flumoxed by flux vector control
flumoxed by flux vector control
(OP)
I am uncertain about my understanding of open loop vector control with a VFD. My take on it is that the drive makes a motor speed calculations/adjustments based on recorded motor parameters and measured I, V and their phase relationships.
Is this close?
We have an application which is giving us some trouble.
The motor speed readback from our open loop vector application is very noisy. At first we investigated instrumentation; then considered noise; It is in fact real variations in the VFD output.
The load is a 2 cylinder, reciprocating gas liquifier. Pistons are connected to a flywheel, which is coupled to the 7.5HP AC motor by 1:6 pulley/v-belt arrangement.
Because of the 300 psia input pressure, the motor functions as a brake. This means the motor is always being over driven. We are using a regenerative braking device but at low speeds(Heavy load) there is almost no regeneration. The apparent speed variation, as reported by the VFD, is ~10% of the nominal speed(300RPM min-1500RPM max)
Is it possible that the over-driven condition of the motor is confusing the VFD's vector calculations?
Am I all wet?
Am I waiting for some thoughtful input?
You betcha.
Is this close?
We have an application which is giving us some trouble.
The motor speed readback from our open loop vector application is very noisy. At first we investigated instrumentation; then considered noise; It is in fact real variations in the VFD output.
The load is a 2 cylinder, reciprocating gas liquifier. Pistons are connected to a flywheel, which is coupled to the 7.5HP AC motor by 1:6 pulley/v-belt arrangement.
Because of the 300 psia input pressure, the motor functions as a brake. This means the motor is always being over driven. We are using a regenerative braking device but at low speeds(Heavy load) there is almost no regeneration. The apparent speed variation, as reported by the VFD, is ~10% of the nominal speed(300RPM min-1500RPM max)
Is it possible that the over-driven condition of the motor is confusing the VFD's vector calculations?
Am I all wet?
Am I waiting for some thoughtful input?
You betcha.





RE: flumoxed by flux vector control
Back to the issue at hand, I am wondering if your problem may be a tuning issue. Are the speed variations periodic? In other words, is there a repeating pattern in the speed variations?
I am not directly familiar with your application, but I imagine that it would be very similar to a 2 cylinder air compressor except that it is a regenerative load(?).
If so, your motor will see a load cycle of 2 periods of high load separated by periods of somewhat light load for every six revolutions. Although a flywheel is present, it's dampening affect will not be complete (to do so would require an impractically large flywheel compared to the actual load). In addition, the drive belts will have some elasticity based on the type used and the amount of tension applied, making the system's response somewhat soft or spongy. If the VFD is trying to speed regulate itself with this type of load it may be overshooting the mark, causing a periodic (repeating) fluctation in speed. If this is the case you may reduce the fluctations by tuning the drive PID loop.
I hope this helps...
RE: flumoxed by flux vector control
The varying load from gas expansion was the cause of over regulation of the VFD output. Fluctuations in the high pressure gas supply and the output sink pressure were complicating matters but when these were factored in, the correlation of "noise" to piston stroke was obvious. We managed to massage the PID loop to the point where regulation, response and readback are all acceptable.
RE: flumoxed by flux vector control
1. Simulation by suitable software could be used to verify your PID fine-tuning
http://www.speedcad.com/
2. A flywheel could be added
3. Caution could be exercised since the fine-tuned PID does not account for potential future irregularity in the system functioning. There are no guarantees.