I need to drive DC motors at 24 V without speed control

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karanmehta93

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In my application, I need to drive DC motors at 24 V without speed control. I have choosen 2 SPDT relays as the solution. When either of them is turned ON , it switches GND at the Common pin of Relay which is otherwise at 24 V (NC). Although this configuration seems to work perfectly, I am facing an unusual issue. Sometimes the relay gets stuck and doesn't trigger even when I try to trigger. When I push or strike the relay with my hands then it starts working. I have tried changing relays but it happens with all of them.
Can anyone provide me an alternative solution for this or a solution to the existing problem?
 

When the current changes in a relay coil, a voltage is developed. this is called back EMF. The same happens when the current is switched off to your motor. So any switch or relay that switched the motor needs some form of protection. One way is to put a diode across the motor/relay coil, so that it does not normally conduct. The other way is to use a snubbing circuit, this is a capacitor and resistor in series this network is again wired across the relay coil/motor. So do this then clean the relay contacts, hopefully the sparking has not damaged them too much and try again.
Frank
 

I had already put the protection diode across the motor. Can you suggest me a schematic of the snubber circuit you just described? For other protection, I am already using tantalum capacitors across the motor terminals and between the relay coil along with protection diodes. 2 additional diodes are also connected, one across NC and COM and the other across NO and COM. Kindly suggest me some other methods.
 

You MUST put a low value resistor in series with your tantalum capacitor, say 10 -> 4.7 ohms. Because the tantalum capacitor have such a low impedance, the inrush current into it will overload any switch that switches the supply on to it. To put some figures out, motor, switched off with no snubber network, causes a 200V voltage spike?, tantalum capacitor when 24V is suddenly applied , inrush current ~ 20 A? So a 10 Ohm resistor in series causes inrush to be < 2.4A + motor start current.
Frank
 

@chuckey
How does your solution relate to my problem..? I have also tried after removing the tantalum cap. but still relay gets stuck any time. There is no direct reason for this.
 

Hi,

I have choosen 2 SPDT relays as the solution. When either of them is turned ON , it switches GND at the Common pin of Relay which is otherwise at 24 V (NC).
Click to expand...

Do not connect 24V to the nc (normally closed) pin. This causes the motor to stop very hard. Short circuit current (sourced by the running motor) destroy your contacts (stick).


If you need it to stop hard, then use a current limiting resistor between nc and 24V. Mind the power dissipation.
If it is ok to stop smoothly, leave nc unconnected.

A schematic of your design helps to help you.

Klaus
 

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