Cycloconverter project: center-tapped or bridge configuration?

[cellularmitosis]

So I have a cheapy drill press, and it would great to be able to run it at lower speed (the pulley ratios were really meant for drilling wood, not metal).



I'd like to keep the AC motor's built-in speed regulation abilities, so that means rather than limiting it's voltage, I should instead change the AC frequency it sees.

The dirt-simple way of doing this is a single-phase to single-phase cycloconverter.

However, there are two basic designs: a center-tapped transformer design:



and a bridge design:



(source: http://dc360.4shared.com/doc/4o-SjR3S/preview.html )

One requires a transformer and four SCR's, while the other requires no transformer and eight SCR's.

(For those unfamiliar with cycloconverters, here is the waveform of a cycloconverter set to run a motor at 1/4 speed:)



Now, most of my electronics knowledge lies in realm of things which hide behind a 12v or 5v wall wart, so when it comes to dealing directly with the mains AC, I'll reach out to the community to ask for safety advice.

First off, I've frequently heard people refer to the notion of "transformer isolation" as a safety feature (i.e., the idea that you would never want to design a circuit which was directly or "galvanicly" coupled to the mains).  E.g, a circuit sitting behind a 1:1 transformer is somehow safer than one sitting directly on the mains.  Honestly, I've never understood why this is safer, because as long as that transformer hasn't melted into a pool of slag, the AC voltage coming off of it's secondary is every bit as deadly as the mains itself.  Please correct / flame / enlighten me on this point, as I fear my lack of understanding on this point is a mental deficiency on my part.

This leads into my second question: is one of the two above designs inherently safer than the other, or is one of the designs inherently unsafe?  I'd assume the transformer-based design would offer some mains isolation, so in theory it should be safer.  However, this brings up another question: why are AC motors one of the only commonly used electric devices which are not transformer isolated from the mains, and why is that considered safe?  And because it is apparently safe to run a motor directly off of mains, does this also mean there is nothing inherently unsafe in the above "bridge" design, which features no transformer?

So this post basically boils down to "why is transformer isolation considered a safety feature, and does that apply to this circuit?"

Thanks!

[Kremmen]

A cycloconverter is anything but dirt simple. First comment would be to seek other solutions for a flyweight motor such as your drill press driver.
My own experience is with 3 phase cycloconverters for ice breaker traction motors in the 10+ MW range. So rather diferent application but the principle is more or less the same.
More specific comments:
-A cycloconverter is a relatively complex way of controlling an AC motor. The firing control system is non-trivial.
-It is not enough to just switch positive and negative half waves. For good results you need to control the magnetization of the motor under varying speeds. This means that you need to regenerate a low frequency approximation of a sine wave by constantly varying firing angle.
-At low speeds the amplitude must further be reduced to avoid overmagnetization further complicating the bridge drive circuits.

[darko31]

Cycloconverters aren't very common for these types of low power motors, and as Kremmen pointed out, control circuitry isn't simple at all. Your best bet is to use speed regulator with a triac, but it will reduce power, or using variable frequency controllers used in industrial machines. With them you can change frequency and retain full power of the motor. Depending on power output, cheaper chinese ones start out at about 100 bucks.   

[Alana]

You never stated if its pure induction motor or is it universal motor [series wound DC-like motor that can run both on AC and DC - typically used in hand held drills and vaccum cleaners].
If its second type cycloconverter wont do you any good but simple triac based light dimmer is your thing.

[cellularmitosis]

Thanks for the feedback everyone, I didn't realize that cycloconverter trigger circuitry was so complicated.  I was inspired by this guy's semester project and thought it would be pretty straight-forward:

[cellularmitosis]

Ah, here's a reason why isolation is important. 

Thanks Dave!

[Kremmen]

The kind of power circuits of which an AC motor supply would be a typical example, are not as a rule galvanically isolated from the grid. Your original drill press motor certainly isn't. Why? - mostly because doing so would require inordinately heavy and expensive transformers for essentially no gain. What can be accomplished with isolation can in practice be equalled by proper grounding and these days, with residual current devices that interrupt stray currents in case of isolation failures.
That logic applies to the main circuit of a power delivery system. The associated control circuit is another matter entirely. There you usually end up needing galvanic isolation because signal levels need to be shifted, connections need to be made to external systems not able to cope with the voltage differentials, and for reasons of safety of life as well as EMI. While there are systems where the control circuitry floats at line potential, average hobbyists should steer well clear of such solutions.

[DenzilPenberthy]

... just put different sized pulleys on it?...

right tool for the job and all that.