motor fan

You're not serious. Are you? " already pass my elect. exams with flyng colors"?

PHC Model SC-3M 300VA Variac
$58.75 new - Electronic City

oops..

sorry.. actually, i got A mark in the electic and electronic paper (only 1). Sorry for the mistakes. Btw, can you solve my problem? Thanks..

Good and informative answer. To the OP - have you considered the use of an extractor fan as used in kitchens many of which has variable speed settings. You might be able to find a suitable unit at kitchen appliance/spares supplier. Good luck. WR

I have serious doubts about either your examiner or your grade in respect of any electronics or that you've done much research - but to add to Ed Weldon's contribution....

Its not clear if you are building a model or something real. First big question is "how to you plan on keeping the motor cool in such a hot environment....and that addressed then to the motor. Is 4 inches the fan size or the motor body diameter or length ...and what volume of gas or fan speed is anticipated. No, its not critical but any informed answer needs to be based on something solid.

If you are talking 4 inches body diameter, then one suitable motor (temperature issues addressed) could be a 240 volt (AC) universal motor out of a domestic spin dry washing machine (probably an older model I suspect these days). The basic motor specs might give you enough, but some additional simple current measurements and schoolboy physics, will allow you o work out the resistor size/rating required to control speed by limiting the rotor current. And I know this easy 'cause I worked up such an arrangement for something else just after Noah came ashore.

Of course resistive control is inefficient and a student/engineer of today would be looking more toward some sort of modulated pulse width control. I have used none of this stuff, but am more than confident its easily acquired (certainly here in Australia) and that would be as easy as going to a local electronic hobby shop for the smaller capacity stuff - failing that there is the web and of course industrial equipment suppliers.

You could use a variac instead of the variable resistor. This will vary the voltage and speed without trying to figure out the resistor value needed. A light dimmer or celing fan speed control would also work. Have FUN! TT3

You cannot vary the speed of such an AC induction motor effectively neither by using a resistor nor a light dimmer. You can only obtain small variations. The motor nearly blocks under the breakdown torque point.

On the other hand you can not know what is the actual speed of the fan at any moment, to decide what reserve power you have, unless you feedback the fan speed by any tacho generator or etc.

I recommend you to use a thyristor controlled motor driver. (We call them frequency converter.) Those drivers are varying the AC frequency, applied to the motor, in paralel with the Voltage. High frequency means high voltage and low frequency means low voltage for the purpose to keep the motor current unchanged at the entire speed scale.

Most of those drivers have an LCD display to show the applied frequency in Hz. and you can easily convert it to RPM in very simple calculation.

Kindest Regards.

What is the furnace back pressure? Is the fan extracting on the hot side or blowing in cold air? Why not throttle or bleed airflow? Is there a servo/control loop involved?

If blowing a few mm water gauge a 12V DC computer fan (junk) might be best; fed from a variable lab power supply, or the scrap computer's power supply.

Suck implies hot side, is the combustion energy small enough to cool with a metre or so of 4" duct, allowing a computer fan. Fan forced domestic cooking ovens might use a suitable unit.

google "speed control of a single phase motor"....This is basic "O" Level Science.... you don´t need to be an engineer.....in short this is NOT Rocket Sicence...

I'm guessing your fan motor is a small shaded pole induction motor, which may easily overheat if you run at reduced voltage. I would let the fan run at full speed and change volume and pressure with a "mechanical damper" manually adjusted. It is not the efficient way, but with a 4" fan, the economical way. And it will be solved mechanically. An electrical engineer.

Hi Dear !

Put a VFD (Variable Frequency Control) which shall give you infinite speed control by controlling the frequency of the ac it supplies to the motor.

The power rating the VFD should be about 50 % or higher than the power rating of the motor. That is necessary because the motor shall draw a very high current in the starting mode and if a safety factor of 50 % higher power rating for VFD is not taken, the VFD shall burn.

In case you want more technical guidance (free ofcourse, but please do not take much time), you may call me on my mobile no : 9810145598

Ashok Shukla

Delhi

Use a Fan regulator manual selector switch or a Dimmer switch for ur project. Next time try to be more specific about the parameters of ur material items.

Dickson Abuja Nigeria