Electrical Question

[3stagevtec]

How can i vary the speed on a 115V, ~20A A/C motor?

I use a centrifuge at work to test the suspended water in oil. The problem is that it starts to spin too rapidly, so oil gets thrown out of the test tubes being used and the test tube holder flys up (under the accleration) and hits the cover. I need a way to make it spin at a slow rate first (to prevent the mentioned problems) and then be able to switch it to max speed for accurate results..

any suggestions?

the centrifuge looks kinda like this:

[APACHE]

i tihnk the best thing to use is the light switch with the dimmer and hook it up to the plug ............. seems most logical thing to use

[olewhip]

it cannot be done the speed of an ac motor is determined by the no. of poles,
for example 2 poles - 3600rpm
4 poles - 1800rpm.

maybe u should consider using a variable speed drill as the rotor and brushes thing allows it to vary the speed without destroying the motor

using a dimmer switch can damage the motor in a really short time.

[3stagevtec]

is that anyway a capacitor can be used to slow the startup speed?

thanks

[shazzz69]

a cap would jump start it even more bro.....

[3stagevtec]

apparently i was led to believe that a capacitor can be used to slow the startup speed.. which i knew didn't make much sense...

thanks..

[ronsin1]

the motor 3ph or single ph thats the ifrst thing you got to know if it a three phase and according the the type you can use a three phase reostat controller to vary the speed but this is expensive and not sure you can get this locally

[5fwd]

delete

[ronsin1]

^^^^ you will burn the motor that way the induction motor is rated at a certain speed and current rating if you slow down the motor it will eventually cause the motor to pull more current trying to get up to the rated speed and you will burn the windings

[TURBOHO]

dats true, i did industrial electrical and i did dat as an experiment to wat happens to an induction motor when some ting is 'holding it back' it burned the windings really fast.But wat u are talkin bout is like a pump motor. but were did u get the motor from, how fast is it 1800rpm or 3600??? wat speed and may be i can help u.

[3stagevtec]

^ i'll see if i can get more specs on the motor tomorrow...

the motor was designed to work with that centrifuge, it's just weird that whoever designed the thing didn't test it out properly before selling it... the speed it starts off at is scary, especially when there is glass test tubes in it...

[ronsin1]

if it was disigned to work with the centrifuge it should have some kind of control see if you could locate a control panel on the machine which should have adjustments for the starting and stoping of the machine.

I am assuming trhat it should start up slow then run fast then slow down before coming to a complete stop.

When you get the specs post all the information of the nameplate of the machine and the motor

[3stagevtec]

sorry i late with the specs, spent the last two evenings by the mech... and he didn't have tuner there..

anyways, this is what i got from the motor:

HP - 1/12
V - 115
RPM - 1725
Hz - 60
PH - 1
A - 1.9

The starting switch is located in an explosion proof box. When i opened the box, there was only a switch in it, nothing else to control the speed.

I am assuming that it should start up slow then run fast then slow down before coming to a complete stop.

this is how it supposed to work..

[ronsin1]

anyways, this is what i got from the motor:

HP - 1/12
V - 115
RPM - 1725
Hz - 60
PH - 1
A - 1.9

The starting switch is located in an explosion proof box. When i opened the box, there was only a switch in it, nothing else to control the speed.

Quote:
I am assuming that it should start up slow then run fast then slow down before coming to a complete stop.


this is how it supposed to work..

At a lost cuz if thats the motor it should have a built in speed controller or some soprt of PLC to tell the motor how to operate

[3stagevtec]

At a lost cuz if thats the motor it should have a built in speed controller or some soprt of PLC to tell the motor how to operate

what you mean by PLC?

so that motor might have a built-in speed controller? maybe that is what bad and causing the problem... (according to my boss, it supposed to start up slower than the way it starts now)

[olewhip]

3stagevtec,
plc means programmable logic control,it is used mainly to reduce the amount of hard wiring with contactors.
the plc box has internal electronic relays, timers etc.
it is widely used in companies as trouble shooting is easier (just plug in the computer, with the right software), and you can change the way, motors behave (foward or reverse motions, slow start up, delayed start up),heaters heat up, light changes (traffic lights) and many other things.

[ronsin1]

what you mean by PLC?

Programable Logic Controllers.

so that motor might have a built-in speed controller? maybe that is what bad and causing the problem... (according to my boss, it supposed to start up slower than the way it starts now)

Hardly likely have never seen a motor as small as that with a built in controller..

Check the machine for something like this

Trace back the wires the was connected to the motor if you find something like the above pic or even an electrionic circuit board that might be your problem

[olewhip]

that device can make a motor do wonders. have something similar in work
you just need to understand its parametric settings and yuh orn.

[3stagevtec]

yeah, now that is what i need...

but sadly it doesn't have a PLC on it. the power switch is wired directly to the motor...

thanks

[olewhip]

then there should be a gearbox or something like that if there is no plc.
that is the only way remaining to start slow and end up at high speed.

[ronsin1]

^^^ yea that the only other thing that would work a gear box

[3stagevtec]

you all remember this question? well i'm back with this centrifuge again..

the centrifuge was never fixed.. i just got a new switch box for it and it came with two 50ohm, 40W resistors & a standard 20A switch...

how could those resistors be used to slow the speed of the AC motor? i don't know how you calculate current etc passing in an AC circuit... it is the same as for a DC circuit? V=IR & P=VI still apply?

e.g. what if i were to put the resistors in parallel (or series) with the motor... how much current will flow through the resistors?


The Specs of the AC motor
HP - 1/12
V - 115
RPM - 1725
Hz - 60
PH - 1
A - 1.9

any help would be greatly appreciated..

[3stagevtec]

btw, we have a DC based centrifuge that had the same problems with a fast startup.. i designed a circuit using relays and resistors to start the centrifuge slow (using a high value resistor) and then using a switch to speed up to full speed...

i forgot the value i used on this resistor..

could this same concept apply to the AC motor or will it damage it...

[nigel1977]

Basically you need a soft start system using a simple scr.

http://www.lmphotonics.com/sstart.htm

http://www.variablefrequencydrives.net/ ... drives.htm

[3stagevtec]

thanks for the links.. i doubt my boss would approve of getting any new equipment just for the centrifuge.. i.e. the Baldor Analog Soft Start

the lmphotonics soft starters is a lil too advanced for me right now... can't really figure out the electronics behind it, and not in the mood to try..

anyways, i'll have to try and get a working centrifuge, dismantle it and see how it is wired / designed..

thanks again though...

[nigel1977]

I have some schematics someplace in my notes for a low current soft start system. No promises, and it prolly wont be soon enough to do you any good. Google is your friend.