calculating motor power output/micrologix to ac inverter with modbus

[eccan]

I am wanting to display the ac motor power being generated by an AC Inverter. I have successfully created a modbus network between a Micrologix 1400 and a Durapulse AC drive, and now get can monitor the drive Output Current, Output Voltage and Power Factor Angle in my PLC. (If anyone wants info on this modbus setup, let me know).
Am I correct in now using the following formula to calculate the output power of my motor?
Power = V x I x 1.732 (for 3 phase) x PF ?

Do I need some adjustment for motor efficiency?

Maybe this formula would be closer:
HP=1.732 x I x E x Eff x PF / 746


Thanks

 

[Tom Jenkins]

Actually, I'd be surprised if the VFD didn't already have power calculated and available. If you have Modbus communications established you should just need to determine the register that has the power value and read it over Modbus.

The first formula is correct, but it will give power in Watts. The convention is to typically measure motor power in kW, and kW = IxVx1.73xPF/1000.

 

[eccan]

Thanks Tom
The drive only has a % load status register, which is just Output Current/Drive Rated Current x 100

My formula doesn't quite work right however, because I think the Power Factor angle the drive is reporting is odd.

With no load at 900 RPM, I have a output voltage of 246.9 Volts, output current of 21.9 amps and a power factor of .791 (modbus reports this as 791, so I am assuming it is 79.1% or .791 for the calculations). This gives me a kW calculation of 7.4

With a load on the motor at 900 RPM, I have an output voltage of 246.9, current of 29.7 amps and a power factor of .496
This gives me a kW calculation of 6.3.

It appears that when loaded, the PF goes down more than the current went up, which gives me a smaller kW value

Am I using this reported power factor angle wrong in the formula? I would think PF would be higher when loaded.

 

[Tom Jenkins]

OK - you are getting way above my level of expertise. I'm hoping DickDV will chime in!

I do know that there are lots of things going on on the VFD output and motor that may impact your calculations. The waveform is non-sinusoidal. The voltage drops as Hz drops. I think some of the current is coming off caps in the DC bus and off the motor inductance(?), so I think there isn't a 1:1 correlation on current into the VFD and current to the motor.

Dick?

 

[sthompson]

OK - you are getting way above my level of expertise. I'm hoping DickDV will chime in!

I do know that there are lots of things going on on the VFD output and motor that may impact your calculations. The waveform is non-sinusoidal. The voltage drops as Hz drops. I think some of the current is coming off caps in the DC bus and off the motor inductance(?), so I think there isn't a 1:1 correlation on current into the VFD and current to the motor.

Dick?

Can you post the exact make and model of VFD you are using? Better yet, a link to the Manual. I am with Tom on this one, the VFD should know how much Power it is delivering to the Motor.

Stu.....

 

[DickDV]

I'd go with Tom and Stu on this. The drive almost certainly knows the actual power and can report it over the bus somehow. Current isn't even a good indicator of torque, much less power. More than likely, the percent load display is percent of nameplate torque. Still not a good indicator of power.

Knowing what brand and model of drive just might get us someone familiar with it.

Please advise.

 

[eccan]

Thanks
It is a Durapulse (Automation Direct) GS3-4030 (30HP)

http://www.automationdirect.com/static/manuals/gs3m/gs3m.pdf

Here is the status info it reports



The only other modbus data It seems to have are it's parameters

 

[sthompson]

Well, it appears your right, there doesn't seem to be an option to display KW or HP anywhere. The closest thing seems to be % Motor Load, if it works dependably. I would suggest contacting their Technical Support for any suggestions and help they can provide.

Stu....

 

[eccan]

Thanks Stu
I thought the power formula would work if I had output volts, output amps and power factor. I wonder if the valus I am reading called "power factor angle" should be used in the power formual directly, or if it is how far away from a PF of 1 the drive is running? Maybe tech support can help me - I will try Monday.

 

[sthompson]

Thanks Stu
I thought the power formula would work if I had output volts, output amps and power factor. I wonder if the valus I am reading called "power factor angle" should be used in the power formual directly, or if it is how far away from a PF of 1 the drive is running? Maybe tech support can help me - I will try Monday.

Sorry we couldn't be more help. This is the first VFD I have personally seen that didn't report either HP or KW. Usually they will do either depending on user selected options. Perhaps Automation Direct can add this to a future Firmware. Perusing their web site, it seems that they created this line of Drives. They brag about their award winning support, so let's see how they do.

Let us know how you make out.

Stu...

 

[DickDV]

Well, it looks to me like "low cost" drives have hit a new low. No kw or hp record anywhere!

And, if it doesn't calculate kw, it probably does not calculate torque either so that "% load" figure is misnamed too. It should then be called "% of FLA".

There isn't much here that will be useful in finding kw or hp. My suggestion would be to add a watt meter upstream from the VFD. This will report steady state power usage with good accuracy (even better if you multiply the reported kw by .97 to remove the losses in the VFD). The watt meter will not be accurate under dynamic conditions as the capacitors in the VFD will smooth the dynamic changes considerably.

I know, the watt meter is expensive. But then, you saved all that money on the VFD, right?

 

[eccan]

Well, Automation Direct tech support came through.
All day yesterday I was kicking myself for changing from the AB Powerflex drive I had originally spec'd, just because the customer wanted to save money, but Bill at AD had just the answer.

The power factor angle that the drive reports is not PF. When the modbus gave me a value of 791 I assumed that was a PF of .791, but in reality I needed to take the COSINE of that number (with 1 dec point) to get the actual PF.

When I was running with no load, the power factor angle from the drive of 791 is actually COS 79.1 = .189, and loaded the angle is 496 which = COS 49.6 = .648. These numbers look much better, as I know my power factor should be closer to 1 when the motor is working.

You never are too old to learn something new

 

[DickDV]

But, in this case, you might have learned a useless thing. The power factor at the drive input on PWM drives is very close to one and does not change with motor speed or loading.

So, whatever the AD guy told you with regards to using this to calculate load power consumption, I'm a bit doubtful that it is in any way useful.

If this is power factor at the motor, then something is wrong with the example you just gave us. At no load, the power factor of an induction motor approaches zero since all power going in is reactive power and none results in any output power. A power factor at no load in the 60%'s is highly unlikely. At full load, you can read the PF off the motor nameplate.

You might want to doublecheck with AD on just what that number stands for.

 

[kamenges]

Originally posted by DickDV:

At no load, the power factor of an induction motor approaches zero since all power going in is reactive power and none results in any output power.

eccan indicated his power factor was 0.189 unloaded. Since we really don't know what he meant by "unloaded" (at this point I assume it to mean motor connected to the drivetrain but not processing material on the machine) it is hard to tell if this is reasonable. But I suspect it is.

While I wouldn't dispute the power company with that calculation, I suspect the equation eccan listed at the bottom of the original post is correct.

Keith

 

[Tom Jenkins]

OK - you are getting way above my level of expertise. I'm hoping DickDV will chime in!

I do know that there are lots of things going on on the VFD output and motor that may impact your calculations. The waveform is non-sinusoidal. The voltage drops as Hz drops. I think some of the current is coming off caps in the DC bus and off the motor inductance(?), so I think there isn't a 1:1 correlation on current into the VFD and current to the motor.

Dick?

As stipulated above, I only know enough to be dangerous. However, in he past I've tried to equate Volts, Amps, and power factor on the output of a VFD with the drive's calculated kW and the kW I back-calculated from a machine's known performace. I can get good correlation to the VFD's calculated power, but I've had lousy luck with the standard formulas. I'd exercise caution in this area at least.