Help identifying suitable replacement resistor. Kitchenaid K45 Stand Mixer

[hdunnicliffe]

First off, I'm a complete beginner when it comes to electronics and posting on forums. So please be gentle.

I'm attempting to restore a Hobart Kitchenaid Stand Mixer (Model K45). This a pre-solid state version, which has a different speed control setup.
As well as a physical restoration (Cleaning, painting etc) and want to convert the unit to run on UK mains power (They only made a US version of this model). I've replaced the motor parts with one suitable for our mains voltage/frequency and this is working well. However, the speed control circuit is giving me some trouble.

The speed control on this model is very simple and is achieved by switching the motor between running directly on the AC line voltage or through a resistor. This video helped me understand how it works
I'm struggling to find much about this resistor but I'm assuming it will only be rated for US mains and I'll need to replace it with one suitable for UK mains.

What I know about the Resistor so far
Name: INDUCTIVE WIRE WOUND FIXED RESISTOR
Part Number: B-89760-1
Rating: 180ohm with centre tapped wire. (Confirmed by myself using a meter, out of circuit)

What I've tried so far
I bought two of these https://www.rapidonline.com/Catalogue/Product/62-0178
I wired them in series to create a 200ohm resistor with a centre tapped wire and I confirmed the resistance measure correctly before installing.
However, when I ran the motor it did not perform as expected and when I retested the resistor they now show as open/the two leads of the resistor are not conected (sorry, not sure what the correct term is here).

So the question is, does anyone know of a suitable replacement part? Or what rating I need for a replacement?

P.S.
I don't expect to find a resistor which is physically compatible. I'll make what I find fit.

[hdunnicliffe]

I've done a bit more research and found this 100w version of the same resistor I tried before. Does anyone know if this is likely to work?
https://www.mouser.co.uk/ProductDetail/ARCOL-Ohmite/AP101-100R-J?qs=sGAEpiMZZMtlubZbdhIBIHWLW9HNIjUk%252be7%252b%2fLwhnZI%3d

[Nusa]

First off, those resistors you're looking at only get their rated power if they're attached to an appropriately sized heat sink. By the time you take that space requirement into account, you might as well have used cheaper wire-wound resistors intended for open-air.

Second, take a step back and apply some math. Look up Ohms law, if you don't already know it.

Let's take the original 120V configuration. If we assume the motor resistance is near-zero, then what power will a 180 ohm resistor drop at 120V? The current will be I = V/R = 120 V / 180 Ohms = 0.67 Amps. The power will be P = IE = 0.67 Amps * 120V = 80 Watts. (Those formulas can be combined to get P = V*V/R = 120*120/180 = 80.)  However, in practice, whenever the control contacts are cycled closed, the current will flow through them instead of the resistor, so the average wattage over time will be significantly less -- worst case would be at the lowest speed. The guy in the video is probably correct in that the primary purpose of the resistor is to reduce wear on those contacts.

Now let's try it at 240V. P=V*V/R = 240*240/180 = 320 Watts! Double the voltage, quadruple the power! I suspect you also want to quadruple the resistor value to get it back down to the 80 Watt ballpark. Adding resistors in series to both sides of the 180 Ohm resistor you already have is one way to achieve that, if you can find space for them.

Disclaimer...top of my head analysis with a minimum of fact. It's on you to think about it and make sure what I said makes sense to your actual situation.

[Ian.M]

Edit: Nusa got there before me and we are basically saying the same ting, but I gave less maths and more details, so I'm posting this anyway.

UK mains is double the voltage of US mains.  That means the motor current of an equivalent motor designed for that voltage will be HALF that of the US motor for the same motor power.    Therefore you would need FOUR times the resistance (as the resistor needs to provide double the voltage droop at half the current).

According to the https://www.scribd.com/document/176382158/Kitchen-Aid-K45-Service-Manual-For-Hobart-Made-Vintage-Mixers]K45 service manual[/url], (page 15) the US resistor should measure 180 ohms between Red and Yellow and 90 ohms between Black and either coloured wire.

