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Voltage rating of stepper motor....torque vs size

Voltage rating of stepper motor....torque vs size

Voltage rating of stepper motor....torque vs size

(OP)
hello,

i am in Maryland, Ohio working with plumbing system.

--first i apologise for length of post......there are many facets but they are all simple and easy to understand..........

We have a valve in a water pipe which is operated with a stepper motor.
The stepper motor is powered by the isolated secondary of an offline SMPS.


-The stepper motor can open or close the valve, restricting/allowing the water flow to be whatever rate the user sets.

When the valve is stationary in a particular position, the stepper motor is obviously stationary and holds the valve still, drawing some electrical power as it does so.

The problem comes when there is a mains power cut, -if the power cut happens when the valve is completely open, then the stepper motor cannot get the electrical power it needs to close the valve off, and water keeps flowing and flowing.

To get round this problem, we have a 20mF capacitor which gets charged up from an isolated 36V winding on the SMPS transformer.......if a mains power cut happens, then the capacitor's energy is used to close the valve by powering the stepper motor to do so.

The problem is that in order to get enough energy into the capacitor, (1/2*C*V^2), we have to charge it up to a quite high voltage (36V).
....The problem is then that the stepper motor runs off 5V.......and when the 20mF capacitor provides energy, it obviously has to do so through a buck converter (36V to 5V).

(UBB1 is our 5V stepper motor.............)
http://www.auf-vertrieb.business.t-online.de/pdf/UBB.pdf

Now , i am just thinking, what is the point in having such a buck converter....why don't we just get a stepper motor which runs off the 36V ?

....my colleagues tell me that we need to use a low voltage (5V) stepper motor because the higher voltage motors would be of a bigger physical size, in order that they can provide the same torque.........................is this true?
 

RE: Voltage rating of stepper motor....torque vs size

Consider using a larger capacitor and stay with the same voltage instead of "charging it up to a quite high voltage".

Since the voltage is lower the large capacitor can cost less.  Look at using a "super cap."

Bigger because the voltage is higher?  No.  Look at the unit you are spec'ing  It comes in four different voltages. How many different sizes does that represent on the data sheet? (Hint: 1)

Are you sure your valve type doesn't need torque to keep the valve closed under water pressure?  Some do and you would still have a problem once the capacitor is depleted in that case.  In that case you need to back up your controlled valve with a normally closed valve that needs power to stay open.  Power fails => valve closes => water stops.

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

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