Everything You Need to Know About Inverter Instability
Hey everyone! It’s Tim, the Head of User Operations at Ecoflow. I decided to create a Medium account to share some of the really cool things that are going on at EcoFlow. I’m really looking forward to talking with all of the EcoFlow fans here!
Recently, some people have been asking me about inverter instability so I decided to put together this article to clear up some of the air around this. Hope you enjoy it and stay tuned for more posts from me in the future!
Analysis of Over Current Protection (OCP) triggered by heavy load
At present, the principle of the inverter is to chop (convert fixed DC power to variable DC power) the DC current through IGBTs with high-frequency switching (frequency about 24KHz), and then filter it into a sinusoidal AC voltage through inductors and capacitors. 24KHz means that a duty cycle of 1/24000S=41.6uS. The IGBT has a high current limiting ability and excessive current will cause heat and damage. Therefore, a current limiting circuit for protecting the IGBT is added to the machine. The working principle of the circuit is that when the current flowing through the IGBT exceeds the threshold value, the IGBT will be forcibly turned off, and the IGBT will be turned on after a working cycle
The user’s faced the waveform problem due to the fact that the current was relatively large when the load first started, and it exceeded the current protection threshold therefore the current protection circuit was triggered to turn off the IGBT this is when the current began to drop. In the next working cycle, the IGBT will continue to turn on and the current will continue to rise. After rising to the current protection threshold, the IGBT is turned off, and the cycle is repeated.
This kind of protection is adopted to trigger the current working cycle one at a time, instead of turning off the whole machine once the current threshold is triggered. Due to some heave duty devices, there will be a large current (inrush current) when the load is just started, and through the cycle-by-cycle overcurrent After a period of protection, the load can be brought up. And once the over-current protection is triggered to turn off the machine, it will cause some loads to be unable to be brought up. Through cycle-by-cycle protection, the product can be adapted to larger loads.
The figure below is the waveform simulated in the laboratory with the Delta Max when the current protection circuit is triggered with a heavy load. The green is the AC current passing through the inductor inside the machine, and the blue is the AC output voltage. After the current protection circuit is triggered, the current changes sharply, resulting in relatively large and high-frequency fluctuations in the output AC voltage.
When the current does not rise to the protection threshold after the load stabilizes, the entire inverter will work normally and output a sinusoidal waveform.
The figure below is the AC output voltage waveform when the cutting machine is loaded. There are 12 sine wave voltage distortions when the cutting machine is put on. After the load stabilizes, the voltage also stabilizes and becomes a sine wave, and the cutting machine works normally.
The analysis of the cause of damage to the Kwh meter is as follows: the schematic diagram of the power supply circuit of the Kwh meter is as bellow, the capacitor is connected in series on the AC voltage circuit, and then the diode bridge rectifier. is connected in series, and the output is connected in parallel with the filter capacitor. Circuit performance is regular when the AC voltage is in a sinusoidal waveform. When the AC voltage is distorted, it becomes a square wave or high-frequency clutter is superimposed on the sine wave. Due to the characteristics of the capacitor itself, the voltage at both ends of the capacitor changes rapidly, the current passing through the capacitor will become even larger, so in the image below a large current will flow through the red capacitor of the lower circuit, causing damage to the circuit.
Other portable power station performance:
Brand J 1800W: It performs similarly to the Delta Max. When it’s overloaded suddenly, cycle-by-cycle current protection turns on automatically. After a few working cycles, the voltage waveform returns to normal.
( Green line: AC current; Yellow line: AC voltage)
Brand A 535 (220v/500W): the product may be damaged after charging the 1800W chainsaw. Cause it can’t restart. From the test waveform, there is no cycle-by-cycle protection function similar to our product to deal with overloading. Without this protection, the battery is easily ruined.
( Green line: AC current ; Yellow line: AC voltage)
