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Universal motor DC drive?

Ralphxyz

Aluminum
Joined
Nov 22, 2015
I am "thinking" of Using DC to power my Universal motor.

It is rated for AC or DC on the label.

So I was thinking of a fuse, a Bridge rectifier and a capacitor.

What else do I need? At 120 volt AC what would the DC voltage be.

I have seen other threads where people were insisting connecting a bridge rectifier directly to AC was dangerous.

How could that be any more dangerous than the AC voltage itself?

Any help/comments welcomed.

Ralph
 
At 120 volt AC what would the DC voltage be.
The commonly used voltages (115VAC, 120VAC, etc.) are Root Mean Square (RMS) voltages. The peak voltage can be calculated by multiplying the RMS voltage by the square root of 2 (1.414). The no-load output of a bridge rectifier with a filter capacitor will be nearly the peak voltage, i.e. 120VAC in, 170VDC out. However, that is the no-load voltage and if the load draws more current during the non-peak portion of the AC half cycle than the filter capacitor can make up for the output voltage will "droop" resulting in ripple. Depending on what the bridge rectifier is driving, the ripple may be more or less important.
 
Thanks for the replies.

Jraef, I do not have a 120 volt DC power supply, so I have to rectify AC to get DC.

The reason I am "thinking" of using DC is to reverse the motor.

I am open to any and all suggestions.
 
Thanks for the replies.

Jraef, I do not have a 120 volt DC power supply, so I have to rectify AC to get DC.

The reason I am "thinking" of using DC is to reverse the motor.

I am open to any and all suggestions.

Why not 'Splain a wee bit more on what your trying to do.
 
You reverse a universal motor by switching the brush leads. Switching the polarity of the supply will not change anything because you are reversing both the field and armature at the same time. Reverse the armature and run it on AC. On simple motors with no series coils, reversing the field also works, but is there are series and parallel fields, they will in effect buck each other.

Bill
 
Thanks again everyone for the replies, I really need this help.

Bill re:You reverse a universal motor by switching the brush leads.

I was going to switch the brush leads not sure what switching the polarity of the supply would be. I suppose I should draw up a schematic.

As far as what I am trying to do and why.

I have a newly acquired Dumore Tool Post Grinder #44.

For internal grinding they say to grind against the spin for external grinding grind with the spin. I need to be able to reverse the motor for internal grinding.

Ralph
 
I need to be able to reverse the motor for internal grinding.
I'm no expert on tool post grinders (mine isn't even running yet) but I assumed that one could just reverse the direction of the lathe spindle in order to get the relative motion of the work piece and grinding wheel as wanted.
 
Ha, just reverse the spindle, wonder why I didn't think of that?

OH maybe because my lathe chuck screws on and if you reverse it it has been known to screw off.

But thanks I will try that to see what happens but there are many stories about people having to chase their chuck around their shop while they tried to work with it in reverse.

Ralph
 
Ralph

Reversing the DC voltage on a universal motor will not reverse direction as stated by other responses. You need to reverse the brush leads, nothing else.

From a safety perspective, AC voltage is 0V 120 times a second and your body will pull away at those times. Rectified and capacitor filtered will be high voltage all time.

CarlBoyd
 
my lathe chuck screws on and if you reverse it it has been known to screw off.
If the chuck is firmly tightened on the threaded spindle I don't think that the forces generated in a grinding operation will work it loose. The danger you mentioned arises, I think, when heavy cuts are taken with the spindle running in reverse or when reversing the spindle under power. Of course, you are unlikely to be able to do the latter if your lathe is single-phase powered.

I suppose that another situation where the unthreading risk might arise is if you were to have a very large work piece with a lot of inertia and your spindle comes up to speed fast. The physics involved in this case are exactly the same as in the case of reversing the spindle under power.
 
Thanks Don, I wanted to be sure certainly makes my life simple to just reverse the spindle. I wondered how much load grinding .001 might produce. I will not be using the grinder to hog out rough cuts.

Ralph
 
Well just incase someone might wonder.

I successfully ground the jaws of my three jaw chuck by reversing the spindle. ta dah!!

Thanks everyone for all of the help and advice.
Ralph
 
Another method to reverse the Tool Post Grinder.
Reverse.jpg

Got some large rubber bands off ebay.

Now I have to make up a belt guard and wheel guard, I have no idea how to mount a belt guard.

Ralph
 
I have no idea how to mount a belt guard.
The attached photo shows the "belt guard hanger" for my Dumore 44-011. They want more than $50 for it (p/n 578-0020) but it certainly wouldn't be difficult to fashion one like it.

Dumore-Belt-Guard-Hanger.jpg

The threaded end (left side of photo) screws into a threaded hole on the base casting and the flange on the right side holds the guard. I see that yours doesn't have a hole but you could add one no doubt.
 
You can download the manual for your grinder from the Dumore site-it will have a drawing showing how the parts fit together.









This photo is from the page for the 44 series grinders:

06acc597ea667ba69607bfabf41eabde.thumb.jpg

However, unless I'm mistaken, the mount as shown in the above post would not allow you to cross the belt as you did.
 
Another method to reverse the Tool Post Grinder.
View attachment 223727
Running the spindle opposite of the normal direction may lead to loosening of the nuts on the spindle ends in operation.

Mine has a left hand thread on the motor shaft and right hand threads on both ends of the spindle. I haven't thought through the forces involved but I suspect that the spindle nuts are the handedness needed to keep them from loosening in operation when the spindle rotates in the normal direction. That would mean, then, that running the spindle in the opposite direction would apply a loosening force to the nuts.
 
Thanks for the picture of how it is mounted. Now that makes sense.

My #44 is a older one so the current parts manual is not 1 to 1.

I probably would never run with the belt twisted but it might be a option if really needed.

I could see from the current parts manual they were holding the belt guard in the middle but could not see how. Now I can make up something
Dumore wants to take retirement on parts.

This is so cool.
Thanks again everyone for all of the help.

Ralph
 








 
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