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I Forge Iron

Need help reversing a motor (if possible)


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I followed that Rockstar, made perfect sense. One of both of us should be frightened? 

I'm rigging the 2HP motor I pulled out of our old soon to be tossed off the upper deck hot tub for my belt grinder. Fortunately once I figure out how to get the pump's impeller off the shaft it turns the right direction so I don't have to play with the ohm meter. I have two but that's another story.

What I'd like is a speed control and I get different stories for if and how. No rheostat, that I know. 

I figure my best bet is to hire or bribe one of my more electronically adept friends to take a look. I know I sure as heck am NOT connecting the shop's power to the meter box!

Professionals can be handy.

Frosty The Lucky.

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Frosty,

A 2HP motor from a residential installation like a hot tub is probably going to be a single phase AC capacitor start/ run motor.  Since there's no reason for the hot tub to need multiple pump speeds, it's unlikely that they used anything more complex or costly.  Sadly, that rules out the relatively few Variable Frequency Drives that exist for single phase motors.  I'm not aware of any "good" electrical answers for speed controlling that motor.

Were it me, I'd look into variable sheave systems like you might find on an old drill press or milling machine.  

 

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Yeah, that's what I'm gathering from my uneducated reading on the things. It's 2HP,  220v single phase, I thought I mentioned that but . . . I can do step pullys but being able to turn a knob would be so nice. <sigh>

I'll let you know if I discover something interesting in electrical connection point on the motor. 

Frosty The Lucky.

 

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49 minutes ago, rockstar.esq said:

Sadly, that rules out the relatively few Variable Frequency Drives that exist for single phase motors.

Rockstar, I only just recently found out about single phase to single phase VFDs, but haven't yet been able to find out what the appropriate and inappropriate applications are for them. Care to elaborate?

24 minutes ago, Frosty said:

I can do step pullys but being able to turn a knob would be so nice. <sigh>

Ever thought of trying a variable speed pulley? I remember that the wood lathe in shop class had a big knob on the front that gave almost infinite adjustment to the headstock speed.

 

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1 minute ago, JHCC said:

Ever thought of trying a variable speed pulley? I remember that the wood lathe in shop class had a big knob on the front that gave almost infinite adjustment to the headstock speed.

Yes I have. Those are actually torque converters, ever look at the price?

Frosty The Lucky.

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JHCC

Single phase VFD's do not work with motors that have a centrifugal switch.  The centrifugal switches function is tied to RPM's so anything that slows the rotation significantly, will bring the start windings online.  The start windings can't handle continuous operation without overheating.  VFD's also don't work with "universal" AC/DC motors like you'd find in power tools. If it's got brushes, chances are good that VFD won't work with it.  That's a major bummer for the folks trying to convert their abrasive disc chop saw into a cold saw with a VFD.

There are single phase motors without centrifugal switches.  Shaded pole motors are generally limited to 1/4 HP (think bathroom fan motor). Sub horsepower AC Synchronous motors often have "Squirrel cage" rotors which aren't visible unless you take the whole thing apart.  On larger horsepower rated synchronous motors, there will often be an "excited core" which requires DC current to maintain a strong enough field to synchronize. 

Permanent Split Capacitor motors will work as well, but they're less common in high horsepower ratings.  The main visible difference between a PSC and a "normal" capacitor start/run motor is that the PSC uses paper capacitors instead of electrolytic.  Electrolytic capacitors are shaped like a round prism with two terminals on one end, with the polarity clearly identified.  Paper capacitors are not polarized and their leads can be on opposite ends.

HVAC and refrigeration equipment will sometimes use PSC motors, but it's a little tough to find anything above 1 Horsepower in a residential, single phase application.

For grinding applications, I wouldn't recommend this approach because the compatible single phase motor probably wouldn't have enough torque at low speeds. 

I hope that helps. 

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I am currently using a variable transformer (variac) to control the speed of my blower’s universal motor (a repurposed vacuum cleaner), and it’s interesting to note that I might to be able to use that to slow down a chop saw sufficiently to use a cold-cut blade. Or would that not have enough torque?

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JHCC,

It's doubtful that the chop saw would have enough torque, plus the integral cooling fan on the saw's motor won't be spinning fast enough to do much good.  Long story short, it will probably overheat the windings while delivering lackluster performance.  The full horsepower rating at synchronous speed of an AC motor operating at a fixed frequency will depend on the number of poles.  Everything that follows is based on 60 HZ

2 Poles is 3,600 RPM -This is the most common motor

4 Poles is 1,800 RPM - This is the second most common motor

8 Poles is 900 RPM

10 Poles is 720 RPM * Note, I've never actually encountered an AC motor with more poles than this in real life.

12 Poles is 600 RPM

14 Poles is 514 RPM

16 Poles is 450 RPM

18 Poles is 400 RPM

Just for reference, a cold saw runs at roughly 88 RPM, which suggests to me that there is some reduction gearing involved.

 

 

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