110VAC voltage dropdown

[dawsonaudio]

I was wondering if someone could help me design a simple drop down circuit from 110vac to 80-60vac or so.  I've got some fans on my Ampex MM1100 16 track tape machine that I'm trying to slow down and putting them in series slowed them down too much.  The issue is I've consolidated into a really small room and the fan noise is an issue.  If I could build something like a "variac" so I could slow them down to find a good speed/noise ratio or a fixed voltage would be fine.  Thanks for any advice here.

There are 3 fans and are rated at 115vac/7watts so a 1/4 amp draw between the three fans, I think that's right.

Nate Dawson

 

[Ethan]

It's much cheaper and easier to just buy a variac than to roll your own:
http://www.amazon.com/3-Amp-Variable-Transformer/dp/B003O3N3BS/ref=sr_1_1?ie=UTF8&qid=1324146616&sr=8-1

3A * 120V = 360Watts

 

[gemini86]

Use a standard light dimmer switch, then you can have variable fan speed.

 

[Kagliostro]

An old simply technique

buking transformer

the secondary of the transformer must be chosen in accordance to the voltage you want to obtain

for documentation

http://sound.westhost.com/articles/buck-xfmr.htm

or google  buking transformer

Kagliostro

 

[dawsonaudio]

Could I use this transformer to drop the voltage?  I found it in some parts that I ordered a while back for a project I never did.

http://parts.digikey.com/1/parts/1000054-transfrmr-22v-2-272a-wires-62085-p2s02.html

Could you walk me through the hookup/wiring procedure if I can use it?  I'm trying to drop from 115 to 80 or so volts.  Any voltage would be good at this point. 

Thank you,
Nate

 

[Andy Peters]

Could I use this transformer to drop the voltage?  I found it in some parts that I ordered a while back for a project I never did.

Could you walk me through the hookup/wiring procedure if I can use it?  I'm trying to drop from 115 to 80 or so volts.  Any voltage would be good at this point.

Sorry to be a wet blanket here, but if you're unfamiliar with AC mains wiring, you shouldn't be messing with it, and I doubt that anyone would be willing to provide such advice. You're dealing with potentially lethal voltages, and miswiring something could lead to an unwelcome outcome.

-a

 

[dawsonaudio]

I'm alright dealing with this.  I'm just asking for some guidance/help here.  I've always used a transformer as a step down and never have "bucked" one.  That didn't sound right.

So my understanding is that I'll feed the transformer with 110vac, take the negative secondary winding and then add that with the incoming neutral leg and that will give me less voltage.  Is that correct?

http://www.geofex.com/article_folders/vintvolt/vintvolt.htm

Nate

 

[PRR]

> just buy a variac

Costly, bulky.

> Use a standard light dimmer switch

Lamp-dimmers can do AWFUL things to motors. (I once filled the house with smoke.)

> to 80-60vac ....putting them in series slowed them down too much

Two in series on 120V IS 60V. You want more, right?

> 115vac/7watts

Then 115V at 0.06A each.

Pretend the motor is a resistor (it isn't). Then 115V/0.06A is 1,889 ohms.

Let's assume 2/3 or 80V. 944.5 ohms and 1889 ohms gives a 2/3 split.

120V/(1889+944) = 0.042A current.

0.042A current at 40V drop is 1.7 Watts.

944.5 ohms 1.7W needed.

Ohms is wild-dart guess; experimentation is needed.

Above 1/2W, resistor prices go up fast to 2W and slow to 10W; always round way up.

1K 10W is standard stock at Radio Shack, and low-price (two/$2?).

That's a starter value. The motor is NOT a fixed resistor. As it comes down from sonic speed it will turn easier, pull less current. Current drops faster than voltage. At 80V it may be well over 2K equivalent resistance. Get a handful of 1K resistors so you can try 1K 1.5K, 2K.

Run separate resistor for each motor. Else if one motor stalls it may hog most of the current and the others will hardly turn.

AC fan motors can be very wacky at part-blow. They are intended to run at some slip from 60Hz; gross slow-down may make them unhappy. Perhaps a better plan is lower-CFM same-size AC fans, or same-size 24VDC fans and a 24V C supply which can be trimmed down.

 

[dawsonaudio]

Well I was able to 'buck' the transformer.  I wired all fans into their own power supply system.  I bypassed the voltage from the Ampex that normally powered the fans and ran a Toroid with a 24(26) volt secondary.  This effectively dropped the voltage from 118vac to 92vac and the fans are just right.  The airflow is slightly less.  The noise level is WAY down though.  What a difference the voltage change did to the sound level coming from the fans.  Thanks again for the suggestions and help here.

Nate