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I have a motor speed controller which changes the motor speed according to the analog voltage input of a potentiometer. I want to set a fixed value for the the motor speed which can not be changed by the operators. Can I use fixed resistors for this purpose and how to do it? Hope you all kind enough to help me.

Yes.

With the circuit switched on, set the pot to the speed you want.

Measure the voltage across all three combinations of terminal pairs.

Note the terminal pairs and the voltages.

You will have 3 voltages. Call the largest, Vsum.

Call the next largest VR1 and the last (smallest) VR2.

Vsum should equal VR1 + VR2 (given the tolerances on the measurements).

If not, check your measurements.

Without altering the pot position, disconnect it.

Measure the resistance across all three combinations of terminal pairs.

Note the terminal pairs in the same sequence as before and note the measured resistances across each.

You will have 3 resistances. Call the largest, Rsum.

Call the next largest R1 and the last (smallest) R2.

Rsum should equal R1 + R2 (given the tolerances on the measurements).

If not, check your measurements.

Try to find fixed resistors with values as close as possible to R1 and R2.

Ensure that the resistor power ratings PR1 and PR2 meet the following constraints:

PR1 > VR1^2*R1

and

PR2 > VR2^2*R2

Fix the resistors in place of the pot so that they are across the terminal pairs as defined by your measurements and notes.

Done.

Or

Try to find something like this:

http://uk.farnell.com/colvern/a43/locking-device/dp/351970

:)