Motor EMF protection? - Page 1
Forum section: electronics / beginners

author: Kizrito
author_rank: Newbie
timestamp: December 02, 2015, 02:42:40 pm
content: Hello,First I would like to say thank you for the help. now on to the issues I am having.I am using this cheap PWMhttp://www.amazon.com/gp/product/B00XRRODEW?psc=1&redirect=true&ref_=oh_aui_detailpage_o04_s00with a 10 amp, 36v , 350 watt power supply on a DC motor. Everything works with my setup in normal use but when I put to much load on the motor and I "stall" it out it blows up my PWM capacitors.From what I read it is because of back EMF? because of the sudden stopping?I would like to put in some kind of fail safe to protect my circuits. I am getting mixed reviews from google, some saying to use diodes, batteries, snubber, ect...it is my understanding that I just need to find a way to dissipate the remaining energy?Can I please get some recommendations on what is the correct way to approach this problem.any information would be greatly appreciated I am a novice just getting into working with this kind of stuff so please be plain in how you describe your answer.thank you all again very much.

author: Srbel
author_rank: Frequent Contributor
timestamp: December 03, 2015, 07:18:43 am
content: Put a diode parallel to the motor (opposite to the polarity of the power supply).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 03, 2015, 09:37:22 am
content: Blowing up due to LR (locked rotor) condition is a fault of the shitty-ass drive circuit, not the motor or anything about its physics, I'm afraid. Rather quite the opposite, it's showing you just how bad it is.Tim

author: Kizrito
author_rank: Newbie
timestamp: December 03, 2015, 01:20:07 pm
content: Would it be possible for me to put a small circuit breaker between the motor and the PWM or is that not how they work?what kind of diode would you suggest? I see the internet talk about many options.I am in fact talking about when I locking the motor.unfortunately I might be stuck with this option for a little while.thank you very much for the information.

author: Ian.M
author_rank: Super Contributor
timestamp: December 03, 2015, 01:46:31 pm
content: Depending on the PWM control module, adding a diode at the motor may make the problem worse as it could interfere with sensing the motor current.If the module blows caps (not semiconductors) either they weren't rated for sufficient ripple current to sustain full load operation, or the module's overload protection is defective (or not implemented), though that would usually cause catastrophic switching transistor failure.If the caps can survive with the rotor locked for >5 seconds, a circuit breaker on the feed to the module might be an option.

author: Kizrito
author_rank: Newbie
timestamp: December 03, 2015, 04:34:10 pm
content: There is a fuse on the PWM. a F20a250VSays it is rate at 20 amps and 250 volts.The fuses have never popped before the board. is the fuse just too large?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 03, 2015, 04:42:30 pm
content: Fuses protect wiring. Transistors die in 10 microseconds, fuses die in 10 milliseconds!Like I said... this is why circuits need to be properly designed in the first place. Alas...Tim

author: Seekonk
author_rank: Super Contributor
timestamp: December 03, 2015, 04:51:56 pm
content: On any type of import boards I figure the capacitors are only sufficient to keep the board itself from oscillating. If the capacitors are blowing up it is likely they are seeing too much current. Additional capacitors should be placed close to the boards. Inductance and resistance of the power leads to the supply is likely an issue. Adding extra caps should solve the problem. A look at the power with a scope would be telling. So what is your gut felling about those caps handling 20A spikes.

author: Ian.M
author_rank: Super Contributor
timestamp: December 03, 2015, 05:50:03 pm
content: Quote from: Seekonk on December 03, 2015, 04:51:56 pmIt depends on the repetition rate and the ESR, but if the PWM frequency is in the Kilohertz range, not a gnat fart's chance in a cat. five hurricane.One issue is that the semiconductors are probably similarly underspecified, so adding enough high ripple current, low ESR decoupling to stop the caps puking their guts may simply cause the chopper transistor to grenade as the peak current through it will no longer be limited by the caps..As caps rated for enough ripple current aren't going to be small or cheap, its probably a better idea to start over with a higher quality branded motor speed controller from a supplier that stands behind their product.

author: TimFox
author_rank: Super Contributor
timestamp: December 03, 2015, 11:41:34 pm
content: There is no back-EMF with a locked rotor, since the back-EMF is proportional to rotational velocity.When the motor stalls, it looks like the copper loss (resistance) plus the winding inductance.If the driver is not properly protected, the load will draw too much current from the switches and possibly smoke them.

author: mich41
author_rank: Regular Contributor
timestamp: December 04, 2015, 10:13:13 am
content: I looked at this controller on some ebay auction with more photos. The heatsinked semiconductor near motor connectors probably is a freewheeling diode, so check this before wasting time on adding another diode.As others wrote, it seems that the capacitors are boiling due to exceeded current rating.