Mains transients caused by air conditioning units on the mains?

[treez]

Hello,
Does the switching ON and OFF of **all ** Air conditioning units result in mains transients?, or is it only old AirCon units that exhibit mains transients when switched ON/OFF?

We ask because our garden lamp products seem to be failing due to mains transients. Our new factory has four new airconditioning units protruding out of the side wall, and so If these are a convenient source of mains transients, then we can put our lamps on the same mains supply, and see if they fail.
I would get the scope out and look on the mains myself, but the boss won’t let me do this kind of investigation unless I provide some proof up front.

The following article, on page 2, states that mains transients to 800V are caused by Air Conditioning Units switching on and off….
http://www.seered.co.uk/sunvic_capacitor_information.pdf

 

[barry]

Does switching on "all" AC units cause transients? Yes.

How much depends on your circumstances. Wiring. Other loads on the line. Specific AC. etc.

There is absolutely no way to answer your question intelligently.

P.S. Your boss sounds like an idiot.

 

[treez]

Thanks, i think most modern aircon units would surely have circuitry included in them to limit the units propensity to push high voltage spikes out onto the mains?.....i mean, surely, companies wouldnt install aircon if it was going to put spikes on your mains electricity?

 

[schmitt trigger]

It is as simple as v= L*di/dt

Aircons are significant current loads, and when that load is released, all of the wiring’s stray inductance will cause a voltage spike.

 

[treez]

Thanks, you'd think they'd have a parallel shunt power resistor switched in across its input terminals to absorb the energy of the inductive current.....switching this resistor in to the circuit just before the on/off switch is opened.....then quickly switching it out again...perhaps even an RC damper switched in instead.
Either way, its surprising that things that can cause voltage spikes are allowed to be sold. -And surprising that customers dont ask for the damping circuit as described to be added?

 

[FvM]

Mains surge events are mainly caused by switching inductive loads with contactors. From an electronic design viewpoint, the surge events can be considered as a fact, respectively they are reflected in test standards for electronic devices.

Devices designed to withstand standardized surge events are unlikely to fail in real power supply networks.

 

[treez]

Mains surge events are mainly caused by switching inductive loads with contactors.

Switching off i can see the problem.......because the inductive current sufddenly has nowhere to go
Switching on would only be a problem in the case of stationary rotors at startup?...and the resultant inrush?

Whilst i agree, ....it is surprising that these products which switch inductive loads with contactors dont have extra circuitry to damp resultant overvoltages.

I am amazed the standards bodies allow this.

I mean if a product puts a 1kV overvoltage spike into the mains, then it should be illegal?

 

[FvM]

Switching on can causes surges too, if the contactor is bouncing, which is often the case.

Whilst i agree, ....it is surprising that these products which switch inductive loads with contactors dont have extra circuitry to damp resultant overvoltages.

Inductive loads (motors, lamp ballasts) operated by mechanical switches or contactors are out there since about 100 years. Electronic devices appeared later and had to adapt to the existing electric noise environment. Those that don't manage are eliminated by the evolution, respectively the market.

Better make your products surge proof.

 

[Kajunbee]

I was under the impression that the inductive spikes came from the motor that was switched on or off. But Schmitt trigger speaks of stray inductance in the wiring. Unless I'm misunderstanding he's saying that the wiring that powers the motor introduces the spike and not the motor windings themselves. Am I correct in assuming this.

 

[KlausST]

Hi,

every inductance - when the current is changed - generates a voltage.
Thus both - the motor windings and the stray inductance will cause a voltage spike.
If this voltage is high enough there will be a spark at the just opening contact. This additionally causes EMI because of the high di/dt.

***
I´ve done a lot of mains voltage measurements.
With 3600 samples/s (cutoff at about 1200Hz) ... and not a single sample within a couple of weeks was above 335Vp (230V RMS = 325Vp). This is in a small village with some minor industry around, even in our building there is a lot of 3PH motors and a about 50kW grid tied photovoltaic.

Now I wonder why in other areas there are a lot of spikes on mains voltage, but here we don´t have them.
One cause may be because here all the wiring is underground, thus the whole gird has capacitive character. Capacitive means low impedance for high frequencies. Voltage spikes will be suppressed automatically.

But in other areas with mostly overhead transmission lines - the grid has inductive character. High impedance for high frequencies. This makes it prone against voltage spikes.

Klaus

 

[andre_luis]

There could be ways to reduce the effect of these transients on the local electrical network, for example by ensuring that these AirCon were not switched simultaneously (eg with a kind of relay delay timer), or even by installing a power line filter in the socket of each of these equipments; However, there is a possibility of an overload or even some other problem in the local electrical installation that may be undersized. By the way, if the quality of the electricity grid is a critical item, I would consider researching refrigeration appliances based on Peltier technology.