Kitchenaid Phase Control Board Repair (2022)

Kitchenaid Phase Control Board Repair (1)It's a story of how I spent three days solving problems on a 9 element circuit when 7 of them are passive. I haven't found what's wrong, but I fixed it.

Preface:
I have a control board marked W10354309, it is a European model 220V phase regulator.
I don't really understand the rules of how Kitchenaid marks their parts, because I found several part numbers for the 220V version: 3184417, 4163707, 4163712, 9701269, 9706596, W10217542, W10538289, W10911442, W11174552, WPW9053828
(110V version has the same idea and schematic, just different values ​​and ratings for the same items)
I'm not sure why they do this. Probably because they use the same part on different models and/or brands.
So I have a 5ksm125 mixer and a W10354309 phase control.

The service manual says that in first gear the planetary shaft should be around 60 RPM, but in my case it was around 120 RPM and I couldn't get it down by adjusting the control board.

I must digress here, these mixers have the ability to maintain constant RPM under different loads. I was surprised when I found out how they do it. One of the main components comes from the era of steam engines, it is the centrifugal governor that is placed on the motor shaft, here it is:

The yellow is the governor itself, the black cylinders highlighted with green weights, the center pin outlined in blue is a pin that provides feedback to the control board. It works in a simple way, the more RPM the motor has, the more pins are spread out.

The next component is called the control board, it actually has a simple main switch and a T-shaped contact. The main switch only breaks the circuit when you move the switch handle to the off position. The T-contact plate simply shorts the contacts on a plate in 3 different configurations. The white tab in the picture above is a dielectric tab on the T contact, the center pin on the regulator pushes this tab and changes which contacts are closed on the control board. Here is the control board on the other side:

And the last component is a phaser board, it's basically a dimmer if you google 'dimmer circuit' you'll find the same schematic used on the phaser board with one exception dimmers usually have a Variable resistor for smooth regulation, phase control board has resistor network where resistors are shorted across the control board in 3 different configurations. You can see it behind the top edge of the control board in the photo above and in the close-up of the photo below:

So how do they work together? Here is the repair manual schematic with comments and ratings added by me:

As I said before, the control board can be in 3 different states:
First: Engine RPM is too low or does not turn at all. Totally closed T contact, completely short-circuits the resistor network (R1, R2, R3, R5) and feeds the motor with almost full sine wave (DIAC Q2 opens around 30V, so the start of the wave is cut off a bit)
The third:the engine has too many RPMs. T contact fully open, the resistor network has maximum resistance, the phase control board supplies the motor with the minimum amount of power (the manual says it should supply 40V RMS, I don't understand why this is true for the 110 versions). V and 220 V, but it seems that it is).
The second: this state is between very low RPM and very high RPM, short on control board R1, equivalent resistance is ((R5+R3)*R2)/(R5+R3+R2), manual says to supply 80V RMS.

The higher RPM the engine has, the more center pin of the centrifugal governor extension, the more the T contact shifts. When the T contact changes, it opens the circuit with first the lower contact and then the upper contact (see diagram above) . Selecting the speed of the mixer changes the distance between the control board and the governor, the greater the distance, the faster the motor must rotate to achieve the balance between the first and third states.

I can finally talk about my problem.
Usually when the phasing control is broken the mixer won't cho-cho or goes to full speed my story was a little different I had a near constant 120 RPM's on the first 3 speeds the next speed or two increased the Full RPM, and other speeds did nothing.

When I saw the schematic I was sure that I just needed to replace the DIAC. In circuits like this, if something goes wrong in 99 cases out of 100, it's caused by a broken semiconductor. When the TRIAC fails, it usually stays open or shorted (the motor should not run or run at full speed).

I changed the DIAC but nothing changed, engine was above rated RPM but not maxed out. The RPM was enough to fully extend the governor center pin and open both control board contacts.

The next suspect was the TRIAC, here it's just two semiconductors, if one of them is good the other is broken right? Wrong. I have tried two different TRIACs without success. BTA12-600SW (has the same features as the original. Logic level gate, 10mA gate current, no snubbers, but rated for 12A instead of 6A) and BTA06-600CW (no logic level and had a current gate current around 35mA, it produced visible sparks during resets on the control board, so don't use it).

What should be suspected next? Capacitors? Both had less than 5% capacitance difference from their nominal values. I've tried other capacitors, the motor RPM changed but not significantly (in theory, failing capacitors can have a noticeable different capacitance under high voltage, but I tested them with a low voltage LCR meter).

After that I went crazy, I even desoldered all the resistors but they had the correct values.
I spent almost 3 days trying to figure out what is wrong.
I had many theories: faulty resistor that gets hot when voltage is applied and changes its resistance, half broken wires, half broken motor, etc.
I even found a thread where people had the same problem but no one could find the solution:https://www.electronicspoint.com/forums/threads/kitchenaid-mixer-phase-control-board-problem.241021/page-2

Poco después de comenzar mis experimentos, descubrí que todo funciona como se esperaba cuando puse R4 con un valor mayor, pero quería averiguar por qué el circuito que tenía los elementos correctos no funcionaba como debería.
At the end of the third day, I gave up. I tried to change all the resistors, all the capacitors in the circuit and it didn't work, I tried to solder wires in parallel with the existing ones,
In the end I decided to put 3.6KOhm R4 instead of the original 560Ohm.

Here are my observations:

  • The manuals say you can check the phase control by putting a sheet of non-conductive material (like paper) between the T contact and the contact it touches, if that's ok it should give around 40V but I have 50V. When I amed the voltage to 40V, I got a response from the control board regulation.
  • The motor starts spinning around 9V DC.
  • The stator coils have a resistance of 7.8 Ohm each, the rotor has a resistance close to 4 Ohm between the closest contacts, the resistance of the motor (between the red and white wire) close to 40 Ohm.
  • The circuit is sensitive to element values, even when I tried to put capacitors with the same value, I got slightly different RPM's. My circuit has 1% R5, the old manual schematic has 3 resistors in series, usually this approach is used when the resistors have a breakdown voltage less than the voltage drop or when you want to use some cheap 5% 10% resistors instead ofneed1.
  • Phase control boards probably more stable with different part numbers. I found photos of other boards and saw that the resistors have different values ​​than the values ​​I observed. Here is an example from amazon:

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