Looking for a replacement capacitor

[Brian]

Hi, the timer on my heating hast stopped running, its a mechanical
driven by a small motor conected to 230Vac, the problem is a capacitor
(the only other component) that is in series with the motor. I shorted
the cap with a multimeter and the motor started running again.

The cap has the following info printed on it:

..1/10
250V ~
MKP1.70
..1/10
250V ~

What should I use as a replacement? What is the value exactly .1uF/10?
Which I assume is 10nF, true?

What exectly is the cap for?

Cheers Brian

 

[Art]

10 UFD @ 250 VAC is the device specs. It is used as a start capacitor,
function is to lessen the electrical current through the start winding of
the motor. Should be able to appropriate capacitor at any industrial motor
service or HVAC facility. Even able to be ordered off the web if you google
for "start capacitors".

 

[Brian]

10 UFD @ 250 VAC is the device specs. It is used as a start capacitor,
function is to lessen the electrical current through the start winding of
the motor. Should be able to appropriate capacitor at any industrial motor
service or HVAC facility. Even able to be ordered off the web if you google
for "start capacitors".

Hey Al, thats great. I posted few days ago on a german NG but still
didn't get any answer. So does the .1/10 mean 100uF / 10 ?
I have an electronics catalog, but I wasn't sure what value I needed.

Thanks very much.

 

[Graham]

Hi, the timer on my heating hast stopped running, its a mechanical
driven by a small motor conected to 230Vac, the problem is a capacitor
(the only other component) that is in series with the motor. I shorted
the cap with a multimeter and the motor started running again.

The cap has the following info printed on it:

.1/10
250V ~
MKP1.70
.1/10
250V ~

What should I use as a replacement? What is the value exactly .1uF/10?
Which I assume is 10nF, true?

What exectly is the cap for?

Cheers Brian

If it's a clock then it will be a synchronous motor, and will have two
terminals only (no separate start winding).

I would have expected that the motor would be connected directly to the
supply, I can only suppose that the capacitor serves as a watt-less
voltage dropper.

Is there anything on the motor that suggests it runs on less than 230v?
120v maybe?


--
Graham.



%Profound_observation%

 

[Brian]

If it's a clock then it will be a synchronous motor, and will have two
terminals only (no separate start winding).

I would have expected that the motor would be connected directly to the
supply, I can only suppose that the capacitor serves as a watt-less
voltage dropper.

Is there anything on the motor that suggests it runs on less than 230v?
120v maybe?

Graham,
looks like you are correct. It says 70V~ 50Hz on the motor. All the
start caps I have seen are pretty large things. The Motor here is very
small about 3/4 inch diameter and 1/2 inch deep. I assume that the
important point here is the frequency controlling the speed, not the
voltage. Is that correct?

I still haven't found a good replacement part yet.

Cheers Brian

 

[Brian]

If it's a clock then it will be a synchronous motor, and will have two
terminals only (no separate start winding).

I would have expected that the motor would be connected directly to the
supply, I can only suppose that the capacitor serves as a watt-less
voltage dropper.

Is there anything on the motor that suggests it runs on less than 230v?
120v maybe?

Forgot to say, yes only two wires go to the motor.

 

[Brian]

If it's a clock then it will be a synchronous motor, and will have two
terminals only (no separate start winding).

I would have expected that the motor would be connected directly to the
supply, I can only suppose that the capacitor serves as a watt-less
voltage dropper.

Is there anything on the motor that suggests it runs on less than 230v?
120v maybe?

Would the replacement cap need to be 250V Ac? I can find various ones at
250 VDC, 250V AC seems to equate to 630V DC in the specs.

Would an electrolytic capacitor be correct? How does that work with
polarity (seeings its getting AC)?

Cheers

 

[Graham]

If you have a selection of capacitors you could find the approximate value
empirically.

Most generic capacitors will show their max voltage rating as a DC value,
not an AC RMS value.
A 250V RMS capacitor should be replaced with at least a 400 volt DC rating.

My guess is the value is 0.1uF (same as 100nF)

Maybe the "10" just means +/- 10% tolerance


--
Graham.



%Profound_observation%

 

[Graham]

Would an electrolytic capacitor be correct? How does that work with
polarity (seeings its getting AC)?

Definatly not!

see my last post, a 400V DC capacitor is approximatly equivilent to a 250V
AC capacitor

--
Graham.



%Profound_observation%

 

[Brian]

If you have a selection of capacitors you could find the approximate value
empirically.

Most generic capacitors will show their max voltage rating as a DC value,
not an AC RMS value.
A 250V RMS capacitor should be replaced with at least a 400 volt DC rating.

My guess is the value is 0.1uF (same as 100nF)

Maybe the "10" just means +/- 10% tolerance

Great, those are a lot easier to find.

Cheers

 

[Michael A. Terrell]

Definatly not!

see my last post, a 400V DC capacitor is approximatly equivilent to a 250V
AC capacitor

He would be better off with an AC rated capacitor designed not to
short out, The "X" or "Y" rated AC capacitors would be a good choice.
If the capacitor does short the motor will burn up, and could start a
fire.

