im useing a 12 v motor out of a old tapedeak to power my model problem iv got is the rpm s keep changeing between 750 - 730 rpms little bit of load on the motor , i need a way to make it run at one speed without changeing so i can record the data.

 

What kind of speed control circuit are you using now? You will, at minimum need a way to either measure the change in load or change in RPM. Depending upon your method of sensing change, there are multiple methods to regulate speed. Attached is a PDF I found with GOOGLE.

 

One easier way if you only need a certain speed and don't need to vary it, is not electronic. A speed reducing gear train or even pulleys and belt is a better constant torque method. With an electronic speed control with out feed back speed varies with load. With a mechanical speed reducer the torque goes up as speed is reduced, and the speed doesn't vary as much as load changes.

 

apart from the baty power no way to controll rpm , im useing a non contact laser photo tachometer to show me the rpms, i think electronic is what it needs as i need the rpm to be controled as much as i can, billb3857 looks like what i need so speed is independent of load, i dont know much about electronics can i buy one of these from some shop, my brains worked on thiss on and off for 28 years so i built it thiss year the data iv got so far is telling me not to stop (good things take time)

 

There is 2 types of old cassete deck motors. One type has inbuilt govenor contacts that open & close to keep speed. These will probably vary in speed as yours does. Next type has in built electronic govenor sensing back EMF of the motor & are more acurate at speed control. This type has a hole in the back of the motor to adjust the speed over a limited range. Electronic controled cassete motors are not expensive new about $8.

 

Pic of the Motors.

 

If you are trying to use a cassette motor, you must not need much torque. One method of regulating speed that is changing due to loading is to measure the motor current as a feedback parameter. (Current varies with torque requirements). Speed is primarily controlled by voltage, so by monitoring current and adjusting voltage electronically, regulation can be obtained. For real accuracy you will need to have an accurate means of measuring the RPM and converting that to a usable signal for feedback to create a closed loop system. Without it, it would be like trying to hold 35 MPH without a speedometer in your car and only the cop with the radar gun knowing how fast you are really going.

Check these sites for more info......
http://www.joliettech.com/dc_drive_fundamentals.htm
www.datasheetcatalog.org/datasheet/BurrBrown/mXysswv.pdf
http://www.electrocircuitdiagram.com/21-small-dc-motor-speed-controller/

 

A motor using a Tacho feed back & circuit. Other motor is out of an old Panasonic VCR capstan motor with tacho windings, with a bit more grunt.

 

thanx guys think i understand dc motors better now, motor im useing nows hadet think im going to have to take thomas edisons aproch , try everything till i get it