App Note #1
Running Manzanita Micro PFC chargers on Generators.
Joe Smalley
7/30/2002
Introduction:
Two popular uses for Manzanita Micro PFC series charger are
1) to make the maximum use of a portable generator in stationary charging and
2) to carry a portable generator to allow charging while in motion.
Both concepts have been tested and found to have a situation were the charger is damaged by the genset.
Problem:
The power transfer maximization concept is valid and works well as long as the peak voltage of the input waveform does not exceed 400 volts. If the peak of the waveform exceeds 400 volts, the transorbs in the logic power supply start absorbing the excess energy and they dissipate the excess energy as heat. The problem is that they can only absorb so much energy before they overheat and fail.
Effect on warrantee:
Operating the chargers in excess of 400 volts peak will blow the transorbs and the logic power supply. We cannot warrantee this abuse of the chargers since it under the control of the user and not under our control. We have seen this happen and know the symptoms. Chargers that have been damaged in this way will cost $400 to repair. If you have doubts about your generator, either constrain your charging to the 120 VAC outlets or perform the following test to confirm the 240 VAC source is safe to use with your charger.
How to tell if your generator is safe:
If you want to test your generator to see if it will blow your charger, you will need a peak voltage test jig. These are simple to built using a full wave bridge and a capacitor. Both must be rated for over 400 volts. The bridge must be rated at least 2 amps and the capacitor should be about 1 microfarad. Larger bridges and capacitors will work but are overkill.
Figure 1: Peak voltage detector schematic.
1.Build the above circuit.
2.Connect the AC male plug to your generator outlet.
3.Connect a DVM across the capacitor.
4.Connect several loads to the generator with their switches off.
5.Start the generator.
6.Read the voltage across the capacitor while the generator is starting.
7.Change the load on the generator by switching the various loads on and off.
8.Read the voltage across the capacitor at each power level.
9.Turn off the generator.
10.Wait until the voltage across the capacitor drops below 10 volts.
11.Disconnect the circuit.
If the voltage surges above 400 volts only between step 5 and step 6, then you have a startup transient as shown in Figure 2 below. If the charger is not connected to the generator until after the generator is running for a few seconds, then it will survive. If you start the generator with the charger connected, the surge may be long enough to overheat the transorbs and kill the power supply. This is not a reliable solution. You should lower the voltage to make it reliable.
Figure 2: Startup transient
If the voltage goes above 400 volts under any load condition, then the voltage is too high and the charger logic power supply will fail.
Buck transformer solution:
The easiest way to reduce the voltage is to use a buck transformer. This is simply a high voltage to low voltage transformer that subtracts from the line voltage. The secondary of the transformer needs to be rated for the current you intend to draw. The primary can be run off either the 120 or 240 lines coming out of the generator. A schematic is shown in Figure 3.
Figure 3: Buck transformer schematic
If the output voltage is greater than the input voltage, the transformer is phased wrong. Change the polarity of either the primary or secondary of the transformer.
Run the test over again to verify that the transformer does not overheat and the peak output voltage never goes over 400 Volts.
If the peak voltage still goes over 400 volts, use a transformer with a higher voltage secondary to get more reduction in the line voltage.
Optimal setting for the charger input voltage is 300 to 330 volts peak to peak.
Autotransformer solution:
If you have a 120 Volt Variac autotransformer and want to reduce the 240 line to 220 to make your charger safe from the peak voltage you can hook it up as shown in Figure 4. The Variac must be rated to carry the charger current for the Variac to survive.
Figure 4: Variac autotransformer schematic.
This connection will allow you to adjust the voltage from 120 to 240. To maximize the power and minimize the current, the voltage needs to be set to keep the transient peaks to less than 400 volts.
Optimal setting for the charger input voltage is 300 to 330 volts peak to peak.