Discharging a motor through relay contacts

Thread Starter

ArakelTheDragon

Joined Nov 18, 2016
1,327
Hi!
I want to discharge a motor through the relay contacts, what bothers me is that when I reverse the direction by switching the DPDT relay the motor will be polarized, after I turn the contacts to the default position will that polarized current get out through the diode in parallel with the contacts?

Is the DC motor coil(rotor) polarized?
 

Dodgydave

Joined Jun 22, 2012
8,867
Fo you want to Stop the motor rapidly, or use forward and reverse direction using a relay??

Post a diagram of your idea...


To stop it rapidly just short the motor terminals together .
 

Alec_t

Joined Sep 17, 2013
10,918
will that polarized current get out through the diode in parallel with the contacts?
Whether the motor is in forward or reverse, when the contacts of the SPDT relay open there will be current through the diode back into the supply. That is the purpose of the diode, to suppress the voltage spike which occurs when the motor current is interrupted.
 

Thread Starter

ArakelTheDragon

Joined Nov 18, 2016
1,327
I needed to confirm this "Whether the motor is in forward or reverse". I was worried will it be ok if the motor is in reverse.because after turning off the DPDT relay, the contacts will go back to the default position.
 

Alec_t

Joined Sep 17, 2013
10,918
Don't use the DPDT relay to change the motor direction while the motor is powered. It's bad for the motor and you will get a massive current surge and arcing across the relay contacts.
 

Thread Starter

ArakelTheDragon

Joined Nov 18, 2016
1,327
So it really is a problem and has to be program realized by turning off the power for "10mS" until the motor is discharged and then turning off the DPDT relay. Will "10mS" be enough?
 

Thread Starter

ArakelTheDragon

Joined Nov 18, 2016
1,327
It's a door control motor which will be closing and opening the door, I have not picked one and I don't know its specs. Can you suggest a minimum and maximum time based on the motor's current?
I was also wondering will the mechanical relays be all right and how long will it be until they need to be replaced.
 

djsfantasi

Joined Apr 11, 2010
6,196
I’d add a diode between connections 11 and 21 of the DPDT relay. The cathode should point to connection 11. This diode would replace the diode across the motor power contacts. By doing this, the back EMF of the motor will short through the diode, protecting the power supply and braking the motor.

I’d find a way to use the motor control signal to disable the direction relay when the motor is turned on. Use the signal to drive a p-channel MOSFET that connects 12VDC to the direction relay coil.
 

Thread Starter

ArakelTheDragon

Joined Nov 18, 2016
1,327
I see a power diode in between. That will work, its normally used for batteries, it will discharge the coil into the source even if the contacts are switched, its a reverse polarity protection.

EDIT:
Can you suggest a diode?


Shouldn't there be a shunt to burn in this case?
 
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Danko

Joined Nov 22, 2017
875
I’d add a diode between connections 11 and 21 of the DPDT relay. The cathode should point to connection 11. This diode would replace the diode across the motor power contacts. By doing this, the back EMF of the motor will short through the diode, protecting the power supply and braking the motor.
Yes, current, caused by back EMF, will go through two gaps (HV arc)
and diode (they are in series) while contacts will trip.:eek:

TS can protect contacts using diode bridge:
upload_2019-2-10_6-38-54.png:)

EDIT:
Single diode:
upload_2019-2-10_7-53-50.png:eek:
Contacts are unprotected.
Can you suggest a diode?
As in source, diodes can be 1N4007.
 
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