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How do I stop buzzing and humming type noise?

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justinterested

Computer
Jan 11, 2006
5
Hi I am looking for a little information on the motors they use in the room type evaporative air conditioners, these are normally on trolleys and it’s the type with centrifugal fans that I an most interested in.

I have noticed that some have an audible hum to them when running, what causes this and how is it stopped.

Second how would the running speed be reduced without using an electronic speed control, again these cause an audible hum.

Bottom line is I want a fan that can move good airflow that is almost silent from buzzing and humming type noise.
 
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Does the motor hum when not connected to the fan?

Barry1961
 
As Barry says,you need to be more specific about the source of the noise.

But in general, any energized ac motor will hum. The perceived noise can be reduced by baffling and noise insulation of the entire unit.

You can probably get noise data from various suppliers to help in making a selection.
 
Turn the TV up louder so you don't hear it.

Seriously though, as dpc says, any AC motor will hum a little. The hum is caused by the induction taking place between the stator windings and rotor of the motor, which is the cause of the rotating magnetic fields and thus the work that the motor does. So the simplistic answer to how to make it stop humming is to turn it off. After that, it's all about the degree of humming you are willing to tolerate. Most of the noise is created by the vibration of the steel laminations used in the motor construction. The better the design and materials, the less audible hum you will perceive. Ergo, cheap motors have a tendancy to make more noise than expensive ones, and consumer products tend to be built as cheaply as possible.

Eng-Tips: Help for your job, not for your homework Read faq731-376 [pirate]
 
Hello justinterested

The reasons for the noise are as above.
If the motor is operating at lees than rated output, it is possible to reduce the voltage. This will reduce the flux in the iron and will reduce the line frequency noise. You must be very carful not to reduce the voltage too much.
Voltage reduction will not significantly reduce the speed or the energy consumption, but the reduction in flux may be sufficient to noticably quieten them down.

Best regards,

Mark Empson
 
LOL Marke, I can't believe I just saw you indirectly advocate a Nola circuit!

Eng-Tips: Help for your job, not for your homework Read faq731-376 [pirate]
 
Hi jraef

I did nto say to use a solid state device, infact what would be required is a fixed voltage, not a variable voltage. A reactor or auto transformer would be the best way, but it would need to be installed elsewhere or it would provide the noise removd from the motors. A solid state phase controller would introduce a higher frequency noise due to the harmonics and could make things worse rather than better.

You can take your toungue out of your cheek now!!

Best regards,

Mark Empson
 

Thanks for the reply’s guys, a few more please.

I have already found that the solid-state type speed controllers introduce more hum as the speed is turned down and are not suitable.

How is the speed controlled in a normal three-speed motor?

With the hum would stripping the motor and varnishing the windings be useful or a waste of time?
 
hello justinterested

A vsd changes the speed by changoing the voltage and frequency of the supply to the motor. The only way that you can alter the speed of a three phase induction motor is by controlling the frequency applied to it.

Reducing the voltage using a transformer will reduce the magnetic flux and this can reduce the noise.

Dismantling the motor and revarnishing the windings etc may help, but you would probably need to vacuum impregnate thw windings to be of much use. Talk to your local rewind shop and they may give some help.

noise can also be associated with bearings couplings and the like as well.

Best regards,

Mark Empson
 

Hi Mark this is not a three-phase motor, from looking around I believe it’s a High Slip Induction Motor.

Its just one of those little room type evap coolers, I want to run it at about half speed of its low setting and as silently as possible for night time use when sleeping.


.
 
Hi justinterested

If this is a "fan" motor, then it could be a high slip motor in which case, you can reduce the speed by reducing the voltage. To do it quietly, use an transformer or a series reactor. - you need a quiet transformer or reactor, or put them out of sight and sound.
Fan motors are sometimes designed for high slip so that they can be speed controlled by variable voltage. There is a high slip loss in the rotor, so the rotor is usually on the outside of the motor and is in the center of the fan so that it is air cooled by the fan airflow.

Best regards,

Mark Empson
 

Thanks Mark I have just been messing around with a patch lead with some large light globes in series and this seems to do the job of slowing it down without inducing any hum to the motor.

I guess I could just play around until I was happy with the speed (noise) and simly measure the voltage and source a suitable transformer.
 
I think you're looking for a variac. This is an autotransformer with a variable output winding you adjust with a knob. You can then just dial in whatever voltage (and thus speed) you want.

 

k4cadd.jpg
 
You may find the cost of that variac distasteful. In the old days, you could have used an old fashioned rheostat (a variable resistor / potentiometer) to do this, that is what fan speed controllers were. Unfortunately in the age of smaller-cheaper-cooler, rheostats gave way to electronic dimmers which use high speed switching to accomplish the same goal. They run cooler so they fit into tighter places making the entire package smaller, but along with that came the high pitched hum that you have noticed. If you can find a true rheostat somewhere it will work.


Just be sure to find the wattage rating of the motor and get a rheostat with that much or more capacity. It will give off some heat so package it accordingly. Also realize that if there are other controls on the cooler, they will still need full voltage, so you most likely will NOT want to put this in front of the entire unit, only the motor circuit.

To answer your other question, 3 speed motors worked by having a special winding configuration and a set of switches to change the configuration for each speed. Since your motor has an electronic speed controller on it, it is a single speed winding so you would need to replace the motor if you wanted to go that route.

Eng-Tips: Help for your job, not for your homework Read faq731-376 [pirate]
 
If your motor is mounted directly on the chasis or directly hard mounted to anything solid the humming will transfer to all hard (metal etc.) components might even be amplified.
You could incorporate rubber vibration dampers at the mount point to attenuate some of the humming.
 
If you slow the motor down the fan will not move as much air in a given time. You will in effect be making a big change to the air conditioners design and as a result its performance and reliability could be affected. Another thing to consider is the fact the motor may require the airflow generated by the fan to cool it and a reduction may cause overheating. All motors generate noise even under normal running conditions but If you think the noise the motor generates is abnormal and intrusive it could have a bearing problem. If it uses bush-bearings you could try a few drops of oil in them or some grease if the bearings are rolling element types and see what effect it has, it certainly wont do any harm if you are confident enough to dismantle the motor. You could also consider a replacement motor and fan combination that has the same performance as the original but runs at a lower speed.
 
There is a type of small three speed fan motor that is so common in Central America, that the number sold may exceed the population. It is a 4 pole high slip induction motor with capacitor start. Many of the motors look like a shaded pole motor without a shading pole.
The winding is tapped and used for speed control. The different taps produce different torque values and resulting different sip frequencies.
Three speeds are common. A four position switch is built into the fan support.
There is also a three speed controller with three steps of impedance that is used in series with cielling fans.
In North America, three speed high slip motors with tapped windings are quite common. Many residential and furnaces and remote air conditioners have a fractional horse power fan motor. The installer picks the speed at the time of installation and connects the appropriate tap on the motor. If you live in North America, you may have one or more such motors in your dwelling without being aware of it.
Re the problem at hand;
You can look for a small transformer and use an auto-transformer connection. This is after you have found the voltage you need with the series lamps.
Primary voltage rating: Line voltage
Secondary voltage rating: The amount of voltage you want to drop. eg; 120 volt line, 100 volts required, look for something close to 20 volts secondary (120-100=20.
Secondary current rating: The motor full voltage current.
This will be a lot cheaper than a Variac.
 

Thank you all for your time and effort in answering my questions.

I have the information I need to move forward in my pursuit of quietness now.
 
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