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How to Prevent a DC Motor From Turning?

03/30/2015 1:41 PM

Hi --

We have an application that uses a 120VAC motor to move a gate. Sometimes the location isn't level, and the gate rolls when the gate is open and the power is not applied. We fix that by applying 12VDC to the motor. This works fine.

The problem is that we want to change to a 0.5HP, 24VDC brush motor. Shorting the leads when the motor is off makes the motor harder to turn, but doesn't provide enough stopping power. Can we use the same approach of applying a very low voltage to this motor?

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#1

Re: How to Prevent a DC Motor From Turning?

03/30/2015 2:12 PM

Motor brake.

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#2

Re: How to Prevent a DC Motor From Turning?

03/30/2015 2:13 PM

I think you would not want to have to run power to a system just to maintain a static position, unless that condition is only a momentary state.

If the gate remains open for any length of time you are wasting power and unnecessarily heating the motor windings.

Worse, what happens when there is a power outage?

A better way to do this is to add a brake that engages when the motor is in the off state (again assuming that the gate spends more time in a static position than it does opening and closing) and no power is applied.

A solenoid would be used to disengage the brake whenever power is applied to the motor.

An even better way to skin the cat is to use a worm drive gear system to drive the gate. The motor can turn the gear set with very little friction, but it is virtually impossible to back drive the motor by pushing on the gate. That would be most likely a cheaper alternative and probably how commercial and residential motorized gates are done anyway.

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#3
In reply to #2

Re: How to Prevent a DC Motor From Turning?

03/30/2015 2:16 PM

A worm drive controlling a gate. The position of the gate will not change after being set. From Wiki: Worm Drive

Anonymous Hero said: "An even better way to skin the cat is to use a worm drive gear system to drive the gate. The motor can turn the gear set with very little friction, but it is virtually impossible to back drive the motor by pushing on the gate. That would be most likely a cheaper alternative and probably how commercial and residential motorized gates are done anyway." And he is correct.

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#20
In reply to #3

Re: How to Prevent a DC Motor From Turning?

03/31/2015 3:44 PM

Anonymous Hero is right in his suggestion. The picture you used is actually of a Stationary Worm gear used on a cam as a locking device on a toothed flat cut gear. Worm gears as driving gears will drive an angle cut gear and the worm will have a motor attached to it, as the name suggest "worm drive gear system." Common use on wiper motors, Sliding Gate Motors, Conveyor belt drives, angle grinders, jigsaws, and many other devices. Two advantages/disadvantages depending on the use. 1. Output shaft is 90 Deg. angled to the motor and 2. Gearbox result, output shaft speed is always slower than the speed of the motor depending to the slots on the Worm per revolution, to the number of gear slots on the driven gear.

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#7
In reply to #2

Re: How to Prevent a DC Motor From Turning?

03/30/2015 5:01 PM

The gate usually stays open for only a few seconds (<~20). In very rare circumstances, it can stay open much longer than that (hours), but there is a solenoid lock that probably could be used in that case. The lock is normally used only when the gate is closed and someone tries to force it open.

We are going to 24VDC specifically so that we can add a battery backup, solving the power outage question.

We have products with worm drives; but there are legal requirements for manual overrides in case of emergencies, and that has issues of its own, so we try to avoid them when possible.

It sounds like nobody has done this before; so maybe I will hook one up and find out what happens.

Thanks for your interest, and all the responses.

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#9
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Re: How to Prevent a DC Motor From Turning?

03/30/2015 7:25 PM

Have you considered a spring assist or counterweight?

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#18
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Re: How to Prevent a DC Motor From Turning?

03/31/2015 11:42 AM

Most people buy a gate opener and don't bother making or putting something together.

I paid $350.00 for our 20' sliding gate opener (FAAC - Estate slide -e.g.) in 1999 and it is still working silently.

It has a plastic teeth track, and a motor module with back up and programmable functionality. And no one can push it open (slide)

It is probably the most silent and functional on the island here. Al the other gates with chain, stumble, screech and are easy to break in ( cut the chain)

The safety is also available by mean of a hex- key.

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#4

Re: How to Prevent a DC Motor From Turning?

03/30/2015 4:19 PM

You could use a magnetic lock....

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#13
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Re: How to Prevent a DC Motor From Turning?

03/31/2015 4:56 AM

Firstly, great idea, LIKE IT!!

But you neglected to mention exactly how it works and how it would provide an easy solution to his problem of holding the gate both open and closed....(assuming he uses two!!) But can easily be over ridden by a sharp pull....assuming DC power is off for some reason!

(Info for the OP only, NOT YOU!, DC is used to NEGATE the magnet only. No DC, then the magnet holds firmly! Opposite to what you may have imagined!)

Here in Germany they are often used to hold open spring loaded fire doors in company offices, they are then demagnetised when the fire alarm goes off......and the door is released and shuts automatically, but can be opened against the spring if urgently needed to....

They may need some weather protection for outside usage if an outside version is not available. But that should be easy....

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#15
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Re: How to Prevent a DC Motor From Turning?

03/31/2015 7:33 AM

My garage door is overridden by a pull cord that simply disengages the door from the chain drive. It's very mechanically simple.

However, I don't know his specific application, but I would bet that there is almost a 100% chance that there is a preexisting standard solution out there with all the bugs already worked out of it. Doorman could probably back that up.

While it is fun to be creative and inventive, it is often better to use an existing wheel rather than reinventing it.

