PaulKraemer
Electrical
- Jan 13, 2012
- 145
Hi,
I am trying to troubleshoot a problem on a printing press which has a variable frequency drive that controls an AC Motor that drives the main drive shaft for the press. This shaft drives all of the driven rolls on the press. The speed reference for the drive is set by a potentiometer (pot) that is wired to an analog input on the drive. There is no encoder or other type of speed sensor wired to the drive.
Anyway, this press (and the drive system), has worked fine for many years. Recently, we have started having an issue where initially, we set the pot so that the press runs at our desired speed. It runs at this speed for 5-10 minutes, but then without changing the pot, the drive starts to gradually slow down. After about 15 minutes, it is about 50% of its original speed.
The drive is an ABB ACS 601. I have been in touch with ABB tech support and they have asked me to check a few things. There are read only parameters that allow you to monitor measured motor current and calculated torque. The displayed values fluctuate rapidly, with the current ranging between 13-15 amps and the torque between 25-30%, but I see no noticeable change in these when we experience our gradual slowdowns. I monitored the 0-10V signal from my pot to the analog input to verify that this does not decrease. I also monitored the output to the motor, setting my meter to display AC voltage and frequency. The AC voltage was very steady at 115 VAC and I saw no gradual decline in this. The frequency fluctuated rapidly between 350-450 Hz, but I never monitored frequency from a VFD like this before. Correct me if I am wrong, but I believe these drives operate at a high carrier frequency in the way that they use a DC bus to create/simulate an AC sine wave at the desired frequency. When I try to monitor this with my meter, I wonder if the meter is thrown off by the high carrier frequency combined with the simulated AC sine wave frequency. In any event, even though my measured frequency did fluctuate quite a bit, I saw no change in this as we experienced our gradual slowdown.
I will continue to follow up with ABB, but I wonder if anyone here can help me get a better conceptual understanding of what I am dealing with. My understanding is that a typical VFD varies both voltage and frequency linear fashion, with higher voltage and higher frequency resulting in faster speeds. Considering that our drive does not make use of an encoder or other type of speed sensor, I would consider this open loop control. I would assume that there is a more or less linear relationship between the analog input from my pot (0-10 VDC) to both voltage and frequency. If I adjust my pot to get the speed I want and don't touch it, I assume that the drive will put out a fixed voltage at a fixed frequency (Voltage A at Frequency A). What confuses me is, what happens if the load changes? For example, if Voltage A at Frequency A results in Speed A at Load A, what would I expect if the load increases. Would I expect the Speed to decrease? Or is a typical VFD without a speed sensor able to detect this situation and compensate to keep speed constant?
I am just wondering if something mechanical is going on that is causing the load to increase over time (maybe as a component heats up), and if this is the case, does our drive have the capability to compensate for this or would a slowdown be expected. If load was increasing, my gut feeling is that I would see increases in current and torque, but I am not entirely sure about this. Also, as this drive system is pulling a web (paper) through the printing press, I would think a load increase would lead to increased tension on the web, but I see no sign of this.
Any input will be greatly appreciated.
Thanks in advance,
Paul
I am trying to troubleshoot a problem on a printing press which has a variable frequency drive that controls an AC Motor that drives the main drive shaft for the press. This shaft drives all of the driven rolls on the press. The speed reference for the drive is set by a potentiometer (pot) that is wired to an analog input on the drive. There is no encoder or other type of speed sensor wired to the drive.
Anyway, this press (and the drive system), has worked fine for many years. Recently, we have started having an issue where initially, we set the pot so that the press runs at our desired speed. It runs at this speed for 5-10 minutes, but then without changing the pot, the drive starts to gradually slow down. After about 15 minutes, it is about 50% of its original speed.
The drive is an ABB ACS 601. I have been in touch with ABB tech support and they have asked me to check a few things. There are read only parameters that allow you to monitor measured motor current and calculated torque. The displayed values fluctuate rapidly, with the current ranging between 13-15 amps and the torque between 25-30%, but I see no noticeable change in these when we experience our gradual slowdowns. I monitored the 0-10V signal from my pot to the analog input to verify that this does not decrease. I also monitored the output to the motor, setting my meter to display AC voltage and frequency. The AC voltage was very steady at 115 VAC and I saw no gradual decline in this. The frequency fluctuated rapidly between 350-450 Hz, but I never monitored frequency from a VFD like this before. Correct me if I am wrong, but I believe these drives operate at a high carrier frequency in the way that they use a DC bus to create/simulate an AC sine wave at the desired frequency. When I try to monitor this with my meter, I wonder if the meter is thrown off by the high carrier frequency combined with the simulated AC sine wave frequency. In any event, even though my measured frequency did fluctuate quite a bit, I saw no change in this as we experienced our gradual slowdown.
I will continue to follow up with ABB, but I wonder if anyone here can help me get a better conceptual understanding of what I am dealing with. My understanding is that a typical VFD varies both voltage and frequency linear fashion, with higher voltage and higher frequency resulting in faster speeds. Considering that our drive does not make use of an encoder or other type of speed sensor, I would consider this open loop control. I would assume that there is a more or less linear relationship between the analog input from my pot (0-10 VDC) to both voltage and frequency. If I adjust my pot to get the speed I want and don't touch it, I assume that the drive will put out a fixed voltage at a fixed frequency (Voltage A at Frequency A). What confuses me is, what happens if the load changes? For example, if Voltage A at Frequency A results in Speed A at Load A, what would I expect if the load increases. Would I expect the Speed to decrease? Or is a typical VFD without a speed sensor able to detect this situation and compensate to keep speed constant?
I am just wondering if something mechanical is going on that is causing the load to increase over time (maybe as a component heats up), and if this is the case, does our drive have the capability to compensate for this or would a slowdown be expected. If load was increasing, my gut feeling is that I would see increases in current and torque, but I am not entirely sure about this. Also, as this drive system is pulling a web (paper) through the printing press, I would think a load increase would lead to increased tension on the web, but I see no sign of this.
Any input will be greatly appreciated.
Thanks in advance,
Paul
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