D.A.(Tony)Stewart
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If you always start the PWM cycle off from a zero crossing, soft start comes for free.
Even with no external load, flux doubling in the transformer can give the inverter quite a significant loading at start up if the the transformer core saturates.
A non soft start would be hitting 100% duty cycle hard in one direction.Are you saying that starting the inverter in such a way that the first output of a 50/60 Hz waveform starts at a zero crossing is automatically a soft start? Why is this? What would be an example of non-soft startup?
Not particularly. I didn't believe that your verbose previous post just suggests to switch on the inverter at the voltage zero crossing, which is the perfect way to drive the transformer into saturation. The question has been extensively discussed at the start of this thread, I won't repeat the details. To avoid saturation, the voltage must be either switched on at maximum (90° phase), or ramped.Surely you do not need to see a picture of a sine wave to understand this ?
If you always start the PWM cycle off from a zero crossing, soft start comes for free.
Even with no external load, flux doubling in the transformer can give the inverter quite a significant loading at start up if the the transformer core saturates.
If you also incorporate a very fast current limit, the two should cope very well together in most transient situations.
A standard EI transformer certainly works, but magnetizing current, and zero load inverter current will be higher than with a silicon steel toroid.
Your suggestion of placing a capacitor directly across the secondary is fairly common practice in many commercial designs that use EI transformers.
It will need to resonate with the secondary transformer inductance "near" 50Hz but not close enough to set off a huge troublesome 50Hz resonance at zero load.
One thing to watch out for will be high amplitude current spikes in the primary, if your transformer leakage inductance is not as great as you expect.
Its trying to switch at 16Khz straight into a large shunt capacitor across the secondary.
It may need some help from slight additional external series inductance with the primary. But you are not going to know until the beast is up and running.
This acts as a turn on snubber, and slows the rate of current rise, only a very few uH can work wonders ! It can dramatically reduce turn on switching losses.
Cheers, Tony.
Not particularly. I didn't believe that your verbose previous post just suggests to switch on the inverter at the voltage zero crossing, which is the perfect way to drive the transformer into saturation. The question has been extensively discussed at the start of this thread, I won't repeat the details. To avoid saturation, the voltage must be either switched on at maximum (90° phase), or ramped.
Zero transformer leakage inductance would cause the switching devices to turn on into a virtual dead short circuit, because of the relatively large filter capacitance connected directly across the secondary.please can you suggest a range of leakage inductance that is suitable for this kind of application and also if i have to include a series inductor at the primary low voltage side of the xformer, at the 16Khz switching frequency and what, any idea of value of the inductor?.
As the original poster, I could share my recent experience.
The soft start procedure for the 5kW toroidal transformer turned to be the easiest part of this project.
Taking account of @FvM and @BradtheRad suggestions, I've just designed a simple software procedure to start generating the sPWM signal at 90 degree, with a significant initial attenuation.
I must say that I'm using a sine wave look-up table with 50 rows, corresponding to a sine wave voltage attenuation range of 45-95% (1% stepping).
So, if you have 128 sine wave interpolations during every half wave, you have to set the look-up table index to 64 (90 degree) and the attenuation to 50% or something.
In my software routine, I check the output voltage (230V) during each sine wave zero crossing (I read an average value, anyway) so for the very next sine wave, the attenuation is changed accordingly.
If the battery is in a good state of charge, the attenuation stays in 70-75% range so it takes 20-25 half waves from the startup to normal operation (25 * 10ms = 250ms).
With the above parameters, there's almost NO inrush current from that big 5kW toroidal transformer.
To further tune up the soft-start procedure, the inverter is turning off (due an alarm or a manual/remote action) when the sine wave is at -90 degree thus the remanent core magnetization it's opposed to the one developed at startup (+90 degree).
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Yes, that is the same me.@Warpspeed:
Welcome to edaboard! You must be the same guru member from thebackshed/energymatters boards, right? .
secondly You said i could force a higher attenaution at start - up to further reduce the inrush current, will this not negate the soft - start procedure, or am i missing something.
Nope, you're still generating a sine wave starting at 90 degree (maximum voltage) but with a lower amplitude. Being a PWM signal, it might look the same (by applying the full DC-link voltage across the primary anyway) but that 50% duty-cycle (at 90 degree) it's still smaller than the 95% one (at 90 degree, too) thus you're applying the DC-link across the primary for a shorter period.
Btw, I might have been using the wrong terms before: by 45-95% attenuation I was talking about 45-95% of the maximum duty-cycle (but I guess you get it right).
You just start at 90 degree then the sine wave is generated normally. You only need to progressively decrease that duty-cycle attenuation after each zero crossing point till it reaches the rated output voltage.
Actually, you just have to let the inverter run into normal operation, as it will self regulate the duty-cycle (attenuation) to keep the output constant.
It happens that I've designed it to change the attenuation (during normal operation) with a step of +/-1% after checking the output voltage at every zero crossing point. Maybe you could increase it by a large factor - I've not tested that.
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