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I've got a fan that is way too powerful and would like to find out how to slow it down. Variable speed would be nice but I would be just as happy if I could say reduce it to a fixed speed i.e half speed. It currently only has an on/off switch.

The specs on the fan are: 240Volt 50Hz 1500 Watt Single Speed: 2900rpm

My ability level is enough to pull things apart and solder but I have no education in electronics.

Any insight into this would be great thanks everyone!

Toni


I'm just going to live with the fan as is and reduce air flow by other means to suit my needs.

Thanks everyone for educating me a little more about fans and electric motors.

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  • \$\begingroup\$ High RPM fans can easily be controlled with a Triac wall dimmer. But not so for low speed ceiling fans \$\endgroup\$ – Sunnyskyguy EE75 Jun 22 '18 at 23:59
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The fan most likely has an asynchronous AC motor and this means the speed of the arrangement is bound to be near the synchronous speed (of 3000rpm at 50Hz). You cannot lower it a substantial amount without having substantial losses within the rotor. Which means much additional heat. As the fan motor is cooling itself by rotating the fan, this is double bad.

So the only reasonable way to change the speed of the motor (and the power of the fan arrangement) is lowering the frequency. You would need a VFD for that. For 1.5kVA, it would be quite expensive. Better use a smaller motor/fan arrangement.

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  • \$\begingroup\$ I had no idea what VFD devices were so I did a quick google yes they are ridiculously expensive for what I'm trying to do so looks like this has come to an end :) \$\endgroup\$ – Toni Feb 9 '17 at 12:06
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In addition to what Janka said, if your fan's motor isn't directly connected to the blades, but uses a belt instead, you could alter the size of one of the pulleys to change the speed that it spins at. The motor will still be spinning at the same speed but less stress would be put on it, as it won't be moving as much air.

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  • \$\begingroup\$ Yeah connected to the blades unfortunately thanks for the suggestion though \$\endgroup\$ – Toni Feb 9 '17 at 12:07
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I'm going to assume this is a single phase motor.

So simple speed control CAN be done on CERTAIN TYPES of single phase AC induction motors, called Shaded Pole (SP) motors and as it so happens, that is a common type used on fans because in general, SP motors are not very powerful. With a Shaded Pole motor, you can simply reduce the voltage, say with a light dimmer and the motor produces less torque, then with less torque the air friction on the fan blades causes it to slow down, which in turn moves less air. Shaded Pole motors are inherently current limiting so in slowing them down, the motors WOULD overload, but can't because the current cannot increase so they just lose torque. If your motor is Shaded Pole, it should say so on the nameplate, or it should at least say "SP".

There are some types of fans that use what is called a Permanent Split Capacitor (PSC) motor, they too can be controlled by simple voltage reduction, but are at greater risk for long term damage if they over heat. Many overhead ceiling paddle fans are PSC type and can have their speed reduced with a dimmer because the case is designed to dissipate that extra heat. If you are not talking about a ceiling paddle fan, I would not attempt it.

Basically if your motor has a big "bump" on the side, like it is about to give birth, that's an indicator that it is not SP or PSC and you cannot change the speed.

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  • \$\begingroup\$ While this is generally true, we are talking about a fan motor with frick'n 1500W. First, "dimable" asynchronous AC motors have to use a massive laminated iron rotor with an aluminium cage to have that "triangle"-block-shaped M/n characteristic needed for a good "dimming" effect, second they usually can be "dimmed" only in an intermittent way and need some time turned off (or full speed) to release the excess heat. That fan rotor pretty sure is only an iron bell encapsulating the coils and the thing should to be run continously. \$\endgroup\$ – Janka Feb 15 '17 at 2:14
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I wonder if there is such thing as XY-answer ;)

If the goal is to reduce air flow then it can be easily done without slowing the motor by using smaller blades.

I would look for ready-made fan blades with different diameter. If that is not feasible here are some DIY options:

  • if you have even number of blades you can cut two opposite blades off;
  • if the blade material is not too thick at the hub you can twist them to reduce pitch;
  • as the last resort you can shorten the blades, but be careful to cut exactly same length, otherwise you'll get nasty vibration.

BTW, if by "reduce air flow by other means" you meant to restrict it somehow (e.g. by covering part of the opening) then it is not a good idea, most likely result will be motor overheating.

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