[SOLVED] PWM Motor control problem

[nimeshasilva]

I have designed a PWM motor controller for 220V DC 8.3A shunt motor. and I have attached the power section of the circuit which is isolated by a 6N137 highspeed IC output photocoupler (Proteus has no model for it and so that I have simulated it with a generic NPN photocoupler, but the connections are exactly the same). and the PWM signal is issued to the photocoupler from a microcontroller.

The circuit is working and the problem comes only when the circuit is powering up and turning off. when the entire circuit is powering up, the motor is turned on momentarily and stops. and when the power is removed, the motor again turned on momentarily and stops, this time the motor is turned on longer than in powering up scenario (I suspect this is due to the smoothing capacitor charge at 220V DC rail).

I'm very thankful to you if anybody can help me out on solving this problem?? I need to keep the motor from turning on momentarily when power is applied and removed.

Thank you
Nimesh

 

[WimRFP]

Hello,

I think that "12VDC" comes faster then "voltage on R1", so Q1 is not conducting before "12VDC" reaches the threshold of Q2. During power down, maybe "voltage on R1" disappears too fast while "12VDC" is still present.

I think "voltage on R1" must be the first present and the last present so that Q1 conducts during 220V power up and down cycle

 

[nimeshasilva]

The problem is solved. it was in the MOSFET IRFZ44, all i have done is, replacing it with a bipolar transistor. that's all

 

[WimRFP]

Hello,

Did you also checked why it didn't function with the IRFZ44 (and does with the BJT)?

For me it was somewhat strange that you took such a heavy mosfet for driving the final one.

 

[nimeshasilva]

I choose IRFZ44, because in here small MOSFETS are somewhat hard to find. but power MOSFETS can be found easily.

I have no idea how the BJT solves the problem. but I think, because it is current operated, and voltage independent, can be the reason.

 

[WimRFP]

Hello,

BJTs start to conduct at around 0.5V, mosfets need several volts. I would check the voltages (see previous posting) during powering up and down to see whether that maybe the root cause.

Now you don't know your design margin.