Bait and switch motors.. And a VFD question for good measure. 1

[itsmoked]

I'm looking at a panel job for running a spindle on a boring machine. I was lead to believe the spindle motor, that's bolted to a gear box, was a 4HP 230V 3ph unit. (I should've known something was amiss by the obscure "4"). The plan was to run the motor with a VFD for variable speed AND to provide current feedback for "effort monitoring".

Turns out the motor is actually a universal motor!!

And they still want effort feedback..

Does this make sense to you guys? Why a universal motor? Why would a gearbox company provide that? Why would it be worth dealing with brushes? Torque profile?


I believe the plan is to use the motor with 1P 230VAC. Since this is a universal motor can it still be run with a VFD? I suspect all VFDs would scream about phase balance.


Perhaps I could rectify a VFD's output to DC and run with that? That way we could get some speed control and load feedback?

Then, when they come to their senses, we could easily sub out the universal for a 3ph motor.

Your thoughts would be most welcome.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

Universal motor - it's easy to implement crude speed control by voltage variation. Relatively small for power output - can run at high speed with a reduction gear.

Rather than a VFD you could implement a back-EMF-sensing controller from one of the dozens of cookbook controller circuits to give something resembling speed regulation, and the 'effort' signal would be available quite simply from such a circuit.


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

 

[waross]

I would be checking a 180 Volt DC motor controller for suitability. Going by old memories, welding machines often had a 90 VDC outlet for running 115 Volt angle grinders with universal motors. I believe that universal motors run faster on DC and 180 Volts DC may give you the same speed as 230 Volts AC.
Before I started I would consider selecting a motor-VFD combination and pitching that to the client.

http://www.sprint-electric.com/products.php?cid=7&sid=17I

don't suggest this as a final solution but as an illustration of the type of controller that I visualize. Up to you to research the suitability for driving a universal motor.
Yours
Bill

Bill
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"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

As Scotty and Bill says. There are lots of "controllers" - I would rather call them dimmers for single phase AC. But if you are going to build something, you could just as well throw in a PIC or similar, plus a triac. At 230 V, you need less than 20 A for 4 HP shaft output. Use isolation (pulse) transformer between PIC and triac gate. Parallel a few pins to get enough gate drive, if necessary.

The universal motor is a series wound motor and has the typical characteristic with lower speed and more torque when loaded. An induction motor will not give you that and there's a definite risk that, if the customer expects that torque/speed relation, it will stall in high-torque situations.

Since speed is load sensitive and you probably cannot have a speed sensor or tach attached, it would be better to just control the firing angle and call it "Effort". As a prostitute, you cannot always tell the customer how you would like it. Better adapt to his wishes and, perhaps, hint that there are subtle variations that he might appreciate once he gets the hang of it.

Brush wear isn't such a big deal on boring machines that are used intermittently. If running continuously and heavily loaded, thats quite another matter.

Happy New Year to you all!


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.

 

[btrueblood]

Ohh...a boring machine, i.e. like a horizontal drill press?

Not a tunnel boring machine (awful small hp for that), nor a <yawn> machine...

 

[MikeHalloran]

I have a portable air compressor that cycles automatically without dimming lights or blowing a fuse on a 115V circuit, ostensibly because of its universal motor (which is also much lighter than an equivalent induction motor, not germane here).

I think the easy speed control and decent torque at low speeds are important for a boring machine. Boring tools are often rather badly unbalanced, and many useful materials do not like being cut at high speeds, so there is a hard limit on how fast the spindle should turn.

Some sort of effort indication is highly desirable, because to gain cycle time, a machinist will push the depth of cut to just short of stalling the motor; an actual stall can break the tool or damage the workpiece, so that would be a bad thing.


Mike Halloran
Pembroke Pines, FL, USA