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Old 07-26-2017, 11:41 AM   #1
momaka
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Thumbs up KitchenAid KFP0924CU0 food processor repair

Not sure if there is a known problem with this food processor or this was just a random blowout. Either way, I’m making a thread here, just in case it is the former. Luckily, this was another simple repair that only involved changing a TRIAC again. This time, it’s a KitchenAid model KFP0924CU0 food processor.


Label:
http://www.badcaps.net/forum/attachm...1&d=1501086923

Pressing the “Low”, “High”, and “Pusle” buttons did nothing other than the processor producing a relay click sound. Pressing the “OFF” button would de-energize the relay (another click sound). And that’s it.

Tore it apart, and this is what it looked like inside:
http://www.badcaps.net/forum/attachm...1&d=1501086923

Motor model information:
http://www.badcaps.net/forum/attachm...1&d=1501086923

Top of the logic board (with old TRIAC already removed by me):
http://www.badcaps.net/forum/attachm...1&d=1501086923

Bottom of the board (with old TRIAC removed and a jumper placed between T1 and T2 by me):
http://www.badcaps.net/forum/attachm...1&d=1501086923

Original TRIAC was a BTA16-6CWG. It was open-circuited between terminals T1 and T2, but had a short circuit from Live (L) to Gate. So I started checking the driving circuit and found an open resistor. If I remember correctly, it was a 10 KOhm SMD. The driving transistor (TR2) was fine, however. So it looked like this food processor could be repaired.

Before purchasing any parts, however, I decided to check the rest of the circuit. It turns out that this food processor only uses the TRIAC for varying the speed on the motor (motor is a simple DC one, with permanent stator magnets). And in order to maintain the same speed, there is a magnetic tachometer on the motor’s shaft that gives feedback to the main controller: a PIC16F716 microcontroller. (On that note, I find it amusing how nowadays even simple devices like a food processor use microcontrollers. ) The power to the TRIAC is switched ON and OFF by a relay, which is why I heard the clicking noises when I tried to run the food processor (the relay also shorts the motor’s power wires when the food processor is OFF / not used, so that the motor blades will stock quickly and not move very much.) Thus, it looked like the microcontroller was also okay. Despite that, I decided to remove the old TRIAC and temporarily put a jumper across terminals T1 and T2 on the board to just test the motor. If everything was working okay, the food processor should run at full speed on both the “Low” and “High” settings. So I put the jumper on there and tested. Motor came running at full speed when the speed buttons were pressed, as expected… and turned OFF when the “Off button was pressed

Funny enough, I had the same exact TRIAC in my spare components box that I bought long time ago as a spare for fixing a busted cheap light dimmer from eBay (damaged by a “pro” technician way back when I worked in the small repair shop). So with that and a spare 10 KOhm SMD resistor from a busted CRT TV board, I got this food processor going quickly again.

Here is a picture with the new TRIAC installed:
http://www.badcaps.net/forum/attachm...1&d=1501086923

And since this is badcaps.net, here is a shot of the capacitors inside this food processor :
http://www.badcaps.net/forum/attachm...1&d=1501086923
As you cans see, they are some cheap, no-name brand. The logo looks like “LM” to me. Series is “GM”, and the caps are general purpose, rated for 85°C. From what I remember, they were used for smoothing power to the microcontroller. Power for the microcontroller is derived in the usual cheap manner: primary-connected film capacitor in series with a resistor in series with a Zener diode to primary neutral (ground). I imagine when the film cap goes leaky and cooks itself, this food processor will be coming my way again. But for now, it is fixed and working.
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Old 07-26-2017, 03:58 PM   #2
eccerr0r
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Default Re: KitchenAid KFP0924CU0 food processor repair

<rant>Whatever happened to AC motor kitchen tools? Is the day coming everything will be DC and thus can be easily be part of the IOT?
</rant>

Good job, I'm shocked that they've started apparently phasing out AC motors...
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Old 07-27-2017, 10:54 PM   #3
momaka
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Default Re: KitchenAid KFP0924CU0 food processor repair

Quote:
Originally Posted by eccerr0r View Post
<rant>Whatever happened to AC motor kitchen tools? Is the day coming everything will be DC and thus can be easily be part of the IOT?
</rant>
Well, there is nothing wrong with DC motors in kitchen tools and appliances. After all, DC motors have a high starting torque and thus can be much smaller and cheaper than an equivalent-spec AC motor.

We have an AC motor -based food processor here leftover from a family friend when they moved back overseas. It is an older model and much much heavier than the food processor shown above. Also only has a simple maintained ON/OFF button and nothing else. But it works, and probably will do so for a very long time due to its simplicity.

