Monday, November 5, 2012

Improvised Motor Reversal Switch With Dynamic Braking

Sometimes tracking down an unusual replacement part can be almost impossible and you just have to use what you can find.  When the main switch on a 12 volt DC winch failed recently I found myself in this situation.  The problem is that it's not a normal switch.  It's a rocker switch that short circuits the load terminals when in the centre position.  When connected to a motor this creates a dynamic braking effect which gives the winch a holding torque.  The other two positions simply apply 12 volts from a battery to the motor with different polarities to give forward and reverse motion.

At first I tried to replace the switch, which didn't go too well.  When you don't know what to Google for, and salesmen tell you that the thing in your hand doesn't exist, you kind of hit a dead end.  Luckily after cleaning the switch contacts it worked again and was put back into service, but I still wanted a solution in case it failed in the future.  I've come up with something, but haven't been able to try it, so for now it's just a theory.

All of the functionality apart from the dynamic braking can be done using a Double Pole Double Throw (DPDT) rocker switch and a little wiring.  In the centre position the switch disconnects the winch motor from the battery and leaves the motor open circuit allowing it to turn freely.

Motor Reversal Switch Dynamic Braking Wiring
Motor Unpowered - No Braking

By flicking the switch one way, voltage is applied to the motor and it turns.

Motor Reversal Switch Dynamic Braking Wiring
Motor Powered

Flicking the switch in the other direction applies the voltage again, except with the polarity reversed causing the motor to turn in the other direction.

Motor Reversal Switch Dynamic Braking Wiring
Motor Powered

To brake the motor while the switch is in the centre position a relay needs to be added.  The relay needs to be rated to operate at the same voltage as the load you are trying to control.  By connecting the motor to the common and normally closed terminals the winch motor will be short circuited when the relay isn't energised.

Motor Reversal Switch Dynamic Braking Wiring
Motor Unpowered - With Braking

When the switch is in the up or down position, the relay is energised and the motor's second terminal is connected to the DPDT switch via the common and normally open terminals of the relay.  As before, the position of the switch determines the polarity of the voltage applied to the motor, which controls the motors direction.

Motor Reversal Switch Dynamic Braking Wiring
Motor Powered - Braking Disengaged

Motor Reversal Switch Dynamic Braking Wiring
Motor Powered - Braking Disengaged

The dynamic braking that the motor experiences when its terminals are short circuited can be thought of in a couple of ways.  I tend to think of the motor acting as a generator, which drives a current through the motor, this current tries to drive the motor in the opposite direction.  These opposing motions lock up the motor and create a braking effect.  You can also think of things in terms of Lenz's law.  When moving a magnet though a coil, the current induced will generate a magnetic field that opposes the motion of the magnet.

There is one really important thing to make sure of before trying this.  The terminals on the relay coil need to be able to handle a positive or negative voltage.  If the relay is controlled by a single coil you should be fine, if however it's a solid state relay you may need to look into things a bit more.  Even if your relay is controlled by a coil you need to be make sure it doesn't have a diode across the terminals to protect the rest of the circuit from back EMF.  If you skip this step there'll be a race between the diode and the fuse for the first to fail.  In my situation protecting the rest of the circuit isn't necessary.  If however protecting the circuit from the back EMF created when voltage is removed from the relay coil is important, you could place an ordinary diode before the DPDT switch, between the battery and the switch.  You could also put a bidirectional TVS diode with a working voltage of at least 12 volts directly on the relay coil.

In theory this should all work, I can't see any problem with it.  So when the time comes I'll be prepared and have a solution ready to go.

3 comments:

  1. I wanted to implement this type of dynamic braking to my lathe motor, but I can't find a suitable relay. Any idea where to get one that works on 120v on both sides of the relay like the one you described?

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  2. I'm not sure this will work on a lathe motor. If it's a DC motor it should be work, but if it's an AC motor it might not work. This kind of dynamic braking doesn't work well on AC motors. I'm not sure of the exact setup but I'd recommend element14 as a place to get a relay.

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