H-Bridge for single phase AC motor drive.

I am working on single phase ac motor drive project.

The schematic is attached only for H-bridge.

I have generated SPWM signals using opamp. When I connect resistive load I get the AC output voltage as expected. But when I connect 150 watt AC motor of washing machine, the motor rotates for few seconds and then my input fuse BLOWS.

While testing I give SPWM signals to H-bridge first, then slowly apply the AC input voltage through variac. At AC input of around 60-65 VAC the fuse blows with driver IC damaged.

Sine wave is of 50 Hz , Triangular wave is of 15 KHz. Driver Vcc= 15V. I am first trying to run motor without any feedback.

I guess the values of gate resistors should be optimized. But these are as per to limit current capacities of IR2104 driver.

Can anybody please help ....

hi
that is not surprising .
you have put an LC filter in order to take the sine signal from SPWM . that looks fine but did you ever think every filter deals with an impedance ? out put impedance i mean !
when you put your motor , it can change impedance of that filter and short circuit or overload then bridge .
by the way . what kinda ac motor you've used ? i hope universal yeah ? please tell me you didn't use squirrel cage motor ! because for squirrel cage motors the only solution is VFC inverter . i hope you have heard of it .

Good luck
Goldsmith

Hi,

I have generated SPWM signals using opamp

Click to expand...

I can´t imagine how you can generate SPWM with OPAMP.
And there is no OPAMP in your schematic...

Klaus

KlausST said:

Hi,



I can´t imagine how you can generate SPWM with OPAMP.
And there is no OPAMP in your schematic...

Klaus

Click to expand...

Sine wave generated using opamp (wein bridge), then triangular wave from opamp astable multivibrator and RC integrator.

These waveforms are modulated using comparators. The modulated output gives SPWM of fixed frequency sine wave which is used to drive the H-bridge. Using this method I run the 150 Watt AC motor.

Now I am trying to build circuit of variable frequency constant amplitude sine wave oscillator whose frequency to be varied by potentiometer.

Can you help me in this regard???? (in variable frequency oscillator)

Hi,

I see, an analog solution.
You need to avoid DC in the output. Only a small part will cause problems.

Klaus

Hi KlausST

I can´t imagine how you can generate SPWM with OPAMP.

Click to expand...

I presume he means comparing a triangle wave with a sine wave however i didn't saw it in his schematic too .

You need to avoid DC in the output. Only a small part will cause problems.

Click to expand...

if with resistive load it works fine then the problem is either out put impedance and resonant frequency of LC filter with motor or motor type ( universal or squirrel cage )
in H bridge if we bias gate sources properly ; amount of DC in outlet would be negligible ( approximately around zero ) .


Best Wishes

Goldsmith

Hi,

If it works with resistive load, but makes troubles with an AC motor (inductive), then this sounds like a DC problem.
This is one of the disadvantages of the analog style SPWM generation.
He definitely needs a curcuit to avoid DC. Best if he uses load current as feedback signal.

Klaus

then this sounds like a DC problem.

Click to expand...

Hi
where this DC should come from ? switches only would be either on or off . they are not working in linear region hence DC signal won't be applied but of course if message signal
(sine wave ) is dealing with DC offset then it can happen but i seriously don't think he summed his sine wave via a DC voltage . isn't it ? oscilloscope and measurement would show this issue .

Best Wishes
Goldsmith

Hi,

where this DC should come from ?

Click to expand...

* Unsymmetric/distorted sine wave
* unsymmetric/distorted triangle
* unsymmetric excitation of both half bridges
***

they are not working in linear region hence DC signal won't be applied

Click to expand...

No need for linear region to get DC output.
***

but i seriously don't think he summed his sine wave via a DC voltage

Click to expand...

I assume he is using single supply... then it surely has about halve VCC of DC offset.
***

oscilloscope and measurement would show this issue .

Click to expand...

I doubt this.
A motor rated with 230V, 150W maybe has 250Ohms ... 300Ohms of impedance. So the current will be around 800mA
But it may have a DC resistance of less than 1Ohms.

Therefore a duty cycle unsymmetry of 0.4% may cause the same DC current than the full load motor AC current.

I doubt you can see this 0.4% deviation ... I even doubt you can see 10% deviation in an SPWM signal with a scope.
If you are experienced and know how to find out, then you may see down to some percent....

Klaus

* Unsymmetric/distorted sine wave
* unsymmetric/distorted triangle
* unsymmetric excitation of both half bridges
***

Click to expand...

You are absolutely right

I assume he is using single supply... then it surely has about halve VCC of DC offset.

Click to expand...

Single supply but we are talking about full bridge which can provide a pure AC signal if the control signal (SPWM ) is designed well .

I doubt this.
A motor rated with 230V, 150W maybe has 250Ohms ... 300Ohms of impedance. So the current will be around 800mA
But it may have a DC resistance of less than 1Ohms.

Click to expand...

In this matter , i was meaning across the resistive load we can simple measure it .
And as i know every usual digital oscilloscope , has FFT menu too . it can tell us about the SPWM modulation ration and it's spectrum and it's DC rating too .
i still believe he used an squirrel cage motor which caused that problem because they are supposed to be controlled by VFD not simply changing the Duty cycle but changing frequency as well .

Best Regards
Goldsmith

Hi,

Single supply but we are talking about full bridge which can provide a pure AC signal if the control signal (SPWM ) is designed well .

Click to expand...

Yes, correctely. It can. But how to ensure this?

In this matter , i was meaning across the resistive load we can simple measure it .

Click to expand...

Is it that simple? You have to measure a relatively small DC voltage (or current) within a relatively high AC voltage (or current).

***
Does the OP have a digital scope with these functions and is he used to use them?

it can tell us about the SPWM modulation ration

Click to expand...

With SPWM the duty cycle is continously changing.
Maybe there is, but I don´t have a scope that can show the SPWM modulation ratio. Or I´m just not aware of it....

Squirrel cage motors have very low DC resistance...


Klaus

Try to replace the motor with an incandescent light bulb.

If the fuse no longer blows, then Klaus is correct: there is a small DC offset which will saturate an induction motor.

Yes, correctely. It can. But how to ensure this?

Click to expand...

Hi
So he has to check it if the SPWM is correctly generated .

Is it that simple? You have to measure a relatively small DC voltage (or current) within a relatively high AC voltage (or current).

***
Does the OP have a digital scope with these functions and is he used to use them?

Click to expand...

Yes that is simple . why you think measuring an small level DC is that hard ?
About Digital oscilloscopes , i guess everybody has a good digital oscilloscope these days , of course everybody who wants to work with inverters and controllers and ... . am i right ?

With SPWM the duty cycle is continously changing.
Maybe there is, but I don´t have a scope that can show the SPWM modulation ratio. Or I´m just not aware of it....

Squirrel cage motors have very low DC resistance...

Click to expand...

When we look at SPWM , fft , then amplitude of other harmonics and amplitude of main sine wave and amplitude of DC voltage are separate from each other and we can simply judge about it . isn't it ? or perhaps i'm missing something ?


About squirrel cage motors , and their DC resistance , i couldn't catch your meaning . of course their DC resistance is low .

Try to replace the motor with an incandescent light bulb.

If the fuse no longer blows, then Klaus is correct: there is a small DC offset which will saturate an induction motor.

Click to expand...

Hi
Using incandescent bulbs is not a very good idea because they have a very low resistance when they are cold hence they will take some more inrush current rather than resistors . they could blow the fuse too .


Best Wishes
Goldsmith