Electric Motor Control

[jei 0]

Hi. I have decided to learn and play with motor control using my FX2N plc. I want to connect to a three phase 240v 0.5hp induction motor. I may be able to get my hands on a telemechanique altivar 28 vsd which outputs 3 phases, 208/230V, 0.5HP and has 3.3A ouput current. My plc outputs are 2A relays, 30vdc/240vac. Can someone tell me if im on the right track, if there is an issue applying this setup with relay outputs and also, what do i need to look for when selecting a contactor to go in between the the plc and vsd. Thanks.

[Leitmotif 0]

THe nice thing with PLC controllling a VFD is that you have complete separation between the power (3 phase) and contrl circuits which are low current or low voltage such as 4 20 mA and 0 to 10 volt. Standalone the VFD can do all motor control start stop speed control reversal and protection ie motor overload. Normal applicatins require a branch circuit with proper over current protection sized IAW with VFD ratings. So you do not need relays between PLC and VFD. In your case I would recommend a 3 pole switch to act as E stop in case you make a mistake - if you are not making mistakes you are not learning. You may need only two pole if you are feeding VFD with single phase. Do NOT put a relay between VFD and motor. Read both the VFD and PLC manusals to see how to get the PLC to control the VFD. Dan Bentler Edited 20 Mar 2011 by Leitmotif

[Sergei Troizky 26]

PLC operates AC Inverter (a.k.a. VFD- Variable Frequency Drive) by logic level control signals, typically 24VDC, not by controlling the inverter input power. Status feedback signals from inverter to PLC are also of the logic voltage. Relay PLC outputs can produce control signals for inverter, as well as transistor ones of proper polarity. However, a relay unit has no high-speed outputs, and will not allow to use PWM signal from PLC for inverter speed control. A contactor on the drive power input is a safety disconnect, and as such should be controlled by hardware circuits, not by PLC. Edited 21 Mar 2011 by Sergei Troizky

[jei 0]

Thanks heaps for the explanation guys, you gave me a much clearer picture. I will find my AC inverter and study the manuals.