What I am more amused with is the fact that almost everything has a micro controller chip nowadays.

The food processor above, for example, could still have 2 speeds and not use a micro controller. All they had to do was run the DC motor on either full rectified AC or half-rectified AC. We actually had a cheap blender done like that about 10 years ago. Very crude implementation, but effective and reliable. No TRIACs, controllers, and associated BS. It did burn out, but that is because my dad overloaded it. And when it burned out, it really just needed a new fusible resistor. Motor is okay. If the case wasn't cracked, I would have fixed it.

As for maintaining the food processor ON/OFF state, that could have easily been implemented with a simple self-biasing relay circuit. (This is like PLC 101 basic stuff).

That said, if the above food processor comes my way again due to bad TRIAC, I will modify it and simplify it as mentioned above.
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Old 07-29-2017, 01:44 AM   #4
eccerr0r
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Default Re: KitchenAid KFP0924CU0 food processor repair

Well, usually nothing goes wrong with AC motors (except that shaded pole motor that I have...grr...) whereas all these oh-so-reliable switching power supplies to feed these DC motors...

Yeah, must be switching else all that weight would be lost out on a line frequency transformer....

I think the microcontroller indeed is superfluous but I suspect a PWM controller is still needed for low speed and high torque, half rectified AC probably would lose a lot of torque on DC motors.

Crazy designs these days...
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Old 08-01-2017, 08:42 PM   #5
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Default Re: KitchenAid KFP0924CU0 food processor repair

Quote:
Originally Posted by eccerr0r View Post
Well, usually nothing goes wrong with AC motors (except that shaded pole motor that I have...grr...) whereas all these oh-so-reliable switching power supplies to feed these DC motors...
This food processor does NOT have a switching power supply though. The TRIAC is there just for the low speed. At full speed, it is fully conducting. There is a full bridge rectifier after the TRIAC. The motor is DC, but it is rated for 120V, so no need for SMPS.

That said, AC motors do fail often as well. It just depends on the type of motor.

For cap-start AC motors, you can expect the cap to fail. If thermally protected, of course, that is all that should go wrong with it.

Then there are the centrifugal switch start AC motors - with those, when the switch fails, the motor will fail to start (and burn itself out too, sometimes - even when thermally protected).

And universal AC/DC motors have brushes, so that's what can fail on those.

Really the only AC motors that are built to last a super long time and at 100% duty cycle, are 3-phase induction motors.
... and shaded pole motors too. But I guess yours was an exception.

Quote:
Originally Posted by eccerr0r View Post
I think the microcontroller indeed is superfluous but I suspect a PWM controller is still needed for low speed and high torque, half rectified AC probably would lose a lot of torque on DC motors.
No, you still get decent torque on a DC motor, even when fed with half-wave rectified voltage. It just tends to make the motor buzz and vibrate a lot.

That said, the microcontroller in the above food processor does not use PWM but pulse-skipping - that is, on low speed, it only fires the TRIAC on the zero-crossings of the AC wave. - Much less noise/RFI/EMI fed back into the line that way. Not like standard light dimmers that can cut off power to the device at any point on the AC wave.

Last edited by momaka; 08-01-2017 at 08:46 PM..
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Old 08-22-2018, 04:36 PM   #6
elvis1965
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Default Re: KitchenAid KFP0924CU0 food processor repair

hi how do you take off the little gear of the motor as the new motor did not come with one
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Old 08-23-2018, 05:37 AM   #7
momaka
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Default Re: KitchenAid KFP0924CU0 food processor repair

Can you post a picture of your old and new motor? I can't remember what mine looked like. Normally I take pictures of everything. But here I didn't take a picture of the motor's gears, because the motor was good and I didn't have anything to do with it. Unfortunately, I don't have the blender anymore as it belonged to family friends.

In any case, if the motor has a brass or steel gear, the gear is probably heat-pressed onto the shaft. Might need to heat it to a high temperature of 70-150C (either with a heat gun or in some cases even boiling water works - just be careful with that), and then it may come out.

That said, are you sure your motor needs replacing?? I expect the TRIAC and other driving components to burn out way before the motor does, even if the motor was supposedly stuck. The motor is a simple DC type with permanent stator magnets. So for testing it out, you can actually plug it in straight to the (+) and (-) terminals of the bridge rectifier on the board. Then plug the food processor cord in the wall. This should make the motor turn. (Though before doing that, I highly recommend putting a 60-100 Watt incandescent light bulb in series with the food processor's live rail to limit current, should the motor really turn out to be bad/shorted).

Last edited by momaka; 08-23-2018 at 05:45 AM..
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