Introduction: Kenwood Chef A902 A904 AC Motor Speed Control Module

I collected some good informations about dimming motor speed using feedback.

This will give you a regulated speed.

Also I added my experiance and all my availble data for free which took about 50 hours of work.

Step 1: Using Magnet to Control the Motor Speed



The last day I received a damaged Kenwood chef mixer A902.

I noticed that this old appliance uses a tricky method to control the rotation speed.

It uses a feedback from the pick-up magnet.

Step 2: Faradays Law of Induction

This is how the magnet makes fulx control.

Step 3: How Kenwood Controls Motor Speed?

As soon as the magnet begins to close-up to the coil, so it induces more magnetic flux by rotation, so the circuit reaches the trigger value in slower speed.

So, when we need to increase the motor speed we pull the control board away from the magnet (which is a ring fixed beside the cooling metal 12 blades fan hub).

Step 4: How to Calibrate the Motor Speed Knob.

After troubleshooting the speed control module. Special care should be carried out when calibrating the speed of the motor.

The aging of the magnet will reduce the generated magnetic field induced by rotation. So, the board should be calibrated to a closer position to the motor. You can detect that in case the motor reached its maximum speed before the main speed control knob reaches the highest position (6).

You can adjust that by putting the knob to the highest position (6) then turning the two calibrating screws clockwise until the speed starts to slow down.

N.B. These two screws which are fixed by two springs, should be balanced. Also no need to disassemble the unit for calibration. Just turn the machine upside down, and look at the adequate place inside the protective perforated black cover. They should be located on the opposite side of the speed control knob.

Step 5: Using Triac - Diac to Make the Dimmer

The Triac will cut the main power sine wave as soon as it reaches the trigger level. (Dimmer)

Now, I read a lot about damaged Triac PH 71719. Any Triac with the same pin assignments and the same rate can work. Like: BT137. (8A, 600V)

This machine consumes no more than 600 Watts at start up. Or in case of using hard pastes.

The written specifications are: 240V 375W. So BT136 (6A) is enough.

Step 6: Why the Triac Could Be Damaged?

The main cause of damaging the Triac is Brushes' issue.

The Carbon brushes wear by time. And they produce conductive black smear on the holding sockets and the motor copper collector which increases the power consumption, spikes and reduces the efficiency. So to prevent such issue it's better to maintain the appliance by a qualified person each 5 years by normal home use.

Step 7: How to Clean and Check the Brushes?

1- Slide the brown cover as illustrated.

2- Push the holding tooth

3- Then release and rotate the cover and remove it as shown

4- Use a screw driver to unscrew the holding screws.

5- Remove the brush from its holding spring. And mark one side of the brush.

6- Clean the brushes and the internal and external sides of the holding sockets. Also clean the copper collector by Alcohol. And leave them to dry.

7- Take care to replace the brush in the same direction in reverse order, when finished cleaning. Because the brush sides wear differently.

8- Tighten the screws and put the brawn cover in reverse order.

Step 8: Other Components Which Should Be Tested or Replaced

Some capacitors could be damaged by aging but please take care not to change them with commercial types as these capacitors are from type X2. Which is long surviving, but the better one is from Y type which is safer in electric insulation if damaged., By the way and as these capacitors are embedded and protected into the machine so X type is enough.

The speed control module also contains a 275 v 593 BC varistor, it will be damaged if the main current reached more than 275 volts. In normal condition, it should give high impedance.

Step 9: Kenwood A902 A904 Schematic Diagram and PCB

I added a top view of the PCB (component side) to help you solving the issue.

Also I added the cable colors the the scematic

There are 4 wires for the motor:

Two are coming from the motor brushes (marked as Orange - Gray)

and two are coming from the field coils (Red - Blue)

the main power (light Blue - Brown)

Step 10: What Is the Green Hidden Button Used For?

If you raised up the main arm of the appliance you can detect a green button.

This is the overheating protection. it will be released in case of overheating.

In such case:

1- Just turn off the system

2- and wait for about 10 minutes

3- Then tightly push this green button.

4- And restart the appliance.

In case of reoccuancy, so you should look for hardware issue such as smears on the brushes, worn brushes. or unsufficiant grease.

Step 11: Maintaining the Bearings - Kenwood A901 Service Manual

Maintaining the bearings:

Grease degrade by time. And you should clean it by

1- Disassembly of mechanical parts

2- Then using grease solvent such as gasoline then

3- Waiting to dry before adding new suitable grease.

N.B. Please, be careful not to contaminate the plastic parts with gasoline

4- Then reassemble by reverse order.

More helpful information can be found inside the attached Kenwood A901 Service Manual

Step 12: Add or Replace Rubber Feet for Your Appliance

Kenwood mixer usually comes with rubber feet.

They will be damaged by time.

I addad an llustrated pictuers to explain how to add non-standard rubber feet.

This will help you adding rubber feet for any other appliance using threaded-Inserts collected from old laptop covers. by the use of heating gun.

I fixed these inserts as shown in the attached pictures using dental self curing Acrylic. which comes as (Liquid - Powder)

Ask a dental lab to teach you how to use this. :)

You can google it, "DIY using self curing acrylic resin"

Prepare the resin mixture then put the screw with the threaded insert in place.

Also you can use two components fast set Epoxy.

Wait to harden

Then unsrew and clean all excess by the aid of knife. then reassemble.