shouldn't current be the same?
- Thread starter ritelec
- Start date
- Status
robbietan
Senior Member
- Location
- Antipolo City
this is becuz the vfd alters the voltage to change the speed of the motor it is controlling. change the voltage, current changes.
- Location
- Massachusetts
Are you using a true RMS meter?
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Doesn't a Variable Frequency Drive change the frequency to change the speed?
- Location
- Massachusetts
infinity said:
Yes it does change the frequency but it also adjusts the voltage level as well.
mdshunk
Senior Member
- Location
- Right here.
Your amprobe probably cannot accomodate the frequency at which the VFD secondaries were operating at during the time you measured the current. Thus, you got a somewhat false reading. The current may well be more, but I sorta have my doubts about double. The voltage doesn't change that much on VFD secondaries, does it?
Just an afterthought.... was this VFD supplied by 240, but controlling a 120V motor? (or similar arrangement).
Just an afterthought.... was this VFD supplied by 240, but controlling a 120V motor? (or similar arrangement).
- Location
- Massachusetts
mdshunk said:
A true RMS meter can deal with this within its limits.
No I don't think it does, the goal is to try and maintain the 'stock' hertz to voltage ratio.
Here is a hard to read graph that shows this.
mdshunk
Senior Member
- Location
- Right here.
Fantastic! That has got to be about the most concise explanation I have ever heard on this aspect of VFD operation. Thank you for that.iwire said:
adrian33773
Member
With VFDs the voltage changes in proportion to the frequency. For example if you have 360V@60Hz you will have 180V@30Hz and 90V@15Hz, etc. With a VFD, current is normally at 150% of the motor rating until you reach the desired speed. Current on a regular motor can be 300% or more of the rated current until it gets up to speed. In other words less stress on the motor and circuit when you have a VFD.
ok, thank you for the replies. I still don't get it. the voltage is 480 and all the readings where taken with a new fluke by the maintenance mechanic at the facility. I can't give him an answer but I did see that the measurements did double. did some reading on line and was wondering if it had to do with "contol" amperage in the armature that the vfd uses to "monitor" the speed. wacky stuff.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- Semi-retired engineer
iwire said:
That is not actually true.
The voltage supplied to the motor is a series of PWM DC pulses of the same amplitude, both positive and negative. The duty cycle of the pulses approximates the same voltage as a sine wave.
Is it possible your VFD is set up to run off a 480V supply while driving a 208V motor? Or maybe single phase supply to 3 phase motor.
And again, I haven't seen where you said you do have a true RMS meter. If the VFD has a 30KHz or higher output frequency, that might also be stretching the limits of the meter like Bob said.
Steve
And again, I haven't seen where you said you do have a true RMS meter. If the VFD has a 30KHz or higher output frequency, that might also be stretching the limits of the meter like Bob said.
Steve
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
A VFD in normal operation takes the input AC, rectifies it to DC, and then uses transistors to chop this DC into variable voltage, variable frequency AC.
By controlling the AC frequency, you control the speed of the motor. However in order to properly drive the motor you must also vary the voltage being supplied to the motor. The 'constant V/Hz' graph that iwire posted is the simplest control method that is used, and in practise is quite effective.
The internal DC supply is called the 'DC rail', and current is circulating between the DC rail, the transistors, and the motor. There is no direct relation between the current circulating to the motor and the current coming in from the AC supply. In fact, if the motor gets tugged along by an 'overhauling load' then it can start to regenerate, and the AC supply current can fall to zero, with current circulating between the motor, the transistors, and the DC rail, and a braking resistor acting to limit the voltage on the DC rail.
Measuring the output voltage is next to impossible without a tool that can deal with the funky nature of the AC output. As petersonra noted, the transistors in the VFD don't supply a smoothly varying AC voltage to the motor. Instead they switch the full DC rail voltage on and off at high frequency (2kHz to 30kHz and above, depending upon the specific VFD; some old drives or very large drives work at the output frequency directly, however). The high frequency 'square' output is modulated by adjusting the relative positive versus negative duration; it is this modulation of the high frequency pulsing that the VFD can control and adjust.
If you stick a volt meter on the output terminals of the VFD, it will quite likely see the full DC rail voltage, or perhaps 1/2 of this value, and totally ignore the modulation values.
The rather interesting thing about all of this is that the motor windings themselves act as a filter, and pretty much don't respond to the high frequency component of the VFD output. If you look at the motor current on a scope, you will see a pretty good sine wave at the modulation frequency, with a little bit of 'wiggle' a the 'carrier' frequency. So the output of the VFD is high voltage, high frequency, but as far as the motor is concerned you are applying low frequency controlled voltage.
