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Locking the movement on an electrical motor
3

Locking the movement on an electrical motor

Locking the movement on an electrical motor

(OP)
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

RE: Locking the movement on an electrical motor

How many turns of the motor are acceptable?

RE: Locking the movement on an electrical motor

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

RE: Locking the movement on an electrical motor

3
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
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Locking the movement on an electrical motor

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

RE: Locking the movement on an electrical motor

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

RE: Locking the movement on an electrical motor

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

RE: Locking the movement on an electrical motor

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

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Locking the movement on an electrical motor

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/products/clutch-brak...

RE: Locking the movement on an electrical motor

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

RE: Locking the movement on an electrical motor

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.

RE: Locking the movement on an electrical motor

(OP)
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.

RE: Locking the movement on an electrical motor

worM.

RE: Locking the movement on an electrical motor

Probably your best solution will be to use the brake suggested by compositepro.
http://www.warnerelectric.com/products/brake-produ...
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

RE: Locking the movement on an electrical motor

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

RE: Locking the movement on an electrical motor

(OP)
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.

RE: Locking the movement on an electrical motor

(OP)
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.

RE: Locking the movement on an electrical motor

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

RE: Locking the movement on an electrical motor

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]

RE: Locking the movement on an electrical motor

Buy a "C" face brake. Remove the motor from the reduction gear. Bolt the brake to the reduction gear. Bolt the motor to the brake. Follow the wiring diagram. This should solve 98% of the issues.
You may chose to install a brake on the final hoisting shaft. A brake on the final shaft will have to handle much more torque and will be much larger and more expensive.
If you build another similar system, consider using a final reduction gear that has a hole in the final gear so that it may be mounted directly onto the final hoisting shaft.
This avoids the cost of chain and sprockets and, more importantly, avoids the possibility of a broken chain dropping the load.

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

RE: Locking the movement on an electrical motor

A gear motor can have any kind of gear in it. Is there a reduction gear, 1:1, a bevel gear, a hypoid gear, any other type where the RPM of the motor is different than the output of the gear box?

RE: Locking the movement on an electrical motor

It is interesting to note back in the day when servo systems were very expensive, we used a VFD and two position switches to perform indexing operation on case packers. When the index operation started, the motor ran at full speed. As soon as the first position switch was made, the VFD shifted to a slow speed, usually around 25% until the second switch was made. The VFD was then commanded to stop. No brake on the motor. Using this system we were able to perform an index of 30 inches in less than 3 seconds with repeatability of 1/16". Perhaps something similar could be used here?

RE: Locking the movement on an electrical motor

You mention it runs on "contrary", which I take to mean it reverses when stopped. That's called "backlash" and is likely the result of poor selection of your gearbox. If the idea of a mechanical brake bothers you, then another solution would be to change your gear box to a zero backlash version, something like the worm drive mentioned earlier.


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

RE: Locking the movement on an electrical motor

Maybe I missed this, but does the brake have to hold a load?

RE: Locking the movement on an electrical motor

Or you could borrow a couple of truck batteries and a contactor and try Gunnar's suggestion of DC injection.
You can try 12 Volts and then 24 Volts. If you get decent results and determine the best voltage to use you can then spend the money on a suitable DC supply.
Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Locking the movement on an electrical motor

(OP)
BrianE22, no. The load is not lifted, it's dragged. The wire rope that drags the load gets stretched out and when the operator stop to press the button, the wire rope pulls the shaft where the gearbox is connected.
jraef, the spin that the gearbox do is kind of a lot. Don't know if a backlash can be responsible for all that.
I think that an electromagnetic brake will be the solution.

RE: Locking the movement on an electrical motor

Now I like Gunnar's DC injection suggestion even more.
With DC injection you may use a higher current DC to provide a rapid stop and then drop to a lower level DC current to let the backlash unwind slowly. A timer should be adequate. X seconds at higher current and then Y seconds at lower current.

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

RE: Locking the movement on an electrical motor

You don't by chance have a VFD on this motor do you? If so, you likely have DC Injection Braking available already, it's a feature of almost every VFD out there. If not, then there are lots of stand-alone DC injection braking units available on the market. two features that you will want to look for:
1) Automatic detection of the motor emf being zero before applying DC. This is missing on cheap units and leads to failure of the DCIB.
2) Automatic shut off when the motor has stopped. Cheap ones don't have that or just use a timer and that can cause unnecessary added thermal stress to the motor.


