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Fish_in_a_Barrel
Joined: 30 Sep 2006
Posts: 673
Location: Perth, Western Australia
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@Grotto: Arcing is caused by the inductance in the motor.
[Long_Post]
[Educational]
Inductance is the basis of a switch mode power supply (don't leave home without one.) An inductor is a current storage device, just like a capacitor is a voltage storage device. Unfortunately, unlike voltage on a capacitor, the current is always moving. The current can NOT be changed instantaneously.
In a single direction speed controller (+ve - motor - Switching device - -ve) the switch is closed to get a current flowing through a motor. The current slowly ramps up in through the inductor, because it cannot change instantaneously. Then the switch opens. The current in the inductor WILL FLOW, until all the energy stored in it dissipates. There is no way to stop it. If there is nothing else in the circuit, then the current will cause a voltage increase until the current flows. This means that it will arc over the nearest thing to make a circuit. This could be your motor terminals, or your switch. If it's a physical switch then it will cause arcing across the contacts, with possible welding. If it's a a solid state switch, then it could be destroyed by the voltage spike (It blows a hole in the di-electric, which could short it out permanently). Bad .
So in the simple uni-directional controller, they place a diode in anti-parallel with the motor. This doesn't conduct when the motor is powered, since it is put in backwards. What it does is provide a path for the current to circulate, when the switch is opened. Therefore there is no arcing, and the switch survives to switch another day.
Now back to the speed controller with relay reverse. You can't use the diode trick, because, when the poles are reversed, the diode is a dead-short across the motor, causing excessive current to flow, destroying your switch.
Here you have two options: a few diodes in a different arrangement, or some capacitors. Diodes first because thats what I use . Instead of putting the diodes directly across the motor, they need to be put backwards (where the don't usually conduct) from the negative supply, to the motor terminals (one to each terminal). Then another set from the motor terminals to the positive rail. This will allow the current to flow around the motor, no matter what state the contacts are in, (even if they're switching).
The other way, with the capacitors, is to place them over the pole / contactor junctions. This allows a temporary current to flow through the contacts, as they are opening. This leaves a charge on the capacitor. When the contactor closes again the capacitor is discharged.
Both methods have their uses in industry, and their converts. The capacitor method can be seen in cars that still have distribution caps. It wouldn't last long without one. The diode method can be seen on most H-bridge configurations, ie the IBC. Also the mosfets in H-bridges have inbuilt diodes which aid in this process. (External ones are used to reduce heating.)
As you can see Aaron and Andrew like the capacitor method, and I like my diodes. In both cases, the components need to be sized correctly. If they are then your laughing
[/Educational]
Any questions?
As you may have noticed, I love this topic
[/Long_Post]
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Fri May 18, 2007 10:09 pm |
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