-Jon
By controlling the AC frequency, you control the speed of the motor. However in order to properly drive the motor you must also vary the voltage being supplied to the motor. The 'constant V/Hz' graph that iwire posted is the simplest control method that is used, and in practise is quite effective.
The internal DC supply is called the 'DC rail', and current is circulating between the DC rail, the transistors, and the motor. There is no direct relation between the current circulating to the motor and the current coming in from the AC supply. In fact, if the motor gets tugged along by an 'overhauling load' then it can start to regenerate, and the AC supply current can fall to zero, with current circulating between the motor, the transistors, and the DC rail, and a braking resistor acting to limit the voltage on the DC rail.
Measuring the output voltage is next to impossible without a tool that can deal with the funky nature of the AC output. As petersonra noted, the transistors in the VFD don't supply a smoothly varying AC voltage to the motor. Instead they switch the full DC rail voltage on and off at high frequency (2kHz to 30kHz and above, depending upon the specific VFD; some old drives or very large drives work at the output frequency directly, however). The high frequency 'square' output is modulated by adjusting the relative positive versus negative duration; it is this modulation of the high frequency pulsing that the VFD can control and adjust.
If you stick a volt meter on the output terminals of the VFD, it will quite likely see the full DC rail voltage, or perhaps 1/2 of this value, and totally ignore the modulation values.
The rather interesting thing about all of this is that the motor windings themselves act as a filter, and pretty much don't respond to the high frequency component of the VFD output. If you look at the motor current on a scope, you will see a pretty good sine wave at the modulation frequency, with a little bit of 'wiggle' a the 'carrier' frequency. So the output of the VFD is high voltage, high frequency, but as far as the motor is concerned you are applying low frequency controlled voltage.
-Jon
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Jon, you're describing a digital audio amplifier.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
Larry,
They are pretty similar really. A VFD is a three channel, extremely low fidelity but very efficient amplifier, combined with signal generator that produces the correct output values to drive the motor.
If you could tolerate the poor efficiency, you could build a VFD based upon any amplifier technique, including a pure 'Class A' linear vacuum tube amplifier
The cooling load would be terrible, the efficiency in the sewer, and the cost astronomical...but the output would be _very_ quiet.
Oh, and in the description above I used the term 'transistor' in a very broad sense, to encompass things such as SCRs, IGBTs, GTOs, etc.
-Jon
They are pretty similar really. A VFD is a three channel, extremely low fidelity but very efficient amplifier, combined with signal generator that produces the correct output values to drive the motor.
If you could tolerate the poor efficiency, you could build a VFD based upon any amplifier technique, including a pure 'Class A' linear vacuum tube amplifier
The cooling load would be terrible, the efficiency in the sewer, and the cost astronomical...but the output would be _very_ quiet.Oh, and in the description above I used the term 'transistor' in a very broad sense, to encompass things such as SCRs, IGBTs, GTOs, etc.
-Jon
robbietan
Senior Member
- Location
- Antipolo City
yep, tried to measure the VFD output with a fluke 41B, but with the settings in DC mode. unless the load is constant all the time, the readings measured will always be changing.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Just because the Fluke is "new" and says "True RMS" does not mean that it can read the output of a VFD. Only the more expensive models are capable of that because they need to have a low pass filter built-in. In addition to all the issues raised by others, the output of a VFD is extremely rich in harmonics, which tend to confuse even the average True RMS meters, and they emit very strong RFI. Think about it, what is the difference between a VFD's modulated frequency and frequency modulation, aka FM radio? Not much!
Also, iron core CTs typically used in clamp-on ammeters can saturate on VFD outputs. For that reason, you should really use the flexible CTs, known as Rogowlski Coils, when reading current. Naturally, those are the most expensive option as well.
Without spending a fortune on a Fluke 189 and i2000 Flex coils, the best way to determine the output current of a VFD is to read it from the VFD's display.
Also, iron core CTs typically used in clamp-on ammeters can saturate on VFD outputs. For that reason, you should really use the flexible CTs, known as Rogowlski Coils, when reading current. Naturally, those are the most expensive option as well.
Without spending a fortune on a Fluke 189 and i2000 Flex coils, the best way to determine the output current of a VFD is to read it from the VFD's display.
- Location
- Massachusetts
Jraef said:
Without a doubt you are much more knowledgeable about this subject than I.
That said I have had very good luck with a Fluke 36 RMS clamp meter.
The readings where very close to the VFDs LCD readout and matched very closely with my Fluke 387 equipped with a flexible current ring.
For anyone buying or specing VFD's the 2005 code requires that ther be some type of feedback to the VFD from the motor to monitor the motor temperature (NEC 430.126(A)). The simplest way to do this is with a VFD that models the temp of the motor based on what the VFD is putting out to the motor. They also have a memeory that retains this information even if the power is lost. If you use this method be sure that your VFD has this feature - many don't.
- Status