Hi,

I got one of the above PCBs at a Repair Cafe that run the motor at full speed for a fraction of a second and then tripped the house electrics. I replaced the Q6 IGBT G4PC40K as it was shorted (replaced with STGW30NC60WD) and Q3 SCR S8025L.

The machine now runs, but the owner says -

"I assembled it all, and had a try. Speed setting is very weird now, as soon as I set speed to 6mph it jumps to 9mpg and then slowly reducing to 8.1-8.2 . Could it be the consequence of our IGBT substitution?

"I press speed of 7.5mph it jumps to 10.6mph. There is a speed potentiometer on the board, i tried reducing it a bit, the higher speed gets better but lower speed gets worse.

"But the problem is that board can see that speed is too high, but seem to struggle reducing it. On higher speed it goes overspeed, on low speed it only runs empty, as soon as i step on it, it stops."

I did some searching here (it's how we knew which IGBT and SCR to get) and others have had similar speed issues, but I haven't seen anyone finding a resolution.

The machine isn't here (I'd like the avoid that!) but I can't really think of any easy way to diagnose this with just PCB and motor on my bench. I can inject a PWM signal, and I'm sure the motor will spin, but I can't understand why the speed won't be corrected as a result of the tacho feedback.

I'm inclined to suspect the 0.01ohm current sense resistor R63 as these die in SMPS when MOSFETs short, but it's a high wattage one so has probably not died.

All feedback welcome

 

There are variations on these models pertaining to Belt speed control, some have a slotted wheel tach on the belt motor itself, while others have a small proximity switch at the belt location .
The sensor can often work loose.