Braking

[Pierre]

OOOFF! And in my day job some even come to ask me about motors...

If they only knew Dick.

Tancks for yet another dose of humility

 

[Rick Densing]

The one miracle the ACS550 couldn't overcome was a grounded phase. One day both drives kept getting bus overvoltage faults with the motors off. Me and tech support struggled with it unitil I decided to check phase to ground instead of phase to phase voltage. Turns out we had a heating unit on the other side of the building with a gound fault. Pulled the breaker and the problem went away.

I have to say the ABB's tech people were very responsive and helpful. It was my first experience with them and was happy with them.

 

[DickDV]

Pierre, thanks again, but, really, my purpose is to share information so we can all do a better job.

There is plenty in this world that baffles me. For example, while I understand ladder logic, I couldn't program a PLC to save my life. That's why I came to this board originally, to learn a bit about PLC's.

I have a project coming up in a year or two that involves my hobby and will require a PLC to do the logic. I am dreading dealing with that and am hoping to get some help for the programming. Then, the tables will be turned!!

And anyway, what's that saying "Everything I know, I learned from someone else!" I try to keep that in mind every day.

My advice is to find your specialty and focus on being good at that. I know that's dangerous these days but, if you have to be a generalist, for me, it just isn't very satisfying.

Good luck on finding and getting comfortable with your "specialty", whatever it is!

 

[elevmike]

Hey Dick,

I dont get out west (past Ann Arbor) much. Is it true that Elvis is pumping gas in Kalamazoo? Or is it Niles? Oh well..tell him I said hello.. just in case... Say HI to Tony also.

P.S. with your dirve expertise you should be working on elevators.

 

[Sliver]

You wish to post graphics DickDV?

I wish to see your graphics, although your text descriptions are excellent,

check out this thread, it worked for me


http://www.plctalk.net/qanda/showthread.php?s=&threadid=7062

 

[DickDV]

Yes, elevmike, Elvis is pumping gas in Kalamazoo! But, I've seen him at two different gas stations at the same time! How does he do that? More stuff to be baffled over!

And as for Tony the Tiger, he met the wrecking ball I believe a couple of years ago when Kellogg downsized (read that "demolished") their facility in Battle Creek. Or maybe they just moved him downtown to the visitors' center. You'd think a guy with a voice that deep would be able to walk that short distance but, no. Baffling, baffling!

My main office is on Brown Road in Auburn Hills. My boss lurks there. I am obliged to show up there maybe once a month. Otherwise I pretty much stay west of US23.

 

[elevmike]

Dick,

I'm very disappointed to hear about Tony's demise. When I was a kid, (Elvis was at Graceland then), my Dad had a project there, so one summer day I took a ride to Battle Creek with him and got an up front and close personal tour directed by the facilities manager.

I was ruined for life as I watch a spinning roll of blue cardboard over here, and a big roll of wax paper over there, and my favorite snack on a conveyer up there all come together in a big blur of blue and orange boxes going that way. And blinking little lights everywhere!

If I hadn't of made that trip with my Dad, I could have been a doctor or lawyer, or big time stockbroker or something..

Oh well...I guess I'll have to make the most of it...

 

[Eric Nelson]

Life is full of ups and downs. Some people (like Mike) choose to make a career of it...

Sorry, Mike. You're probably tired of hearing elevator jokes...

By the way, we're about a mile from the old Westinghouse Elevator building (which closed in '89). When the leaves are off the trees, I can see it from the corner of our complex. I don't know who bought it, as I can't see a use for a 300' tall building in the middle of nowhere...

beerchug

-Eric

 

[elevmike]

Eric,

I'd tell you why Westinghouse Elevator closed, but I dont want to go around bashing on forums. Besides I'm gratefull for them. We make a hefty profit from tearing out their equipment and replacing it.

Hey, that guy on the ADC forum says theirs 25 hours in a day! And of all things you helped him correct his display to show it that way!?

Your a funny guy!

LOL, Mike

 

[Eric Nelson]

Just wait 'til he figures out he's getting one hour less sleep each night!...

beerchug

-Eric

 

[elevmike]

Well its 24:42 and I have to go to bed.

Wait till I tell the guys about this.lolis

G'Day mate!

 

[shooter]

is mechanical construction impossible?
like a ratchet or something like it.
A motor will have big problems as the amps will go very high, without cooling.
why not use mechanical break or clutch?

 

[DickDV]

shooter, mechanical braking is definitely used in many applications especially elevators and similar machines.

The problem with mechanical braking is the maintenance required to replace the friction elements. For that reason, electronic motor braking is preferred for dynamic braking and mechanical braking is often added for holding or parking purposes (static). That way, the benefits of mechanical and electronic braking are optimized and maintenance of the mechanical brakes is minimized.

There is no known electronic substitute for a spring-applied safety brake or for the various safety rachet systems used in hoists and elevators (lifts).

If you depend on electronic braking exclusively, you can get a nasty and even deadly surprise when a power failure occurs. Some have suggested that DC motor dynamic braking is safer in power failure situations but that is only true if the motor has a permanent magnet field. If a DC motor with a wound field looses its field current as in a power failure, dynamic brake resistors don't do much braking because the armature can't produce any CEMF without a field.

Finally, scooter, in AC motor braking, the current only rises as a function of braking torque. So, if you control the torque, you also control the motor current. This is commonly done and the results are very good for dynamic braking. Of course, if the control system is poorly designed, all kinds of problems can result including excessive motor current.

 

[elevmike]

Dick,

You really should be in the elevator busniess.

We do alomost exclusively use dynamic bracking all the way down to the stop. I prefer the closed loop method with and encoder, or gentach for feed back. A few years ago ABB came out with the concept of feedback from the motor (through the lines), but this didnt work properly for us in very low speed conditions; back to the tach.

Anyway as a protective device we use a field current sensing relay to drop the mechanical brake under emergency conditions.

A story: One of the major manufacures for a long time used a single horizontal plunger/actuater mechanical braking system. The single steel core would slide sideways, inside a brass sleave. After a while the sleave would wear and the core would tend to catch on the sleave. No big deal if the brake didnt pick, you would just loose a motor, of if you got lucky the protective timer would shutdown in time. But one time the core locked when the brake was picked. The motor held the car at all stops and nobody was the wiser, but when traffic demand slowed and the elevator became parked for a time, the motor protective device tripped letting the elevator zip to the top of the hoistway. I cant stand to look at the photos. If the elevator had passengers on it at the time we would have made the front page of every perodical, every day for a month. Fortunately, a few months before the accident, I wrote the customer a unsolicited regestered letter proposing to change out the brake. Whew!

 

[APB]

AS Dick indicated, you need the copper to generate the torque. You also need the copper to act as a heat sink for the I^2R heating that occurs.

You may still have problem with cooling the motor if the duty cycle is high.

Most AC motors use the motor shaft to spin the cooling fan. Cooling decreases rapidly with speed. If you use DC injection (which works very well in many cases) you will get IR heating which can be a problem.

As for what to due in a power outage. It depends. I have used a spring type motor mounted friction brake to hold a load after it was stopped, but have never had to stop a load using a brake. If this is required, then you will have to find or design a brake large enough.

In a total power outgate you might be able to short the motor windings together. (I am not sure I like some of the control design and reliability issues that go with this)