25.5vac to varible DC

[greenBean]

I am looking for a relatively easy circuit that will be used to control a dc motor (cooling fan) with the ability to control speed. I have put together a circuit using a dedicated rectifier, LM317T, linear pot., and a handful of diodes, capacitors, and resistors. It blows the 3A slow-blow fuses quite nicely. I would like to use the 25.5vac transformer i have if possible.
...man, am i a noob... but I do have persistance.
Thanks in advance.

 

[House_Cat]

You need to specify the voltage and current rating of the fan motor before any meaningful help can be given. It would also be nice to know what kind of motor the fan has - brushless DC, series wound, shunt wound, etc. You may be blowing fuses because you are trying to apply too much voltage across the motor windings, and/or because of the type of motor that it is. If you persist, you may smoke your motor windings.

In general, a simple circuit that just varies the voltage to control DC motor speed is not the best way to go. The motor needs some current to generate torque to overcome the inertia of the fan blades and any bearing friction to get started. Until the voltage and current is high enough to get the rotor moving, you will just heat up the windings in the motor; how high the current will be and how much heating occurs depends on what type of motor you have. Once the rotor is moving, it generates a counter EMF that reduces the running current, which in turn reduces the heating in the windings, etc.

A good motor control circuit, that really isn't too complicated with inexpensive IC's, is a pulse width modulated motor control. This works by applying a constant operating voltage to the motor, but varying the speed by controlling how long the current to the motor is turned on. It switches on and off very rapidly so you don't notice the cycling, but the motor speed is proportional to the average time it is 'ON'. The on time is determined by the width of the pulse applied - hence the name pulse width modualation (modulation is a variation in a signal).

An example of such a circuit can be found at:
http://www.solorb.com/elect/solarcirc/pwm1/

Note that you could use your 25VAC motor to power the above circuit by putting a full wave rectifier on the secondary of the transformer. That would give you approximately 24VDC, which is the alternate power supply recommended for the circuit. A note of CAUTION - whether or not 24VDC is appropriate depends on the rating of your motor. This takes us back to the first paragraph where I said we need to know the motor ratings.

 

[IanP]

Blowing 3A slow-blow fuse with 1.5A voltage regulator doesn't look good, does it?

Anyhow, depending on the power/voltage rating of the fan you are going to use, you have several options to choose from: PWM, mentioned in the previous post (for simple fan speed regulatin a 555-timer based circuit is the simplest solution), switchmode voltage regulator, such as LM2575ADJ or similar, or linear regulator, as you have already tried ..

Bear in mind that a 25Vac transformer with rectyfying bridge and smoothing capacitor will produce ≈35Vdc unregulated output, and that will be much to high for a 12V or 24V fan ..
From that point of view a switchmode voltage regulator will be the best option as you can easily limit the output voltage to required level (by adding a couple of resistors in series with the potentiometer), it is highly efficient (very low power is converted into heat) and electrical "noise" is contained to the regulator itself .. have a look at the pictures below ..

Regards,
IanP

 

[greenBean]

Thanks for the clarifying info..
The motor has no markings on it at all. It did come originally on a hot air popcorn popper with a power wire coming from the secondary heating coil and a diode bridge (not sure if thats what it's called, 2 diodes around each terminal, 2 terminals.) it does appear that it has brushes.
again, i appreciate the info. on what I was hoping to be a not-too-involved project..

 

[IanP]

Whatever you do next, you have to estimate the voltage/current rating ot this fan ..
For purely dc application you don't need these diodes attached to the fan's termianls .. so, remove them ..
If you can, get a 12V battery and a 10Ω/5W (or similar) resistor, and connect this fan in series with this resitor to this battery (or alternative 12V power souce) ..
If you think the fan is not overdriven by 12v+resistor remove resistor and connect the fan directly to 12V; at the same time measure current drawn by the fan ..
Once you find out what voltage/current is required by your fan, you can start thinking about a PWM controller suitable for it ..

Regards,
IanP

 

[greenBean]

ok. Now that the holiday is over...
12vdc doesn't run the motor fast enough. I tried 26vdc from the full wave rectifier and it didnt run the motor at all. I just tried it for a second..
Maybe 18vdc?

 

[House_Cat]

12vdc doesn't run the motor fast enough. I tried 26vdc from the full wave rectifier and it didnt run the motor at all. I just tried it for a second..
Maybe 18vdc?

If it is a DC motor, increasing the DC voltage should have caused it to run faster. If it didn't run at all on the higher voltage, then something was wrong with the connection. If it ran on 12VDC, 26VDC should have caused it to take off at high speed immediately when connected.

Did you keep the polarity of the DC the same at 26VDC as it was when you tried 12VDC? Some DC motors are constructed to run in only one direction.

If the diodes you mentioned above are still in place, they probably are a bridge rectifier to supply the DC motor from the appliance AC source. Unless you remove them, you will either have to apply your external DC to the original AC input, or you will have to connect the DC source to the output of the bridge in the proper orientation - a backwards connection to the bridge output will short the external supply through the diodes. DC can be applied to the input of a bridge in either orientation, and the bridge will conduct through the motor in the proper direction.