single phase AC motor question

[HoosierBuddy]

So...

I've got an old U.S. Burke 'Millrite' milling machine with a single phase motor that very well may be original to the mill. That would make it roughly 40 years old. It's configurable as either 240 or 120 V, and I have it set up for 120V. (Had a electrician switch it over from 240 when I bought the mill).

This old mill has 5 pullies that let you adjust the spindle speed.

I've noted that when I set the mill up for one of the fastest 2 speeds, it's like it has a hard time getting up to speed. It switches on, starts the spindle, advances to a certain RPM, hums and then kicks up to full speed. It takes maybe 2 seconds.

If I switch off the motor for a couple of seconds and then re-energize the switch, with the spindle still turning, it will immediately spin up the the correct RPM.

Is this anything to be worried about by itself.

After it gets up to speed, it works fine and is not making any unusual noises.



Phil

 

[uniballer]

OLD! But use till dead. But still look for another in the process.

 

[Gary S]

Did you pay attention in 7th grade science class? They taught how pulleys work, and from that we all know that any motor will start slower when the load is geared up than it will when it is geared low.

 

[A_Pmech]

Hi Phil,

A couple thoughts on the subject:

1) Most milling machines take several seconds to come up to speed when in top gear. It may be normal for your machine. Mine takes about 6 seconds.

2) If the spindle doesn't turn freely by hand you may have a sticky motor or spindle bearing.

3) The start capacitor may be in the process of failing, or the centrifugal starting switch is opening up early.

 

[kbs2244]

“Old” often means stiff, gummy grease.
Both in the motor and the machine.
Check that first.
Then check the centrifugal switch on the motor for freedom in the linkage.

 

[HoosierBuddy]

Thanks Pmech and kbs2244....I was wondering if it could be a symptom of a failing capacitor. The centrifugal switch was not on my radar. I will check it out!

And Gary, I don't remember a lot from 7th grade science, but it seems like when I picked up my degree in Mechanical Engineering we covered leverage in all its forms the first day. Communication...not so much. What I was attempting to articulate with my question is a condition where the mill accelerates to a rotational velocity (lets call it v=0.5 theta), pauses for 1 to 3 seconds and then accelerates to a higher rotational velocity, v=1.0 theta. From there, my question boils down to, 'Is this a normal condition given a single phase AC motor?'

From the comments above, I'm going to assume the answer to that question is: 'Not normal'.

Phil

 

[larry_g]

I agree with the above in that the start cap or switch is not up to snuff. If you check it out now and fix the problem you may avoid a total failure.

lg
no neat sig line

 

[Zeke]

...............................................

And Gary, I don't remember a lot from 7th grade science, but it seems like when I picked up my degree in Mechanical Engineering we covered leverage in all its forms the first day. Communication...not so much. What I was attempting to articulate with my question is a condition where the mill accelerates to a rotational velocity (lets call it v=0.5 theta), pauses for 1 to 3 seconds and then accelerates to a higher rotational velocity, v=1.0 theta. From there, my question boils down to, 'Is this a normal condition given a single phase AC motor?'

From the comments above, I'm going to assume the answer to that question is: 'Not normal'.

Phil

I love it.

 

[G'ord]

Support the Idea of the cap.

Why did you switch to the 120v? and are you truly getting 120V at the unit?

 

[HoosierBuddy]

Support the Idea of the cap.

Why did you switch to the 120v? and are you truly getting 120V at the unit?

I bought the mill from a garage that was going out of business. They had 240V at the mill's location...but I didn't in my home's garage.

It was cheaper to switch the motor on the mill from 240 to 120 than it would have been to run 240V to the location (finished walls).

I may be reading something into your question: Are you indicating that the mill would "run better" on 240V?

My outlets typically check about 125V AC. I haven't sat there and watched it as the motor fires up...but I guess I could do that within the limits of my el-cheapo multimeter to react. If there's a transient voltage drop that only last 1/10s of seconds...I don't know how I'd see it.

Phil