# 3 Phase Inverter & 3 Phase Logic problem

#### cloud623

Joined Dec 16, 2007
9
below has a circuit diagram, in this circuit have many problems.

first is the pwm connect with the D flip-flop then the output Duty cycle cannot be change. because i use the D flip-flop to split the pwm in 3. have anyone can give another ideal or circuit?

the second is the SCR connection correct or not?

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#### beenthere

Joined Apr 20, 2004
15,819
There is a problem with your approach. If you are trying to vary the speed of the motor, then it is done with a VFD (variable frequency drive) circuit. Varying the duty cycle on an induction motor only loses torque. The PWM method is for brush DC motors.

You also cannot drive SCR gates directly from logic. Here is an article that talks about phase control and SCR's - http://www.st.com/stonline/books/pdf/docs/3566.pdf. The thyristor data sheet will always give the conditions for triggering.

Here is an explanation of SCR's in out Ebook that explains about triggering - http://www.allaboutcircuits.com/vol_3/chpt_7/5.html.

#### SgtWookie

Joined Jul 17, 2007
22,210
There's another problem; you're trying to use SCRs when they're powered from a DC source. Once you get them turned on, you won't be able to get them turned off until the dead short you'll create across your power supply (due to both sides of the H-bridge being turned on) either fries the SCRs, blows your rectifier or your power source, whichever is the weakest link.

Since you're dealing with less than 200v, look at using power MOSFETs. You'll need to use P-channel for the high side, and N-channel for the low side (unless you use a driver IC).

You also must ensure that both high and low sides of the H-bridge are never on simultaneously. This is not quite as easy as it sounds. Semiconductors don't turn on and off instantaneously, and their turn-on times are very frequently different than their turn-off times. You must plan for this, or there will be bright flashes, loud pops and smoke in your future.