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If I short the three terminals of a bldc motor after switching off the supply to the motor, will that give me braking? And if it can how can I implement this?

I'm working on a project called Efficycle, which is human-electric hybrid trike.

this is how an efficycle looks

this is the set of things we getting from a supplier

enter image description here

enter image description here

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    \$\begingroup\$ "bring the motor to a immediate halt" I don't want to ride this bike........... \$\endgroup\$ – Trevor_G Aug 24 '17 at 16:23
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    \$\begingroup\$ Welcome to EE.SE. Please add relevant details into your question and don't expect everyone to do a web search just to save you adding it once. (It's probably one of the reasons you're being down-voted.) \$\endgroup\$ – Transistor Aug 24 '17 at 16:26
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    \$\begingroup\$ Immediate halt: meh.ro/wp-content/uploads/2012/11/meh.ro10637.jpg \$\endgroup\$ – Janka Aug 24 '17 at 19:21
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    \$\begingroup\$ @asr: Since you have a freewheel you cannot stop the trike using the motor. When you stop the motor the trike will continue to roll. \$\endgroup\$ – Transistor Aug 25 '17 at 0:52
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    \$\begingroup\$ @asr: Get on your bike. Pedal. Stop pedalling. Listen and you'll hear click-click-click from the freewheel mechanism in the rear wheel. missionbicycle.com/how-do-freewheels-work. \$\endgroup\$ – Transistor Aug 25 '17 at 1:05
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Yes, it is possible to do what you are asking (remove power and then short across the motor to cause a braking effect). The short allows high current to flow creating a magnetic field that opposes the rotor's field and produces the braking effect.

Of course, you must design it well otherwise it will be dangerous.

1) If you simply apply it at speed, the braking effect is sudden and can cause the rider to be thrown off, or mechanical damage.

2) Accidentally applying the shorting switch while power is on will result in short circuit, and at the levels you are talking about (400 Watts), fire.

The best way to design is to either custom design the BLDC controller or work with a manufacturer or find a BLDC that implements braking.

The BLDC controller already has MOSFETs electronically controlled by logic circuits and a microcontroller than can be sequenced to short the motor windings, so no additional parts are needed; just proper logic and firmware in the controller.

The BLDC controller can provide proportional braking control, not just ON/OFF.

Also, the BLDC controller should provide heatsinking, current monitoring, and temperature monitoring. Braking current can be very high and the BLDC needs to make sure that its own current and operating temperature limits are not exceeded.

So basically, look into a BLDC controller that can do braking if you want a good design, or use a shorting switch if you want something possibly dangerous and unsafe.

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    \$\begingroup\$ the short doesn't cause a high back emf, the emf is proportional to the motor's speed as it always was, the short makes a high current flow, and so a high braking torque be generated. \$\endgroup\$ – Neil_UK Aug 24 '17 at 18:15
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    \$\begingroup\$ The inductance of the stator may result in less braking torque than expected especially at high speed as the signal being shorted is AC. In electric cars under some fault conditions the windings are shorted (3-phase short) and that is considered preferable to the uncontrolled back-emf of the motor. At low to moderate speeds the torque can be very high and cause vehicle instability though. I would think it would be undesirable on an electric bike. \$\endgroup\$ – Kevin White Aug 24 '17 at 19:38
  • \$\begingroup\$ Thanks @Neil_UK. I updated the answer with a better explanation. \$\endgroup\$ – Vince Patron Aug 24 '17 at 21:15
  • \$\begingroup\$ @Vince Patron so this braking mechanism is not possible without designing a proper controller.. isn't there any other way the idea could be implemented?.. because we getting a motor controller set from a supplier \$\endgroup\$ – asr Aug 25 '17 at 0:53
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You can only brake a 'small' motor by shorting the terminals. A small motor is inefficient enough so that the winding resistance provides a reasonable limit to the braking current.

For an efficient large motor, say a 400W BLDC motor, you need external resistances to limit the braking current, and to dissipate the energy somewhere other than the motor windings. In a vehicle, bringing to 'an immediate halt' is neither possible nor desirable.

Your text talks about a BLDC motor, but your schematic shows a brushed motor, which is it?

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  • \$\begingroup\$ So is the use of a shorting switch which connects a external resistance in series with the motor safe? \$\endgroup\$ – asr Aug 25 '17 at 3:43
  • \$\begingroup\$ @asr yes, but if your motor is BLDC, then your diagram shows you doing that upstream of the ESC, which won't work. \$\endgroup\$ – Neil_UK Aug 25 '17 at 6:16
  • \$\begingroup\$ I didn't get you \$\endgroup\$ – asr Aug 25 '17 at 6:29
  • \$\begingroup\$ @asr a brushed motor has two terminals, a BLDC three terminals. Can you count? \$\endgroup\$ – Neil_UK Aug 25 '17 at 6:59
  • \$\begingroup\$ yes it does have three terminals \$\endgroup\$ – asr Aug 25 '17 at 7:16

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