brucechase
Member
PeterW said:The only contactor required for an inverter, would be the power on contactor, which could be the safety contactor, which would be BEFORE the inverter.
Doug is quite correct, you should not put a contactor between the inverter and the motor. You use the control signals to RUN/STOP the motor, not the contactor.
Overloads also go before the inverter as you are protecting the inverter, the inverter protects the motor.
Where the inverter supplies multiple motors, each motor should have its own protection after the inverter (in addition to the inverter protection), but if one trips the inverter should be stopped.
We had a HMI selection where we selected if a drive was enabled or not, therefore if one had a fault and tripped the feedback would be different to selection and the inverter commanded to stop. If the motor had a fault, then the HMI would be used to de-select that drive to enable restart without it. The breaker would be locked out. If someone turned on a breaker that was de-selected then again the inverter stopped.
Completely disagree on this!
Several drive manufacturers have instructions on putting contactors AFTER the inverter and how to control them. I do this when safety application demand it. In some situations, I've installed 2 contactors after an inverter. I have NEVER had an inverter fail because of this.
All overloads go between the inverter and the motor - not before the inverter. (I'm not sure why you would use one if you are only controlling 1 motor since all drives I've seen have an overload function built in.) As a matter of fact, most overloads won't work properly before an inverter since it has a dc bus that can charge and discharge. If there are multiple motors, each motor get an overload and that is between the drive and motor.
A circuit breaker or fuse goes before the inverter to protect the inverter in case there is a SHORT CIRCUIT. The circuit breaker protects the wiring and components in the drive and motor.
There are many other things I disagree with when it comes to the OP. Switching the negative in NOT illegal, especially if the DC common is not grounded (as I have in many cases). How would this apply in sinking/sources applications if you can't switch the negative? A sensor or other I/O device is nothing but a switch to the plc.
Stationmaster:
At least the plant I was referring to requires a bell to be rung 3 times before starting. I guess if no one yells during the startup bell they figure it's ok to start the system. I know they have power lockouts near the motors and conveyors....if they use them. I try to keep my head buried in a panel and stay out of the machine area if I can. It's out of my control......I just get called when needed. The owners/managers make those decisions, I just try to make the control system do what they want it to.
That used to be the way the conveyors were controlled here. I changed them to sound the alarm for 4 seconds and then require another push of the start (after the 4 seconds but before 8 seconds) before they start. My reasoning is if someone yells, then the conveyors won't start automatically so someone would have to hit the stop button. We also have estop ropes (with slack detection) and a red light on each switch so when it is activated, it can be seen for hundreds of feet away.
I'm also not sure what prompted this but my general warning is to take what is said with a grain of salt. I don't follow Doug's "rules" in many cases and think that some of his rules would violate modern safety practices. As Ron point out, Risk Assesment wtih the proper applicaiton of safety categories is the "proper" way to design a system.