Locking the movement on an electrical motor 3

[Gustavo Silvano]

Hello there!

I'm an electrical engineer and I just mounted a new machine on my industrie. It's composed of a motor turning a shaft. This turn is made when the operator press a button. But due to the inertia of the shaft, when the motor stop spinning, the shaft make a half of a turn. And this is caused by the fact that the motor does not have a break.
This inertia was not taked into account when I was making the project for this improvement, but I want to fix it without changing the motor. Is there a way to do this? An electrical or mechanical one?
I don't know if it's necessary, but here's the project that was implemented.

Best regards.

Gustavo

 

[LionelHutz]

How many turns of the motor are acceptable?

 

[waross]

Without knowing speeds, HP or gear ratios we are guessing, but:-
A large clutch-brake assembly on the drive from the gear reduction to the shaft. This will stop and hold the shaft and allow the motor to coast down, without driving the shaft.
One successful application of this solution has been alligator shears used to cut re-bar. One clutch-brake that I serviced was air pressure operated.
It may be cheaper to replace the motor with a motor with a brake.
Many years ago we wold have plugged the motor to a stop. (momentary DOL reversing).
Plugging worked well with "U" frame motors, but when "T" frame motors came in, plugging went out.
You may consider using an auto-transformer and plugging at reduced voltage.
Comparing the cost of an auto-transformer, a reversing starter and a plugging switch or timer to de-energize when the motor stops, a new motor may be cheaper and have less failure modes.


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

 

[Skogsgurra]

DC braking is a simple method. You can either use a rectifier/resistor/timer/zero speed switch or a dedicated unit. Like this:

https://en.wikipedia.org/wiki/DC_injection_braking

Gunnar Englund

http://www.gke.org

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

 

[Gustavo Silvano]

Awesome! Didn't know about the DC braking., Gonna try it out. Thanks!

 

[Compositepro]

There are also electric brakes that you can mount between your motor and gearbox.

http://www.warnerelectric.com/products/brake-products/motor-brakes/double-c-face-power-off-brakes

 

[jraef]

DC braking turns the motor into the heat sink for the kinetic energy in the moving mass, so you must take that into account in your Starts/Hour rating of the motor. Basically, consider each Stop as having the same thermal effect on the motor as a Start. So if your motor was rated for 10 starts/hour, it is now rated for 5 starts and 5 stops.

And because it appears that English is a second (or 3rd, 4th, etc.) language for you, here is today's free lesson:

Break = the act of making something broken.
Brake = the act of stopping something.

It's important not to break something when trying to brake something...


" We are all here on earth to help others; what on earth the others are here for I don't know.
" -- W. H. Auden

 

[waross]

Dc injection braking is not an instant stop but it does stop the motor faster than letting it coast to a stop.

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

 

[Skogsgurra]

Instant stops do not exist IRL. DC braking can be made as fast as the starting. Even faster.

Gunnar Englund

http://www.gke.org

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

 

[3DDave]

Since the shaft makes half a turn this is probably a worm drive gearbox. If so, you either need to brake the motor directly (mechanically or electrically) or, as suggested above, add a clutch between the motor/gearbox and shaft.

Check with the motor maker to see if they offer a brake for the motor. You may be able to get them to replace just the motor and not pay for a new gearbox.

I looked at clutch-brake assemblies; even if they are cheaper than changing the motor, the mounting and additional installation expense will probably be more costly.

Example, no affiliation, not a recommendation:

http://www.warnerelectric.com/produ...ucts/clutch-brake-modules/iec-mount/em-var-12

 

[waross]

I agree. Instant stops do not happen IRL.
There is always a measurable time to stop.
With electrical braking, plugging, Dc injection or other means the stop time is often measured in revolutions to stop.
I have worked on mechanical brakes where the stop time may be measured in a few degrees.
Then there is the Saw Stop System where the time to stop/retract is claimed to be one millisecond, once!
But not instantly.

https://www.youtube.com/watch?v=E3mzhvMgrLE

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

 

[zeusfaber]

This is another of those things where it would be good to understand more clearly what you actually want to achieve.

Are you desperate for the motor to stop really quickly whenever the operator releases the button (in which case, you're looking for one of the things that are being discussed above) or are you actually more interested in making the shaft stop in the right place (in which case, what you really want is a decent control system).

A.

 

[Gustavo Silvano]

jraef, how can I know the starts/hour of my motor? And thanks for the grammar point you've made.

3DDaves the system is new. All components that was installed are new. I don't think that there's an worn on the gearbox. You said to install a clutch between the motor and the shaft, but I'm using a chain to connect then. The clutch will be installed on the shaft?

zeusfaber, in fact what I want is to make the shaft stop as soon as the operator release the button. More degrees it takes to stop, more problems I'll have.

 

[3DDave]

worM.

 

[waross]

Probably your best solution will be to use the brake suggested by compositepro.

http://www.warnerelectric.com/products/brake-products/motor-brakes/double-c-face-power-off-brakes

Putting the brake on the higher speed shaft is often the most effective.
If you need faster stopping, you may add a clutch between the motor and the brake.

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

 

[drawoh]

Gustavo Silvano,

Your picture looks like a worm drive. Have you worked out the inertias of your system? If if is a worm drive, t is possible that most of your system inertia is your motor.

Regardless of how you stop your system, it won't stop instantly. If you decelerate hard to a stop, you will impose high stresses, and something likely will break. What you need to do is stop the system at a predictable rate. If you decelerate at a known, constant rate, you can work out when to start decelerating.

--
JHG

 

[Gustavo Silvano]

It can decelerate. What is happening is that the system, when the operator stop pressing the button, run a little to the contrary. This is caused by the inertia of the system and the load. What I want to avoid is just that. With a brake, when the operator stop pressing the button, the system will stop and don't let the inertia routate it a little.
So the problem here is to avoid the system to rotate to the contrary.

 

[Gustavo Silvano]

Just to be clear, the system is not a worm gear. Is just a gearmotor with a chain connecting the gearmotor's gear to the shaft gear.

 

[waross]

Have you considered retraining the operator to anticipate the coasting time?
That is a common solution to coasting, with and without brakes on many cranes.

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

 

[crshears]

Gustavo,

How frequently is this stopping action required? Also, you have begun using the word "contrary," something you never mentioned in the OP or for a while since...

Is there actually some momentary reverse rotation in the opposite direction you are trying to eliminate?

If there is but it's always by the same amount, the operator can learn to anticipate it and switch off the power supply at the correct instant required to achieve the desired final positioning. [Admittedly this could become problematic if the coasting time constant is so great that a lot of time is wasted while the machine comes to rest, reverses, then finally settles in the right spot.]

If there is and it's never by the same amount twice, a mechanical brake is a virtual necessity, IMHO, since in that situation such anticipation is impossible and therefore unreasonable to expect.

Providing all of the pertinent information right from the outset can save everyone so much time...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]