Motor control with relay - Page 1
Forum section: electronics / beginners

author: ramonest
author_rank: Contributor
timestamp: August 22, 2017, 07:22:28 pm
content: I have a mechanical device that is powered by a DC motor at 72 Vdcand it takes a maximum of 6 A.It is supposed to be controlled by 3 relays. The relays only turn on and off the 72 V source and they will not be commuting too often, and no PWM control or anything just the supply to the device,it takes some seconds to change position and the relays stop by disconnecting the supply.I already have the schematic and it allows the motor to turn in both directions.Now I have the problem of choosing the relays. I've never worked with relays and I'm not familiar with the terminology and what you can and cannot do with them.I understand that a DC motor is still a highly inductive load(is that correct?) and it sure will play an important role on choosing the correct relay.I started looking for what i thought were some overdimensioned devices (8-16A) and about 100+ Vdc. But then the graph for the maximum break capacity isn't nearly as much(attached 2 graphs). Does that mean that it'll arch over and not work properly or that it'll reduce its life-span or the maximum commutations it's designed for?I'm having a hard time finding a proper relay for the application. They whether have very low current switching capacity at 72 Vdc(if I understand the graphs properly) or a constant large current (over 20 A) for up to the maximum voltage (over 200 for example).Some places also use the L/R time constant of the circuit you intend to use the relay with to specify more accurately the breaking capacity of the relay. Can I know this constant for my motor?Finally, I do believe that putting a parallel diode to the motor will make it look like a resistive load for the relay perspective?. That would be impossible since I want to drive the motor both ways, so I thought of putting two 50 V zener diodes reversed parallel to the motor, so at least when the relay interrupts the current flow the inductance won't create a peak higher than 100 V. Would that work? Improve something?I think these are some simple/stupid questions but... If you know some source about these topics I'd appreciate is as well!!Thanks!

author: Benta
author_rank: Super Contributor
timestamp: August 22, 2017, 08:07:06 pm
content: DC motors are basically resistive loads. There is a small amount of parasitic inductance, but this should not influence your choice of relay.The main problem with DC motors is the starting current. When closing the contacts, your motor will almost be a short-circuit until the anchor is rotating.Instead of searching for relays, use the term "contactor" instead. That should bring relevant hits.

author: ramonest
author_rank: Contributor
timestamp: August 22, 2017, 09:19:35 pm
content: Quote from: Benta on August 22, 2017, 08:07:06 pmWhy? I mean maybe the intensity will be somewhat constant,but it's basically current passing through the coils of the motor and a coil is an inductance if you try to stop that current flow you will create a voltage spike anyways, which is(at my understanding) the problem that might have some influence on the relay/contactor choice.I guess I don't understand why a dc motor is a resistive load.

author: Assafl
author_rank: Frequent Contributor
timestamp: August 22, 2017, 09:30:54 pm
content: The model for a DC motor consists of a back emf generator and D.C. Winding Resistance. The faster it rotates, the higher the back emf and the lower the voltage difference between the applied emf and back emf (and hence the current through the motor winding).The D.C. Resistance is the coil resistance.Some inductance is there for AC applications Like PWM and startup.

author: Benta
author_rank: Super Contributor
timestamp: August 22, 2017, 09:57:48 pm
content: Quote from: ramonest on August 22, 2017, 09:19:35 pmThey are resistive loads, as they are NOT (or almost not) reactive. The main things happening in a DC motor are mechanical.But for a contactor, treat them as resistive. And take a course on DC motors. It's quite simple.