cdosrun
Enlightened
Hello, I don't think I have asked any non-light questions before but there are so many very capable people on here, I have no doubt there is a good answer.
I have been given an old Kenwood Major kitchen mixer. The model is a 707a and I have finished overhauling the mechanics and have turned to the motor. The brushes seem quite good still and I have finished cleaning, lubricating and reassembling the motor. The only difficult I have is with the speed controller. The motor is a series-wound type, rated at 450w, and has a electro-mechanical speed controller; there is a centrifugal mass on the bottom of the motor which extends along the plane of the rotor when driven at speed and this pushes on a contactor on the speed controller. The speed is varied by moving the contactor up and down so the centrifugal mass operates the contactor at higher and lower speeds.
The motor has 4 wires, with the series circuit interrupted between one of the field coils and the armature and the contactor switch is placed here. There is a 450r resistor shunted across the switch (I presume to reduce the switching frequency, as the motor would slow down more slowly) as well as a spark suppression capacitor and a couple of small inductors to quench the spark and reduce noise.
The contactor plates were, as one can imagine, heavily pitted over around 40 years of use and the resistor is showing signs of heat damage. I have cleaned up the contacts which have a heavy bi-layer contact material and could take some resurfacing and could replace the other components and leave as is. However, I am contemplating whether I should supplement or replace the speed controller. The service manual for the machine shows another version (presumably later) which used a triac that was switched by the contactor but given the low RDSon of modern MOSFETS, I was considering using 2 in series to switch the AC in place of the triac. However, the triac would have a much lower switching frequency given that it would hold on for a half cycle whereas the MOSFET would be switching on and off all the time.
If anyone has deciphered my ramblings, would you please let me know if you think it would be best to simply insert the triac or whether the MOSFET is a better bet or, ideally, if it would be better to use a micro with PWM (and crossover switching) together with a speed pickup? I am certainly keen to remove the need to have the 450r resistor as it runs remarkably hot when the mixer is used at low speed (such as for kneading bread) and this is something that can be done for a long time. I would upload the relevant pages from the service manual but presume there would be copyright issues so a crude circuit diagram will follow.
Thank you,
Andrew
I have been given an old Kenwood Major kitchen mixer. The model is a 707a and I have finished overhauling the mechanics and have turned to the motor. The brushes seem quite good still and I have finished cleaning, lubricating and reassembling the motor. The only difficult I have is with the speed controller. The motor is a series-wound type, rated at 450w, and has a electro-mechanical speed controller; there is a centrifugal mass on the bottom of the motor which extends along the plane of the rotor when driven at speed and this pushes on a contactor on the speed controller. The speed is varied by moving the contactor up and down so the centrifugal mass operates the contactor at higher and lower speeds.
The motor has 4 wires, with the series circuit interrupted between one of the field coils and the armature and the contactor switch is placed here. There is a 450r resistor shunted across the switch (I presume to reduce the switching frequency, as the motor would slow down more slowly) as well as a spark suppression capacitor and a couple of small inductors to quench the spark and reduce noise.
The contactor plates were, as one can imagine, heavily pitted over around 40 years of use and the resistor is showing signs of heat damage. I have cleaned up the contacts which have a heavy bi-layer contact material and could take some resurfacing and could replace the other components and leave as is. However, I am contemplating whether I should supplement or replace the speed controller. The service manual for the machine shows another version (presumably later) which used a triac that was switched by the contactor but given the low RDSon of modern MOSFETS, I was considering using 2 in series to switch the AC in place of the triac. However, the triac would have a much lower switching frequency given that it would hold on for a half cycle whereas the MOSFET would be switching on and off all the time.
If anyone has deciphered my ramblings, would you please let me know if you think it would be best to simply insert the triac or whether the MOSFET is a better bet or, ideally, if it would be better to use a micro with PWM (and crossover switching) together with a speed pickup? I am certainly keen to remove the need to have the 450r resistor as it runs remarkably hot when the mixer is used at low speed (such as for kneading bread) and this is something that can be done for a long time. I would upload the relevant pages from the service manual but presume there would be copyright issues so a crude circuit diagram will follow.
Thank you,
Andrew
