# Question about an electrical motor and fan **URL:** \ **Category:** Factual Questions\ **Created:** [January 22, 2009, 10:20am UTC](https://boards.straightdope.com/t/question-about-an-electrical-motor-and-fan/482431 "2009-01-22T10:20:18Z")\ **Posts on this page:** 18\ **Page:** 1
**Author:** ![Indian](https://avatars.discourse-cdn.com/v4/letter/i/c0e974/32.png) [@Indian](https://boards.straightdope.com/u/Indian)\ **Post date:** [January 22, 2009, 10:20am UTC](https://boards.straightdope.com/t/question-about-an-electrical-motor-and-fan/482431/1 "2009-01-22T10:20:18Z")
You have a 230V,50 Hz electric motor with shaft extending both sides. One side you have a fan (with certain blade angle) attached . When the motor runs and air is blown out through the fan away from motor. Motor spins in one direction only. 1. When the blade angle is changed 180 degrees, will the fan become suck through instead of blow through ?? 2. When the fan is put on the other side of shaft without any blade angle change, will the fan suck through instead of blow through ?? ---
**Author:** ![minor7flat5](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/minor7flat5/32/258_2.png) [@minor7flat5](https://boards.straightdope.com/u/minor7flat5)\ **Post date:** [January 22, 2009, 12:38pm UTC](https://boards.straightdope.com/t/question-about-an-electrical-motor-and-fan/482431/2 "2009-01-22T12:38:39Z")
Not an expert on these at all but… The only industrial motors I recall had straight blades like [these](http://www.etimotors.com/images/blower_poly.jpg). The fan is at the end of the motor and is covered by a shroud. The fan works as a sort of “centrifugal pump”, sucking air in the center and throwing it radially, and the shroud directs the breeze over the cooling vanes of the motor. Of course, you said angled blades, perhaps like [this](http://www.etimotors.com/paddlewheel%20blower.jpg). I imagine that those would simply act better in one direction than the other, but if the motor has the same shroud arrangement, the air would probably still go in the center of the fan and be flung outward, guided down the sides of the motor. The one I linked to appears reversible, so one could simply flip it over if it’s pointing the wrong way. ---
**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\ **Post date:** [January 22, 2009, 2:14pm UTC](https://boards.straightdope.com/t/question-about-an-electrical-motor-and-fan/482431/3 "2009-01-22T14:14:00Z")
1. No. The blade angle relative to the shaft will be precisely the same. This applies whether you take the entire fan off the shaft and rotate it 180’ before reinstalling, or (magically?) rotate each individual blade on the fan hub. In the latter case, it should be very easy to visualize that the blade will return to its exact original position. In the former case, there might be a change in efficiency **if** the blades are curved, but still blade angle and direction of airflow will be the same. 2. No. The airflow direction will be unchanged. It will change to blowing across the motor instead of sucking across the motor, but it will suck from the same side of the motor/fan assembly as before and blow from the same side as before. If you want to change the direction of airflow, you need to rotate the motor 180’ (relative to the line of its shaft) or reverse the direction of shaft rotation. ---
**Author:** ![minor7flat5](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/minor7flat5/32/258_2.png) [@minor7flat5](https://boards.straightdope.com/u/minor7flat5)\ **Post date:** [January 22, 2009, 2:52pm UTC](https://boards.straightdope.com/t/question-about-an-electrical-motor-and-fan/482431/4 "2009-01-22T14:52:25Z")
I have no idea why I was thinking about the motor’s _own_ fan and not a regular fan…😃 Carry on! ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [January 22, 2009, 5:06pm UTC](https://boards.straightdope.com/t/question-about-an-electrical-motor-and-fan/482431/5 "2009-01-22T17:06:59Z")
A squirrel cage fan in a shroud will move air in at the center and out at the end of the shroud no matter which way you spin it, but it will be less energy efficient if you have the shroud pointed the wrong way. I once got to investigate a very large fan of this sort that was drawing too much power. While it was off, I got to crawl through the intake duct, going around corners and everything. No, my friends didn’t turn it on when I got there, to make sport. ---
