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Author Topic: Braking resistor / back EMF protection for motor driver  (Read 304 times)

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Offline hlavac

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Braking resistor / back EMF protection for motor driver
« on: September 24, 2017, 08:59:22 AM »
Hi guys,

I have a 24V power supply that will drive a BLDC motor that I want to protect from back-EMF feeding reverse current into it, it most probably cannot handle that.
I'm looking to kill about 10A inductive back-EMF spike.

I was thinking about a series diode and buffer capacitor for the motor rail that would stop the reverse current, and a brake resistor that would be switched onto the motor rail when the rail voltage goes above the supply voltage to stop the caps from blowing up when the motor is braking.
Anyone has a good simple design for this? I doubt I can get a beefy enough 240W zener that could eat it :)

Maybe 24V zener into base of a beefy BJT that has braking resistor in the collector? Or do i need to go full opamp with hysteresis and MOSFET to keep the power dissipation on the braking resistor?
Good enough is the enemy of the best.
 

Offline hlavac

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Re: Braking resistor / back EMF protection for motor driver
« Reply #1 on: September 24, 2017, 11:13:58 AM »
Something like this maybe?

Good enough is the enemy of the best.
 

Offline floobydust

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Re: Braking resistor / back EMF protection for motor driver
« Reply #2 on: September 24, 2017, 11:29:58 AM »
What is the mechanical load?
Just to get an idea of the power and energy the brake has to withstand. Usually the inertia of the rotating mass, but back-spin from a pump etc. can make it worse.

Short-term (1-3 second) brakes I've used an active zener like in your schematic, on the DC bus. So the power transistor has to be cooled well enough and have an SOA rating to take the current and voltage. How did you get the 10A number?

Clamping voltage- I went a comfortable margin below the BLDC mosfet's rating and considered the capacitor's voltage rating.

 

Offline hlavac

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Re: Braking resistor / back EMF protection for motor driver
« Reply #3 on: September 24, 2017, 07:43:24 PM »
I'm working on a DIY BLDC servo to use on my 3D printers to replace noisy NEMA17 steppers.
I want to try use the 42BLF01 BLDC motors as they have a similar form factor (NEMA17).
They need about 1.7A per motor and I will have up to 5 of them.
Not very heavy load but pretty dynamic.

Cheap power supply from Meanwell: 240W or 480W.

Its not a one time braking event in the normal operation, but can be in some cases: It will be 3 phase H-bridges PWMing around, plus catastrophic events like sudden stop/crash/power failure while the axis are moving/user moving the carriage while the machine is off.

With large enough caps I can absorb the normal back-emf in a sort of regenerative fashion. But high voltage caps are expensive and I hate electrolytic caps for their unreliability/limited life.
I was thinking of putting about 100uF in ceramic caps (10x10uF) on each driver PCB and backing it up with some large electrolytic ones on the power supply protection/cap buffer module.
But I can't go above ~35V voltage rating without everything getting extremely expensive. So I'm looking for a way to get away with 35V rated caps on the 24V rail and not having it blow up everytime something does not go according to plan :)
Good enough is the enemy of the best.
 

Offline C

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Re: Braking resistor / back EMF protection for motor driver
« Reply #4 on: September 25, 2017, 05:08:29 AM »
hlavac

You might think of how good a servo driver you are creating. The poor drives just power the motor, but when you get real fancy the breaking power gets recycled into a battery bank.
The great driver is a Bi-directional buck-boost converter. So after boosting back to power rails, the protection is just an extra channel with a resistor to keep power bus from going to high..
Your 3 ph servo driver becomes a 4 phase driver with the 4'th connected to a resistor.

So ideas in a great servo driver can be used as protection for DC bus.
A chip that you could look at is the LTC3780 which is special for DC bus. You might also want to look at some fancy BLDC driver chips as some have the 4'th channel also.

A buck converter that has it's voltage sense connected to it's input  instead of it's output. The input voltage needs to be inverted such that low is off.

Just a different way of looking at problem.

« Last Edit: September 25, 2017, 05:11:33 AM by C »
 

Offline moffy

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Re: Braking resistor / back EMF protection for motor driver
« Reply #5 on: September 25, 2017, 12:04:27 PM »
Used to work on diesel/electric locomotives, they had massive dynamic brake resistors mounted on top, cooled by massive variable speed fans. The drivers loved the dynamic brake for controlling the speed down hill. Your solution is a good idea, just use a comparator to control the brake resistor, make sure you have some hysteresis, and size the resistor suitably.
 

Online anishkgt

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Test Gear: Rigol DS1054z, Hakko Fx88D, Brymen BM869s
 


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