Breaking DC motor

[mahmoud shendy]

Dear All, my problem is that I have a large DC motor ( permanent magnet DC motor ), it operates at about 180VDC / 8 Amp.
When it rotates and reaches its speed ( 7200 rpm ) and I turned it off it still rotates for about 6 minutes..

I need to break it even this take HALF or ONE minute..

What is the best and safe way to make that..

By the way, when the motor is turned off it becomes a generator ( magnet rotates in a coil ), can I connect a load such a 200W
lamp to break it or connecting such load will not affect?!!

is the principle of connecting load will break the motor TRUE or NOT?

thanx in advance.. SHENDY

 

[Nigel Goodwin]

Yes, connecting a load to a generator will brake it, you might also consider a mechanical brake as well?.

 

[Valence_4]

You can consider using a relay that will, when energized, connect the motor to the supply and when de-energized, connect the motor/generator to a 22 ohm 200 watt power resistor.

Your motor, when running idle will generate close to 180 volts. The 22 ohm resistor will consume around 8 amperes (the motor's rated current) and quickly stop it.

BTW, it will rotate for 6 minutes ???? Does it have a flywheel attached to it ? If so, consider a much larger 22 ohm brake resistor. That resistor will have to absorb and dissipate all of the flywheel's energy and a 200 watt resistor may become red hot !

 

[Boncuk]

Connect a 1,000W floodlight to the motor. This will brake it rapidly.

Boncuk

 

[MrAl]

Hi again,

Sometimes DC motors are shorted out to cause braking. Of course
you have to use components that can stand the entire motor current,
but many times this is just the same H bridge that drives the motor.

 

[Willbe]

You could also reverse the voltage to the motor, either for a fixed time or based on actual motor speed. A higher-than-normal reverse voltage will make for more deceleration.

 

[Ubergeek63]

relays will burn out in no time switching those levels. a resistor or a bulb will work and if you do not mind bringing it all the way down an SCR is a good choice for the switch.

 

[Boncuk]

You could also reverse the voltage to the motor, either for a fixed time or based on actual motor speed. A higher-than-normal reverse voltage will make for more deceleration.

That's about the same as braking your car shifting to reverse gear at full forward speed. Don't forget to take along a basket to collect all the spare parts suddenly available.

 

[audioguru]

He doesn't want to brake the motor. He says he wants to break it.
A hammer might work. Or drop the motor from the top of a tall building.

 

[Boncuk]

He doesn't want to brake the motor. He says he wants to break it.
A hammer might work. Or drop the motor from the top of a tall building.

You have to tolerate that small typo. Anglo-saxons do it, Germans don't.

 

[MrAl]

He doesn't want to brake the motor. He says he wants to break it.
A hammer might work. Or drop the motor from the top of a tall building.

Hi,

That's funny, but when i first read the topic title i thought maybe the
guy/gal actually 'broke' the motor and maybe needed some help fixing
it. I read the post though and that cleared things up.
I try to be tolerant of people who dont speak my language and i hope
they will be equally tolerant of me when i try to speak their language.
In the past i've taken up a few different languages and i found that
it really takes a while to understand all the different verb forms for
past and present and things like that, so i guess you could say i
am very willing to give them a 'brake' (he he, pun intended).

 

[audioguru]

We speak English in this forum. People who don't speak English should go to a forum in their country where they can speak their own language. I don't speak their language so I don't go to their forum.

 

[Valence_4]

Hi again,

Sometimes DC motors are shorted out to cause braking. Of course
you have to use components that can stand the entire motor current,
but many times this is just the same H bridge that drives the motor.

This method is used by pratically ALL battery operated drill manufacturers.

What i don't like in this method is the high current surge that peaks during the motor braking. You can actually SEE it while stopping a battery drill running a full speed. You'll see a bright spark at the motor's commutator while the normal sparking of the commutator is MUCH lower when the motor is running, even under a heavy load. The resistor (or floodlight) method generates MUCH LESS current surges. Furthermore, it's the resistor that absorbs (and dissipates) the whirling motor's kinetic energy, not the motor itself. The downside of the resistor method is that the motor takes longer to brake.

BTW, the reverse voltage method is an excellent method to torture a motor

It will stop... but i'm sot sure it will re-start !

You could have the same effect by using a sledgehammer.

 

[Burnt]

You can use a Resistor Pack similar to what locomotives use or instead of wasting the energy use it to charge batteries maybe, you just want a load to do the job.
You could make a small eddy current retarder for this out of a car brake disc & feed the generated power into it & this will quickly stop it without friction, the problem i suppose with this is as the rpm goes down so will the braking Torque.

The 1000w bulb idea is along the same lines but you may need more of a load depending on your motor.

One very simple method although crude is to have a saline solution--salt water etc in a small tank & have a small suspended Vertical sheet of steel cut to size & to a point at the bottom.
Insulate where the suspension points are with some small second hand ceramic insulators from an electric fence setup.
The plastic lined drums are good as small tanks.
Immerse the sheet steel about 3/4's of the way into the solution & connect your positive & earth wires to each side at the top & you have your perfect load.
Playing with the Steel sheet having more or less of it in the solution & also the total size or drilling holes in it can tune it to perfection.
Cheap but very effective, in this case it would be a small setup, Power stations in some places use this method with huge tanks etc to dump excess power into & once again cheap & effective.