I left out. Or the centrifugal control screws that I need to adjust. How to get it fix? Pls help.
Paul Bird
after all these years since I last dealt with them I wonder if it could
be the contacts that after all suffer the most wear? Or to do with the
centrifugal action that makes and breaks the contacts?
After all the parts you have replaced seem to me to be the most reliable
not the least reliable and if possible I would be looking at the sprung
plate and the centrifugal switch.
Feel free to contact me directly if you like although I'm not sure I can
add too much to what I've said here.
Cheers
Paul
---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus
The Natural Philosopher
--
I would rather have questions that cannot be answered...
...than to have answers that cannot be questioned
Richard Feynman
Paul Bird
<snip>
>
> Triac gone 'soft' maybe
>
You're 'aving a laugh aren't you? I don't think the circuit reached that
level of sophistication. It was a suppressed centrifugal on/off switch
controlling a non-continuoslly rated 1/3hp motor.
PB
Paul Bird
The Natural Philosopher
> On 23/05/2018 18:33, The Natural Philosopher wrote:
> <snip>
> >
>> Triac gone 'soft' maybe
>>
>
> You're 'aving a laugh aren't you? I don't think the circuit reached that
> level of sophistication. It was a suppressed centrifugal on/off switch
> controlling a non-continuoslly rated 1/3hp motor.
>
Chances are that the centrifugal weight mechanism is binding then
> PB
>
> ---
> This email has been checked for viruses by Avast antivirus software.
> https://www.avast.com/antivirus
>
husto...@gmail.com
> not the least reliable and if possible I would be looking at the sprung
> plate and the centrifugal switch.
>
> Feel free to contact me directly if you like although I'm not sure I can
> add too much to what I've said here.
husto...@gmail.com
> Chances are that the centrifugal weight mechanism is binding then
Is there a standard or prefered distance for the Two Spring Screw that adjust the centrifugal gap?
Anyway, many thanks for your help. Appreciate it.
Paul Bird
> Is there a standard or prefered distance for the Two Spring Screw that adjust the centrifugal gap?
on the minimum setting is 60rpm of the whisk holder. The two springs
have to be equal distances i.e. the switch plate has to be balanced.
husto...@gmail.com
Hope you're well. so sorry to bother you.
Required your Kenwood chef A701A expertise yet again.
Not sure if you have encounter Motor jerking whenever it sparks at the copper contacts at the contact bar at lower speeds 3-4 knob. I had clean the copper contacts. check the Fan grub screw. adjust the 2x spring screws. Change capacitor/resistors from KenwoodChefrestoreuk. Problem still persist at 3-4 speed. But once I go beyond speed 5 knob, everything seems normal...i think. wonder it will spolit the motor if i continue using it in this state. It's still usable to me. Anyway thank you. And here's my video of my CHEF motor
https://www.youtube.com/watch?v=ecNzzUohJCM
Paul Bird
First of all that's a good video to demonstrate to interested party the
operation of the centrigual switch.
Secondly, from memory, and it has been 40 years, that twitching and
jerking movement and sound was perfectly normal at low speeds. It it
just the nature of the beast. The designers had used a very crude and
cheap method of controlling the motor speed. Even back in the case and
driving a K beater it would behave like that.
From what you have shown me very eloquently by way of a video I cannot
see anything to worry about. Certainly at no stage of my working life
1978-1980 did I ever get to see the motor in operation at that close range.
PB
Paul Bird
Hi Huston,
First of all that's a good video to demonstrate to interested parties
the operation of the centrifugal switch.
Secondly, from memory, and it has been 40 years, that twitching and
just the nature of the beast. The designers used a crude and cheap
a K beater it would behave like that.
From what you have shown me very eloquently by way of a video I cannot
see anything to worry about. Certainly at no stage of my working life
1978-1980 did I ever get to see the motor in operation at that close
nothing to worry about.
PB
husto...@gmail.com
I loosen up the 2x Spring screw evenly and created more gap inbetween the Centrifugal switch and the Contact bar. Lower speed at knob 3-4 has much more torque now and overrides the motor on/off jerking action. Now i know that seeing sparks going off at the copper contacts and carbon brushes is completely normal. The motor jerking on/off is greatly minimise or overwrite. Problem solved. I can now use this Vintage classic kenwood chef once again.
Thank you and have a great weekend.
Paul Bird
pushing the machine to its limits since it's only a 1/3 hp motor so I
don't recommend using it to make bread. If you want to make bread by a
breadmaker, you will prolong the life of the Chef.
PB