Dc Motor Brake - Page 1
Forum section: electronics / projects

author: anishkgt
author_rank: Frequent Contributor
timestamp: August 30, 2017, 10:33:54 pm
content: Hi All,I've been doing some reading and googling for a simple circuit for a DC motor brake and from what i understood. i've put down a simple schematic. How practical can this be ?So the idea simple, In Fig 1 the switch (S1) is ON and motor (M1) turns, when switched OFF power to the motor is cutt-off and the free spining motor is like a generator now and the voltage generated goes through the MOSFET (Q1) and triggers its gate which shorts the motor's positive terminal to GND through the power resistor (R1) there by bringing the motor to sudden stop like when a brake is applied.Figure 2 is similar without the MOSFET.Could somebody please advise me if this would be ok and no serious damage would occur.

author: firehopper
author_rank: Frequent Contributor
timestamp: August 30, 2017, 11:03:51 pm
content: depending on what the motor is driving you dont want a sudden stop ever. a slow gradual stop is better.

author: anishkgt
author_rank: Frequent Contributor
timestamp: August 30, 2017, 11:08:06 pm
content: On second thought yes. A sudden stop would bring disaster but would this schematic work as intended and to gradually stop the resistor should be lower value ? and would it make difference if a thyristor is used ?

author: schmitt trigger
author_rank: Super Contributor
timestamp: August 30, 2017, 11:38:18 pm
content: As long as you don't exceed the rated armature current, your motor should be safe

author: anishkgt
author_rank: Frequent Contributor
timestamp: August 30, 2017, 11:43:58 pm
content: Well would a dc brushed motor such as thishttp://r.ebay.com/fPil0khave an armature current ?Is Armature current the Ratted current here of 13A ? from the ebay listingManufacturer brand: JOHNSONMotor type: RS-755Rated voltage: 18V DCWorking voltage: 12V -20V DCIdling speed: 18400 rpmNo load current: 1.92AStarting current: greater than 7A (no load)Rated current: 13ARated power: 180WProduct quality: 350g

author: mjkuwp
author_rank: Unknown
timestamp: August 31, 2017, 01:46:53 am
content: Try re-drawing it with the flyback diode in there ... free-wheeling diode. The motor is an inductor and while it is running current flows. When the circuit is opened the current will still try to flow and the energy exists in the form of a magnetic field. A big potential will develop and this can lead to damage. Note, I am not a physicist so I might have the understanding a tiny bit wrong but there's no denying these circuits always have a diode that allows current to flow when the circuit is opened. I am not sure but it may change things...

author: jmelson
author_rank: Super Contributor
timestamp: August 31, 2017, 03:39:42 am
content: Quote from: anishkgt on August 30, 2017, 10:33:54 pmFigure 1 has no way to surely turn the MOSFET off. If the switch was tapped to the brake position and then flipped back to run, the MOSFET would stay on. You should have a resistor to drain the gate charge when the switch is not in the brake position.Otherwise, the MOSFET really does nothing in this case. The switch can connect the braking resistor as well without the MOSFET.Jon

author: anishkgt
author_rank: Frequent Contributor
timestamp: August 31, 2017, 06:59:28 am
content: Thanks Jon.I've tested figure 2 and it works. But it's a sudden stop with sparking at the brushes. How do I gradually stop the motor ? Would adding a resistor in parallel do the job ? I checked to see the negative volts during braking is about -72volts. Would that help in determining anything ? Or is there a better way to do it.Www.Georgehobby.wordpress.comEquipments: DSO104z, Hakko FX888D

author: Gyro
author_rank: Super Contributor
timestamp: August 31, 2017, 11:46:25 am
content: If it stops too quickly then simply increase the value of R2 to reduce the braking current.BTW. Treat the schematic with the MOSFET as a 'block diagram', in practice the MOSFET Gate would require much more protection from spikes and overvoltage.P.S. You might always see a bit more sparking from the brushes as they will be passing current in the opposite direction to what they have bedded in with during normal running. This is harmless unless you exceed rated current.

