P
P E Schoen
As an adjunct to the DC-DC converter I want to make, is a means to implement
regeneration from the AC induction motor and the VFD used to control it.
This is presently for a small EV (lawn tractor). I found a very detailed
treatise on the subject:
http://scholar.lib.vt.edu/theses/available/etd-02052008-122048/unrestricted/Dissertation_jhz.pdf
(3.4 MB)
However, my idea is to use a separate switching type battery charger on the
high voltage DC link that will feed power back from the motor when in
regenerative mode. The DC link will be nominally about 250-350 VDC and the
batteries will be most likely four 12 VDC SLAs in series for 48 VDC nominal.
Thus, a standard 240 VAC switching battery charger should be able to accept
the DC level during regeneration (which will raise the voltage depending on
the bus capacitance and the amount of braking being applied).
The bus capacitance could be sized to accept the energy from maximum braking
which would be perhaps 1.5 kW for 10 seconds, or 15 kJ, so the capacitors
would need to be sized for that energy from nominal 250 VDC to maximum 400
VDC. I calculate:
C = 2 * 15 / (Vmax^2 - Vmin^2) = 308 uF
This is easily and reasonably obtained, and probably could be increased to
something like 1000 uF 450 VDC, which is available for about $25. It could
go in parallel to the high frequency low ESR capacitors that provide the
main energy storage for the PWM. So that is not a problem.
The battery charger would ideally handle the full regeneration power, but
that could be expensive and excessive for the batteries, which may be as
small as 12 Ah and ideally charged at 0.1C, or about 15 watts, but short
time charging at up to 1C (150W) may be OK. I have a 12V SLA charger rated
at 1.3 amps and it was only about $6 from China on eBay. Four of these would
probably work well.
This form of regeneration would come at no extra cost or complexity, since
the batteries need a charger anyway. Thus these chargers could be wired in
parallel and switched between the VFD link and AC line power. For a tractor,
a 10 hour overnight charge is perfectly acceptable.
Any suggestions on ways to improve this system? If it works well, I may
scale it up to a 12V-48 VDC module of 500-2000 W and capable of external
connection in parallel or series to obtain 150 to 600 VDC for AC VFDs or DC
PWM motor controllers. Each module could have internal fault sensing and
emergency shut-down so that the maximum exposable voltage will be a "safe"
48 VDC. The modules could have 12V SLAs or LiFeO4 cells with their own
charging systems and BMS.
Thanks,
Paul
regeneration from the AC induction motor and the VFD used to control it.
This is presently for a small EV (lawn tractor). I found a very detailed
treatise on the subject:
http://scholar.lib.vt.edu/theses/available/etd-02052008-122048/unrestricted/Dissertation_jhz.pdf
(3.4 MB)
However, my idea is to use a separate switching type battery charger on the
high voltage DC link that will feed power back from the motor when in
regenerative mode. The DC link will be nominally about 250-350 VDC and the
batteries will be most likely four 12 VDC SLAs in series for 48 VDC nominal.
Thus, a standard 240 VAC switching battery charger should be able to accept
the DC level during regeneration (which will raise the voltage depending on
the bus capacitance and the amount of braking being applied).
The bus capacitance could be sized to accept the energy from maximum braking
which would be perhaps 1.5 kW for 10 seconds, or 15 kJ, so the capacitors
would need to be sized for that energy from nominal 250 VDC to maximum 400
VDC. I calculate:
C = 2 * 15 / (Vmax^2 - Vmin^2) = 308 uF
This is easily and reasonably obtained, and probably could be increased to
something like 1000 uF 450 VDC, which is available for about $25. It could
go in parallel to the high frequency low ESR capacitors that provide the
main energy storage for the PWM. So that is not a problem.
The battery charger would ideally handle the full regeneration power, but
that could be expensive and excessive for the batteries, which may be as
small as 12 Ah and ideally charged at 0.1C, or about 15 watts, but short
time charging at up to 1C (150W) may be OK. I have a 12V SLA charger rated
at 1.3 amps and it was only about $6 from China on eBay. Four of these would
probably work well.
This form of regeneration would come at no extra cost or complexity, since
the batteries need a charger anyway. Thus these chargers could be wired in
parallel and switched between the VFD link and AC line power. For a tractor,
a 10 hour overnight charge is perfectly acceptable.
Any suggestions on ways to improve this system? If it works well, I may
scale it up to a 12V-48 VDC module of 500-2000 W and capable of external
connection in parallel or series to obtain 150 to 600 VDC for AC VFDs or DC
PWM motor controllers. Each module could have internal fault sensing and
emergency shut-down so that the maximum exposable voltage will be a "safe"
48 VDC. The modules could have 12V SLAs or LiFeO4 cells with their own
charging systems and BMS.
Thanks,
Paul