Measuring current after VFD and DC-drives

[Kev77]

Today one location ran tests on all the motors and found that 8 motors were pulling 22 amps and the other 2 were 16 amps and 13 amps. This is one reason for the under performance in the kiln. Now we need to find out why. Possible fan pitch or baffles.

The kiln motors are $$$ so I want to play it safe. Each motor has overloads so as long as there set correctly I should be good to get close to FLA. The problem is that people who generally don't know much about electrical know more then the people that do, just ask them.

Thanks Paul

 

[Stig]

My experience

I have done some tests to measure the current at the output of a variable frequency drives. Of course you should look at the current that the drive displays, but we had some doubts if the drive was still ok and wanted to compare the current indicated
by the drive with the current measured with a measuring clamp.
I could see that if the motor is running at speed (and not blocked mechanically or at speed 0), the current measured with the clamp is approximately the same as the one indicated by the drive. And this was even with a non true rms clamp.
However if the drive was blocked the current measured with the clamp was completely different from the current indicated by the drive.
Could I conclude that if the drive is running at speed the current measured with a clamp is approximately accurate, and the influence of a true rms or not a true rms clamp will be minimal??
What if the motor is not blocked but running at lower speed then the setpoint (torque reduced), will the measurement then be accurate???
How about measuring at the output of a dc-drive, is a true rms clamp then a real necessity, or will a non true rms clamp also do the job (like for the vfd).
Share me your experiences in measuring current with a measuring clamp.
Thanks.

We have 2 mixers that we wanted to shut down at a certain motor current for consistency of product, but it also had to be adjustable through a panel meter. We used a true RMS because the frequency drive used it for readings which made it easier to do scaling in the panel meter. Ran it through a controller with some timers to open the e-stop circuit shutting the drive down.

 

[DickDV]

Part of the problem here is that you are trying to use amps to measure motor loading. On sine wave power and much moreso on PWM power, this is hopelessly inaccurate.

Instead, measure the motor slip and compare it to the full load slip on the motor nameplate. For example, a motor labeled 1760rpm would have 40rpm slip (1800-1760=40) at full load. This is true at all speeds under the motor's base speed of 1800rpm.

So, set the drive for some high end frequency. Determine the synchronous speed at that frequency. Then, using a strobe lite, find the actual shaft speed of each motor. Compare the actual slip to the nameplate slip and you will know exactly how each motor is working.

 

[Kev77]

Correct me if I'm wrong but you are saying that the amount of slip will tell me what the true brake horsepower is on that motor? Measuring the shaft RPM could be a tough thing to do on the 60" fans that are direct coupled to the motors. Could go in for a hair cut!

Thanks DickDV,

Kevin

 

[DickDV]

Slip is directly proportional to shaft torque assuming the motor has the correct operating voltage.

But, it sounds like the motor is completely enclosed in the air box. That surely is a problem.
However, using amps for loading is not going to be accurate under about 90% load. That is due to the total amps in the motor leads being the vector sum of magnetizing amps and torque-producing amps. If a premium efficient motor, you can figure the mag amps at around 22% of FLA or you can measure it with the motor running at full speed unloaded.

Now, since you know one side of the triangle (the mag amps) and the hypotenuse (total amps measured), you can solve for the torque-producing amps with the Pathagorean Theorem. You have to do that at full load (nameplate FLA) and at measured total amps to find the percent of full torque.

As you can see, this is not pretty. Unfortunately, this is what you have when you use amps for measuring load in AC machines. Again, you'll be ok at FLA +/- 10% but outside that, you will need a calculator.

 

[OkiePC]

We have 2 mixers that we wanted to shut down at a certain motor current for consistency of product.

This is exactly what we are using the UPC for. We are able to chill batches of ground beef or sausage in a mixer to within 0.5 degrees of the set point by monitoring the load on the blender paddles. As the meat cools the current climbs. Of course the size of the batch needs to be considered. Our blender operators keep a chart that they have accumulated good amp target settings for different batch sizes.

You really need to be looking at drive torque amps or power as displayed or communicated from the VFD, or use a specialized monitor to get that level of repeatable consistency. We use both methods. We monitor Torque Amps from PF70 and 700 drives on networks and use UPC Load Control on motors that are DOL started.

 

[Stig]

Our operating procedure depending on product is mix in speed 3 (use the FU output on the VFD to show the mixer is at speed, sends a signal to the controller I1 as soon as frequency is reached), once the panel meter reaches 190 amps (read from L1 on the VFD's outfeed) it outputs to I2 of the controller, once it has maintained or exceeded amperage for 2 seconds (inside the controller) it shuts down the mixer. We have had very good luck considering that the machine was not designed for our product type. We are basically making dough in a cutter designed for meat. However we are doing this on 3 phase 480v machines not DC.

 

[OkiePC]

Sounds very similar, and if it works well, I expect the controller is well designed for the purpose. In our case the speed presets and the times are in the PLC, we use a 2 second debounce also, and haven't really ever needed to change it.

 

[DickDV]

Stig, I think that the reason your system works as well as it does is that you are not really measuring 190amps but using it as a set point. When used that way, since error is about the same every time, the setpoint is consistent. It might really be 225amps or 170amps but who cares. It's the same every time and the process works well.

Of course, you could have done the same thing with one of the drive's 4-20ma outputs pointed to output current. You would have monitored the ma output for whatever current you found was suitable and triggered the next step in the process from that.

If you want to accurately measure current, however, a CT on PWM pulses will not be accurate.

 

[Stig]

Yep, thats what I wanted to do but the powers that be overruled me, they wanted a clamp so thats why they got. The 4-20 would have been more accurate from the vfd.