author: schmitt trigger
author_rank: Super Contributor
timestamp: August 31, 2017, 12:51:45 pm
content: Quote from: anishkgt on August 30, 2017, 11:43:58 pmCorrect!

author: anishkgt
author_rank: Frequent Contributor
timestamp: August 31, 2017, 05:07:22 pm
content: Quote from: mjkuwp on August 31, 2017, 01:46:53 amThe Fly Back diode has no use here as the motor terminals are shorted with the resistor. I am guessing as the resistor value is increased the sparking should reduce. I tried a 30Ohm and that did not reduce the braking. Before i go any further with that i will put in a load and try these. There should be some change as a load is added.

author: viperidae
author_rank: Frequent Contributor
timestamp: September 02, 2017, 09:17:35 am
content: The diode will be passing most of the current. The current through the resistor is only going to be the forward drop across the diode/ohms

author: anishkgt
author_rank: Frequent Contributor
timestamp: September 02, 2017, 11:24:14 am
content: Quote from: viperidae on September 02, 2017, 09:17:35 amSo would you suggest adding a diode would keep sparks to a minimum and resistor would slow the momentum of motor ?Www.Georgehobby.wordpress.comEquipments: DSO104z, Hakko FX888D

author: Gyro
author_rank: Super Contributor
timestamp: September 02, 2017, 01:26:57 pm
content: Quote from: viperidae on September 02, 2017, 09:17:35 amI can't see how that works - if the diode clamps the voltage to ~0.7V then trying to adjust the braking effect with the resistor value will be pointless. The diode acts as an effective short across the motor.

author: anishkgt
author_rank: Frequent Contributor
timestamp: September 02, 2017, 01:32:55 pm
content: Well i did some test withand without a diode. I am just learning to use an oscilloscope, from what i learned i understand adding a diode has reduced the negative volts. The load here is a saw blade basically the project is a table saw.

author: anishkgt
author_rank: Frequent Contributor
timestamp: September 02, 2017, 01:52:44 pm
content: Here is how it is wire.

author: Zero999
author_rank: Super Contributor
timestamp: September 04, 2017, 07:54:53 am
content: The relay won't switch instantly, so the diode is still required.I'd be more worried about exceeding the relay's maximum current rating, than the motor's. The motor will have a large thermal time constant but the relay's contacts could weld together, if the current is too high.

author: anishkgt
author_rank: Frequent Contributor
timestamp: September 04, 2017, 07:56:42 am
content: Well is not a relay. It's SPDT switch.Www.Georgehobby.wordpress.comEquipments: DSO104z, Hakko FX888D

author: Zero999
author_rank: Super Contributor
timestamp: September 04, 2017, 08:12:56 am
content: Quote from: anishkgt on September 04, 2017, 07:56:42 amThat makes no difference. The current rating of the switch, is more important than the motor.

author: anishkgt
author_rank: Frequent Contributor
timestamp: September 04, 2017, 11:20:06 am
content: The switch is rated for 16A and checked with DMM that the current during brake is about 23A that is just for milliseconds. So question is a diode required ?I will latter post a video of the working.

author: Damianos
author_rank: Frequent Contributor
timestamp: September 04, 2017, 09:18:33 pm
content: Hello!I am curious what is the behavior of the motor (without a load) when you simply open the switch? It continues to rolling or stops almost immediately?What type is this motor? Do you have a data-sheet?Is it with permanent magnets on stator, with stator coils in series or parallel with the rotor or something else?If it has permanent magnets, then there is a chance to demagnetize them, if abused. I don't know how it happens but I have seen that (in reality and some relative manuals)!Relatively to the contacts of the switch that you use and the peak current:- The contacts are not like resistors that have mass and a thermal capacity to need time for damaging them. What happens is on the surfaces that make or break the circuit so can happen on less than milliseconds. So always we select them by the voltage rating, the maximum/minimum current, the type of the load ...- If your DMM does not have a "peak detect" function, then your measurement is irrelevant. The current, during braking, starts with a value of Vback-EMF/Rtotaland is reduced as the rotation speed decreases.By the way: a usual DC motor is not an inductor, it is a generator that produces the back-EMF (of course while it is rotating and not stalled/blocked). It has a "parasitic" inductance due to imperfections...Regards,Damianos

