Better Idiots and VFD 0hz

[ptonsparky]

I went to one of my projects today that was sending me alarms. After a few minutes of looking at the PLC I asked them how & the golly gee darn were they operating at all. Answer was something like "well this part here quit on this, but it's still good on that, but that is bent on that so we took the hydraulic output from this to run that". I left. Told them there was no fix. They were on their own.

However, I am curious about part of the results. The programming is commanding the VFD to operate at 0 hz. How tough is this on the motor? Or is it at all?

 

[briank]

Zero is Zero so as long as the motor is not being spun by the load its no big deal.

 

[Smart $]

...

However, I am curious about part of the results. The programming is commanding the VFD to operate at 0 hz. How tough is this on the motor? Or is it at all?

Depends on how much current.

Is it an AC/DC hybrid?




If it is, iwire will get right on it...:rotflmao:

 

[Jraef]

Depends on what "this and that" was. There are some drives out there which have an option to use the drive as a way to keep a motor warm to prevent condensation, a form of "motor winding heater" by putting DC power through it at controlled low current values but at zero Hz. As long as that current is low enough to not do damage to the insulation, no problemo.

There is also a control option with many drives on centrifugal pumps when using their internal PID control loop called "Sleep Mode". In a pump, at some point as you lower the speed you have lost the head pressure output from the pump needed to overcome the system head pressure, so your pump stops pumping even though it is still spinning and consuming energy, so there is no point in having it run any more. So the drive goes to "sleep" meaning it is still thinking (dreaming?) and monitoring the PID loop, but commanding zero Hz to the motor so that it doesn't waste any energy by spinning uselessly. But when the PID loop detects a change of enough magnitude to demand a response, it doesn't have to begin from an Off state, it just changes the speed to some minimum value and takes off.

 

[ptonsparky]

The this and that were other pieces of equipment that relate to the commanded hz, but 0 was not supposed to be a result. :blink:
The load of the motor does not coast. Motor stops, load stops.

 

[iwire]

If it is, iwire will get right on it...:rotflmao:

:lol:

Sorry buddy, maybe the next post will need some vectors and phasors worked out.

 

[Jraef]

The this and that were other pieces of equipment that relate to the commanded hz, but 0 was not supposed to be a result. :blink:
The load of the motor does not coast. Motor stops, load stops.

Ah. Depending on programming in the drive then, it may be that it has detected a loss of speed reference, and is programmed to go to zero Hz as a safety precaution. If the PLC is communicating to the drive over a network, it might be a network loss or bad addressing. If the speed command is coming from an analog signal it might just be the loss of the signal, or even if the signal value is too low (again, a form of "sleep mode"). On many drives you can find a parameter that tells you what the analog input value is.

See of what the PLC is saying to the drive matches what the drive thinks it is hearing. Sometimes drives and PLCs are like a married couple in that way...

 

[ptonsparky]

Ah. Depending on programming in the drive then, it may be that it has detected a loss of speed reference, and is programmed to go to zero Hz as a safety precaution. If the PLC is communicating to the drive over a network, it might be a network loss or bad addressing. If the speed command is coming from an analog signal it might just be the loss of the signal, or even if the signal value is too low (again, a form of "sleep mode"). On many drives you can find a parameter that tells you what the analog input value is.

See of what the PLC is saying to the drive matches what the drive thinks it is hearing. Sometimes drives and PLCs are like a married couple in that way...

Looking at the programing this morning and 0 hz should be what is called for under the conditions they have managed to obtain.