In this part I’ll describe restoring and rebuilding the motor assembly of a Kenwood Chef model A700-A. See part 1 for a description of removing and disassembling the motor.
Motor and Electrical Restoration
I probably spent more time on repairing and rebuilding the motor and electrics than anything else – the bit you don’t even really see! The first job, as with most of the mixer, was cleaning the oil and degraded grease off the parts. The field coil was particularly bad as it was soaked through with oil. I didn’t want to damage the winding wire insulation with aggressive cleaning solvents such as hexane, so I cleaned it by flushing it many times with isopropanol. This removed a lot of oil and did no damage to the coils. I also used isopropanol to clean up the armature.
The commutator was dirty and quite badly scratched, so I polished it back using a strips of emery paper (600 and 1000 grit) whilst turning the armature with a drill. You can see a demonstration of this technique in this YouTube video. If you do this, be careful to clean away all of the copper dust so that it doesn’t short the commutator bars.
To finish the exterior metal parts of the motor assembly, I rubbed back the existing paint using a synthetic “steel wool” pad and painted them with satin black with Rust-Oleum Universal spray paint.
When putting the field coil back into the brush holding (bottom) end-bell, as illustrated in the above photo, there are two important things to note. Firstly, the orientation of the two parts (end bell and coil) should be the same as it originally was in order for the motor to run optimally (or even at all). Secondly, each brush wire must be connected to the brush on the same side as the wire as shown. This is what determines the rotation direction of the motor. Note that I have kept the brush wires short and positioned them out of the way of the rotating armature.
It is important to correctly orient the non-brush (top) end bell as well, or you will have problems getting the correct relationship of this end to the shroud (see below). Orient the bell so that when you look directly down on it, a line between the two bolts is 90 degrees to the two brush housings. This will ensure that the bolts are straight. Because the bolts are long, it is easy to assemble the motor with the bolts slightly twisted (i.e. not parallel to the motor spindle) if you’re not careful.
The original brushes were worn down less than half way, but they seemed very hard compared to new ones (when rubbed on a piece of paper), so I decided to replace them with new ones bought from eBay. The new brushes had a different spring arrangement with a brass cap joined with braded copper wire (see upper brush in photo below). I did try installing them like this, but it was very difficult to get them in, and once in, they seemed to push too hard against the commutator such that they were binding and squeaking when the armature was turned. So I cut the copper braid near the brush, and soldered the braid stub to the original springs (lower brush in photo below). This fixed the issue.
The next step was to fit the assembled motor back into the shroud. The motor must be oriented in the shroud such that the two motor bolts are in line with the speed-control-knob shaft as illustrated in the drawing below.
Before putting the fan and wishbone assembly back on the motor shaft, I bolted the bypass resistor to the outside of the shroud. I replaced the original exposed-wire resistor with a ceramic encased 20W, 470 Ohm wire-wound resistor. This particular resistor, pictured below, came with a handy mounting bracket attached which I bent 90° to allow the resistor to sit in the best position. The resistor is made by TE connectivity, part number SQBW20470RJ. The function of this resistor is described below.
Now we can refit what I call the fan and “wishbone” assembly to the motor shaft. I ended up pushing it up as far as it would go so that the view of through the round port-hole was as shown below. This gave me a minimum running speed that was a bit high, however I had tested the motor with the wishbone a millimetre or two lower, and the slowest speed wasn’t perfectly smooth (it pulsed a bit). I think that under load this may smooth out a bit, but too bad, I’m not pulling the whole thing apart to adjust it now!
Next it was time to finish the wiring. Below I provide a diagram I made from the original mixer wiring:
The wiring’s pretty simple, so let’s discuss its workings as I understand them. First we have the mains wiring coming in, with a 50nF EMI suppression capacitor between the live and neutral, know as an “across the line” configuration. The original capacitor was a paper capacitor which had blown out of its metal can at some point in the past. Note that for safety, the replacement across-the-line capacitor should be “X2” class and have an appropriate voltage rating.
The neutral line connects directly to one of the motor wires. The live line connects to the other motor wire via the on/off switch and the governor switch. The governor switch is bridged by what I call the bypass resistor (because it bypasses the switch). It also has an RC snubber pair across it, consisting of a 5 Ohm resistor and a 100nF capacitor. A snubber is a circuit designed to reduce or eliminate arcing across the switch caused by switching the inductive load of the motor.
Speed control of the motor is by way of a centrifugal mechanism involving the wishbone and the governor switch. As the motor accelerates after switch on, the curved copper arms which hold the wishbone and have little weights at the ends, move outwards through centrifugal force. As they do so, they protrude further through the holes in the wishbone, forcing the wishbone downwards (because of the curvature of the arms).
The pointed bottom of the wishbone will move down until it pushes on the governor switch and opens the switch. When this happens, the current can only flow to the motor via the bypass resistor, which impedes the current flow and thus slows the motor. As the motor slows, the wishbone moves back up, allowing the switch to close and restoring full power to the motor. This on/off sequence happens rapidly such that the overall effect is that the motor sits at a more-or-less constant speed set by the position of the governor switch. The speed control cam moves the governor switch up and down as the speed dial is turned, thus changing the speed of the motor. The function of the bypass resistor is to provide a smoothing action by reducing the deceleration rate of the motor on switch opening. The downside is that a lot of energy ends up being dumped in the resistor, especially at slow speeds, which ends up getting very hot. I measured a surface temperature of around 220-250°C for my replacement resistor. That’s one reason it’s mounted on the outside of the shroud – to prevent all this heat passing directly into the motor. The other is size – resistors that can handle this power tend to be large.
I wanted to make a tidy job of the wiring and not just have components hanging in the wind by their leads, so I designed a PCB for the small resistor and capacitors (which are a lot smaller than the paper originals) which would sit in place of the old solder lug board.
As originally wired, the mains earth wire was connected to the metal shroud via one of the connection board mounting screws (now one of the PCB mounting screws). I wasn’t completely satisfied with this, so I piggybacked another earth wire off this connection and took it back out to connect it directly to the mixer body via one of the cable clamp screws under the mixer base. This is the blue wire in the above photo (the green/yellow earth wire I had was too heavy).