Massive voltage spikes with transformer

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Old 03-29-20, 01:38 PM
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Massive voltage spikes with transformer

So I have a 220-110 (in reality 230-115, but people call it that) step down transformer that is supposedly rated for 2kw. Itís a massive mechanical transformer, not one of those electronic ones.

Today I noticed that when I turned on my 110v kitchenaid (250w) in high the voltage goes wild. At a low speed nothing appears to happen.

Is this normal? is the transformer broken?

here is a short video:

https://youtu.be/BWE4tfnUFHk
 
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Old 03-29-20, 06:25 PM
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I am not believing the meter. Wire an incandescent lamp across the load then post results. Those mixer motors are quite noisy, electrically.
 
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Old 03-29-20, 06:46 PM
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It appears you would be stepping down a 220 volt non-North American power supply.
Most foreign 220 volt power is 50 cycles and asuming your mixer is American it needs 60 cycle power.
The speed control on those is electronic and likely the source of your problem.
 
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Old 03-29-20, 08:01 PM
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GregH, yes it is 50hz here, I had understood that that would just mean that the motor spun at 5/6ths itís normal speed.
 
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Old 03-29-20, 08:09 PM
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Here is why I ended up measuring the voltage. the powerstrip had a 15a rating and would trip every time I plugged in an instant pot (1100w), so today I excised the breaker switch from the power strip and was testing things out when I noticed this.

after I made this post, I tried again with the instant pot and didnít get below 113v at any point. There is a 16a fast blow ceramic fuse wired in-line before the transformer and it didnt blow so Iím not sure what was wrong with the powerstrip.

I then tried my coffee grinder (also 110v -60hz) plugged in to the powerstrip and the multimeter was again all over the place from 260v to 43v.

is this a question of mechanical va resistive loads and is this something I should worry about if everything appears to work now that I removed the breaker switch?
 
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Old 03-29-20, 08:19 PM
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Wow, telecom guy, Iím not sure what to say other than you are a very smart man.

this video seems pretty conclusive. But why is it that the multimeter seems to defy what we see with our own eyeballs in this vid?

https://youtu.be/I0Y2Cy5QAd4
 
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Old 03-30-20, 03:04 AM
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Where are you located that you don't have 120 vac and 60 cycle power directly available in the home? Only electrical losses in the transformer are affected by frequency so the transformer is not your problem. When did the problem start?
 
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Old 03-30-20, 03:37 AM
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I tried to find a link to a decent explanation of this that would make sense to you but could not decide on one.
So perhaps you should Google "voltage readings inductive to resistive loads" and take a look.
The following made sense to me
http://acra.co.za/Pages/Blog/2610/2610.pdf

It all has to do with back EMF (electro magnetic field)
Basically when one wire has AC current on it it will induce an AC current in another wire that is close to it.
This is how your transformer works.
But in a motor this induced AC actually fights/resists the AC in the powered wire hence back EMF (inductive load)
 
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Old 03-30-20, 09:38 AM
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Wow, telecom guy, I’m not sure what to say other than you are a very smart man.

this video seems pretty conclusive. But why is it that the multimeter seems to defy what we see with our own eyeballs in this vid?
Not sure about the "smart guy", but I do a lot of work in EMC labs, looking for strange electronic behavior with external stimuli. In your case, the brushed motor is electrically noisy. This means that there is energy both radiated out of the mixer (listen to an AM radio nearby with the motor on), and conducted noise outbound from the mixer into the DMM. DMM are notoriously sensitive to things they shouldn't be, including some expensive Fluke meters that I have tested. When in doubt, reduce the complexity of the measurement system. ie; go very low tech, my favorite meters are old time VOM, non electronic type, to resist these types of noise interferences.

I have the same KA mixer in my kitchen, looks like an electronic speed control. Meaning, the motor is getting pulses of voltage, more pulses, more speed. These pulses have fast risetimes, and do have unintended consequences when sensitive equipment is nearby.
 
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Old 03-30-20, 09:49 AM
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Smile

The "back-EMF" manden mentioned is what keeps motor windings from being a dead short. It is an opposition to current flow, acting like a resistor. The same is true for the primary of a transformer.

Uninteresting things in a course of study often underlie important concepts introduced later. (Don't ask how I learned this.)
 
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Old 03-30-20, 04:49 PM
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Your KA is a universal motor that has the field and rotor windings in series, hence a large inductance. Speed control is usually a phase fired SCR. A conducting SCR stops conducting when the applied voltage crosses zero . Since this is the time of highest current rate of change, the back EMF is the highest A larger inductance also results in a larger back EMF. The time periods of the back EMF here are milliseconds so I doubt a DMM would detect the voltage spikes. My guess is the voltage spikes are causing uncontrolled firing of the SCR causing the observed motor speed changes.
 
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