How to reduce modified square-wave inverter's humming?

[baghu]

I have bought a Chinese 250W 'SUVPR' brand car
inverter guaranteed to deliver 150W. It is 10 times cheaper than a pure
sinewave inverter and so I chose it for backup
against infrequent power-cuts. It outputs step
approximated sinewave at 220V, 50Hz AC. It
works great, but the only problem is the humming on fluorescent ballasts and fan motors.
The buzzing is extremely irritating. I have noticed
that if I connect a non-polarized capacitor of
440V 8uF capacitance, the humming almost goes
away and isn't noticeable. Is this a good idea to
keep the capacitor connected? Or do I need a low pass filter to block the hum? Can you kindly tell
me the values of the capacitor and inductor if I
need to build a low pass filter?
Also please help me with the circuit. Output: 220V
Frequency: 50Hz
Waveform: Step approximated sinewave.
Load: Maximum: 100Watts, minimum 15watts. Please help. Thanks.

 

[FvM]

Is this a good idea to keep the capacitor connected?

Why not? You should check if output voltage and power losses are in an acceptable range.

 

[baghu]

Why not? You should check if output voltage and power losses are in an acceptable range.

Someone told me that capacitors in output can induce back surge which can damage the MOSFETs inside the circuit. So I was just afraid to keep the capacitor connected.

 

[FvM]

It depends on the ouput stage circuit. A low pass filter will have basically the same effect.

 

[baghu]

It depends on the ouput stage circuit. A low pass filter will have basically the same effect.

Thank you so much for your reply. Can you please help me design a 'safe' low pass filter to reduce the humming? I don't mind using large inductors. Thank you.