**Author:** ![Really\_Not\_All\_That\_Bright](https://avatars.discourse-cdn.com/v4/letter/r/e8c25b/32.png) [@Really\_Not\_All\_That\_Bright](https://boards.straightdope.com/u/Really_Not_All_That_Bright)\ **Post date:** [January 22, 2009, 5:37pm UTC](https://boards.straightdope.com/t/question-about-an-electrical-motor-and-fan/482431/6 "2009-01-22T17:37:12Z")
> [@Gary\_T](#): > > 1. No. The blade angle relative to the shaft will be precisely the same. This applies whether you take the entire fan off the shaft and rotate it 180’ before reinstalling, or (magically?) rotate each individual blade on the fan hub. In the latter case, it should be very easy to visualize that the blade will return to its exact original position. In the former case, there might be a change in efficiency **if** the blades are curved, but still blade angle and direction of airflow will be the same. I tried visualizing this and I’m pretty sure you’re wrong. Picture an aircraft propeller. Focus on one blade, that slant up toward the engine. When you flip it, it will slant down and in. ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [January 22, 2009, 5:52pm UTC](https://boards.straightdope.com/t/question-about-an-electrical-motor-and-fan/482431/7 "2009-01-22T17:52:09Z")
Pick up the fan, motor and all. Rotate it 180° about a vertical axis. Set it back down. Which way does the air flow? Swapping the blade from one shaft to the other will have exactly the same effect. Now if you take the blade off, rotate it 180° about a vertical axis, and slap it back on either shaft, you _will_ change the direction air moves with respect to the motor housing. ---
**Author:** ![AndrewL](https://avatars.discourse-cdn.com/v4/letter/a/3d9bf3/32.png) [@AndrewL](https://boards.straightdope.com/u/AndrewL)\ **Post date:** [January 22, 2009, 5:55pm UTC](https://boards.straightdope.com/t/question-about-an-electrical-motor-and-fan/482431/8 "2009-01-22T17:55:50Z")
> [@Really\_Not\_All\_That\_Bright](#): > > I tried visualizing this and I’m pretty sure you’re wrong. > > Picture an aircraft propeller. Focus on one blade, that slant up toward the engine. When you flip it, it will slant down and in. The question didn’t say flip. The question says to rotate the blade 180 degrees, which will result in no change in the direction of airflow. If you mirror-reverse flip the blade, you will reverse the direction of airflow. ---
**Author:** ![Really\_Not\_All\_That\_Bright](https://avatars.discourse-cdn.com/v4/letter/r/e8c25b/32.png) [@Really\_Not\_All\_That\_Bright](https://boards.straightdope.com/u/Really_Not_All_That_Bright)\ **Post date:** [January 22, 2009, 6:00pm UTC](https://boards.straightdope.com/t/question-about-an-electrical-motor-and-fan/482431/9 "2009-01-22T18:00:18Z")
> [@AndrewL](#): > > The question didn’t say flip. The question says to rotate the blade 180 degrees, which will result in no change in the direction of airflow. So it does. However, **Gary T** said it would remain the same whether the fan was removed from the shaft and turned 180 degrees-ie., flipped- in which case the airflow would be reversed, or if you turned the individual fan blades 180 degrees, in which case it wouldn’t. ---
**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\ **Post date:** [January 22, 2009, 6:02pm UTC](https://boards.straightdope.com/t/question-about-an-electrical-motor-and-fan/482431/10 "2009-01-22T18:02:40Z")
> [@Really\_Not\_All\_That\_Bright](#): > > I tried visualizing this and I’m pretty sure you’re wrong. > > Picture an aircraft propeller. Focus on one blade, that slant up toward the engine. When you flip it, it will slant down and in. Flip it 45’, yeah. Flip it 180’, as specified in the OP, and it will be oriented with the very same angle it had before. ---
**Author:** ![Really\_Not\_All\_That\_Bright](https://avatars.discourse-cdn.com/v4/letter/r/e8c25b/32.png) [@Really\_Not\_All\_That\_Bright](https://boards.straightdope.com/u/Really_Not_All_That_Bright)\ **Post date:** [January 22, 2009, 6:05pm UTC](https://boards.straightdope.com/t/question-about-an-electrical-motor-and-fan/482431/11 "2009-01-22T18:05:07Z")
> [@Gary\_T](#): > > Flip it 45’, yeah. Flip it 180’, as specified in the OP, and it will be oriented with the very same angle it had before. Huh? If you flip it 45’ (or 90’), you won’t be able to put it back on the shaft. I don’t think turning something 45’ counts as “flipping” it, incidentally. ---
**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\ **Post date:** [January 22, 2009, 6:09pm UTC](https://boards.straightdope.com/t/question-about-an-electrical-motor-and-fan/482431/12 "2009-01-22T18:09:27Z")
> [@Squink](#): > > Pick up the fan, motor and all. > Rotate it 180° about a vertical axis. > Set it back down. > > Which way does the air flow? > Swapping the blade from one shaft to the other will have exactly the same effect. No it won’t. Let’s say when you start, you’re looking one the end of the shaft (end A). And let’s say from that orientation, the shaft rotates clockwise, and air is blowing towards you. Rotate it as you’ve described, and now (from the same vantage point) you’re looking at end B, the rotation is counterclockwise, and the air is blowing away from you. Now let’s go back to the starting position, and move the fan from end A to end B. The rotation is still clockwise and the air is still blowing towards you. Rotating the motor does **NOT** have the same effect as swapping the blade to the other end of the shaft. > [@](#): > > Now if you take the blade off, rotate it 180° about a vertical axis, and slap it back on either shaft, you _will_ change the direction air moves with respect to the motor housing. No you won’t. The blade angle is the same, the rotation is the same, and the direction of airflow is the same. ---
