Interrupting Power To Slow Down A Motor - Electrical - DIY Chatroom Home Improvement Forum


Go Back   DIY Chatroom Home Improvement Forum > Home Improvement > Electrical

CLICK HERE AND JOIN OUR COMMUNITY TODAY...IT'S FREE!

Reply
 
Thread Tools Display Modes
Old 10-28-2008, 12:14 PM   #1
Newbie
 
Join Date: Oct 2008
Posts: 1
Rewards Points: 10
Default

Interrupting power to slow down a motor


Here is a project for Halloween, we are building a mini golf course for a party and I wasn't to use a small 20" Box Fan as a windmill, but obviously even the slowest speed is still too fast to put a ball through it. Any ideas?

I was thinking about buying a dimmer switch and splicing it into the box fans cord. Since it is a variable speed fan (3 modes) the motor should be able to take the reduction in power. Or could this be a fire hazard?

Thanks People!

Advertisement

stevobyrnzie85 is offline   Reply With Quote
Old 10-28-2008, 01:58 PM   #2
Member
 
Join Date: Sep 2008
Location: Portland, OR
Posts: 1,186
Rewards Points: 500
Default

Interrupting power to slow down a motor


You will burn out the fan and could start a fire.

What most people don't know is that dimmers are not rheostats that cut the voltage. They just turn the power on/off at slower and slower levels to get the dimmer lighting.

Motors don't like this.

Advertisement

__________________
My idea of a perfect day: No where to go and all day to get there.
Marvin Gardens is offline   Reply With Quote
Old 10-28-2008, 02:31 PM   #3
Member
 
Join Date: Jun 2007
Posts: 3,510
Rewards Points: 2,008
Default

Interrupting power to slow down a motor


Dimmers are rheostats and do control voltage. This is one reason dimmers and CFL's do not mix.

Note: OP: If you can find the two wires connected to the slowest speed, try putting a dimmer in series with them. Caution: This could cause the motor to overheat and fry the motor as Marvin stated above.
I doubt you can get it slow enough for the application without ruining the fan.

Last edited by J. V.; 10-28-2008 at 02:36 PM.
J. V. is offline   Reply With Quote
Old 10-28-2008, 02:47 PM   #4
Member
 
Join Date: Sep 2008
Location: Portland, OR
Posts: 1,186
Rewards Points: 500
Default

Interrupting power to slow down a motor


Quote:
Originally Posted by J. V. View Post
Dimmers are rheostats and do control voltage. This is one reason dimmers and CFL's do not mix.
I don't have any dimmers that cut the voltage. They are all 120 at any dim setting. Most are run by tiracs that cut up the wave form. Very few are true rheostats because of the heat dissipation.
__________________
My idea of a perfect day: No where to go and all day to get there.

Last edited by Marvin Gardens; 10-28-2008 at 03:09 PM.
Marvin Gardens is offline   Reply With Quote
Old 10-28-2008, 05:46 PM   #5
Newbie
 
Join Date: Oct 2008
Posts: 9
Rewards Points: 10
Default

Interrupting power to slow down a motor


hmmmm...how about a VFD...just kidding...
motorcity11 is offline   Reply With Quote
Old 10-29-2008, 12:41 AM   #6
DIYer
 
Join Date: Oct 2008
Location: Virginia
Posts: 910
Rewards Points: 500
Default

Interrupting power to slow down a motor


Marvin is right about that, all modern dimmers work off a duty cycle principle, cutting the waveform out.

If he put the fan on a variac he could truly lower the voltage, but that's still not that effective for speed control. A rheostat/resistor would burn up unless it was massive... too many watts.

If you can afford to buy something, buy a rotisserie motor, they are already geared down and slow. Failing that, go to radio shack and buy one of the little DC toy type motors and run it on a big reduction belt.
Gigs is offline   Reply With Quote
Old 10-29-2008, 12:54 AM   #7
Idiot Emeritus
 
Join Date: Mar 2008
Location: Fernley, Nevada (near Reno)
Posts: 1,643
Rewards Points: 1,080
Default

Interrupting power to slow down a motor


Try a fan speed control. It's specifically designed to control the speed of a motor.

Rob
micromind is online now   Reply With Quote
Old 10-29-2008, 02:55 AM   #8
DIYer
 
Join Date: Oct 2008
Location: Virginia
Posts: 910
Rewards Points: 500
Default

Interrupting power to slow down a motor


A ceiling fan speed control? I'm not sure that would work well. He wants this fan to spin at like 60rpm or less. If you turn the fan on low and the fan speed control on low, it probably will not turn at all. When he turns it up enough to actually rotate, it'll immediately go to like 300 rpm or whatever, with weak torque.

He'll really need either a geared down motor or a small motor with a big pulley to get these low speeds. Without a VFD you can't take an induction motor down that low.
Gigs is offline   Reply With Quote
Old 10-29-2008, 07:56 AM   #9
the Musigician
 
DangerMouse's Avatar
 
Join Date: Jul 2008
Location: I'm right here!
Posts: 10,404
Rewards Points: 2,000
Default

Interrupting power to slow down a motor


easiest thing to do is have someone stand there and flip the blades by hand when someone is taking their turn.... lol
lots of hassle to do what he wants.....

