Two motors one shaft.

[itsmoked]

I'm looking for opinions here. I think engineers sometimes have them.

Dough machine.

There are two motors that run the beater. One is 'Slow' and is about 3HP and the other is 'Fast' and is 10HP. They both co-drive the same shaft. I'm completely replacing the ancient French manual control panels with a PLC run touch screen solution. The machines are timer based and the slow timer runs the Slow motor for some minutes and once timed out runs the Fast motor for a bunch more minutes.

My question. Can I simultaneously drop the slow contactor and pull-in the fast contactor to allow the the slow-fast transition Fast motor to avoid a standing-start or do I have to put in a long enough delay for the whole works to grind to a stop and then start the Fast?

What say you all?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[electricpete]

I say it is not a problem. No delay needed.
IF you were reenergizing the same motor, then you might need a slight delay to avoid reclosing out of phase with residual voltage.
But in your case you are energizing a motor (fast motor) which was previously spinning but not energized... it will have no significant residual voltage.


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

 

[wayne440]

I claim no dough machine expertise. But it seems as if the transition to "fast" would be less dramatic if done while the machine was already turning as opposed to starting from a standstill. What method did it use with the old controls?

 

[wroggent]

Might be interesting to measure what voltage develops across the slow motor contactor as the slow motor residual voltage moves out of phase with the supply.

 

[Skogsgurra]

Would it be very awkward to skip the 3 HP motor altogether and drive the 10 HP from a VFD? That would teach the old dough new tricks.

Gunnar Englund

http://www.gke.org

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

 

[waross]

I agree with Pete. The residual would be negligible.
I agree with Gunnar. VFD unless there are some unique loading characteristics.
I remember a "Red-neck" solution in a sawmill years ago. A motor used a a vee belt to drive a gear reducer that ultimately pushed logs through a pair of saws.
It was decided that production could be increased by pushing the smaller logs through faster.
A second motor was belted to the gear reducer with a higher pulley ratio.
A reversing contactor was used to switch between motors safely without inadvertently energising both at the same time.
No delay.
Worked well.
Motors were "U" frame in the range of 5 HP to 10 HP. (memory fails)

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

 

[itsmoked]

electricpete; Thanks. That was exactly my thinking since the Fast motor isn't energized.

wayne440; Less wear on the Fast contactor was my hope. The old controls are so screwed up it's hard to tell. They'd hacked out the old timers and replaced them with bathroom fan twist TICKTICKTICKTICK timers... which makes me think they're currently doing it all manually. When the slow timer times out someone goes over and cranks ON the fast timer. Also asking the absolute simplest questions takes a week for me to get a response so after a couple of weeks I made the executive decision to plow ahead with, 'how I'd do it'.

wroggent; That may be but I'm NOT bringing my scope into THAT place.

Guunar and Bill;....... where to even begin... This place is very budget. They've never had issues with the motors. I'm the follow-on act of another EE who talked them into TWO (2) VFDs and got so far as to have bought all the cr@p and assembled it onto a panel before deciding not to continue and disappearing. This means they've already spent a bunch of $$ for nothing. The only real issue is the failed switches/cracked acrylic control panel faces/worn out contactors and an aged transformer. The switches break frequently due to heavy use. The timers don't cascade. And to reverse the machine for slow jogging they have to turn the disconnect OFF and then ON backwards to swap two phases coming in. I figured I'd just get the controls going correctly with them entirely built on the back of a new aluminum face plate so it can all be assembled elsewhere and brought in and mounted to the enclosure face with just the power and motor leads needed to be hooked up. There are severe up-time constraints not lending this to a long pull-out the guts and replace them in situ. I have to do this 4 times. Adding four 10HP VFDs... would certainly torque up the cost a bit and make the panels a heavier. I also don't know what the torque needs of a three foot wide beater in a hundred pounds of dough actually are and know has a clue as to the two actual speeds are so maybe a 10HP couldn't actually turn-down and do the job.

I also had the need to get them happy about EEs again since when they invited me over they thought I wasn't an EE and their faces all dropped when I responded I was an EE. I think they now have a very negative picture of EEs and I need to win one for the team. On time, under estimate, and effective. That's the plan anyway. LOL.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

My point may have been the pun with learning old dough new tricks. But, of course - and as always with me - my technical reasoning was flawless. Or nearly...

Gunnar Englund

http://www.gke.org

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

 

[flexoprinting]

Hi Keith

I like the reversing contactor idea, or you could use a one way bearing on the slow motor so the fast one can just take over. I know mechanical idea, sorry about that.

Chuck

 

[itsmoked]

Mechanical

Hey! I do mechanical. I was at client's place and after fixing a lathe was walked by a huge CNC machining center that was completely undressed (no sheet metal). "What's up with this", I asked. The guy promptly fired it up and using rapid on a hand wheel transited the 2 ton Y axis. A nerve bending keening sound poured out of it. They'd exasperatedly torn the whole thing apart twice replacing the bearings, ways, and had the screw re-ground to no avail. After some contemplation I told them to replace the coupling between the servo drive and the screw. Skeptically they ordered a new $200 one. They called me a few days later to tell me I was a genius. I now have mech creds there.

Interesting one-way bearing idea, but would it even work since the Slow motor is the one that does the reverse jogging? Besides I've never been able to see the motors since they're buried inside a huge casting under a completely clogged screen. :/

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

I think your concern for EMF caused by the collapsing magnetic field might be related to this issue in 2 speed motors, where it is definitely a problem. But in this case since the motors are separate, not in the same core, I don't see the issue being of much concern. The only thing you might have is residual magnetism inside of the unused motor core steel, but without something like caps to allow it to amplify*, it's negligible.

* I would have never mentioned that, but I've seen stuff on the internet regarding people utilizing the residual magnetism in AC motors amplified with capacitors to make them self-exciting induction generators for wind machines.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[electricpete]

Yes, good point. If there were caps on the load side (motor side) of the fast-speed contactor, that would create a potential concern to be considered.

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

 

[itsmoked]

No caps.

Keith Cress
kcress -

http://www.flaminsystems.com