author: Zero999
author_rank: Super Contributor
timestamp: September 04, 2017, 10:03:26 pm
content: Quote from: anishkgt on September 04, 2017, 11:20:06 amYes, the diode is required, as I said before, switching takes time. It is also irrelevant to this discussion.When the current flowing through the motor is suddenly interrupted, there will be a brief high voltage spike, but this has nothing to do with breaking. The spike is due to the field inside the armature collapsing. If the motor continues to rotate, it will act a dynamo, producing a DC voltage, proportional to the speed. It is the break, which is dissipating the power generated by the motor, when it's moving due to external mechanical energy. The diode just gets rid of high voltage pulse, when the power is interrupred.

author: amyk
author_rank: Super Contributor
timestamp: September 04, 2017, 10:54:04 pm
content: What you're doing is known asdynamic brakingand is a common method of slowing down DC motors.

author: anishkgt
author_rank: Frequent Contributor
timestamp: September 04, 2017, 10:55:19 pm
content: Yes I do understand that but my concern is if the return emf would burn the coil in the motor.Www.Georgehobby.wordpress.comEquipments: DSO104z, Hakko FX888D

author: anishkgt
author_rank: Frequent Contributor
timestamp: September 04, 2017, 11:10:16 pm
content: Just posted the video here. This is without a diode connected.I will post another one with a diode. So the diode would be reverse biased correct. Anode to negative of the motor and the cathode to the positive of the battery.Www.Georgehobby.wordpress.comEquipments: DSO104z, Hakko FX888D

author: Zero999
author_rank: Super Contributor
timestamp: September 05, 2017, 08:17:52 am
content: It will work without the diode and it isn't so critical, if you're using a mechanical switch, rather than a transistor.The diode is still a good idea, because it reduces arcing on the switch contacts, thus prolonging their life and reliability.I doubt you'll observe any visible difference between diode and no diode, unless you actually show the switch contacts to see if the arcing or put an oscilloscope across them measure the voltage.In any case, diodes are cheaper than switches, so it's worth it.

author: anishkgt
author_rank: Frequent Contributor
timestamp: September 05, 2017, 08:48:20 am
content: Ah ok so the diode was actually to protect the switch and not the motor. Would a 1N007 be ok here ?And to check the voltage at the switch , the probes should go at the anode and cathode, correct ?Www.Georgehobby.wordpress.comEquipments: DSO104z, Hakko FX888D

author: Zero999
author_rank: Super Contributor
timestamp: September 05, 2017, 10:02:13 am
content: Your previous schematic is correct, except you'll need a resistor in series with the shorting contacts, to limit the current to a safe level.I would have expected the negative pulse to be higher, than the results given by your previous experiment, but it will depend on the how fast it's turned off, which seems to be pretty slow. Going by the negative voltages, you've measured, it should be fine without a diode.https://www.eevblog.com/forum/projects/dc-motor-brake/msg1293204/#msg1293204

author: anishkgt
author_rank: Frequent Contributor
timestamp: September 05, 2017, 10:15:48 am
content: Quoteas in the switch off time would generally be more than 30 seconds after switch ON. The blade has to reach its full speed before anything can be cut.And a resistor in series, would that be just for test with the OSC or it has to be their as part of the circuit ?Www.Georgehobby.wordpress.comEquipments: DSO104z, Hakko FX888D

author: Zero999
author_rank: Super Contributor
timestamp: September 05, 2017, 10:39:29 am
content: Quote from: anishkgt on September 05, 2017, 10:15:48 amYou've misunderstood my comment regarding switching speed. It has nothing to do with how long the motor is turned on or off for. The switching speed, is how long it takes for the switch's contacts to physically open and close, thus switching the current on or off. It is a property of the switch itself.The faster the switch contacts open, the higher the reverse voltage spike. In this case, the contacts open quite slowly, so there isn't much of a voltage spike.QuoteThe resistor is there to limit the current through the switch and to control the deceleration of the motor, nothing to do with the test.

