VFD operated from battery
VFD operated from battery
(OP)
Hi,
I saw recently a peculiar system, this is a 3ph custom VFD + 4 induction motors in parallel. The motors are labeled Vn=600Vrms
The curious thing is that the VFD can work in an "emergency mode" supplied from a (really big) 48V battery.
They use that to allow moving the thing quite slowly to a "controlled" position even if power fails.
It puzzles me so big difference of supply voltage (600 to 48), but even so it makes the work quite well.
The VFD has no boost circuit or DC/DC, just typical PWM VSI.
I understand that a high current can be supplied by the battery to the DC bus (it uses a 200A breaker), but voltage is very low, and as far I understood, motor flux depends mainly on peak voltage (and freq.). In a vector controlled drive I understand that torque is proportional to flux and "load" current so torque should be quite low comparing the nominal voltage (48/600?).
Just want to know your opinion on the topic about how affects so big voltage change to the motor performance. Maybe there is some control strategy to overcome that?.
Regards
I saw recently a peculiar system, this is a 3ph custom VFD + 4 induction motors in parallel. The motors are labeled Vn=600Vrms
The curious thing is that the VFD can work in an "emergency mode" supplied from a (really big) 48V battery.
They use that to allow moving the thing quite slowly to a "controlled" position even if power fails.
It puzzles me so big difference of supply voltage (600 to 48), but even so it makes the work quite well.
The VFD has no boost circuit or DC/DC, just typical PWM VSI.
I understand that a high current can be supplied by the battery to the DC bus (it uses a 200A breaker), but voltage is very low, and as far I understood, motor flux depends mainly on peak voltage (and freq.). In a vector controlled drive I understand that torque is proportional to flux and "load" current so torque should be quite low comparing the nominal voltage (48/600?).
Just want to know your opinion on the topic about how affects so big voltage change to the motor performance. Maybe there is some control strategy to overcome that?.
Regards





RE: VFD operated from battery
f=48x60/600
f=4.8
(Assuming 60Hz in the 600V mode.)
So with the battery V=48V, the "creep home" frequency is V/Hz limited to 4.8Hz.
No control strategy needed. This is the same thing the drive would provide if it was fed 600V. The only difference is that the drive doesn't need to PWM at an extremely low duty cycle to provide the equivalent 48V from the 600V.
The drive will change its modulation scheme because it would no longer be needing to synthesize 48V out of 600.
Keith Cress
kcress - http://www.flaminsystems.com
RE: VFD operated from battery
RE: VFD operated from battery
A Non-vector vfd will keep basic v/hz ratio at this 48 vdc input, so assuming 2v drop in drive, it will make a (48-2)/1.414 or 32Vac rms output, so hz out can be 60hz*32/600 or 3.2hz. so if assume a 1775 rpm motor then it will run 1800*3.2/60 or 96 rpm if very lightly loaded. Since this is above its slip speed of 25 rpm, it will run down to as slow as 71 rpm if putting out full nameplate rated load.
RE: VFD operated from battery
Your torque will be the same as long as the VFD outputs 32V RMS and 3.2Hz.
Basically, the PWM has to be adjusted to account for the lower buss voltage. Also, the drive has to switch it's control power source from the ~850VDC bus when running on line power to the 48VDC bus from the battery. I doubt there is a power supply that can handle that big a range so it's likely switched some how when the main power is lost.
Keith, you forgot to divide by sqrt(2) to convert the 48V peak from the battery to the AC RMS output voltage you would get.
RE: VFD operated from battery
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RE: VFD operated from battery
I checked a VFD of similar size, a Siemens Micro Master 420, and it won't work at all at low DC link voltages.
http://www.gke.org/pub/files/VFD%20control%20volta...
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: VFD operated from battery
RE: VFD operated from battery
The drive is vector controlled, including speed sensor. It uses a separate power supply for the control, coming from the battery.
Then following your explanation, the motor would be working about the nominal flux (I now remember that one the objectives of v/f was to approx. get a constant flux), and the torque current will be vector-controlled by the command.
Regards
RE: VFD operated from battery
From thread237-193052: VFD and induction motor power factor, I understand than reactive power is normally drawn from the DC bus cap, and due to this reason PF should be quite high even with low load.
In a regular AC installation you can get some cap-banks that must be sized to supply the needed reactive power, but with the DC bus cap it just doesn´t seem to need this "fixed" sizing. I guess that the motor gets what it needs from the cap, and it shall be sized for maximum power drawn.
The doubt is, it makes any difference supplying from the low voltage battery directly to the DC bus? (the cap will store much less energy than with nominal voltage)
Regards
RE: VFD operated from battery
"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: VFD operated from battery