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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

asr
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    "bring the motor to a immediate halt" I don't want to ride this bike........... – Trevor_G Aug 24 '17 at 16:23
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    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.) – Transistor Aug 24 '17 at 16:26
  • Does the motor have a free-wheel built in? If so then stopping the motor will not stop the wheel. – Transistor Aug 24 '17 at 16:28
  • @asr, please draw a circuit diagram using the tool and put all relevant info in the question. – Voltage Spike Aug 24 '17 at 16:54
  • Probably need a 1_ohm 10 watt resistor to absorb the energy, not a short. – analogsystemsrf Aug 24 '17 at 16:56
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    Immediate halt: http://www.meh.ro/wp-content/uploads/2012/11/meh.ro10637.jpg – Janka Aug 24 '17 at 19:21
  • based on experiments with an electric scooter, it's not going to be an immediate halt, but it will noticeably slow down whatever it's attached to; try turning a shorted motor vs an unshorted one. – dandavis Aug 24 '17 at 21:38
  • @transistor this is my first time posting a question,so.. I think only a circuit diagram was all that was missing, I've put it now – asr Aug 25 '17 at 00:23
  • @laptop2d I have now – asr Aug 25 '17 at 00:23
  • @dandavis yes basically Lenz's law – asr Aug 25 '17 at 00:25
  • @Trevor by immediate halt try to bring the efficycle to a stop asap :) – asr Aug 25 '17 at 00:27
  • @asr: I see the schematic. Someone else added the link to Efficycle for you. You didn't answer my question regarding freewheel on the motor (or anywhere else between the motor and wheel). Regarding stopping ASAP, you need to start thinking about the required deceleration rate and what power that would take, given mass and velocity. – Transistor Aug 25 '17 at 00:30
  • If the goal is to get maximum efficiency don't you want some sort of regenerative braking? – user253751 Aug 25 '17 at 00:34
  • @Transistor there is a flywheel between motor and the wheel. Haven't thought about the deceleration rate yet – asr Aug 25 '17 at 00:38
  • @immibis yes but we getting this bldc motor and controller set from a supplier.. so I think for regeneration some kind of designing is required in the controller nd may be even the motor.. i was thinking of a circuitry that would implement this braking action without using the controller – asr Aug 25 '17 at 00:42
  • @asr: The trike looks great. I'm not asking about a flywheel - I'm asking about a "freewheel". On most bikes if you stop pedaling (ASAP) the bike continues to roll because there is a freewheel mechanism in the rear sprocket. Does your trike have this? Or if you push the bike forward with your hands does the motor turn? My first electric bike had a front hub motor with freewheel. If the motor stopped the wheel could still turn in the forward direction. – Transistor Aug 25 '17 at 00:45
  • @Transistor sorry nd yes we do have a freewheel between motor and wheels.. we haven't built the trike yet, that was just a picture I put for reference :p – asr Aug 25 '17 at 00:49
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    @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. – Transistor Aug 25 '17 at 00:52
  • @Transistor and if there was no freewheel?.. nd can u refer me to a website or video that helps understand the freewheel – asr Aug 25 '17 at 00:55
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    @asr: Get on your bike. Pedal. Stop pedalling. Listen and you'll hear click-click-click from the freewheel mechanism in the rear wheel. https://www.missionbicycle.com/how-do-freewheels-work. – Transistor Aug 25 '17 at 01:05
  • @Transistor if There was no free wheel would it be possible to design a circuitry that Is not a part of the motor controller? Or is it a must to have the circuitry as part of the controller? – asr Aug 25 '17 at 02:42
  • Sorry @asr. You are drip-feeding information all the time. Now you have added a photo of the equipment you are using but haven't provided a link to the datasheet. Nobody can answer your question when you do this as we don't know what the specification are. "*Haven't thought about deceleration rate yet.*" Well you need to and should have calculated the power required to stop the vehicle by now. It will take a few minutes once you find the formulas. I hope the project is a good learning experience. Bye. – Transistor Aug 25 '17 at 05:49
  • regenerative charging generates load. shorting both windings to V+ 0V or V- brakes even harder. Series heat loss regulates the brake. – Tony Stewart EE75 Aug 25 '17 at 06:31
  • @TonyStewart.EEsince'75 so controlling the series resistance can control the amount of braking.. but is that practically possible? – asr Aug 25 '17 at 06:38
  • it is done with (well designed ) PWM controlled FET Bridge between battery and motor. https://electronics.stackexchange.com/questions/56186/how-can-i-implement-regenerative-braking-of-a-dc-motor – Tony Stewart EE75 Aug 25 '17 at 13:47

2 Answers2

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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.

Vince Patron
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    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. – Neil_UK Aug 24 '17 at 18:15
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    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. – Kevin White Aug 24 '17 at 19:38
  • Thanks @Neil_UK. I updated the answer with a better explanation. – Vince Patron Aug 24 '17 at 21:15
  • @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 – asr Aug 25 '17 at 00:53
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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?

Neil_UK
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  • So is the use of a shorting switch which connects a external resistance in series with the motor safe? – asr Aug 25 '17 at 03:43
  • @asr yes, but if your motor is BLDC, then your diagram shows you doing that upstream of the ESC, which won't work. – Neil_UK Aug 25 '17 at 06:16
  • I didn't get you – asr Aug 25 '17 at 06:29
  • @asr a brushed motor has two terminals, a BLDC three terminals. Can you count? – Neil_UK Aug 25 '17 at 06:59
  • yes it does have three terminals – asr Aug 25 '17 at 07:16
  • @asr then why have you shown it with two terminals on your schematic? – Neil_UK Aug 25 '17 at 07:19
  • because I am dumb and forgot it had three terminals – asr Aug 25 '17 at 09:01
  • @asr ... so update your schematic to show all three terminals, and you might learn something, and potential answers won't get confused about what you're trying to do, because if they don't understand what you intend, you might get advice that's inapprorpiate, wrong, irrelevant, so generally not useful. – Neil_UK Aug 25 '17 at 09:54
  • This actually depends on the motor. Shorting all three phases in some cases does not lead to excessive torque or heating of the motor windings. As speed increases, the phase current reaches a limit determined by the winding inductance. The torque decreases with speed because the current is out of phase with the back EMF. – user57037 Dec 08 '22 at 17:56