Thread: Heat Pump Air Handler noise

What might cause a humming noise, similar to the hum you hear near an electric transformer box, coming from an idle heat pump's air handler? The thermostat was set to cool, auto and was not calling for cooling. I had lowered the t-stat and the A/C operated for a few minutes, raised the t-stat to stop the A/C. About 15 minutes later the humming noise continued. It is a 1987 York air handler. The condenser plate is not readable so it's probably the same age.
Thanks in advance,

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Was the transformer in the indoor air handler?
That is the usual source for the hum on all units when off.

Thanks Jim,
I don't know. I was only there briefly to set radon canisters. I'm inspecting the home tomorrow. I only turned the A/C on briefly because the outside coils looked so damaged. I just wanted to see if it came on. So besides that damaged coils, age and for my future knowledge, what is wrong when the transformer hums?
Thanks and have a good day!

Originally Posted by Jim Kasterko

what is wrong when the transformer hums?

Transformers are made from thin pieces of metal, insulated from each other by being dipped in an insulating varnish, then stacked together and bound tightly together. That is what the magnetic fields goes through and what makes the transformer effective - that ferrous core.

The reason for the thin pieces being laminated together is to reduce any eddy currents which would otherwise set up within that ferrous core itself, not only robbing power from the transformer, but creating quite a bit of heat within the transformer, causing it to overheat and burn itself up.

The hum is from those tightly held together pieces becoming not-so-tightly-held-together, and the magnetic field expanding and collapsing around and through them causing them to vibrate.

Really bad ones will 'rattle', others just 'hum' some. Neither is a good sign.

Nothing is wrong when you get humming from a transformer unless it is unusually loud.
They all hum.
Now if it changes to a hum to a rattle, then change the transformer.
Sometimes the noise is not the transformer but the mounting is loose and just amplifies the noise.
Sometimes they get louder just before burning out... but not always.
Just like people, some are quiet and some are loud but that says nothing about how well they work

Originally Posted by Jim Luttrall

Nothing is wrong when you get humming from a transformer unless it is unusually loud.
They all hum.

Not all hum noticeably.

Those that do have something wrong ... the laminations are not held tightly together. As to whether or not that has a measurable and noticeable effect on the operation of the equipment, probably not. Putting out 23.5 volts instead of 24 volts due to losses in the core is not going to affect it anymore if the supply voltage dropped from 240 volts to 235 volts. It happens all the time and there is no effect on the equipment.

However, once it gets noisy enough to be bothersome ... it is still bad, but now you replace it, not because "it is still bad", but because it is bothersome.

How much the losses within the coil affect the voltage, I don't recall, that was from 40 years ago.

Thanks gang! I always learn something here whether posting or just reading.

Have a good evening!

I guess the way to describe it is like this:

If it is humming 'Way down upon the old Swanee River', that's okay, but when it gets to 'Rocky Top' ( Rocky Top ), it's got to go.

How much the losses within the coil affect the voltage, I don't recall, that was from 40 years ago.

Hysteresis losses only reduce the efficiency of the transformer, the voltage step-down ratio does not change. Voltage is not affected.

Originally Posted by Vern Heiler

Hysteresis losses only reduce the efficiency of the transformer, the voltage step-down ratio does not change. Voltage is not affected.

Vern,

I'm not talking about hysteresis losses resulting from the reversing magnetic fields, I'm talking about eddy current losses from the current which is induced into the ferrous metal core.

Although, you have a good point about step-down / step-up voltage, it is a result of the number primary windings versus the number of secondary windings, all windings being equal. Start changing winding wire sizes and types and you start to get into other territory, as I recall. (Been a long time since I did much with transformers.)

Jerry, I was just trying to give a little information about what was happening regarding the transformer noise. The eddy currents are a direct result of the hysteresis fields inside the core. The point I was making is that those magnetic fields and currents do not reduce the voltage output of the secondary winding. The noise is objectionable but not a problem. As you stated the noise comes from loose laminations of the core. The 24 volt transformer used in the HVAC low voltage circuit steps the 220 volt supply down to 24 volts ( a step down transformer), the ratio of primary turns to secondary turns is directly proportional to the voltage ratio desired, 9.166:1. Easyer to wind full turns at 9:1, that is why the actual voltage out is closer to 24.5 vlots than 24.

Originally Posted by Vern Heiler

The 24 volt transformer used in the HVAC low voltage circuit steps the 220 volt supply down to 24 volts ( a step down transformer), the ratio of primary turns to secondary turns is directly proportional to the voltage ratio desired, 9.166:1. Easyer to wind full turns at 9:1, that is why the actual voltage out is closer to 24.5 vlots than 24.

That's based on the old standard voltage of 110 volts / 220 volts. Now it is more typically 120 volts / 240 volts - which makes the primary to secondary wrap ratio 10:1 for a 24 volt transformer. Frequently, you will find voltage is 123 volts / 246 volts, which makes for the 24.6 volts.

Even when the voltage is only 110 / 220, it is still 10:1 turn ratio, the output voltage fluctuates to match the ratio from the input voltage, thus the output voltage would be 22 volts - still plenty to operate those solenoids, etc., and, with newer equipment using computer boards, I am sure that they have their own 'voltage regulator' built-in to regulate the voltage to a lower voltage to allow for fluctuations in voltage.

I'm not sure that it is the hysteresis which causes the laminations to vibrate - the collapsing and expanding magnetic fields are what causes the laminations to move back and forth (if there is room for them to move back and forth). Hysteresis is the difference in electrical current flow caused by the collapsing and expanding magnetic fields and those collapsing and expanding magnetic fields themselves.

It is not a 1:1 time relationship between the collapsing magnetic field and the flow of current, then the magnetic field expands, with a time lag between that expanding and the reversal of current flowing in the other direction. That 'time-lag' is dependent on several things, one of which is frequency (in power line cases that would be 60 Hz).

Remembering back to my days when I used to calibrate and repair oscilloscopes and other things, and when I used to use oscilloscopes for testing purposes, I remember measuring the hysteresis of power wave forms from small mW output electron tubes to multi-MW output electron tubes. Another hysteresis curve we would measure on the oscilloscopes was the tuning curve hysteresis - that was where the frequency the electron tube was tuned to would not exactly follow the tuning adjuster. I.e, let's say you had the frequency adjusted to a given level, then tuned to a slightly higher frequency, the output frequency would actually drop before increasing, and vice versa. That difference was measurable, was critical, and could be seen on the oscilloscopes. That was the hysteresis of the tuning curve.

This discussion brings back interesting and good memories.

Ah ... but back to transformers ...

It is not a 1:1 time relationship between the collapsing magnetic field and the flow of current, then the magnetic field expands, with a time lag between that expanding and the reversal of current flowing in the other direction.

The plotting of the delayed magnetics, or residual magnitisim, is called the hysteresis loop.

I too am searching old gray matter! The prediction at the time I started carrier in electronics was that everything would have "electronics" in it some day! Car's, home appliances, not to mention TV's, PC weren't eaven a dream yet. Wow! a carrier that will always require technical repair. Too bad they didn't mention everything would be disposable....