 

[James Sweet]

10 UFD @ 250 VAC is the device specs. It is used as a start capacitor,
function is to lessen the electrical current through the start winding of
the motor. Should be able to appropriate capacitor at any industrial motor
service or HVAC facility. Even able to be ordered off the web if you google
for "start capacitors".

It depends on the type of motor, but it's purpose is to offset the phase of
the start winding in order to create a rotating magnetic field. It does
limit the current somewhat but that's not it's primary purpose.

I usually get motor caps from HVAC surplus dealers, they're a fraction of
new cost.

 

[James Sweet]

Graham,
looks like you are correct. It says 70V~ 50Hz on the motor. All the
start caps I have seen are pretty large things. The Motor here is very
small about 3/4 inch diameter and 1/2 inch deep. I assume that the
important point here is the frequency controlling the speed, not the
voltage. Is that correct?

I still haven't found a good replacement part yet.

Cheers Brian

Hmm interesting, yeah it sounds like the cap is just to reduce the voltage
then, and yes frequency is the important part. The cap should be non
critical, just don't go too big or you'll overheat the motor.

 

[James Sweet]

Would the replacement cap need to be 250V Ac? I can find various ones at
250 VDC, 250V AC seems to equate to 630V DC in the specs.

Would an electrolytic capacitor be correct? How does that work with
polarity (seeings its getting AC)?

Cheers

No, you'll want a mylar or other similar capacitor, at least 250vac rated,
but personally I'd look for something with a 400vac rating. If you can't
find one I probably have one in my junk stash I could send you for the cost
of postage.

 

[James Sweet]

i'll put my thoughts in on that.
i think it's set up that way as a torque regulator which can effect
speed.
that would mean that proper size of the cap is required.

Not on a syncronous motor, there's no need to regulate the torque on a time
clock, and you can't really do it, the motor will either run at the correct
speed or it won't run at all.

 

[Franc Zabkar]

If you have a selection of capacitors you could find the approximate value
empirically.

Most generic capacitors will show their max voltage rating as a DC value,
not an AC RMS value.
A 250V RMS capacitor should be replaced with at least a 400 volt DC rating.

My guess is the value is 0.1uF (same as 100nF)

Maybe the "10" just means +/- 10% tolerance

The Xc of an 0.1uF cap at 50Hz is 32kohm, whereas for a 10uF cap it is
320 ohm. If the motor is to be driven at 70V from a 240V (?) supply,
then the latter option would draw a current of ~0.5A and dissipate 35W
in the motor. The first option requires only 5mA and a 350mW motor.

-- Franc Zabkar

Please remove one 'i' from my address when replying by email.

 

[Jamie]

Hmm interesting, yeah it sounds like the cap is just to reduce the voltage
then, and yes frequency is the important part. The cap should be non
critical, just don't go too big or you'll overheat the motor.

i'll put my thoughts in on that.
i think it's set up that way as a torque regulator which can effect
speed.
that would mean that proper size of the cap is required.

 

[Jamie]

Not on a syncronous motor, there's no need to regulate the torque on a time
clock, and you can't really do it, the motor will either run at the correct
speed or it won't run at all.

your referring to a shaded pole motors ?
my education and experience tells me that synchronous motors need the
rotor energized with DC to force the rotor to lock with the stator which
gives you a good steady speed.
in many cases, you have a squirrel cage type rotor that is wound with
heavy copper configed so that it acts like a Async type motor on start
up getting the rotor to start turning with a very low torque and once
the rotor gets near the stator speed using a tach or maybe a centrifugal
type switch then applies some DC source via slip rings to the rotor.
it is true that synchronous motors do lock their rotors to the stator
speed but like i said, there has to be something else to help the rotor
to do so.
so what ever..

 

[James Sweet]

your referring to a shaded pole motors ?
my education and experience tells me that synchronous motors need the
rotor energized with DC to force the rotor to lock with the stator which
gives you a good steady speed.
in many cases, you have a squirrel cage type rotor that is wound with

No, shaded pole motors are different, syncronous motors as used in timeclock
drives normally have a permanent magnet rotor and a single field coil with a
slotted metal stator, you'll find one in any mechanical lamp or appliance
timer.

 

[Brian]

The Xc of an 0.1uF cap at 50Hz is 32kohm, whereas for a 10uF cap it is
320 ohm. If the motor is to be driven at 70V from a 240V (?) supply,
then the latter option would draw a current of ~0.5A and dissipate 35W
in the motor. The first option requires only 5mA and a 350mW motor.

-- Franc Zabkar

Please remove one 'i' from my address when replying by email.

Frank,

very, very, thin wires were used and in use the thing did not get in the
slightest warm so IMHO it has to be the first option. All the start caps
I have seen in my online searches were obviously for powerful motors,
this thing is tiny!


Hey thanks to everyone who answered this thread. I have learnt a lot and
it should be no problem getting it working again. A normal timer that
goes in a socket would not work as this timer controls a three pin
microswitch. Socket timers here always switch the mains (230v) on or
off, otherwise I would have just bought such a timer. I could have
attached a small circuit with a 230V relay to such a timer though!

Cheers