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

RE: Locking the movement on an electrical motor

Quote (Gustavo Silvano)

The wire rope that drags the load gets stretched out and when the operator stop to press the button, the wire rope pulls the shaft where the gearbox is connected.
jraef, the spin that the gearbox do is kind of a lot. Don't know if a backlash can be responsible for all that.

If that is the case an electric bake or switching to a worm gear drive gearbox is the only solution.

RE: Locking the movement on an electrical motor

Some of the Rotary Friction Welding machines shown on the 'net have fairly impressive 'sudden stop' capabilities. Hundreds of kg of rotating mass, reportedly 600 rpm. Once the joint reaches the desired conditions: bang. Seemingly a nearly-instantaneous (I know, not really) dead stop. Certainly impressive enough that the videos are all over YouTube.

Not likely applicable in this case. Presented just in case it provides inspiration. smile

[EDIT] Here's an example (cued-up at 12s in): https://www.youtube.com/watch?v=MIYJnd2X9eU&t=...

RE: Locking the movement on an electrical motor

VE1BLL,

There are table saws that stop when you stick your finger in the blade. I am not sure this is good for the table saw. I am not willing to test one.

--
JHG

RE: Locking the movement on an electrical motor

SawStop is pretty impressive, blade will be damaged:

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

-AK2DM

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
"It's the questions that drive us"
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

RE: Locking the movement on an electrical motor

I'm not sure the blade is damaged. The blade stops by jamming a soft aluminum wedge into it. I use table saws to cut aluminum all the time, it's not a problem for the blade. The Saw Stop unit is a one-and-done device however.


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

RE: Locking the movement on an electrical motor

j, the blade is usually damaged in the process. Broken teeth and the like... teeth can be brazed back on, but the integrity of the main disk is then in question after such a shock.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Locking the movement on an electrical motor

I posted this back on 13 Sept. At the 50 second mark you can see the action and the deformation of the stopping device.
https://www.youtube.com/watch?v=E3mzhvMgrLE

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

RE: Locking the movement on an electrical motor

The real question is however, why is this guy always trying to cut his hotdogs with a table saw when a knife works better?

And hey, nice camera work!


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

RE: Locking the movement on an electrical motor

I don't understand why the blade drops from that movie. The movie doesn't show it dropping only the soft-stopper being launched into it and its rotation being arrested.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Locking the movement on an electrical motor

Conservation of angular momentum and a compliant mount that releases the locking support that locates the exposed blade.

Now I'm been down the Sawstop rabbit hole. Lots of anger and dismay about that company for their legal dealings.

RE: Locking the movement on an electrical motor

Keith,

Activation of the capture mechanism releases the axle, allowing the blade (and mechanism) to drop from the table. I cannot recall if there is a spring, as well, but the blade drops quickly, so I imagine there is.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Locking the movement on an electrical motor

Hi s,
The first project that I worked on was a re-and-re of major substations in a refinery. This was way back in the mid 70's so some of the details are a bit fuzzy. But I do recall installing dc-injection braking modules on cooling tower fans. These modules were to prevent a stopped fan from running backwards due to backdraft caused by adjacent fans running. The modules were also used to provide anti-condensation heat in the stator winding, which is a bonus in such a damp environment. These modules were built by Allen Bradley, but I haven't seen any since the late 70's. (I'm sure that Others can provide a similar solution).

Is there any chance of installing an ASD? A 4-quadrant ASD (ie active FE) would be the ultimate solution, but any ASD can be fitted with a braking resistor. Any ASD could be made to work, plus you get the usual advantages of having an ASD in-situ. An ASD option might even be less expensive than monkeying around with an electric brake.

GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

RE: Locking the movement on an electrical motor

Jeff 22 Sep 17 19:53

Oh, is that a Hotdog? I thought that this guy had had an accident and now thinking about how to re-attach his pinky.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Locking the movement on an electrical motor

Howdy Gustavo,
FYI: I was just perusing a few VFD manufacturer web-sites and discovered that many of their products come (standard) with braking systems using dc-current-injection. Check out the Yaskawa and Toshiba web-sites. Depending on the motor size, this might be less expensive that trying to retrofit an electro-mechanical brake to an existing system.
GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

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