Some Key Power Quality issues With CNC and Optimal Solutions:

Computer Numerical control Machines (CNC) are used to shape metal Parts by milling cutting and grinding.

A CNC consists of some critical Parts, Mechanical, electrical, electronic and digital circuits, Like servo motors. Spindle Motors, amplifiers, Programmable logic controllers (PLC) and Drives and various tooling. The machine can be programmed to shape a part by use of a numerically controlled front Panel. Most CNC machines can be uploaded with CAD drawings.

 The Electronic components within a CNC machine are particularly sensitive to the grounding techniques and the quality of Power supply to the machine. The quality of power rich in Harmonics is a major source of disturbance that adversely affect the performance and life span of the CNC machine. The maintenance cost, cost of break down contribute to the Total cost of ownership of the CNC machine.

Some of the critical elements of Power quality are;

a. Grounding and Earthing

b. Harmonics

c. electrical noise (differential and common mode noise)

Grounding and Earthing:

The intent of Grounding electrode conductor and grounding electrode system is to provide a low impedance Path to earth for lighting surge current and to reference the building electrical system to the surroundings. This minimizes the voltage difference between exposed metal Parts of the power including connected equipment and the surrounding parts of the building thus reducing the hazard of electric shock.

The fundamental purpose of equipment Grounding is to provide equipment and personnel safety, and the secondary purpose of equipment grounding is to enhance equipment performance by providing a reference for the electronic components.

A proper combination of equipment and system grounding should ensure that a ground fault with a solidly grounded electric system, the low impedance path provides the fault current flow back to the source. With sufficient ground fault current, the overcurrent protection device will trip quickly and clear the ground fault.

Electrically the AC power is usually converted to several levels of DC volts for the servo systems and computer logic. The different reference points may increase the CNC machines sensitivity to Power disturbances.

Today electronically controlled CNC machines require a common signal reference ground to logical circuits to operate reliably. The term signal reference ground for logical circuits eg. The 5 volts DC above signal indicates a logic level ‘1’ and zero volts DC signal reference indicates ‘ 0’, this signal reference is typically the common computer supply, which is connected to the CNC machine ground plate. The ground plate is bonded to the machine enclosure. Equipment grounding conductor or conduit and subsequently to the building grounding electrodes system. So the equipment ground establishes the local signal reference both within the CNC machine and for any remotely connected device.

Susceptibility of CNC machines to different sources of noise and stray currents will depend on how the machine is designed and installed, Grounding bonding and shielding also play a role in Immunity. Some communication circuits are susceptible to high frequency noise , others are susceptible to stray ground currents. However fiber cables are not susceptible to noise.

Electrical noise in CNC:

Contactors relays, solenoids and motor drives are a large source of noise and transients, which could affect the logic circuits. CNC machines that are contained in single cabinet with good bonding, grounding should be relatively immune to noise. However CNC with remote controllers or data links may be quite Susceptible.

Good Electrical Practice and Optimized Solution:

Installation of phase shifted and Shielded Isolation transformer will the most optimized solution for the following reasons. Commonly Isolation Transformer K rated with harmonic mitigation is commonly Installed. However Phase shifted (30 degree) Isolation transformer will also mitigate the Harmonics (5th and 7th) generated by drives can be mitigated and the need for oversizing and overrating is not required as in the case of K rated Transformer.

RECOMMENDED PRACTICE FOR GROUDING A CNC MACHINES

The Multiple benefits is as follows:

1.      The shielded Isolation transformer helps to filter electrical noise ( common Mode) and reduce voltage transients.

2.      It provides a low impedance path for fault current, allowing the proper function of over current devices.

3.      A new signal reference is created by the connection of the neutral of the transformer secondary to the equipment grounding system.

4.      Large currents such as those caused by lighting and utility faults flowing to and from the earth at the shielded Isolation transformer earth connection should not adversely affect the operation of the CNC, Since the current flow does not pass directly through the CNC machines enclosure.

5.      Phase shifting the secondary of Isolation transformer by 30 degree will mitigate the 5th and 7th Harmonics generate by drives. This improves the performance of the drives by mitigating the 5th and 7th Harmonic by 80%. 5th being negative sequence can cause torsional vibration and premature bearing failure.

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