Your replacement resistor assembly should therefore total 720 ohms, evenly split in two halves of 360 ohms.  Three stubby 120 Ohm axial resistors in series for each half of the ring is probably the best option, and with care and some glassfibre string and fire cement, should be possible to assemble into a ring similar to the original.

With 115V across the original resistor, it would dissipate 73.5 watts.  That would be the worst case dissipation if the motor stalled before the governor acted to short out the resistor.   Four times the resistance across double the voltage would have the same dissipation, but the UK mains voltage (typically 240V) is about 4.3% higher than the nominal EU voltage of 230V, so  worst case, the replacement resistor needs to be able to dissipate 80W.   Whenever the motor is actually running the voltage across the resistor is reduced in proportion to the speed, and the duty cycle for which the resistor is in circuit is also reduced similarly (but increased in proportion to the load) so you don't need much safety margin above 80W as the power in normal operation will be significantly less.  Assuming you go with 6x 120 ohm resistors, they *should* be to be 14 or 15 watts each, but you'd probably get away with 10 watt ones.

The resistors you purchased are intended to be mounted to a heatsink.  They are only rated for 2.25W in free air at 25 deg C, so it isn't surprising they blew as you had them dissipating 72W each (instantaneous power), 20% more than their surge rating of double their 30W nominal rating when heatsinked.  Also they are plastic packaged, with a maximum operating temperature of 150 deg C.  A vitreous ceramic coated wirewound resistor like the original typically has a maximum rated surface temperature of around 300 deg C

The K45 electromechanical governor is known to run rather hot at full load.  If it reaches 88 deg C inside the endcap (at the resistor surface), you'd have to derate similar TO-220 plastic case resistors to only half their nominal power rating.  Vitreous ceramic wirewounds have a much much higher operating temperature, so will only need to be derated to 3/4 their nominal power rating at the same temperature.

Edit: corrected a typo

[hdunnicliffe]

Thank you so much Nusa & Ian.M for taking the time to look into this and post replies.

Your explanations have cleared up a lot of my confusion. I did try work out the wattage rating I'd need but I got stuck because I thought I needed to know how many amps the motor would be pulling through the resistor, this makes a lot more sense.

I like your idea of combining resistors to build a ring and that's definitely something I'll look into. 

Thanks again!

[Nusa]

If you don't reuse your 180 Ohm resistor, it's probably something you can sell on Ebay to a 100-120V country. It's an unobtainium part for the K45, after all.

[Ian.M]

If you can run at its slowest speed, empty, from 120V with the end cap off and the original motor installed. and monitor the ring resistor surface temperature with an IR thermometer up close (or with a bare thermocouple probe with a dab of heatsink compound and a narrow wrap of aluminum foil to hold it in contact), you could measure the max surface temperature after 10 minutes running.   Remove the resistor, connect it to a Variac and slowly bring up the voltage till it holds the same temperature, and you can measure the voltage and calculate the actual average power the replacement resistor needs to dissipate.   It may well be that 80W total is *far* more than it needs to be rated for, and lower power physically smaller resistors will certainly be easier to fit in place of the ring resistor.

Personally, I'd just stick a 1KVA site safety isolating transformer in the back of a kitchen cupboard, and wire its output to an imported US socket strip instead of converting the mixer, but I'm lazy that way.

Another option would be to develop an electronic speed control for it.   All the original K45SS 'solid state' speed control does is replace the resistor with a TRIAC dimmer circuit.  If both contacts make, the TRIAC circuit is shorted out for 100% power, but (with the governor properly 'tuned') if one contact makes it boosts the motor power to around 50%.   The minimum power would need to be tweaked till it didn't overspeed with an empty bowl at the minimum speed setting.   The K45SS TRIAC circuit can be found online - no component values, but they'd all need to be recalculated for 240V anyway. 

It also might be possible to mod it to make the governor operate a drill trigger speed control module.

