over-voltage on universal motor 2

[subsearobot]

Hi. I am trying to source a universal motor for a prototype- with very little luck.

I need 240V, 1/2 hP

I have found a 1/4 hP, 115V. If I double the supplied voltage, any guesses whether I will destroy the motor?

I am running this in cold water, so cooling will be plentiful.
(We have successfully done this with BLDC motors, but I'm unsure about the construction of universals)

thanks

 

[Skogsgurra]

Universal motors are not known to be long-lfe at all. Some of the Black&Decker tools had something like a few hours guaranteed life. If you run a universal motor at double voltage, you will run it at approximately double speed. That will cause lots of arcing and will most likely kill the motor in a very short time.

Aso, running it under water is just dumb. At least if you allow water to get into the motor.

Why not get a motor that is suitable instead of trying hopeless "solutions"?

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[MiketheEngineer]

I think you can get a 1 hp - 240V motor. So it will just run cooler and probably cheaper......

 

[subsearobot]

Gunnar, I am eliminating hopeless solutions. sounds like running at 2x voltage is hopeless. thanks for your response!

Running a motor underwater is neither dumb, nor unique. Contrary to that, it is actually ideal, as water has a much better cooling capacity that air. Sure, it is fair to say letting water *into* a motor is dumb!

the reason for the interest in universal motors is that the power to weight ratio is high, and motor drivers are not required (and cost is low- more on that later). In subsea vehicles, size and weight are both premiums.

As I mentioned in the original post, typically we use BLDC motors. This project is for a scientist and the budget is limited, so we are trying to improvise and give a cost effective solution. And, he can use free labor (grad-student) to change brushes every 300 hours.

thanks again

 

[Skogsgurra]

I am not at all unfamiliar with motors running in water. But those are induction motors, synchronous motors or BLDC motors where no uninsulated parts are exposed to the water.

If you look into a universal motor, you will see a collector and brushes. All of these things are connected to voltage and bringing them into contact with water is the dumbest thing you can do to such a motor.

If you do not believe me - just go try it! But beware of the dangers involved.

A six or twelve volt motor might work, but definitely not a 240 V motor.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[subsearobot]

Yes, I agree that letting conductors come into contact with (sea)water is not a viable solution. The contrary was never suggested or implied. Just assumed. There may be good reason for this assumption- this is a website devoted to helping others with engineering problems, and at times, armatures and hobbyists are involved.

Believe me, a waterproof housing that incorporates a shaft seal is not a problem to design, fabricate or assemble. that was not my question here- I am well versed in marinizing both custom and COTS equipment.

The question was simply about a piece of equipment that I am not overly familiar with- universal motors.

So, thanks for your initial input: doubling the voltage/speed of a universal is likely to cause problems.

have a nice weekend.

 

[BrianE22]

Did you try Groschopp in Iowa? They supply us with armatures and fields for our 230 volt motors. Actually, one of our motors might work for you -1.3 peak hp at about 10,800 RPM, about 3/4 hp continuous.

 

[waross]

Another possible issue with a universal motor is degradation of some components from the ozone generated from the brush arcing.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[burnt2x]

If you want make do with what you have, IMO universal motors can run on half-wave rectifier supply. With a 240VAC supply, you can have a pulsating DC output feeding 240VAC to the half-wave rectifier. The average DC output will be 0.318 X V_input = 0.318 X 240 = 76V. Please bear in mind that this will be a lot less than the motor's rated voltage and the motor torque will be a lot lesser than if the full voltage was supplied. Be sure to have incorporate a free-wheeling diode across the motor to maintain a continuous load current on the motor.

 

[ScottyUK]

Half-wave rectification is a really bad idea because it will introduce a DC component which will saturate the core of any iron-cored equipment such as a transformer, motor, etc which is not galvanically isolated from the rectifier.


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If we learn from our mistakes I'm getting a great education!

 

[waross]

At one time universal motors were suitable for DC operation. Hence the name universal. Older welding machines would supply 90 VDC for the operation of 110 VAC rated grinders. That said, using half wave rectified 240 V to operate a 115 Volt motors still seems like a bad idea.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

While that is true when it comes to transformers and induction motors etcetera. It is not necessarily so with universal motors.

The "universal" in the name means universal in the respect that they can be used on AC and DC. They are, in effect, small series wound DC motors with a laminated core, so they can also work with AC.

But half-wave is one of the worst waveforms you can select for them. The peak voltage will not be reduced and the current crest factor will be really bad. All that while you still won't get any more power out of it.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[electricpete]

What are the application requirements that led to preferring universal motor over other types?

=====================================
(2B)+(2B)' ?

 

[gattie]

I suggest you contact the company Flyght - They manufacture submersible pumps & the motor windings are watercooled.
They may have encountered applications similar to yours.

 

[Skogsgurra]

It isn't Flyght. And it isn't (ITT) Flygt anymore either. It is XYLEM these days.
I know them very well, did consulting work for them just a few weeks ago.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[burnt2x]

Y'all:

I would like to know any experience/s, anecdotal or factual re how bad a half-wave rectifier feeding a "universal" motor is. I bet that type of motor doesn't care if it is fed DC, AC, or a mix of both. Been there, done that, and that setup worked: the job was done (need to run a power tool rated at 115V with the only supply available was 240VAC). Any wild reaction is accepted.

 

[Skogsgurra]

Stand by for measurements. With photographs of the collector when fed with DC, AC and half-wave rectified AC.

The motor will run on all occasions. But half-wave AC is the worst commutator killer. Ever.

I'll be back with a report. May take a few hours. Patience!

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Skogsgurra]

Even worse than I expected. Could not run motor at double voltage and half-wave rectifier. OK at 230 AC - terribly bad at 300 V and half-wave rectifier.

Read here:

http://www.gke.org/pub/files/Universal Motor on AC and HalfWave DC.pdf

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

Great work Gunnar. Thanks.
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[burnt2x]

Thanks for the report Gunnar.

One more question: did you add a free-wheeling diode across the rectifier output/ parallel with the universal motor when you did your experiment? Because if you did, you won't see that much arcing, IMO.