Transformer makes an audible noise with SSR but does not make it without SSR

Question

I have a large transformer (see below) that I wanted to switch on/off from a digital platform using a solid state relay. The equipment works without any issue and makes no noise when the solid state relay is bypassed but the transformer makes a buzzing noise when it is in series with the relay. I tested the equipment with the relay and the noise and it seems to be working but the buzzing noise worries me and I fear for the transformer.

This is my SSR: PF240D25R

My qusetions as follows:

• Why is my transformer making an audible buzzing noise with the SSR, while there is no audible noise without the SSR? Is it caused by the approx 1.6V drop (see the datasheet)?
• How can I stop this noise?
• Is it safe for the transformer or should I stop using this SSR?

UPDATE 1: The control circuit is a Raspberry PI Zero (GPIO: 3.3V) and the resistor is 330R that makes the current 0.01A

UPDATE 2: My SSR is a 'Random Turn-On' type

Answer 1

Your SSR may be too powerful (25A) for that current that has to switch. More powerful triac means also higher latching/holding triac current.

In case the SSR is zero-cross type, it would skip semi-periods, so this type of SSR is not adequate for large inductive loads. Your's seems to be random phase type, so the correct one. Your setup misses the snubber circuit, which is usally a must when switching inductive loads. My recommendation is to try adding a snubber, before making any wrong assumptions.

EDIT:

The input signal of SSR is 3-15V @ 15mA. The SSR has an internal circuit to limit the LED current. It is not recommended to add an additional series resistor, rather you could use an external N-MOSFET as a switch if you want to protect the GPIO.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

You are using a "random-fire" SSR so it can switch on mid-cycle to allow dimming or power control which you probably don't want.

Figure 1. The upper trace shows the trigger delayed close to the end of the cycle. The resultant effective voltage is low. The lower trace shows the trigger close to the start of the cycle. This will result in close to full voltage. The relationship between phase angle delay and resultant RMS voltage is graphed on the right.

A zero-cross triac, when triggered, will wait for the next zero-cross before turning on and will wait for the current to fall to zero before switching off.

Figure 2. With zero-cross switching the result is that the waveform consists of multiple complete half-cycles.

(Both images are mine and taken from Opto-triacs, solid-state relays (SSR), zero-cross and how they work where you can read a little more on the topic.)

At full power there will be a slight clipping of the start of the sinewave due to the turn-on voltage of the triac. This might be the cause but a more likely and severe one would be your control circuit which you haven't shared with us. If this is weak or arrives late in the mains cycle then it would cause triggering as shown in Figure 1.

Answer 3

You can shunt the SSR with a mechanical relay contact. You might have to delay the mechanical relay pull-in, but possibly you can just power the two simultaneously.

This effect is from the SSR thyristor turning off every half-cycle then turning on again after the voltage has risen a bit. Sometimes dimmers made a buzzing sound at close to full brightness in the old incandescent lamp dimmers, especially if the filament is poorly supported.

As well as the acoustic noise caused directly, this can cause interference due to the EMI. The relay will get rid of that, except at the switching.

Answer 4

The cause of the noise may be spurious DC introduced into the transformer primary by unintended rectification in the SSR. It has a significant voltage drop. If that voltage drop is slightly different in the two directions, then it will impress a very slight DC onto the transformer.

This effect will usually not trouble a small transformer, as the winding resistance is relatively high, or an EI transformer, as the core has small inadvertent air-gaps, either of which will tend to keep the DC flux down. In a nice big toroidal transformer like yours, the low primary resistance and excellent flux path can allow a significant DC flux to build up in response to only mV of DC, which will then drive the transformer into saturation once per cycle when the normal mains voltage is superimposed on it. This can cause magnetostriction noise at 50 (60) Hz, which is quite rich with harmonics, and so is audible.

The simplest cure would be to short the SSR with a relay, once the SSR has taken care of the inrush transient. A delay of a few seconds is all that's needed.

I had exactly the same noise in exactly the same type of audio power supply. It took me months to figure it out.

Answer 5

The solution for relay contact welding would be a soft start.

The inrush current would be limited by a series power resistor in the primary circuit which would then be bypassed by the relay contact.

Standard units may be available off-the-shelf.

Answer 6

I think the first part of your question has been answered already. This won’t harm your transformer to any significant degree but could shorten its life a little, although I’d expect decades of service life. Most solid-state devices will necessarily drop a couple of volts and won’t start to conduct instantly at zero-crossing, so a mechanical relay may be a better option if you want to get rid of the buzz.

Answer 7

Using dot board with mains voltages is not particularly safe. Any isolation the ssr gives you has been compromised. I’d suggest you remove the unused dots.

Answer 8

SSR is optotriac. Triac is opened by current to gate. The source of that current is primary voiltage, some commutating device and resistor. Triac is off at zero voltage an on at some voltage, usually 6-10V, every cycle. So load current is not sinusoidal. It may cause the sound. Also your SSR may be low quality, switching happens e.i. at 50V.

Answer 9

Think you got some good answers here, but just like to add transformers can buzz randomly accoring to the point in the AC cycle power gets applied. It's slightly counter intuitive. If power is applied at zero crossing point the inrush current duing the first half cycle sets up a huge DC current. This magnetises the core and the transformer buzzes thereafter. If power is applied at the peak of the voltage the momentary DC set up by the first 1/4 cycle gets cancelled by the second 1/4 in the opposite polarity. The 3rd and fourth 1/4 cycles equal and opposite current occurs. No residual DC magnetisation occurs and the transformer is quiet. So a zero crossing point SSR would be always bad!

A soft start is a good way to mimimise this. In smaller pieces of equipment (preamps, graphics etc.), a series resitor is sometime enough if you can stand dropping a few volts. In the UK this is usually OK as our 230V equipment needs to work on nominal 220V for EU, but we usulaly get a good 240V.