E-stop

15 years 10 months ago #1635352 by bjurobot
E-stop was created by bjurobot
I am trying to decide on the best location to put a safety E-stop disconnect in the power system. Should this diconnect be before the motor contoller or after the controler? I am using the AX2550. Can I do it with one relay before the controller, or would it be better to use an individual relay for both of my motors after the controller?

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15 years 10 months ago #1635502 by cosma
Replied by cosma on topic Re:E-stop
It would probably be best to put the disconnect before the controller. This would allow you do cut power to everything regardless whether the fault is in the motors or in the controller.

If you are just looking for a quick way to stop the motors, remember that there is an e-stop input on the controller that will turn off the power to the motors until the controller is reset or powered up again.

Cosma

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15 years 10 months ago #1635689 by bjurobot
Replied by bjurobot on topic Re:E-stop
As specified on p 26 of the manual, "For safety reasons, it is highly recommended that a way of quickly disconnecting the Motor Power be provided ..." For this reason, we don't think that the E-stop input itself is safe enough. However, p 30 says, "Avoid switching Off the main power cables while the motors are spinning. Damage to the controller may occur." The e-stop has to work when the motors are spinning. We don't see any way to do a safe e-stop without damaging the controller.

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15 years 10 months ago #1636335 by cosma
Replied by cosma on topic Re:E-stop
Disconnecting the controller's power while the motors are spinning is actually quite safe now.

The issue is that if the motors are spinning (and therefore producing a voltage) while there is no battery connected, this voltage will cause a current to want to flow back in the battery. If the controller is pulsing the MOSFETs with a PWM duty cycle other than 100% (all on), or 0% (motor shorted), then there is actually a voltage multiplier effect which can cause it to rise above the maximum supported by the MOSFETs.

The controller monitors this voltage and will turn off all MOSFETs when the voltage rises above 43V.

In the early version of the controller (AX2500), this monitoring did not exist and we have seen a couple units with exploded components because of this.

While we now protect against it, we still do not want to encourage disconnecting the controller for casual on/off operation. For emergency, it is the safest method.

Cosma

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