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Sieg Mini-Lathe DC Motor Controllers

Hugo Holden

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I thought I would post this here, not because the mini-lathe is specifically a computer repair tool, but I have used mine to make spare computer parts, stand offs bushes etc. A mini lathe is such a useful electronics workshop tool for all kinds of mechanical repairs.

In any case, the information in this large article I have recently drafted is about keeping the DC motor speed controllers running.

Sieg actually created what amounts to an analog computer, with a motor control system based on OP amps and they did a really good job on them too.

It is an interesting story:

Initially they used PWM control boards. I watched over the last decade or so, these boards failing and many failed attempts to repair them posted on the net. One big part of the problem was Sieg never published a circuit functional description or any detail on how to set up the multiple pcb adjustments, leaving technicians in the dark.

Also, because the boards were surface mount types, when people attempted shotgun replacements of parts, such as the IC's, the pcb damage escalated until the board was destroyed in many cases.

In the meantime something else happened. Sieg discontinued the PWM design and moved to SCR's using phase control. This system was cheaper to make and eliminated the on board miniature power transformer and the Power Mosfet/s of the previous design. Though IMHO, they don't work quite as well at low speeds.

Now it is impossible to buy the original PWM boards, for the C1 lathe at least, for love or money. Although they were so well done they could be a target for cloning. So the design, starting out in China, could ultimately end up getting copied and cloned elsewhere.

So the idea of this article is to provide the information required to understand, repair and adjust the original PWM pcb. The general principles are the same for other models of Lathes using similar OP amp based PWM controllers. As noted in the article, to understand Sieg's servo system requires that at least 5 principles of OP amp behavior are understood.

Here is the article:

 
This grabbed my attention. I have a Red Chinese 11x26 lathe that has a 1HP 115v AC motor. Lowest speed is 150RPM. Talk about threading up to a shoulder...

What's the possibility of a 3/4HP PWM DC motor run off a 20A 115v AC line?
 
A lot of water has gone under the bridge since these things first came out. I've got one of the red Chinese mini-lathes as well. My first impulse would be to graduate to MCU-controlled PWM (many MCUs have features tailored to this application). I'd probably add a tach pickup to the motor.

Just thinking aloud.
 
This grabbed my attention. I have a Red Chinese 11x26 lathe that has a 1HP 115v AC motor. Lowest speed is 150RPM. Talk about threading up to a shoulder...

What's the possibility of a 3/4HP PWM DC motor run off a 20A 115v AC line?
Given the tendency of Chinese marketeers to overstate their motor capacity, I suspect that your lathe motor really is only 1HP "locked rotor". If it's variable-speed, I suspect that it's also a "universal" AC/DC motor. I've got a shop vac that claims 3HP and it runs from a 120V 15A circuit without even blinking the lights.

Shades of "Peak Music Power", but there you are.
 
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Given the tendency of Chinese marketeers to overstate their motor capacity, I suspect that your lathe motor really is only 1HP "locked rotor". If it's variable-speed, I suspect that it's also a "universal" AC/DC motor. I've got a shop vac that claims 3HP and it runs from a 120V 15A circuit without even blinking the lights.

Shades of "Peak Music Power", but there you are.
Yes, peak power has always been an industrial grade con.

When we talk of power, it is a rate of energy delivery, in Joules per second or Watts. So its more than reasonable to calculate from the product of rms voltage and rms current in AC systems(ignoring power factor and assuming they are in phase). However, if you pick out the peak voltage of a waveform and multiply that by the peak current at that moment, you simply have a much larger number , not representing the true rate of power delivery, but it is great for marketing purposes.
 
Time for a story. Maybe 25 years ago, I purchased a Chinese-made 10" buffer (looks like a bench grinder with two wheels). Good heavy construction, capacitor-start motor, nameplate rated at 1HP. I saw no reason to doubt that--it had more power than I could use. A few years pass and a friend seeing my unit decided to go down to the local Harbor Freight store and purchase its twin--sort of, mine is painted dark green and his was painted shiny red. Same 1 HP nameplate rating.
He came by a few days after his purchase and said that he could almost stop the buffer with a gloved hand--absolutely no power. I offered to have a look--and he was absolutely correct--it was a wimp. It also weighed a lot less than mine. Curiosity got the better of me and I pulled off one of the bell ends to have a look at the motor. The manufacturer had used the same-sized housing, but incorporated a space-filler to accommodate a motor approximately one-quarter the size of mine. He returned the buffer for a refund.

You have to be very careful with this stuff. Becomes a matter sometimes of "what are you going to believe--our nameplate or your lying eyes?". :)
 
The manufacturer had used the same-sized housing, but incorporated a space-filler to accommodate a motor approximately one-quarter the size of mine. He returned the buffer for a refund.

You have to be very careful with this stuff. Becomes a matter sometimes of "what are you going to believe--our nameplate or your lying eyes?". :)
Sounds like the tiny transistor dies in faked 2N3055 transistors.

Recently I saw a picture of a car battery, typical looking size that had been cut open. It had a much smaller battery concealed in the casing and the space around that was filled with concrete to make it feel heavy. There is no end to the possible deceptions and the way people dream this stuff up.

Years ago I saw a funny one, a concrete brick inside a color TV ! It wasn't really a deception. What had happened was the CRT was very front heavy and the cabinet had a tendency to tip forward onto the face of the CRT. So they stuck the big concrete brick in there on the floor of the cabinet, towards the back of the set, to counter balance it. Still it looked pretty unusual in there.
 
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