There are several power failures. Namely:
All these situations can cause severe problems to the electronic equipment of a CNC (especially those brownouts). PCs can be easily and effectively protected with a cheap UPS, but a UPS for the whole CNC is usually out of the question.
So, How can we protect a CNC from power failures?
Here is a simple and cheap method to protect the expensive electrical equipment of a CNC. The idea goes like this: The power of the CNC is delivered through a power relay. The 3-phase mains is monitored by a 3-phase voltage control relay and a 3-phase symmetry monitoring relay. If any of the above controllers detect a power failure, the main power relay is deactivated and the CNC is immediately disconnected from the power.
This will cause a permanent blackout to the machine. This blackout can still cause some problem to the machine, but not a very serious one. Most serious problems occur with a brownout and phase asymmetry. So, a blackout is a risk -usually- only for the workpiece and not for the CNC. It is always better to have a broken power supply than a broken drive or inverter...
First of all, let's see the material that i will be using for the protection circuit:
The schematic is very simple:
A push button will arm the relay, as long as the EU300 and EU302 are armed (no failure is detected). The 13-14 contact of the relay is used to keep the relay armed when the push button is released.
If any of the 2 EU relays is deactivated due to a power failure, the relay will be immediately deactivated. A green LED lamp indicates that the relay is armed, and a red LED lamp indicates that there is power to the first phase.
In the next page i show how make of this circuit