Is a rectified supply inductive or resistive?

[ericmark]

With so many switch mode, or pulse width modulated devices arriving in the domestic situation where the peak current can be far higher than the RMS current is there a rating for rectified load?

It is neither resistive nor inductive, and when we buy switching devices they are rated inductive or resistive but not pulse width modulated or switch mode.

I would consider my freezer and fridge/freezer, and washing machine as using less peak power than in the past due to using inverter motor control. But using rectifiers they tend to be powered from the crest of the wave form rather than even so the peak current is likely much higher than the RMS value.

So in real terms if you have a socket rated at 13A resistive what power can you safely take to supply a freezer? At the moment I supply my old non inverter drive fridge/freezer from a 10A rated thermostat, it keeps my beer to correct brewing temperature. Would that be overloaded if using an inverter drive or not?

With industrial I have turned off the inverter drive before switching supply, but with domestic using items like the lightwave socket referred to in another post you could be switch off while the inverter drive is running. So if rated at 13A resistive what would be the safe power using an inverter drive?

 

[stillp]

13A rms.

 

[ericmark]

I would consider 13A to equal around 18.4A peak with a resistive load. With an inductive load it can exceed that, but the peak is really unknown, however it is normally considered 5A is the limit for a switch designed for 13A resistive. But with a rectified load specially with a capacitor requiring charging we don't see them tripping a B10 MCB so likely less than 30A but without an oscilloscope it is just a guess.

Until the post on RF lightwave sockets I had not considered a situation where a 13A socket could be overloaded, the RCD and Filtered double sockets are rated at 13A but I considered that was to limit heat generated damaging electronic components in the socket. But the RF lightwave socket actually switch and for first time I have seen the resistive load of 13A limit without it would seem any inductive limit being written on the socket.

However the spec sheet states 3000W max per gang. It also states consumption approx 0.5 watt so personally I want something I can unplug or switch off. Also socket needs removing before installation resistance tests which is not really what we want with something permanently installed.

 

[bernardgreen]

Capacitive loading creates current overloads when the switch ( mechanical or solid state ) closes. Inductive loads ( without adequate back EMF suppression ) can create current overloads when the switch closes and voltage overloads ( high voltage pulses from the inductor. back EMF ) when the switch opens.

A bridge rectifier between supply and load will not reduce the current overloads and may only reduce the size of the back EMF spike.

 

[ericmark]

In my house I have one or two sockets which are near enough dedicated, fridge/freezer, freezer, TV and audio system, etc. But most have a range of devices plugged in. To fit a filtered socket to the supply to router may make some sense, and having a 10 mA RCD socket supplying the pond. But in the main things are swapped around and have many items plugged into them.

When I started writing the post I had not down loaded the Spec PDF and from picture it would seem 3 kVA was for pair of sockets, but now I see it is 3 kVA for each socket, so not as bad as I thought. However if I wanted to use a large item of plant in my house, for example a de-humidifier I would not be saying hang on let me unscrew this socket first and see if it is suitable. I would simply plug it in.

It is very unlikely I would plug in two de-humidifiers into the same twin socket, so 3 kVA for a pair is not really a problem. But if the contacts are not suitable for an inductive load then there could still be problems. I would not look at a RF lightwave socket and say to myself can't use that socket let me find another.

And using a welding inverter again I would not auto think I should not use it with that socket. It has a 13A plug so any 13A socket should do.

I see two problems, one the warning notice is hidden once the socket is fitted, and two we expect to be able to use 13A sockets for any normal equipment. OK fitted with a hospital red plug (no fuse) one would be careful, but other than that if fitted with 13A plug one expects to be able to use in any socket without damage, OK again the odd site transformer may trip a B16 MCB but it has not done any damage it has only tripped the MCB.

 

[SimonH2]

The term I don't see mentioned is crest factor. With a rectifier/capacitor, the power is taken during a fairly short, high current spell near the peak in the voltage waveform - and these peaks play havoc through the whole electrical system if there's too much of that sort of load. Hence, it was my understanding that such loads have been effectively prohibited by rules on EMC - and there are switch mode devices designed specifically to "front end" a rectifier/cap and correct the current waveform to make it look more like a resistive load.
Any properly designed system should have a good waveform - and in particular, an inverter driven compressor on a fridge is probably a "better" load than the DOL version (if done right).

 

[ericmark]

I went my son was living in a narrow boat, looked into inverter drives, it seemed washing machines, at that time only LG with inverter drives would work quite happily with a simulated sine wave, but most manufacturers with old type motors said their units would not work with a simulated sine wave. This was when I realised the wave form of the AC supply is important.

Now with my LED lamp it states voltage 85 ~ 265 so one would assume at 85 volt the space is nearly zero time and mark nearly 100%. And the system will start drawing power as the voltage hits the 85 volt mark in the sine wave, however once the capacitor is charged, then it would draw current at a much higher voltage to top up the capacitor, at 5W we don't really worry, but at 3000W then it is a different story.

With my freezer with inverter control I can see when the de-frost cycle cuts in, the power is recorded as around 60 ~ 80 watt on freeze cycle, when the de-frost heater cuts in I see a 120 watt peak. But the energy meter is recording a RMS value. We really have no idea as to peak value.