I should start by saying that I am new to the forum and don't have any formal training working with VFDs. So if I say something foolish, I'll apologize in advance. I have an existing installation that uses an Allen Bradley VFD on an agitator. Because of safety concerns, we have added a local disconnect near the motor. The control system monitors the VFD frequncy and current outputs to indicate that the agitator is running. However if the local disconect is left off, the motor does not run but the contol system shows the agitator is running; i.e. the current output is not equal to zero and the frequency does not show zero. The current output signal from the VFD not being zero when the motor is not connected is confusing to me. I now understand that the frequncy output signal from the drive indicates what it is attempting to do, even though the motor isn't connected. I had wrongly assumed that the frequency output was an indication of the actual speed not the "wanted" speed. My question is: What is the best way for the control system to identify based on signals from the control system that the disconnect has been left in the off position when the control system is attempting to run the agitator, so that it can alert the operator about the issue. Thank you.
Disconnect Switch with Allen Bradley 1336 VFD
- Thread starter WesternPA
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- Massachusetts
I would likely buy an auxillery switch kit for the disconnect switch.
Wire it into the VFD stop or into a PLC input.
Wire it into the VFD stop or into a PLC input.
- Location
- Placerville, CA, USA
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- Retired PV System Designer
If the aux contact opens/closes before the main contacts, it can also serve to protect the motor and VFD if someone opens the disconnect while the unit is running.
Tapatalk!
Tapatalk!
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
Frequency not being zero is normal, the VFD does not know if the motor is disconnected in that respect, that output is just telling you what the speed COMMAND is in the VFD.**
The current not showing as zero when the disconnect is open is however a problem. There are two main possibilities here:
1) You are not using the Analog Outputs of the VFD as the "current" value and instead, someone has added a CT or other current transducer AHEAD of the VFD, which is showing the LOSSES inside of the VFD as if they are motor current.
2) You are using the analog output of the VFD (and assuming it is programmed for "Motor Current"), but someone has not properly adjusted the span and offset of that output signal to truly reflect the actual motor current. In other words, the motor current is zero (has to be if the disconnect is open), but the AO is transmitting something other than 0VDC or 4mA (depending on what the AO is programmed for) to your control system (or the control system is not properly calibrated to know that 4ma = zero speed).
Side issues to #2:
A) The AO you think is "Motor Current" is actually something else. That drive can have the 2 analog outputs programmed to reflect Speed, Current, Volts, Torque, or Power. If it is transmitting volts for example, that would be DC bus volts, which would not be zero even if the disconnect down stream were open.
B) You have poor shielding or shield connections on those analog signal wires and they are picking up noise, which the control system is interpreting as being something other than zero current.
** Depending on the age and version of 1336 it is (there were 4 versions), the drive may or may not be capable of Open Loop Vector Control. If it is not tripping off when the disconnect is opened, that indicates that it is not capable of that, or that feature is not being used. In Vector Control, the VFD would know that the motor is disconnected and trip.
Aleman
Senior Member
- Location
- Southern Ca, USA
As the others mentioned, wire an aux contact on the switch and tie it in with your stop/start or enable. Get a copy of the manual,
available free on the net if you don't have one.
What to use for a run signal?
Typically there will be an aux relay output on drives that can be set for alarms, run, etc. Depending on what your control system uses
for an input, you could add in a disconnect aux contact with that signal so the drive won't show a run when you have the disconnect off.
There are many ways to achieve the same result, it will vary by application.
available free on the net if you don't have one.
What to use for a run signal?
Typically there will be an aux relay output on drives that can be set for alarms, run, etc. Depending on what your control system uses
for an input, you could add in a disconnect aux contact with that signal so the drive won't show a run when you have the disconnect off.
There are many ways to achieve the same result, it will vary by application.
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