author: anishkgt
author_rank: Frequent Contributor
timestamp: September 05, 2017, 10:44:16 am
content: Thanks for clearing that. The resistor in series made it a lot more slower in stoping the blade. Without one was better. I tried an 8ohm 20w and that did not help. Not sure if I should go lower but even if I had I would have to get this from eBay.Www.Georgehobby.wordpress.comEquipments: DSO104z, Hakko FX888D

author: anishkgt
author_rank: Frequent Contributor
timestamp: September 05, 2017, 01:31:08 pm
content: Quote from: Hero999 on September 05, 2017, 10:02:13 amHere it is on the scope. If i have interpreted it correctly, the spike is at 23.60v and less than 50ns and the whole duration after contact is 738ns.

author: Zero999
author_rank: Super Contributor
timestamp: September 05, 2017, 01:41:50 pm
content: Is that without the diode?It should be fine.On second thoughts the lack of a resistor isn't that bad, since the blade doesn't have much mass. The motor also switches on, with nothing to limit the inrush and it's certainly no worse.

author: anishkgt
author_rank: Frequent Contributor
timestamp: September 05, 2017, 01:51:50 pm
content: It is with this diodehttps://www.digikey.com/products/en/circuit-protection/tvs-diodes/144?k=P6KE27A&k=&pkeyword=P6KE27A&pv1291=136&FV=1c0002%2C1f140000%2Cffe00090%2C6540013%2Cb2801b3%2C1b1001f3&mnonly=0&ColumnSort=0&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25and no resistor.To play it safe what would be the best inrush limiter ?

author: Zero999
author_rank: Super Contributor
timestamp: September 05, 2017, 06:05:16 pm
content: Seems reasonable to me.The inrush limiter can be as simple as, a series resistor which is bypassed by a relay, after a certain time has elapsed, or a transistor with an RC circuit, to make it turn on slowly.See below.RL represents the motor. The MOSFET needs to be able to dissipate1/2of the power rating of the motor, for the time it takes to start.

author: anishkgt
author_rank: Frequent Contributor
timestamp: September 05, 2017, 06:09:26 pm
content: That would take some time design and troubleshoot. I will just use a pwm controller like thishttps://www.ebay.com/itm/322417384785Www.Georgehobby.wordpress.comEquipments: DSO104z, Hakko FX888D

author: Zero999
author_rank: Super Contributor
timestamp: September 05, 2017, 06:21:06 pm
content: Quote from: anishkgt on September 05, 2017, 06:09:26 pmThe link you've posted didn't work but I agree, a PWM controller would be a good idea.

author: anishkgt
author_rank: Frequent Contributor
timestamp: September 05, 2017, 06:25:34 pm
content: Here you go. A Chinese by let's see how well it goes.Www.Georgehobby.wordpress.comEquipments: DSO104z, Hakko FX888D

author: anishkgt
author_rank: Frequent Contributor
timestamp: September 06, 2017, 04:10:03 pm
content: A quick update.So if i've not misread the scope, adding a cap and a resistor has significantly reduced the spike at the switch. Is this correct ?

author: Zero999
author_rank: Super Contributor
timestamp: September 06, 2017, 05:41:15 pm
content: Quote from: anishkgt on September 06, 2017, 04:10:03 pmYes, but with 10K there will be very little braking.

author: anishkgt
author_rank: Frequent Contributor
timestamp: September 07, 2017, 07:49:25 pm
content: Oh i got the connections wrong though the schematic was correct.Yes the 10k Resistor would be a bad idea. and the spike at the contacts pritty high. I had thinner wire that connected from the power source and that heated quickly and due to that amps was dissipated as heat.Changed the wire to a thicker cable and now the motor has a bit more torque and as a result the spikes are higher too.