HomeRoast Digest

Topic: Grinder motor start capacitors (15 msgs / 749 lines)
1) From: Edward Spiegel
Hi folks,
My 15 year old Gaggia MDF needs a new start capacitor (it uses a 16 microfarad capacitor rated at 400 volts that also has a mounting bolt coming out of one end). Does anyone have an idea where I might find such a thing. I am having a heck of a time finding it through Gaggia repair places but a friend suggested that maybe other grinders use them, too.
Does the Rocky use a start capacitor?

2) From: Philip Keleshian
You may have to figure out a new mounting method, but such a capacitor =
should be easy to find.  Try WW Grainger (SP?).

3) From: Philip Keleshian
This should be close enough:http://www.grainger.com/Grainger/productdetail.jsp?xi=xi&ItemId=16117=86870&ccitem=

4) From: Ken Mary
If you have no luck at the espresso parts stores, try a local motor repair
shop.http://surpluscenter.com/ has capacitors. Are you sure it is not a
run capacitor?

5) From: Edward Spiegel
At 8:33 AM -0500 3/10/05, Ken Mary wrote:
Not absolutely sure but pretty sure. The motor runs fine if you get it started by turning it by hand. Someone explained to me that that made it a start capacitor. Also it is round rather than oval which the guy at the local components store said made it a start capacitor. I don't know if that is actually true as I don't know tons about electronic components.
I have looked at a couple of online catalogs and so far haven't found an exact match in terms of specs.
Could someone advise me if it is ok to replace a 16 microfarad capacitor with a 15 microfarad or 17 microfarad capacitor and which would be preferable.
Also the capacitor shows that it is rate to 450 V but none of the capacitors that I have seen are rated for that high a voltage. Does that matter?
Thanks for all the help,
p.s. I have been impressed by the Zass box mill that I have been using as a substitute while I look for the start capacitor.

6) From: Philip Keleshian
Sometimes I get accused of telling how to build a watch when asked the =
time, but here goes:
Your coffee has what is called an AC induction motor.  These motors =
operate by producing a rotating magnetic field around the rotor.
To produce such a field more than more than one phase of electricity is =
required.  Most homes have only one phase service. There are several =
induction motor designs to get around this problem. The most common =
(other than small shaded pole motors) is the split phase motor. This =
motor has two winding wound 90 degrees from each other.  One winding =
(the run winding) is wound with relatively heave wire and appears =
inductive as the motor starts. The other winding (starting winding) is =
wound with finer wire and therefore appears more resistive at starting  =
than the run winding.  After the motor comes up to speed the start =
winding is disconnected.  The magnetic field produced now in not truly =
rotating but rather it is pulsating.  Once the rotor is turning it will =
continue to turn with a pulsating field. These motors produce a =
pulsating torque which is why you hear a 60Hz sound coming from them.
The next class of induction motor is the capacitor start motor. This =
motor is similar to the split phase motor except that a capacitor is =
placed in series with the starting winding. The capacitor shifts the =
phase of the current thru the starting winding to be closer to the =
optimum than what can be achieved with the basic split phase motor. This =
motor will produce more starting torque.
The next class is the capacitor run motor. This is the type of motor in =
your coffee mill. This motor also has two windings wound 90 degrees from =
each other, but in this case they are identical.  One winding has a =
capacitor in series with it.  The capacitor is sized to give current in =
one winding shifted 90 degrees from the other winding at the run speed =
at the expected load. This motor produces constant torque and is =
therefore relatively quiet. 
There is yet another class of induction of induction motor.  If very =
high starting torque is required (such as AC compressors) a capacitor =
start/capacitor run motor is used. In this motor a much larger capacitor =
is switched across the run capacitor when starting.
The voltage requirement of the capacitor is a minimum requirement.  A =
higher voltage rating is extra margin.  The physical size of the =
capacitor grows with the square of the voltage rating.  
As far as the capacitance is concerned. I would assume the OEM part is =
correctly sized but +/- 20% difference should hardly be noticed.
If you want to nail it precisely, you can look at the voltage across =
both motor windings with a dual trace oscilloscope with the motor =
running (with beans grinding if you want to be real fussy). The phase =
relationship should be 90 degrees.  You can also "tune" the value for =
the minimum hum from the motor.

7) From: Philip Keleshian
I realized that I didn't give you the short answer.  Your machine uses a =
run capacitor. It is in the circuit all the time.  A start capacitor is =
switched out of the circuit after the motor starts.  Motor starting =
capacitors usually have a higher capacitance (hundreds of microfarads). =
To keep the size and cost down they usually use non-polarized =
electrolytic capacitors.  They are more lossey but since they are used =
only for starting it doesn't matter.  Run caps should be of higher =
quality. (film or oil filled)

8) From: Edward Spiegel
Thanks for both the long and short answers. The capacitor that is in there is 16 microfarad. Is it ok to use a 15 or 17.5 microfarad capacitor in its place? (I can't find a 16 microfarad capacitor here).
If I were to show you a picture of the existing capacitor would it tell you anything? It is about 1.5" in diameter and 2 3/4" long.
Thanks again,
At 12:28 PM -0800 3/10/05, Philip Keleshian wrote:

9) From: Philip Keleshian
Hi Edward,
Either 15 or 17.5 microfarad should be close enough.  If you want to be =
more accurate you can start with 15, then add smaller amounts in =
parallel (.5, 1, 2 microfarad) while the motor is running to see which =
give the least 60 Hz hum.
The dimensions of the cap in your machine are smaller than the one which =
I found at Grangers.  If you have the room the one from Grangers should =
be fine.  If it won't fit, go to their website and search for motor run =
capacitors. Get the closest to the value of your old cap as you can =
find.  The voltage rating must be at least equal to that of your old =
part.  You can't go to high in voltage however since the case size grows =
very fast with increasing voltage.