DM
__________________
[SIGPIC][/SIGPIC]Click here to see some of my original magic tricks and trick boxes!
DangerMouse is offline   Reply With Quote
Old 10-29-2008, 11:20 AM   #10
Member
 
Join Date: Jun 2007
Posts: 3,510
Rewards Points: 2,008
Default

Interrupting power to slow down a motor


I tested a dimmer yesterday. Before I posted. Connected my fluke multi to the load side of a dimmer I had in the basement. It lowered the voltage proportional to the brightness of the incandescent bulb. I could get zero volts at bottom. 120 at the top. Is this because I used a dimmer that is old and are the new ones different?
J. V. is offline   Reply With Quote
Old 10-29-2008, 12:35 PM   #11
Member
 
Join Date: Sep 2008
Location: Portland, OR
Posts: 1,186
Rewards Points: 500
Default

Interrupting power to slow down a motor


Quote:
Originally Posted by J. V. View Post
I tested a dimmer yesterday. Before I posted. Connected my fluke multi to the load side of a dimmer I had in the basement. It lowered the voltage proportional to the brightness of the incandescent bulb. I could get zero volts at bottom. 120 at the top. Is this because I used a dimmer that is old and are the new ones different?
There are true dimmers out there but they are rare. Most are triacs and I haven't seem a non triac dimmer in years.

More than likely you have a true dimmer because a triac dimmer will show 120vdc down to the lowest dim setting.

True dimmers are mostly used for commercial applications such as stage lighting. The reason that they are not found in homes is due to the heat dissipation issue. It is hard to get rid of heat in a closed up wall.

Even the triac heats up and they use the surface of the switch plate to get rid of heat by making them large and use aluminum. Some even have wings that extend beyond the switch but can still be covered by the switch cover.

CF's cannot be used with a triac because they will over heat. The pulses actually cut off the energy to the bulb and then add energy. This causes the CF to restart many times a second and is real hard on them.

There are dimmable CF's but I have never been real happy with them and they are real expensive. In my experience they have a shorter life span and the cost of replacing them over and over is much more than any energy savings. I just use incandescent in all my dimmers (which is everything).
__________________
My idea of a perfect day: No where to go and all day to get there.
Marvin Gardens is offline   Reply With Quote
Old 10-29-2008, 12:43 PM   #12
Member
 
Join Date: Jul 2008
Location: NW of D.C.
Posts: 5,990
Rewards Points: 2,000
Default

Interrupting power to slow down a motor


Put an incandescent lamp in series with the fan. Start with a 100w bulb and work down until you get the desired effect.

A glass jar full of salt water used as a resistor will also work but it is much more dangerous and the water tends to boil away in a few hours.
Yoyizit is offline   Reply With Quote
Old 10-29-2008, 01:59 PM   #13
DIYer
 
Join Date: Oct 2008
Location: Virginia
Posts: 910
Rewards Points: 500
Default

Interrupting power to slow down a motor


Quote:
Originally Posted by J. V. View Post
I tested a dimmer yesterday. Before I posted. Connected my fluke multi to the load side of a dimmer I had in the basement. It lowered the voltage proportional to the brightness of the incandescent bulb. I could get zero volts at bottom. 120 at the top. Is this because I used a dimmer that is old and are the new ones different?
If your meter is a true RMS meter then it was effectively averaging the voltage output.

You need an oscilloscope to be able to see the chopped up waveform correctly.

If it's very old it may well be a rheostat though. Try it on a 9 volt battery and see if it still works the same way as with AC. If it does then it's resistive.
Gigs is offline   Reply With Quote
Old 10-29-2008, 04:04 PM   #14
Member
 
Join Date: Sep 2008
Location: Portland, OR
Posts: 1,186
Rewards Points: 500
Default

Interrupting power to slow down a motor


Quote:
Originally Posted by Gigs View Post
If your meter is a true RMS meter then it was effectively averaging the voltage output.

You need an oscilloscope to be able to see the chopped up waveform correctly.

If it's very old it may well be a rheostat though. Try it on a 9 volt battery and see if it still works the same way as with AC. If it does then it's resistive.
He could just do a resistance test.
__________________
My idea of a perfect day: No where to go and all day to get there.
Marvin Gardens is offline   Reply With Quote
Old 10-29-2008, 05:48 PM   #15
DIYer
 
Join Date: Oct 2008
Location: Virginia
Posts: 910
Rewards Points: 500
Default

Interrupting power to slow down a motor


Quote:
Originally Posted by Marvin Gardens View Post
He could just do a resistance test.


Leave it to me to overcomplicate it. You are right, with a DMM the resistance test voltage isn't high enough to overcome the voltage drop of a SCR/triac/whatever.

Advertisement

Gigs is offline   Reply With Quote
Reply


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
hooking up dryer....bronx ny code SURFBUG Appliances 6 10-14-2008 10:41 PM
Motor drawing too much juice to start? Prince Electrical 6 09-14-2008 11:53 AM
I'm all about saving money on electricity but what is this? imola ghost Electrical 15 09-09-2008 09:36 PM
Replacing a Condenser Fan Motor thothtp HVAC 5 06-08-2008 07:30 PM
Problem W/ Power after Switching Outlets... G19-Fanatic Electrical 17 08-28-2006 02:55 PM




Top of Page | View New Posts