Braking Resistors

I had a machine in a factory I used to work in that had a DC motor on it. It had resistors used as a brake as the motion was causing a generator effect. The factory installed resistors were not working well enough to brake the motion when the button was released. The problem was causing the machine to go past the limit switch and causing the fuse to blow. What I did was put two sets of resistors in parallel and put both sets in series. This worked well to brake the motion.
 
I never had the need to use braking resistors on the few VFDs we installed. I have an older VFD along with 3 phase grain aeration fan in my left over good stuff so I put one of the 62s on for the S&Gs. Manual says 70 ohm, but it can stop that 3500 RPM fan in the 4 seconds ramp. I would have had an ov fault in the past. I haven't tried shorter time yet.
 
Years ago we had a smart chef electrician who.liked to modify & make improvements to equipment. On several 1 to 3 HP 480 volt three phase motors he installed a full wave rectifier thru a NO contact on starters with an off delay timer that would send DC power to motors to quickly stop them. Had to keep time down to bare minimum or would blow rectifier fuses or rectifier.Had to adjust stopping speed with motor fully loaded. Back in the 1990''s we had two machines with 50HP DC motors that had brake resistors. They braking resistors never gave us any problem. 15 years later the machines were overhauled along with AC motors on VFD'S. No more braking resistors.We were having problem restarting maybe a 40 or 50 HP return fans on AHU units that had two 100 or 125 HP supply fans. All three motors were on VFD'S. For some reason when the return fan on a few AHU units were off they would rotate at enough speed that the VFD could not restart them. Tripped out on DC over voltage Buss or another fault. I contacted Danfoss to purchase a brake kit but they told me you have to order it while purchasing a drive. Had to place the two supply fans in hand and ramp speed down to around 30% to slow speed of return fan and then drive was able to restart return fans running backwards.
 
ptonsparky said:
The axial flow fan I'm working with has a relatively heavy blade and while the drive ramps down until the output stops, the blade is still coasting. I haven't found how to stop the blade entirely and it may not be possible with this VFD. It's about 14 years old and not high end.
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The return fans that I mentioned on my post were Axial type with heavy blades. Ours we're inline with at least a 4' round duct and was High in the air and told Union contractors charged $5,000 ten years ago to change a bad motor. They had to install scaffolding 25' high.along with planks. Could never understand while they are building a new building they seldom install an eye bolt or a section of 4" I beam to save several hours replacing motors in hard to access areas.
 
ptonsparky said:
The axial flow fan I'm working with has a relatively heavy blade and while the drive ramps down until the output stops, the blade is still coasting. I haven't found how to stop the blade entirely and it may not be possible with this VFD. It's about 14 years old and not high end.
Click to expand...
Never attempted it but you could try to place drive in hand, go into the program & reverse direction , set the maximum speed to a very low setting, set the ramp up speed to maybe a minute and while somebody is monitoring fan start the drive up. Had several electricians say a motor was bad while they attempted to take resistance readings on motor or meggering amp while a fan or a pump was even just the turning 30 RPM..
 
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