[hdunnicliffe]

Interesting idea, that process requires a few tools I don't currently have (although this could be a great excuse to get them). I think for now I'm going to attempt to build a resistor which can handle the worst case. But its good to know the procedure I could use if I think I can't make the tolerances work and need to push the wattage limit down.

They actually make a 220-240v version of the speed control board (phase board) used in the solid state version. So worst case I can convert the machine to use solid state controls. I've seen a YT video showing that conversion is possible and relatively easy. Clearly, this is the saner approach the solving the problem, but I like a challenge and this project it more about me learning than a practical conversion. Plus I like the idea of having a "one of kind" custom 240v old school k45 mixer.

I'm having some trouble finding short enough resistors for the ring. Everything I've seen is around 50mm long, I'd need it to be a max of 25mm and ideally 20mm in order to build a small enough ring.   However, while on the hunt I found this resistance wire: https://www.conrad-electronic.co.uk/ce/en/product/429074/Resistance-wire-100-m?ref=list.
Its 100 /m and can handle 0.166A @ 60c. So if I double it up it can take 0.332 which if my maths is right is my worst case max amps (240/720 = 0.332). It's going to get hotter than that because it's bunched up and wrapped around a ring but I think 60c is lower enough that even if it ends up double that'll be ok.

So for each half, I should need two wires, 3.6m long. My idea is to take the wire and wrap it around a ring to build a physically compatible resistor. But I'm a bit worried about building an electromagnet. If I use a non-metal ring that should help reduce the problem. If I wrap the two halves (each 360 section) of the resistor in opposite directions, could I cancel out any induced magnetic field? Another option could be to create the ring out of the coiled wire (might need some thicker wire in the bundle to hold the shape), any thoughts on which direction I should wrap the wire? (I've attached a terrible diagram, which hopefully shows the 2 wrapping options clearer).

I think the resistance wire could work, but I'd like to know your opinions and If my maths/understanding of the wires specification is correct?

[Ian.M]

You are most unlikely to  able to build a ring shaped wirewound resistor from enamelled resistance wire to handle the dissipation in that space that's safe with 240V mains applied to it.  You'd need a heatproof former to wind it on (possibly hand-carved from a slab of Steatite), and the wire needs to be locked in place by a layer of heat resistant cement, so you would have to fire the completed resistor in a kiln to cure and sinter the cement.

If you cant find suitable resistors, your best option as you have already paid out for the 230V motor, is to get the electronic control board, or build your own DIY version of it, which could be as simple as point to point wiring between components inserted through tiny holes drilled in a ring of Paxolin sheet.

[hdunnicliffe]

Interesting, If you don't mind me asking why would I need to fire the cement if I built my own resistor but not if I used the stubby 120 Ohm axial resistors in series?

The best option I've found so far are these: https://www.mouser.co.uk/ProductDetail/Vishay-Dale/RH010180R0FE02?qs=sGAEpiMZZMtlubZbdhIBIMow1WQ9W2ngvwqg46UgmOY%3d
They're 12.5watt which is a little low but probably ok. 19mm long, not including the terminals. They are "Chassis Mount" which I think just means they are in an Aluminum housing, I can't find anything which suggests they need a heat sink. I'd have to saw off the mounting tabs to make them fit but that should be ok. Do you see any issue with them?

Thanks again for all the help your providing, it's invaluable.

[Nusa]

The datasheet linked off the mouser page is pretty good. Page 3 has the power rating discussion, including free air rating.
https://www.mouser.com/ds/2/427/rhnh-220018.pdf

[hdunnicliffe]

Ah, 7.5 watts, that's not going to be good enough.
Ok, well it's looking like this won't be possible with my limited skills/knowledge. I'll probably go for the SS conversion.

Thank you both for all your help.

