Would a Variac transformer work on my 110v Rong Fu RF-30?

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upjeeper

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i was thinking about going with a VFD and 3 phase motor to control the speeds on my RF-30 but a coworker told me to look into a Variac transformer. would this kind of thing work to control speeds?
 
I'm no expert but I don't think Variac transformers can be used on capacitor start motors.
 
You can't use a Variac to control an induction motor. It would work with a universal motor; brushed motor with field windings instead of permanent magnets but even then, it won't give the best performance at low speeds as torque will drop drastically, For universal motors such a variable speed drills, an SCR controller works better.
 
Using a variac will not work for a single phase capacitor motor. A VFD with a 3 phase motor or BLDC is a better option.
 
RJSakowski said:
You can't use a Variac to control an induction motor. It would work with a universal motor; brushed motor with field windings instead of permanent magnets but even then, it won't give the best performance at low speeds as torque will drop drastically, For universal motors such a variable speed drills, an SCR controller works better.
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"brushed motor with field windings instead of permanent magnets"? Didn't you mean brushed motor with permanent magnets instead of field windings. Or you could wire the field windings with a constant proper voltage. And then just vari the armature.
 
Like a vacuum cleaner motor- no perm magnets at all, just windings. Or router motor, blender motor, drill motor, etc.
Generally called "universal motors"
 
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Induction motor speed is determined by frequency. A typical single phase induction motor will run at 1750 rpm at 60 hertz AC. If everything was perfect, it would be 1800rpm. A 50 hertz motor will run at 1440 rpm (no slip, 1500 rpm). These motors are designed to operate at specific voltages. If run too low of a voltage, you will end up with problems such as overheating, loss of torque, short life, maybe even maximum smoke.
Even using a variac on a DC or universal motor is not the best way. You lower your voltage; you lower your torque. This is the exact opposite of running a lathe or mill. I'd stick with the VFD and three phase motor. You will have lots of torque with a wide speed range.
 
I've replaced all my single phase motors in the metal shop with 3 phase motors and VFDs. Either because the machines were 3 phase to start with or I wanted to upgrade them for variable speed.
I got 2 kits from these folks years ago.

They are also a good place for affordable VFDs.
If you buy a VFD used off eBay, make sure it's the 200volt class, as the 400volt class will not work unless you have 440-480VAC in you shop. Also make sure you can download the manual from the manufacture's web site, as that's needed. I've never been able to find the manuals for older VFDs from Allen Bradley/Rockwell Automation. Seems they expunge that from their web site. You want to check that the VFD will run on single phase input. I've never had that issue with any of the VFDs I've gotten. Also if you want decent torque at low speeds the VFD must be of the sensorless vector control. That may be a standard now, but when I put the first one on my lathe, it had a scalar type control, and that has no torque at low Hz operation. It was a FM50 (Flux Master 50) Westinghouse VFD (now TECO). I learned from that.
 
metric_taper said:
I've replaced all my single phase motors in the metal shop with 3 phase motors and VFDs. Either because the machines were 3 phase to start with or I wanted to upgrade them for variable speed.
I got 2 kits from these folks years ago.

They are also a good place for affordable VFDs.
If you buy a VFD used off eBay, make sure it's the 200volt class, as the 400volt class will not work unless you have 440-480VAC in you shop. Also make sure you can download the manual from the manufacture's web site, as that's needed. I've never been able to find the manuals for older VFDs from Allen Bradley/Rockwell Automation. Seems they expunge that from their web site. You want to check that the VFD will run on single phase input. I've never had that issue with any of the VFDs I've gotten. Also if you want decent torque at low speeds the VFD must be of the sensorless vector control. That may be a standard now, but when I put the first one on my lathe, it had a scalar type control, and that has no torque at low Hz operation. It was a FM50 (Flux Master 50) Westinghouse VFD (now TECO). I learned from that.
Click to expand...
I did start out with a RPC of my own construction, and would never go back to that, as it was noise running in the shop, that I had to start before using the mill, and only single speed motor operation resulted.
Also I put the V-belts at their middle RPM range, and program the VFD for 120Hz operation on a 4 pole motor that give me the full spindle operation RPM (and much slower).
 
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Go with a three phase and VFD. Here’s mine in process, still need the right pulley and some belts but both my lathes are setup this way and I wouldn’t change back.

John
 
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