PowerFlex drive power output

[EC Dan]

I'm trying to wrap my head around the power output values that three PowerFlex 525 drives are reporting. All three drives are 20 HP. Two of the drives are driving 15 HP motors, one is driving a 20 HP motor. All three motors are driving PD blowers (belt-driven) into a shared air manifold, maintained at constant pressure via PID control. The speed output of the PID goes to each drive without modification. I expected the 20 HP motor to take on a greater proportion of the load than the two 15 HP motors, since at the same speed, the 20 HP motor should be producing more torque and power. However, what I see is all three drives reporting nearly identical power outputs, with varying current. See pic:



Blower 4 has the 20 HP motor. This doesn't show voltage, but they are all similar as well at around 325 V. Drives are configured correctly. Drives for blowers 1 and 3 have not been tuned, but Blower 4 has been. All run SVC.

These values do not align with my understanding of electric motors, although I've been trying to learn more about the topic; I'm currently reading Electric Motors & Drives by Hughes and Drury. I've also consulted Rockwell's tech note PowerFlex 7-Class and 520-Class, Drive Power Output Calculations, which shows how power output is being calculated. As I thought, it's just amps * volts * 1.73 * PF. Power factor is being calculated by firmware. In order for the power outputs to be so similar, the power factors would have to be different from each other in a very specific way, which doesn't seem likely. Does anyone have any insight into this?

 

[petersonra]

The power the motor uses is based on the mass of the air flow. Given that all three vfds are showing the same power, it is likely the air flow is being divided roughly equally between them. If the fans are the same this would be true even if you did not have vfds.

 

[Jraef]

As Bob said, the load determines what the motors draw, not the size of the motor (although that affects the flux current, which affects the power factor).

Side note:
You can’t trust what you see on the drive if it hasn’t been tuned to the motor.

 

[EC Dan]

The power the motor uses is based on the mass of the air flow. Given that all three vfds are showing the same power, it is likely the air flow is being divided roughly equally between them. If the fans are the same this would be true even if you did not have vfds.

The power output should be based on more than just the mass of air flow, right? Differences in the mechanical state of the blowers and air flow restrictions should result in varying power needed to move a fixed volume of air. I agree that each blower is putting out an equal amount of air, since the positive displacement cavity is the same between the blowers and they are all running at the same speed, however I wouldn't think power output would be so similar. And in fact, the differences do seem to manifest in the different amount of current required to move that fixed amount of air. If the calculation is as simple as amps * volts * 1.73 * PF, then the difference in amps and the similarities in voltage should lead to a difference in power output UNLESS the power factors differ in the EXACT way necessary to result in similar power output.

 

[EC Dan]

As Bob said, the load determines what the motors draw, not the size of the motor (although that affects the flux current, which affects the power factor).

Side note:
You can’t trust what you see on the drive if it hasn’t been tuned to the motor.

Can you elaborate on why voltage/current/power can't be trusted? My assumption is that the drive is directly measuring output voltage and current, and calculating the phase angle difference between the two wave forms in real-time for power factor. Is this not what is happening?

 

[petersonra]

The power output should be based on more than just the mass of air flow, right? Differences in the mechanical state of the blowers and air flow restrictions should result in varying power needed to move a fixed volume of air. I agree that each blower is putting out an equal amount of air, since the positive displacement cavity is the same between the blowers and they are all running at the same speed, however I wouldn't think power output would be so similar. And in fact, the differences do seem to manifest in the different amount of current required to move that fixed amount of air. If the calculation is as simple as amps * volts * 1.73 * PF, then the difference in amps and the similarities in voltage should lead to a difference in power output UNLESS the power factors differ in the EXACT way necessary to result in similar power output.

It's all about how much mass of air is moved. There are minor differences due to friction and other factors but they might be small enough to not be noticable.

The power formula does not control how much power is used. It's a reflection of power being used.

 

[EC Dan]

I can see now my assumption from the OP was incorrect:

I expected the 20 HP motor to take on a greater proportion of the load than the two 15 HP motors, since at the same speed, the 20 HP motor should be producing more torque and power.

I should not have expected the 20 HP to take on a greater proportion of the load. I could have put a 100 HP motor on that blower and it would still be moving the same mass of air at the same output power. The difference would be I would have more capacity to add load to the 100 HP motor by increasing the motor pulley or decreasing the blower pulley to drive more air from that blower.

 

[kwired]

I can see now my assumption from the OP was incorrect:



I should not have expected the 20 HP to take on a greater proportion of the load. I could have put a 100 HP motor on that blower and it would still be moving the same mass of air at the same output power. The difference would be I would have more capacity to add load to the 100 HP motor by increasing the motor pulley or decreasing the blower pulley to drive more air from that blower.

Something tells me if you had two 15 HP blowers and a 100 HP blower it won't balance out as well as it does with a 20 HP motor, at least not when near the upper output range of the whole system. Yes you still get more capacity out of it but if all the motors are drawing near full rating, the 100 HP is still adding more air into the system as it likely is a much bigger blower wheel. The difference in blower wheels between the 15 and 20 might not really be all that significant and just a small RPM difference could result in them having about the same load on them as loading is exponentially proportional to speed on a centrifugal fan.

 

[petersonra]

Something tells me if you had two 15 HP blowers and a 100 HP blower it won't balance out as well as it does with a 20 HP motor, at least not when near the upper output range of the whole system. Yes you still get more capacity out of it but if all the motors are drawing near full rating, the 100 HP is still adding more air into the system as it likely is a much bigger blower wheel. The difference in blower wheels between the 15 and 20 might not really be all that significant and just a small RPM difference could result in them having about the same load on them as loading is exponentially proportional to speed on a centrifugal fan.

as long as the blowers themselves are the same size, the motor size is not especially significant to the amount of power used to push the same mass of air.

 

[kwired]

as long as the blowers themselves are the same size, the motor size is not especially significant to the amount of power used to push the same mass of air.

correct. They will still have same mechanical load if the blowers themselves are the same thing and operating at same speed. If the 20 HP blower is actually a little different it could possibly still move the same amount of air with not much of a difference in speed as the 15 HP blower does. But other factors could also restrict air flow making performance a little different as well. Maybe a back flow damper in each one of them so if one isn't running you don't compromise the pressure and flow, but maybe it doesn't open as far as others for some reason, or some gate is intentionally there for adjusting performance and is set to where the 20 HP really has more motor than the blower will ever utilize? several possibilities here I would think that help equalize the OP's setup even though one of them has a larger motor driving it. This was all assuming direct drive blowers, if you have mechanical drive components between motor and blower there is more possibilities of what may be different there.

 

[Jraef]

Can you elaborate on why voltage/current/power can't be trusted? My assumption is that the drive is directly measuring output voltage and current, and calculating the phase angle difference between the two wave forms in real-time for power factor. Is this not what is happening?

When a drive is in SVC mode, it is essentially separating the flux producing current form the torque producing current in real time during each AC cycle, so that it can maximize the torque producing current without saturating the motor core. To accomplish this, the drive needs to fully know all of the electrical properties of the motor so that it can use the motor equivalent circuit in detail, typically done via the Autotune procedure (it could be done manually if you know all of the data points, but people rarely do). Without the drive knowing the parameters of the motor equivalent circuit, it can only guess at what it takes and that usually results in inaccuracies in the motor performance and data.