Supplying current to locked DC motor

[mlambrecht]

I have a question about DC motors. I have some 12vdc motors that operate a pair of butterfly valves, I have a problem with them vibrating open after they have been closed so I was wondering if I made an additional circuit that kept a constant current on the motors with a lower voltage say like 3v would that be enough to keep pressure on the valves with out burning up the motors? They are small motors operating a gear reduction box that will be open to free air flow. If this is possible is there a formula to figure out what voltage to safely supply based on the regular operating amperage?
Thanks

 

[Clyde38]

Do you know the torque constant (torque/amp) of the motor?
Do you know the gear ratio and efficiency?
Do you know the armature resistance of the motor?
Do you know the thermal resistance of the motor?
Do you know the torque required to actuate the valve?
If so, you can.

 

[ScottyUK]

The value you can apply safely without overheating the motor depends on the construction of the motor and its ability to reject heat to the surroundings. This might take the form of conducted heat into a larger piece of equipment such as the gearbox which acts as a heatsink, or by convection into the air. Any shaft driven fan on the motor will be ineffective at standstill. A fully enclosed motor will not be cooled by convection as well as an open frame type. Is there any external air movement, and is this always present and / or constant?

The easiest way to determine the maximum safe current is probably to directly measure the winding temperature under stall conditions. I don't think any simple formula will be forthcoming as there are so many variables.


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If we learn from our mistakes I'm getting a great education!

 

[bklauba]

Another solution would be to use motors with an integral (or possibly even an external) brake.

BK

 

[mlambrecht]

The motor does see constant air flow it is mounted underneath a car. I can find out the winding resitance and gear reduction ratio but I don't know how to find out the other statistics. If I find a part number on the motor is that something I can look up? My non scientific method of figuring this out would be to hook up a rheostat in series and start with a high enough resistance to get no motor movement and then slowly turn down the resistance until the valve just barely starts moving. Then running that resistor in series with the additional circuit. I figure if that is too much current for the motor then it won't hold the valve shut anyways. What are your guys thoughts on that? And thanks a lot for the help.

 

[mlambrecht]

do they make motors with integral brakes in them that are as small as the ones I'm working with? They are only about 1 inch diameter by 1.25 inches long.

 

[itsmoked]

Likely not.

If you can power the motor with a PWM signal you would just turn the duty cycle down to whatever is required to keep the valve closed.

I simple microcontroller could do this automatically (once closed go to low duty cycle).

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[mikek10]

Or to solve the original problem anti-vibration mounts or a cam / toggle mechanism to give mechanical advantage?

 

[Skogsgurra]

I would use a series resistor to limit current to around half rated current. That would not harm either rotor winding or commutator if there is a constant air flow.

If operation is hampered by the resistor, a simple time relay that opens a shorting contact across the resistor after the valve has closed will solve that problem.

May also be combined with a diode that bypasses the resistor when you open the valve.

Of course, the resistor will dissipate some heat. But it can be put where cooling is good and heat doesn't harm.

Gunnar Englund

http://www.gke.org

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