[Gregg]

You may be able to run another wire loop from the back of the mixer to the base with a couple of plastic strain reliefs and mount your resistors to the die cast base for the heatsink. You may want to separate the resistors with suitable material (maybe mica) in case they unexpectedly blow and cause a short to the base. 
Since you already have gone to the trouble of converting the voltage; another probably better hack would be to mount a PWM speed control in the base and make some type of feedback sensor fit the end of the motor shaft.  The motors on the older KitchenAid mixers are brushed universal motors that work quite well with PWM DC input.
Edit: if you put the resistors in the base, you should probably run a separate ground wire between the mains input ground and the base.

[james_s]

A resistor is a lousy way of controlling the speed of a motor under the best of circumstances, and to run a 120V motor that size on 240V you're going to need a resistor with a huge power handling capability, think space heater or hair dryer. I would highly recommend a solid state solution such as a triac speed control, or repair the mixer to original specs and run it on a stepdown transformer. Even one of those electronic travel converters may do the trick, they're intended for purely resistive loads but may work fine with a mixer, they're essentially a fixed setting light dimmer.

[Nusa]

A resistor is a lousy way of controlling the speed of a motor under the best of circumstances, and to run a 120V motor that size on 240V you're going to need a resistor with a huge power handling capability, think space heater or hair dryer. I would highly recommend a solid state solution such as a triac speed control, or repair the mixer to original specs and run it on a stepdown transformer. Even one of those electronic travel converters may do the trick, they're intended for purely resistive loads but may work fine with a mixer, they're essentially a fixed setting light dimmer.

The speed is controlled by a mechanical governor and points system. Even todays "solid state" models still use the mechanical speed control! The difference is instead of points switching the load directly, they switch the triac which then controls the load.

In the K45 model (1962 technology!) the resistor is there to reduce the sparking of the contacts by providing an alternate path when the points are open. The motor should actually run without it installed, sparking madly until the points either burned out or fused. And no, the power used by the mixer doesn't go up if you've properly converted to a different voltage. You need a different value resistor, but the theory of operation doesn't change.

You're right that a stepdown transformer would have been simple and likely the cheapest solution. But he's long past that decision point, and probably not that far from a working mixer.

[james_s]

That's interesting, I've seen the mechanical speed control in hand mixers but I've never looked inside one of those larger stand mixers. I suppose it's a reasonable way of doing things as it provides some closed loop control of motor speed with a simple mechanism.

So the 240V mixer uses the same motor as the 120V model with only a change of the of the resistor?

[Nusa]

I was speaking about the control system, not the motor itself. The resistor has to change because the relationship between voltage and current has changed. But the power (P=IE) stays about the same!

The OP knows more about the motor changes than I do, since he's apparently researched and done it. I'd guess the armature and stator are different for 240V, given it's a simple brushed motor. Not something I'd have to do myself, since I live in the US.

[Ian.M]

Done right - so the TRIAC starts off with a pulse of gate drive at the instant the governor contacts open, so it will conduct till the next current zero crossing,  the solid state 'electronic resistor' speed control will virtually eliminate sparking at the contacts.

I cant see any good reason not to mod it with a TRIAC board to bring it up to the K45SS spec. and if a K45SS 240V TRIAC board is unobtanium, excessively expensive, or impossible to make fit,  designing one on a ring PCB to fit would be an option, though it would be a lot easier to mount the TRIAC circuit remotely, taking care to avoid pinching the extra wiring at any flex points and ensuring it has adequate reinforced insulation for 240V operation.

[hdunnicliffe]

I came to the same conclusion, as most of you have, that retro fitting the triac speed control board (phase board) is the best option.
I took one out of the parts machine I have, tried it last night and it's working well. No sparking at the contacts & the motor runs smooth.

I've got quite a lot more work to do on the physical restoration but I'll try to remember to post some pictures once I have everything back together.

Thanks everyone for your help & advice.

-- For anyone who's found this via a web search --
You can retro fit a phase board to the old K45 mixers without needing to change anything else in the mixer. This video (https://youtu.be /j6Y3QpW1TAY) suggests you need to change the speed control plate, back bearing housing and speed control linkage, this is incorrect).