VFD Output Voltage, Current, Frequency
VFD Output Voltage, Current, Frequency
(OP)
I would like to know what the actual output curves look like from a VFD. I have read several comments and from what I have read there is a difference of opinion. If the PWM voltage is constant and only the frequency is varied what is going on with the volts/hertz ratio? It may be that we must include the motor inductance to be able to see what the curves the motor is seeing. I have read much about VFd's but, it is still confusing to what is actually taking place at the motor. The motor seeing constant voltage with a varying current frequency?





RE: VFD Output Voltage, Current, Frequency
The PWM output Voltage is not constant, the max. value (amplitude) is constant, and equal to DC Link voltage, but first harmonic of the voltage varies with frequency. This, first harmonic, is the voltage seen by motor. Motor bahaviour is dictated by this voltage/frequency combination, and other harmonics cause the parasitic effects.
Regards
Tom.
RE: VFD Output Voltage, Current, Frequency
RE: VFD Output Voltage, Current, Frequency
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: VFD Output Voltage, Current, Frequency
RE: VFD Output Voltage, Current, Frequency
V/f ration is constant up to nominal frequency, after that the voltage is constant and V/f ratio is decreasing.
At the vary low frequences, the ratio is increased to compensate the stator resistance (stator voltage drop)
Tom.
RE: VFD Output Voltage, Current, Frequency
You are right 230V => 30Hz !
RE: VFD Output Voltage, Current, Frequency
RE: VFD Output Voltage, Current, Frequency
Best regards,
Mark Empson
http://www.lmphotonics.com
RE: VFD Output Voltage, Current, Frequency
As others have said, the ratio is held constant for most general purpose applications, and is determined by the motor nameplate voltage and frequency rating (done during setup of the VFD parameters). So for a 460V 60Hz motor, the ratio is 7.6 V/Hz because 460 ÷ 60 = 7.6. So a VFD set up for a 460V 60Hz motor will mantain that ratio throughout the speed range, and preyty much in a linear fashion. There is a slight twist to that in variable torque applications where you want to save extra money. The ratio is tweaked to further reduce energy consumption at lower speeds because less torque is required there.
Coincindentally, a 380V 50Hz motor is essentially the same because 380 ÷ 50 = 7.6V/Hz. That is why you can often use 380V/50Hz motors in the US even if you are not controlling them with a VFD, they just turn faster. On a 230V motor, the V/Hz ratio is 230 ÷ 60, or 3.83V/Hz, but a 230V 50Hz IEC motor is going to be 4.6V/Hz, so they are not as interchangeable.
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: VFD Output Voltage, Current, Frequency
Terms such as 'torque boost' that allow a few more volts at the lower frquency range also act to shift the volts higher than the expected, so measuring volts at lower frequencies can seem higher if you are not aware that the feature 'torque boost or voltage boost' has been enabled.
Ultimately you are trying to provide an optimum volt/frequency to the motor to ensure the magnetising current (flux gap) is adequate to ensure the motor can produce the required torque.
RE: VFD Output Voltage, Current, Frequency
RE: VFD Output Voltage, Current, Frequency
RE: VFD Output Voltage, Current, Frequency
The difference in current reading could come from the motor parameters entered into the VFD. Did you check that motor size, speed, etc. ... entered into VFD are correct?
Tommy
RE: VFD Output Voltage, Current, Frequency
RE: VFD Output Voltage, Current, Frequency
A vector controlled VFD adjusts its voltage to suit the actual load. So it is not at all unusual to see a rather low V/Hz ratio on lightly loaded motors. I have had a few calls from people that were convinced that something was wrong with there drives - took some time to explain this to them over the phone.
Also, to measure the motor voltage it is not enough to have a True RMS meter. It will show the RMS of the PWM waveform and that is a lot more than RMS of fundamental. I use a simple filter with a 1 Hz corner frequency to check motor voltage. The 1 Hz corner frequency makes its output voltage = U/f so I read 8,0 V if the motor is a 400 V/50 Hz one. The good old ITT/mw/Metrix MX210 clamp-on has this filter built in and the measured value is recalculated (times f) before being displayed. There are filters for current as well as voltage.
Many recorders have filters with 1 or 5 Hz corner frequencies that can also be used. But beware of high voltage and short circuit currents!
Gunnar Englund
www.gke.org
RE: VFD Output Voltage, Current, Frequency
you have to register but it is free. A good document is called: "Why true-rms? Non-linear loads need a true-rms
current clamp for accurate readings"
RE: VFD Output Voltage, Current, Frequency
Any decent VFD tech support person should know that you cannot get accurate current readings with most clamp-on ammeters on the output of a PWM VFD. The VFD itself probably uses an internal Hall Effect transducer for that very reason. That's why you get the advice to trust the VFD reading rather than the meter; the issues have already been dealt with in the VFD because it needs accurate current readings in order to function properly and protect the motor.
If it is important for you to verify the VFD's reading then skogsgurra's sggestions are appropriate.
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: VFD Output Voltage, Current, Frequency
RE: VFD Output Voltage, Current, Frequency
What model clamp on do you have?
RE: VFD Output Voltage, Current, Frequency
RE: VFD Output Voltage, Current, Frequency
Can you provide the current and Voltage readings before and after replacing VSD's?
If possible Current and Voltage Input to the driver, current and voltage output to the motor by the driver meter and by Clamp on Fluke.
RE: VFD Output Voltage, Current, Frequency
PWM crest factors can be all over the map (usually high) depending on the frequency/voltage of the drive at that particular time as well as the switching frequency. Most meters on the market can only handle crest factors of 3 with some handling crest factors up to 5. Here is a link that lists some examples of crest factor- http://www.tinaja.com/glib/muse125.pdf
Drive mfgs running amps can be inaccurate as well. I did some testing on a 5HP single phase drive using a clamp-on, scope (with series resistor), and the drives running amps. Neither the drive nor the clamp-on were that accurate compared with the scope. So I still don't depend on the drive too much but this was a small drive (probably cheaply made so they can sell the thing).
Dont forget about PAM drives (these are the ones I am used to working on - used in the oil patch alot).
RE: VFD Output Voltage, Current, Frequency