CNC Repair Basics: Electro-mechanical Relays
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Before you can really do any effective electrical troubleshooting during machine repairs, it would be best to have a basic understanding of relays and how they work. Most importantly, the difference between relay coils and contacts. Hopefully, this will make sense for those who may not understand the difference.
Functions of an Electro-mechanical Relay:
Several reasons exist for using an electro-mechanical relay in a circuit.
1. Isolation of voltage sources
2. Protection of circuit boards
3. Control of several devices simultaneously
4. Latching and breaking circuits
Construction of an Electro-mechanical Relay:
A typical electro-mechanical relay has a coil (rated at a particular voltage and AC or DC) and contacts. The contacts are also rated according to their load carrying capacity. The relay may contain one or more sets of contacts. These contact may be normally open (NO), normally closed (NC) or both may be seen on one relay. “Normally” refers to the condition of the contacts when there is no voltage applied to the coil. Please review Fig. 1 below:
Fig. 1
The contacts shown in this particular drawing are normally open. When voltage is applied to the coil of the relay, a magnetic field is created. This magnetic force pulls the moveable armature and, as a result, the contacts are closed (or connected). When this happens, power is applied to the load. If the contacts of the relay were normally closed (N.C.) the power would be applied to the load until power is applied to the coil. At that point the moveable armature would open (disconnect) the contacts.
Coils and Contacts:
One important concept to remember is that the coil and the contacts are two separate things. The coil may be rated at 24VDC, but the voltage passing through the contacts may be 100VAC. They never cross. One really has nothing to do with the other with the exception of the fact that the energizing of the coil results in a change of state with regard to the contacts (that is, N.C. contacts open and N.O. contacts close).
RELAYS AND BASES
As mentioned earlier, a relay may have more than one set of contacts. The contacts may be normally open, normally closed, or it may have both. The relay will typically have a series of pins protruding from the case. These pins plug into a relay base or socket. The base provides the ability to connect wires to the relay coil and contacts via screw terminals on the base. In most cases, there will be a diagram on the relay showing the contacts. The diagram will show which terminal number on the base corresponds to which contacts on the relay (when the relay is plugged into the base). Also shown will be the connections for the coil, the coil voltage, and the current rating of the contacts. Examine the following:
14 PIN RELAY AND BASE
Simple Relay Logic Circuit:
The following is a simple circuit using 2 momentary push-button switches and 3 relays. This is a latching circuit:
Function of the above circuit is as follows:
1. N.O. push-button PB1 is pressed
2. Relay coil R1 turns on, but only while PB1 (momentary) is pressed
3. N.O. contacts on R1 turn on relay coil R2
4. When coil R2 turns on, the N.O. contact R2 also turns on
5. Even when PB1 is released, coil R2 is “latched” on. As long as power is applied to the circuit, contact R2 will remain closed and coil R2 stays on
6. In order to turn off R2, N.O. push-button PB2 must be pressed to turn on relay coil R3. N.C. contact R3 opens, turning off coil R2. This is the “latch breaker”. The circuit is then reset to the initial condition
When the coil of a relay turns on, its contacts change state as a result. Normally closed contacts open and normally open contacts close. This is the basic concept on which a ladder diagram is written.
This is an excerpt from the book: “Pinpoint Troubleshooting: CNC Machines with Fanuc Control”. For more information go to www.pinpointcnc.com.
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