Vfd help

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bure961

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Which is the correct way to take a amp reading of a vfd. Line or load side. Line side is ok for rating of motor. Load side of vfd amp reading is above motor amp rating. Thanks your help.
 
Best way is to read the display on the drive when it is displaying the parameter you are looking for. I haven't seen one that will not display output amps, but many will not display input amps. But not often is input amps all that important to you either, at least not to any high precision level.
 
bure961 said:
Which is the correct way to take a amp reading of a vfd. Line or load side. Line side is ok for rating of motor. Load side of vfd amp reading is above motor amp rating. Thanks your help.
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Load side is what matters for the motor.
Kine side is usually less. Difference in power factors.
 
kwired said:
Best way is to read the display on the drive when it is displaying the parameter you are looking for. I haven't seen one that will not display output amps, but many will not display input amps. But not often is input amps all that important to you either, at least not to any high precision level.
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Amen to that. Use the drive display, it has to calculate the correct motor amps as part of what it does, so the complexity it needs to be accurate is part and parcel to the drive design. If your motor amp reading as the DRIVE displays it were really higher than the motor FLA rating, then either it is about to trip on Over Load, or you failed to enter the motor nameplate data into the VFD correctly, which you need to fix ASAP.

My rule of digital meter accuracy as it applies to VFD outputs is that if the price you paid for the meter had less than 4 digits to the left of the decimal point, the readings that you get are not worth worrying about.
 
Jraef said:
Amen to that. Use the drive display, it has to calculate the correct motor amps as part of what it does, so the complexity it needs to be accurate is part and parcel to the drive design. If your motor amp reading as the DRIVE displays it were really higher than the motor FLA rating, then either it is about to trip on Over Load, or you failed to enter the motor nameplate data into the VFD correctly, which you need to fix ASAP.

My rule of digital meter accuracy as it applies to VFD outputs is that if the price you paid for the meter had less than 4 digits to the left of the decimal point, the readings that you get are not worth worrying about.
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Don't mean to hijack the thread but a question on line voltage. Schneider Altivar 61, 15 hp drive reads Mains voltage of 450 volts but both of my Fluke 337s read around 470 volts on each phase? And I'm pretty sure they're reasonably accurate.
 
delectric123 said:
Don't mean to hijack the thread but a question on line voltage. Schneider Altivar 61, 15 hp drive reads Mains voltage of 450 volts but both of my Fluke 337s read around 470 volts on each phase? And I'm pretty sure they're reasonably accurate.
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OK. But I'm not sure what that has to do with the Altivar drive?
It's simply supply voltage.
 
Besoeker said:
OK. But I'm not sure what that has to do with the Altivar drive?
It's simply supply voltage.
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Just an indication that the same drive's readout of the motor voltage might be inaccurate too. Or that the Flukes are inaccurate. As mentioned, the drive is under more pressure to determine the output voltage accurately than the input voltage. And there may not even be any common metering components between the two measurements except the digital readout.....
 
I don't know of a standard 2 quadrant AC drive that measures anything other than the DC bus, from which it calculates anything else it displays. Since it CREATES the output RMS voltage itself, that is going to be accurate. But from the drives perspective, incoming line voltage is only relevant in how it affects the DC bus level anyway, so accuracy is unimportant. In fact technically it doesn't care if it's single phase or three phase either, it's just a raw material as far as the drive is concerned. The incoming line will have harmonic distortion on it, probably even before the drive was connected, which will affect the readings on a "true RMS" reading meter, but which is, as I said irrelevant to the VFD so it would not bother accounting for it in displaying it.
 
First I like to say thank you for all the responses to my post. The vfd's key pad is not functioning properly at all so I have nothing to compare my reading to. The line side amp reading is under the motors flc but the load side amp reading indicates about 10 amps higher then the flc. This is 1/3 higher amp reading. Thanks to all of you.
 
bure961 said:
First I like to say thank you for all the responses to my post. The vfd's key pad is not functioning properly at all so I have nothing to compare my reading to. The line side amp reading is under the motors flc but the load side amp reading indicates about 10 amps higher then the flc. This is 1/3 higher amp reading. Thanks to all of you.
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As Jraef mentioned earlier, the load side will have a lower power factor which will mean a higher VA appears on the load side conductors. Line side may have some power factor but will probably be pretty close to unity.
 
bure961 said:
First I like to say thank you for all the responses to my post. The vfd's key pad is not functioning properly at all so I have nothing to compare my reading to. The line side amp reading is under the motors flc but the load side amp reading indicates about 10 amps higher then the flc. This is 1/3 higher amp reading. Thanks to all of you.
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Does this excess current trip the drive or overheat the motor?
 
bure961 said:
First I like to say thank you for all the responses to my post. The vfd's key pad is not functioning properly at all so I have nothing to compare my reading to. The line side amp reading is under the motors flc but the load side amp reading indicates about 10 amps higher then the flc. This is 1/3 higher amp reading. Thanks to all of you.
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You didn't say what brand it is, but there are several VFDs that are notorious for having bad connectors on the HMI (keypad). If you haven't already, try unplugging it and plugging it back in, maybe spray a little contact cleaner on the pins and let them dry first if possible. For most drives, the HMI is replaceable as well, given how important it is to know what's going on, you should consider not living with that issue.

If you have or can borrow one, use an old fashioned analog clamp on ammeter, like an old Amprobe. They tend to be a better choice for reading current on the output of a VFD than many digital ones.
 
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