Nowadays, using variable frequency drivers is the most common way to control AC motor´s speed in industries worldwide, looking to avoid DC motors in the system and therefore eliminated the maintenance of the brushes and the expensive control systems.
Once it’s decided to install a VFD in the process whichever it might be, developing engineering for its proper installation is need, and the following article highlights some of the factors that have to be considered.
In first place is important to have in mind during the engineering process that the installation can be one of the two following:
1. The VFD will be installed with a motor that has been design to run with a VFD. The article will focus on this option.
2. The VFD will be installed with an old motor that used to run without speed control, and it was not design to work with a VFD. Considerations in this case will be analyzed in an up-coming article.
For the first case New Motor – New VFD, the following points shall be taken under consideration:
1. Location of the VFD.
2. Distance from VFD to Motor (total wiring route).
3. Type of cable from the VFD to the motor.
4. Kind of wiring installation.
5. Installation of control wiring.
6. Electrical protection against short-circuits and terminal overload.
7. Harmonics generated by the VFD.
8. Effects of reflected waves.
9. High frequency Foucault currents between the motor and the pump or compressor.
10. VFD autonomy when faults in the main power feeder.
1. Location of the VFD.
A VFD is an electrical equipment with electronic components that generate a lot of heat, therefore it has to be notice that the VFD should have internal fans to help the refrigeration, the environmental conditions play a key factor in the engendering process for this matter.
2. Distance from the VFD to the Motor.
The distance in this case is no relate to the voltage drop that it produces, it is suppose that the cable has been selected accordingly to this considering the distance and the installation type. In this case is important to be aware of the distance in order to eliminate any reflections in the tension wave, which may be present as a result of long distances between VFD and Motor. The distance from the VFD and the motor (wiring distance) should be always informed to the manufacture, depending on the technology used, filters may be needed in the arrangement.
3. Cable used to feed the motor
There are several literatures regarding this matter, at the same time some manufactures design cables to be used exclusively with from the VFD to the motor and is one of the most expensive items in the project.
The cable that goes from the CCM to the VFD can be a regular cable. The cable that goes from the VFD to the motor have to be special for the application, further analysis on this subject will be presented.
Control the AC motor speed using a VFD is something that has become more frequent over time. Let’s reason the cable design more suitable to avoid the troubles that come with it, the manufactures usually present this information to the client but sometimes are not considered as a result of negligence or ignorance of the actual problem.
4. Power Cable Wiring
The cable should be installed apart from other feeders of the system. Different VFD cables to motors can be installed in the same route. It is recommended that the cable from the CCM to the VFD, VFD to Motor and control cables are installed separately.
It’s important to avoid that the cable to the motor goes parallel to other cables for long distances, this to avoid electromagnetic interferences produce by changes in the voltage at the VFD.
If the control cables and the power cables have to crus at certain point, be sure that they do it with an angle of 90 degrees from each other.
The air way trays shall have a good electrical connection between them and respect the grounding system. It’s recommended to use aluminum air ways to level the potential more efficiently
5. Selection and routing of control cables
Similar to the power cables they should go through separated routes.
All control cable shall be shield.
Shall be used a twisted par with double shield cable for analog signals. This kind of cable is also recommended for the signals of the pulse generator. Use an individual pair for each signal. Do not use a combined return for different analog signals.
The best alternative for low voltage digital signals is a cable with double shield, but a several cables with simple shield can also be used.
Analog and digital signals should be transmitted through separated shield cables.
The signals controlled by a relay can be transmitted through the same cable used for digital entries, as long as the voltage is less than 48 V. It is recommended that the signals controlled by relay are transmitted using a twisted pair cable.
24 V DC and 115/230 V AC signals never have to combine in the same cable.
See it in Spanish
E.g. Enrique Paradas
