Dc Motor Brake

[anishkgt]

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.

[firehopper]

depending on what the motor is driving you dont want a sudden stop ever. a slow gradual stop is better.

[anishkgt]

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 ?

[schmitt trigger]

As long as you don't exceed the rated armature current, your motor should be safe

[anishkgt]

Well would a dc brushed motor such as this http://r.ebay.com/fPil0k have an armature current ?

Is Armature current the Ratted current here of 13A ? from the ebay listing

Manufacturer brand: JOHNSON
Motor type: RS-755
Rated voltage: 18V DC
Working voltage: 12V -20V  DC
Idling speed: 18400 rpm
No load current: 1.92A
Starting current: greater than 7A (no load)
Rated current: 13A
Rated power: 180W
Product quality: 350g

[mjkuwp]

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...

[jmelson]

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 ?

Figure 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

[anishkgt]

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.com

Equipments: DSO104z, Hakko FX888D

[Gyro]

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.

[schmitt trigger]

Well would a dc brushed motor such as this http://r.ebay.com/fPil0k have an armature current ?

Is Armature current the Ratted current here of 13A ? from the ebay listing

Correct!

[anishkgt]

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...

The 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.

[viperidae]

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

[anishkgt]

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

So would you suggest adding a diode would keep sparks to a minimum and resistor would slow the momentum of motor ?


Www.Georgehobby.wordpress.com

Equipments: DSO104z, Hakko FX888D

[Gyro]

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

I 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. 

[anishkgt]

Well i did some test with

and 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.

[anishkgt]

Here is how it is wire.

[Zero999]

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.

[anishkgt]

Well is not a relay. It's SPDT switch.


Www.Georgehobby.wordpress.com

Equipments: DSO104z, Hakko FX888D

[Zero999]

Well is not a relay. It's SPDT switch.

That makes no difference. The current rating of the switch, is more important than the motor.

[anishkgt]

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.

[Damianos]

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 V_back-EMF/R_total and 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

[Zero999]

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.

Yes, 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.

[amyk]

What you're doing is known as dynamic braking and is a common method of slowing down DC motors.

[anishkgt]

Yes I do understand that but my concern is if the return emf would burn the coil in the motor.


Www.Georgehobby.wordpress.com

Equipments: DSO104z, Hakko FX888D

[anishkgt]

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.com

Equipments: DSO104z, Hakko FX888D