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 My Job - Mains Power Failure Detection and Protection for CNC AuthorGiorgos LazaridisJune 24, 2012

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The top-half of the CNC cabinet costs one third of the overall CNC price...
Usually, CNC machines do not come with mains power failure protection. The customer is responsible to have stable and symmetrical 3-phase power within a certain voltage limit.

There are several power failures. Namely:
• Voltage spike: (AKA Power surge) When voltage in one or more phases increases rapidly and then returns to normal
• Transient fault: When voltage in one or more phases drops rapidly and returns back again
• Blackout: When voltage in one or more phases becomes 0 never to come back
• Brownout: When voltage in one or more phases is decreased for a long period of time

• 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...

The schematic
First of all, let's see the material that i will be using for the protection circuit:

 These are the two monitoring relays. The Hager EU300 which monitors the phases for asymmetry. And the Hager EU302 voltage monitor relay. This 26KW relay is enough to control the power of the 17KW CNC

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

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