**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\ **Post date:** [January 22, 2009, 6:15pm UTC](https://boards.straightdope.com/t/question-about-an-electrical-motor-and-fan/482431/13 "2009-01-22T18:15:15Z")
> [@Really\_Not\_All\_That\_Bright](#): > > Huh? If you flip it 45’ (or 90’), you won’t be able to put it back on the shaft. > > I don’t think turning something 45’ counts as “flipping” it, incidentally. He was talking about flipping a **blade** , not removing and refitting the propeller. Actually I did mean to say 90’, but either that or a 45’ flip would require twisting the blade on its hub (not practical, of course), and what I was saying was that yeah, with a 90’ flip you’d change the blade orientation and airflow, but with a 180’ flip you would not. ---
**Author:** ![Really\_Not\_All\_That\_Bright](https://avatars.discourse-cdn.com/v4/letter/r/e8c25b/32.png) [@Really\_Not\_All\_That\_Bright](https://boards.straightdope.com/u/Really_Not_All_That_Bright)\ **Post date:** [January 22, 2009, 6:21pm UTC](https://boards.straightdope.com/t/question-about-an-electrical-motor-and-fan/482431/14 "2009-01-22T18:21:47Z")
> [@Gary\_T](#): > > He was talking about flipping a **blade** , not removing and refitting the propeller. Actually I did mean to say 90’, but either that or a 45’ flip would require twisting the blade on its hub (not practical, of course), and what I was saying was that yeah, with a 90’ flip you’d change the blade orientation and airflow, but with a 180’ flip you would not. In your first post you said it would still have the same effect if you removed the whole fan and turned it 180 degrees. ---
**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\ **Post date:** [January 22, 2009, 6:25pm UTC](https://boards.straightdope.com/t/question-about-an-electrical-motor-and-fan/482431/15 "2009-01-22T18:25:28Z")
> [@Really\_Not\_All\_That\_Bright](#): > > However, **Gary T** said it would remain the same whether the fan was removed from the shaft and turned 180 degrees-ie., flipped- in which case the airflow would be reversed, or if you turned the individual fan blades 180 degrees, in which case it wouldn’t. I’m afraid you’re just wrong here. Take the fan off, rotate it 180’, and you’ll see that the blade angle and airflow is unchanged. Let’s say the blade is angling up and back on the left side. That means that the blade across from it on the right side is angling up and **forward**. If you rotate the fan about a vertical axis, the blade that was on the left is now on the right, angling up and forward, and the blade that was on the right is now on the left, angling up and back - everything is oriented just as it was before. ---
**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\ **Post date:** [January 22, 2009, 6:32pm UTC](https://boards.straightdope.com/t/question-about-an-electrical-motor-and-fan/482431/16 "2009-01-22T18:32:09Z")
> [@Really\_Not\_All\_That\_Bright](#): > > Picture an aircraft propeller. Focus on one blade, that slant up toward the engine. When you flip it, it will slant down and in. Yes, but it will have switched sides (e.g. from right to left). Rotate it half a turn and you’ll see that it moves from down and in to up and in - in other words, when a given blade is viewed from the exact same position, it has the exact same orientation, both before and after flipping. ---
**Author:** ![Really\_Not\_All\_That\_Bright](https://avatars.discourse-cdn.com/v4/letter/r/e8c25b/32.png) [@Really\_Not\_All\_That\_Bright](https://boards.straightdope.com/u/Really_Not_All_That_Bright)\ **Post date:** [January 22, 2009, 6:34pm UTC](https://boards.straightdope.com/t/question-about-an-electrical-motor-and-fan/482431/17 "2009-01-22T18:34:06Z")
> [@Gary\_T](#): > > Yes, but it will have switched sides (e.g. from right to left). Rotate it half a turn and you’ll see that it moves from down and in to up and in - in other words, when a given blade is viewed from the exact same position, it has the exact same orientation, both before and after flipping. :smack: You’re right. ---
**Author:** ![Indian](https://avatars.discourse-cdn.com/v4/letter/i/c0e974/32.png) [@Indian](https://boards.straightdope.com/u/Indian)\ **Post date:** [January 23, 2009, 4:49pm UTC](https://boards.straightdope.com/t/question-about-an-electrical-motor-and-fan/482431/18 "2009-01-23T16:49:31Z")
Thanks for the replies . Ignorance fought.🙂