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LAN_403

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Wed Mar 13 2013, 12:04PM

Registered Member #5786 Joined: Tue Jul 17 2012, 04:21PM
Location: China
Posts: 5

How do the brake work when using a typical H-bridge to drive a DC motor?
If both lower or both upper transistors are turned on at the same time, I don't think the motor will be in a short-circuited condition because one of the transistors won't conduct the current.

Can you guys help? Thanks a lot.

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Wed Mar 13 2013, 12:17PM

Registered Member #5786 Joined: Tue Jul 17 2012, 04:21PM
Location: China
Posts: 5

I see, the diode provides a current path. I was using this circuit without the EMF diodes... But silicon diodes (and transistors) require about at least 0.6V to turn on, and 0.6Vf+0.6Vce=1.2V, so the DC motor can't be short-circuited completely. Is that right?

Sulaiman

Wed Mar 13 2013, 01:21PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

If both upper or both lower transistors are turned on
then the motor will be braked by the transistor on one side and by a diode on the other side.
The initial voltage from the braking motor will be almost the same as the running voltage,
but (almost) short-circuited by the diode + transistor.

If the motor and it's load have significant inertia
this simple scheme will destroy either the transistor or diode or both or many components simultaneously !

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Wed Mar 13 2013, 01:32PM

Registered Member #5786 Joined: Tue Jul 17 2012, 04:21PM
Location: China
Posts: 5

Sulaiman wrote ...

If both upper or both lower transistors are turned on
then the motor will be braked by the transistor on one side and by a diode on the other side.
The initial voltage from the braking motor will be almost the same as the running voltage,
but (almost) short-circuited by the diode + transistor.

If the motor and it's load have significant inertia
this simple scheme will destroy either the transistor or diode or both or many components simultaneously !

THX a lot for answering. Due to the voltage drop, I don't think this is the perfect way to stop a motor, either. Do you have any better ideas?

Steve Conner

Wed Mar 13 2013, 02:28PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

When properly controlled using PWM, this circuit gives regenerative braking. During the "off" period, the motor is shorted and its back EMF ramps up current through its inductance. During the "on" period, the current returns to the DC power source through the diodes. The motor operates as a generator, and the H-bridge operates as a boost converter.

If you use MOSFETs, the channel conducts in both directions, so the diode voltage drop doesn't enter into the picture.

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Wed Mar 13 2013, 02:53PM

Registered Member #5786 Joined: Tue Jul 17 2012, 04:21PM
Location: China
Posts: 5

Steve Conner wrote ...

When properly controlled using PWM, this circuit gives regenerative braking. During the "off" period, the motor is shorted and its back EMF ramps up current through its inductance. During the "on" period, the current returns to the DC power source through the diodes. The motor operates as a generator, and the H-bridge operates as a boost converter.

If you use MOSFETs, the channel conducts in both directions, so the diode voltage drop doesn't enter into the picture.

Really appreciate. Should I determine the duty cycle by actual testing? Applying a temporally bipolar PWM signal might also be useful, I guess.

Steve Conner

Wed Mar 13 2013, 04:01PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Regen braking is pretty much automatic. The PWM duty cycle sets the average voltage applied to the motor. The motor generates a back EMF proportional to speed, and the motor current is set by the difference between these two voltages.

If this average voltage is bigger than the motor's back EMF, the motor will accelerate. If the average voltage is smaller than the back EMF, the motor current will change direction and it will brake.

So in other words, the PWM duty cycle sets the speed that the motor wants to turn at.

For more details read this:

Patrick

Wed Mar 13 2013, 07:39PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

typically in large high power circuits, like diesel-electric trains, they use regen braking, recover the energy out of the phases and semiconductors and push the waste electricity into a large megawatt size ceramic resistor.

in high power circuits many of us try to do the same, either recovering the power and storing it for use later (batteries, ultra-caps, SMPS) or wasting the heat away from the H-bridge and motor itself via external load (resistor) ...

else as others have said, you can thermally saturate your semi's real fast, and poof them...

Steve Conner

Thu Mar 14 2013, 10:45AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Shorting the motor out uses the motor's own armature resistance as the braking resistor. You can get away with it in small motors, indeed this is how the spindle brake in a cordless drill works. But as the motor gets bigger, the resistance goes down and the short-circuit current (and braking torque) increases to dangerous levels. So you have to do what Patrick says.

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Thu Mar 14 2013, 02:45PM

Registered Member #5786 Joined: Tue Jul 17 2012, 04:21PM
Location: China
Posts: 5

I'm currently designing a smart braking system for an RC car with my classmate, and this belongs to a part of a high-school project. The car has a ultrasonic module installed in it and we use a MCU to get real - time distance & velocity data. The system is set in order to forbid over speed and car crash. Although it will face a lot of problems in real cars, we still enjoy improving it. Now we've changed the transistors to MOSFETs and found the correct timing for the brake system. Thanks to everyone!

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