Bidirectional DC-DC converters and regeneration from VFD and ACIM

Use a synchronous 48-to-350V DC-DC converter. Who needs link cap? ;-)

Tim

-- Seven Transistor Labs Electrical Engineering Consultation Website:

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

If the VFD you are using has a DB terminal bus on it (most likely does), and you can bring the HV DC Bus (-) side to your common (most can in cases like yours), you then do what we did one time which seem to work well, but I wouldn't recommend it to everyone. We set up a tap on the DB resistor to extra only a portion of the bus voltage and ran that into a charge control circuit. If the batteries didn't require charge the charge controller would then connect the DBR back to common to allow the DBR to fully take the regen.

Essentially, the charge output simply connected back to the input of the DC-DC inverter, Like you're doing, except you need to monitor voltages and current rates etc.

This was done for a 220 Volt 3 phase VFD drive where a DC-DC inverter connected to the DC Bus, the inverter operated from a 24 volt battery source. We tried for a 48volts but the little three wheeler he made wouldn't hold it.

P.S. The VFD that was used was very picky about the bus voltage, I hope yours isn't so bad.. It was a little painful to program the drive for an auto reset when silly little faults of bus over or under voltage occurred. Not all Drives allow you to ignore these errors. What ever you do, please don't use Parker/SSD/Eurotherm AC drives, they are made with border line voltage limited parts. We replace them all the time for the same reasons. Every one else's drives work just fine for the various venders we have tried.

Jamie

batteries

I made a DC-DC converter using a steel core toroid and I ran it on both

12 VDC and 24 VDC producing about 300 VDC for my VFD. It powered a 2 HP 2 pole three phase motor on a converted riding mower. I also converted a 1000W 12V/220V automotive inverter to get 270 VDC output, and a 1200W 24V/220V

inverter that produced only 240 VDC on 24V. I made videos of my adventures:

And a picture thread about the 1200W inverter:

I also found another reference for a bidirectional DC-DC which includes a schematic:

And others on the same site:

*&btnG.x=12&btnG. y=11

an

I have been using a GE/Fuji E11 drive, which I got new for about $65 on eBay some time ago. It has high/low limits of 400/200 VDC.

Paul

Do you have any examples or references to such converters? I can see how

using MOSFETs or IGBTs in place of rectifiers in the output section might be used to drive the conversion in reverse, but will it also work if they are triggered synchronously with the input section? I suppose it might, since the input section uses the body diodes to charge the DC link bus when the load becomes regenerative...

Probably analogous to the regeneration through the three legged H-bridge of the motor controller IGBTs. But I think that operates at the effective motor frequency based on RPM and not on the PWM carrier, so it would not feed back through the high frequency transformer. I started an LTSpice simulation for a bidirectional converter and maybe I need to work on that. I think it may need a full bridge rather than the half-bridge topology, however.

Thanks,

Paul

Of course, the motor inverter won't fully regenerate if it's not at full conduction angle -- any time it's open circuit, it's not doing anything to help, and only freewheel current can charge DC link. Regen is just a matter of keeping the bridges on in whichever state, so each leg is a forced logic value (+bus or -bus), at whatever PWM and modulation the controller is running at. Tell the controller to ramp down RPM and it'll actively suck mechanical energy out.

Once you get regeneration into the DC bus, that energy is free for use; if you've got that DC bus supplied by synchronous DC-DC, it will simply give or take amps as needed to keep its voltage stable (against whatever reference it's regulated to, or not).

Note that both the inverter and synchronous DC-DC have to handle both hard and soft (ZVS and ZCS) switching -- they'll see all quadrants of operation, so plan accordingly (switching losses, snubbers, EMI..).

Tim

-- Seven Transistor Labs Electrical Engineering Consultation Website:

Do you have any examples or references to such converters? I can see how using MOSFETs or IGBTs in place of rectifiers in the output section might be used to drive the conversion in reverse, but will it also work if they are triggered synchronously with the input section? I suppose it might, since the input section uses the body diodes to charge the DC link bus when the load becomes regenerative...

Probably analogous to the regeneration through the three legged H-bridge of the motor controller IGBTs. But I think that operates at the effective motor frequency based on RPM and not on the PWM carrier, so it would not feed back through the high frequency transformer. I started an LTSpice simulation for a bidirectional converter and maybe I need to work on that. I think it may need a full bridge rather than the half-bridge topology, however.

Thanks,

Paul

includes a

Yup, full bridge on each side with a full bridge of power fets accompanying them. Things are kept in sync via the master oscillator , optically coupled.

Many Ups units work on a similar idea. the battery is connected to the center tap on the low side (inverter primary(. A pushPull set up is used.

Diodes are hung on high side of the primary's to provide two operations, one to snub the extra voltage from the opposite side and two, to force a rectification which shows up at the center tap of the primary which is where the battery is connected. This charges the battery from the AC mains. With UPS units like this, there is normally an extra winding where the

120 mains comes in and the inverter simply keeps synchronized with it. The voltage is seen on the inverter primary's and rectified which provides a charging source for the battery.

In your case, you wouldn't have this extra primary winding and the inverter would need to operate on its own but it can take advantage of using the center tap of the inverters primary to generate voltage back to the battery. You would of course, need to provide a switching bridge on the high side around the rectifying diodes.

If you use a center tap transformer to operate in push pull, you'll save on semiconductors for both sides.

If you have some of those power inverters hanging around, You can use the transformers in those. most of them use center tape topology to save on components. I have a collection of dead inverters with nice transformers for many usages :)

Jamie