Monitoring VFD Power

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
I have a ATV71 VFD and need to monitor the speed of the motor, amperage or the power via A01. Constant torque, not variable.

Which will be more predictive of an overload?

At the moment we have three speeds, 40, 50 & 60 Hz.
Empty, the amperage did not change with the Hz and I only know the amperage at 60hz when loaded. I have not looked at Power or Motor speed for those three frequencies,.

What will the Power look like when the motor goes Locked rotor?
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Locked rotor = close to zero power.

What kind of overload are you worried about?

Another question that may matter more is what are you trying to accomplish? If the motor draws too much current it will trip the drive, protecting the motor. This is a built in function of the drive.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Strictly it is zero power at zero rpm
It all depends on whether you are talking about the output shaft power of the motor (zero at zero rpm) or electrical input power to the motor (equal to the resitive losses in the windings and the hysteresis losses in the magneic circuit of the motor.
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
My thread a few days ago, I plugged a drag. Via a major brain malfunction I ran it too slow for the amount of grain that was being forced into it. That particular situation won’t be an issue once interlocks are involved. There will will be times when the drag is gravity fed at lower speeds. I don’t want it to begin an overload during testing let alone after.

I will try monitoring power, or amperage, as I manually change the grain flow to the auger. I’m afraid once motor speed goes on a decline, it’s too late.
 

Besoeker3

Senior Member
Location
UK
Occupation
Electrical Engineer
It all depends on whether you are talking about the output shaft power of the motor (zero at zero rpm) or electrical input power to the motor (equal to the resitive losses in the windings and the hysteresis losses in the magneic circuit of the motor.
A motor is rated at its nameplate power.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Monitoring the output kW with the VFD gives you a true look at the actual shaft power on the motor. Most drives have a "shear pin" feature that you can set to trigger on that (or current only). You can use that as either a trip or an alarm, sometimes both at different levels / delays. In addition, you can often program the VFD as to how you want it to respond to that situation, i.e. go into current limit at the motor FLA, which you can do forever, with hopes that the load situation changes, or reduce the speed command (not what you want in that situation you described), or of course, trip.
 

Besoeker3

Senior Member
Location
UK
Occupation
Electrical Engineer
Monitoring the output kW with the VFD gives you a true look at the actual shaft power on the motor. Most drives have a "shear pin" feature that you can set to trigger on that (or current only).
Most?
I know what they are but not something I have used on variable speed drives. We had a few on the farm in agriculture but I was just a wee boy then.
 

Besoeker3

Senior Member
Location
UK
Occupation
Electrical Engineer
Electronic "shear pin", not a real shear pin. It's just called that on some drives because it replicates the functionality of a shear pin...
Yes, I can understand the functionality and I'm sure there applications. I just haven't seen them on the VFDs.
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
Monitoring the power seems to be working at the moment. Still manual operation. We ran it at 60Hz for an hour last week with power staying at about 64%. Set the alarm notification for 68%. The operator ran it a few hours later and many degrees colder outside at 74%. Buzzer sounding the whole time. Cover my A-- Graphs showed it peaked at about 110% a couple times and did run at 104% for quite a while. I've omitted some details but the amperage of motors feeding and taking product away did not change. Grain flow with this operation sequence is forced and it will never be allowed to run slower than 60Hz. We've passed the information to the millwrights, electrically it's not my issue.
 

gar

Senior Member
211213-1707 EST

ptonsparky:

I think you are really concerned with input current. You are concerned with motor damage and that is related to power dissipated in the motor. This power dissipation is primarily determined I^2 * R. But there is an additional factor, and that is motor cooling. If you have a separate blower on the motor for cooling, then you can go to zero speed at full current. If not, then you need to use an adjustable criteria as a function of motor RPM.

.
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
I’m not worried about the motor exactly. I need to make sure the product does not plug the drag. The cost to unplug could exceed that of the motor. I suspect the VFD will protect the motor well enough.

My next need is to know how the drag will respond to lower speed and gravity feed to the drag, vs a forced feed.

Empty I have 15, 19 & 24% at 40, 50, & 60 HZ respectively. I did not note the amperages.
 

gar

Senior Member
211213-2011 EST

Since motor power and failure of motor to overload is not your concern, then you need to monitor load torque as your primary measure.

I suggest a load torque transducer, and rated full scale 10X expected maximum torque. This you probably don't want to do. So measure both motor power input, and RPM, and pick some combined criteria for them.

.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
211213-2011 EST

Since motor power and failure of motor to overload is not your concern, then you need to monitor load torque as your primary measure.

I suggest a load torque transducer, and rated full scale 10X expected maximum torque. This you probably don't want to do. So measure both motor power input, and RPM, and pick some combined criteria for them.

.
If you know RPM and horsepower you can calculate torque.
 
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