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#19
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Re: How to Prevent a DC Motor From Turning?

03/31/2015 1:26 PM

Those units have been around many, many years. Its nothing new at all.....Solar Eagle obviously knows about them too.......

Also, very simple and easy to include in such a system as the OP mentioned, without changing much!! Basically, just powering the coil of the unit for a few seconds when the motor is powered up....many simple ways to do that.....

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#5

Re: How to Prevent a DC Motor From Turning?

03/30/2015 4:26 PM

DC servo loop would do it. It would certainly increase the cost of the system but you could now gain complete motion control this way.

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#6

Re: How to Prevent a DC Motor From Turning?

03/30/2015 4:45 PM

A bolt. Solenoid to release, spring return to lock.

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#8

Re: How to Prevent a DC Motor From Turning?

03/30/2015 5:21 PM

It will depend on what type of DC motor you are using. The reason it works on the AC motor is because with an AC motor, the electromagnets that make it spin are created by the AC, and move with it around the stator windings, then the rotor follows. So when you apply DC to the winding, the stator electromagnet no longer rotates, so the rotor "follows" it and stays still as well. You use a low voltage DC level to keep from burning up the windings.

With a brush type DC motor, you have DC going to both the stator and the rotor, so since applying DC to it is what it NORMALLY works on, applying lower voltage DC just makes it turn slower. The reason why shorting a winding doesn't HOLD it is because there is nothing to HOLD until you already start moving, because it is the movement that generates the voltage that then retards the movement (which means you must be using a PMDC motor). So you perceive is only as RESISTANCE to moving.

If you have a BRUSHLESS permanent magnet DC motor, such as a servo motor, then the controller it requires to make it work is putting out something akin to multi-phase AC power, which, like AC, rotates around the frame and the rotor follows it. In that case, you can have it power only ONE set of windings, just like you did on the AC motor, and it too will provide a DC hold. You can get a similar effect from a stepper motor as well, just tell the stepper driver to not advance the steps.

How long you can do that is a conversation to have with the motor and drive mfr.

But have you looked into a brake motor? Most are available with a manual release lever on the brake mechanism. I would think that would satisfy any requirements for manual override.

Like this.

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#10

Re: How to Prevent a DC Motor From Turning?

03/30/2015 9:30 PM

Most gates work with 12 or 24 Volts DC.

The applied gears, combined with a permanent magnet motor (as stator) and a brush type rotor will brake when the wires are shortened out.

A robot we designed with 4. 100 Watts motors could hold 120 Kg as ballast.

It you really have a problem, consider a ballast weight that balances the slope.

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#11

Re: How to Prevent a DC Motor From Turning?

03/31/2015 12:18 AM

The simplest, easiest to maintain and probably least costly method is to use a motor brake. Power is on motor and the brake is disengaged, shut the power off to the motor and the brake is engaged. Simple! They are available from a fraction of a ft-lb up to in excess of 1,000 ft-lbs from the standard distributors or a motor shop.

Some of the configurations are shaft mounted, motor mounted, clutch and several others. That type of installation is most likely one that you want to install it and forget it, except for PM. This is simpler and more "dumb friendly" than worm gears and others what need more careful set-up, perhaps lubrication and almost certainly maintenance.

http://www.baldor.com/brands/baldor-dodge/products/clutches---brakes

http://www.globalspec.com/ds/90/areaspec/mount_shaft

are only a few of the many sources. These are especially used on machines that MUST stop in a fraction of a second for example safety. Some would include punches, packaging equipment and others that require hands to be near the pinch points during operation.

Good Luck, Old Salt

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#12

Re: How to Prevent a DC Motor From Turning?

03/31/2015 12:40 AM

If you were happy with the way it worked before then why not just run the gate controller from a true sin wave UPS connected to your existing mains. Don't know how you did the 12VDC (??A) before but that can be sorted similarly.

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#14

Re: How to Prevent a DC Motor From Turning?

03/31/2015 6:52 AM

Yes.

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#16
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Re: How to Prevent a DC Motor From Turning?

03/31/2015 7:38 AM

Simply increase the voltage until the motor does not turn backwards. It will only be a few volts and a small power supply is all that's needed.
The gearbox will also add to the resistive effect of reversing the motor.

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#17

Re: How to Prevent a DC Motor From Turning?

03/31/2015 9:12 AM

Level the gate.

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#21

Re: How to Prevent a DC Motor From Turning?

03/31/2015 9:33 PM

Powered wheelchair motors are available in a range of sizes. They come with a spring loaded brake; power off = brake on. They are also able to be disengaged for manual operation. If you just want one for yourself you could pick one up for almost nothing. If you are a manufacturer you could designate the type of mounting you want and current handling and the physical size etc. They are almost all 24v and brush type. They also work in all weather ( to a point ).
I hope you can see this ebay picture, if not go to http://www.ebay.com/bhp/electric-wheelchair-motor. This motor is $40. To the left is the brake to the right is the gearbox. Sometimes these are worm drive but almost always they have a disengage mechanism to allow the chair to be pushed when the batteries are flat.In short;- everything you want.Jim

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#22

Re: How to Prevent a DC Motor From Turning?

04/01/2015 5:24 AM

You already have a gate motor from a manufacturer that make gate motors with the same fittings and meshing gears. Read the name of the supplier of the motor/system google them and buy the motor that just need replacing on the same mountings and the DC components and battery that goes with it.

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