10) From: Edward Spiegel
The capacitor in the picture on grainger's is an oval capacitor and someone told me to be sure to replace a round capacitor with a round capacitor because the oval ones are 'dual capacitors' or something. There was also web page (whose URL I forgot to bookmark) about troubleshooting capacitors that said something similar. Does that make sense?
The capacitor I have is round and has only two leads on it.
Sorry if I am dense. I don't really know much about this subject.
Thanks for all the help,
p.s. I have posted a picture of the capacitor in case it gives you any ideas. It is found at:http://www.edwardspiegel.org/grindertemp/

11) From: Philip Keleshian
I notice that your capacitor has several voltage ratings. Part of the =
rating has worn off so I don't know what the significance of that might =
be.  My guess is different voltage ratings at different temperatures or =
different methods of measuring it.
The Granger part is a single capacitor with two terminals. Whether it is =
cylindrical or oval is of no consequence other than physical mounting.  =
If the Grainger part will fit I am sure it will work fine.

12) From: Edward Spiegel
At 5:35 PM -0800 3/10/05, Philip Keleshian wrote:
Thank you very much.

13) From: Michael Dhabolt
This is a multi-part message in MIME format.
Generally capacitors of a higher than necessary voltage rating are the norm.
If the unit is a 120 Volt grinder, I wouldn't hesitate replacing the cap
with something rated for 220/240 Volt and these should be readily available.
A lot of commercial equipment runs at 220/240 and local motor/electrical
shops should have what you need.
The starting capacitor acts like a quick charge/discharge battery that gives
the motor a quick shot of extra energy to get it moving from a dead stop
(sort of like that first morning shot of your favorite caffeine extraction
does for your physiognomy).  Capacitors are cheap, an easy way to get it
running and hold you over until you find the specific part that will fit in
and mount to the equipment correctly is to:
Remember that you can add in parallel capacitors to get the correct Farad
rating, I.E. -  a ten microfarad capacitor and a six microfarad capacitor
wired in parallel with each other gives you a capacitance of sixteen
microfards.  Many capacitors (not usually starting caps) have a positive and
negative terminal.  Always connect these with the polarity of all capacitors
the same.  A little extra capacitance is not a problem, a little less is not
Once you have connected the correct quantity of capacitors together, connect
wires long enough to get to the terminals where the original capacitor was
connected to the equipment.  NOW THIS NEXT STEP IS IMPORTANT. Insulate the
wires - use shrink tubing, tape etc. - DO NOT leave either of the wires
uninsulated in a location where they could be touched or bumped or
accidentally contact you, the equipment or anything else.  The capacitors
store a substantial amount of energy and will knock you on you're a$$, if
not worse.  You can hang it out the side (tape it up, use tie wraps etc. so
it isn't just swinging around).
If you don't have a work bench set up for doing this kind of work a good way
to test is to:  Use a wall receptacle that you are sure has a functional
breaker,  do not plug in the equipment until you have set it on something
that is not easily flammable (clear counter top),  turn the off/on
switch/timer to 'on', stretch the cord out to the receptacle and plug it in
while looking the other way.  If the breaker doesn't trip and the equipment
functions for a few seconds turn it 'off' and feel all around it to see if
it is generating excess heat (if so something else is wrong),  If everything
seems OK, load it (add beans) and run it again for a bit and recheck.
For any of you other techies out there any suggestions, caveats, additions
etc. to the above would be helpful.  I've only had one cappa and a shot so
far this morning, and I'd hate to get Edward to a Full City +.  Edward you
might want to wait until someone else reviews my instructions before taking
it for gospel.
Hope this helps get the grinder back up and running while you wait to find
the exact replacement piece.

14) From: Philip Keleshian
This is a multi-part message in MIME format.
The notes about polarized capacitors raised a flag for me.  With a =
polarized electrolytic capacitor the plus terminal must not be negative =
with respect to the negative terminal.  This means no AC without a DC =
bias.  Failure to observe this may cause the capacitor to explode.  Do =
not used a polarized capacitor!
Non polarized electrolytic capacitors are available for use as motor =
starting caps.  They are in the circuit for starting only.  The coffee =
mill uses a capacitor run motor which has the capacitor in the circuit =
whenever the motor is running.  The capacitor should be of high quality. =
 Oil filled or film capacitors are suitable.

15) From: Philip Keleshian
This is a multi-part message in MIME format.
I forgot to mention this, but I would hesitate to replace the capacitor =
with a 240 volt unit.  I might try one on the bench to verify the =
capacitance value but I wouldn't leave it.  Ideally the voltage produced =
across the motor winding with the capacitor in series with it should be =
the same as the other winding but in this circuit you have a capacitor =
in series with the inductance of the motor winding.  The resonant =
frequency of the combination isn't far from the line frequency.  It is =
not inconceivable that under some conditions voltages exceeding 250 =
Volts can be produced.

HomeRoast Digest