BRUSA Elektronik AG
Neudorf 14
Postfach 55
CH - 9466 Sennwald
[email protected]
www.brusa.biz
Tel. +41 81-758 19 00
Fax +41 81-758 19 99
NLG5 output current specification
Version 3.0
Contents
1. ... Preface...............................................................................................................1
2. ... Voltage, Current and Power flow ..................................................................2
2.1.
2.2.
Single Phase AC............................................................................................................ 2
3-Phase AC.................................................................................................................... 3
3. ... Conclusion ........................................................................................................4
1. Preface
The NLG5 charger is a device that's designed for converting single phase AC into DC current at the highest possible
efficiency while providing full galvanic isolation between mains and battery side. Moreover it contains comprehensive functionality to measure, control and monitor parameters like battery voltage/current, amp-hours, temperature
etc. Safety and efficiency, ruggedness, compactness and low weight were important design goals for the units,
hence energy storage and filtering was kept at a level necessary to achieve low EMI.
The following document explains, how power and current flow through the NLG5 takes place, why it works like this
and what the impact on the battery side is.
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2. Voltage, Current and Power flow
AC
UAC, IAC, PAC
AC
UDC, IDC, PDC
DC
DC
mains outlet
2.1.
NLG5xx charger
battery
Single Phase AC
AC side
DC side
UDC
IAC
IDC
Voltage
UAC
Current
PAC
PDC
Power
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The diagrams above show the voltage, current and power on AC and DC side of the charger during a complete sine
wave of the single phase AC outlet 230V/50Hz. In order to achieve a good power factor, i.e. cos (phi) close to 1, AC
current must exactly follow AC voltage. Hence the power function is of the form
p = c * sin(ω)2
Since the charger does not contain any means of massive energy storage, the instantaneous power on either side must
be the same at any time. The mean power being 3300W at full performance, the amount of energy which would have
to be stored in order to provide flat power on the output side would be about
W = 5ms * 3300W ≈ 16 Joule
To store this amount of energy e.g. in a capacitor would require a heavy, bulky and expensive device and is therefore
considered impractical. So it's inevitable that the DC output current oscillates between nearly zero and maximum current, following the given sine square form of the AC side power.
Example:
UDC = 330V (battery voltage)
IDC = 10A (average)
PDC = 3300W
→ IDC will oscillate between nearly 0 and 20A, whereby the current will have a sine square form, i.e. it will not contain any steps or sharp changes. The current is measured and integrated over time, so the average battery current,
power, Ah etc. are measured accurately.
2.2.
3-Phase AC
In 3-Phase AC systems, the behaviour is quite different: if several charger units are connected symmetrically to a 3phase outlet (i.e. the same number of charger units to every phase) and outputs are paralleled, then the power flow on
the AC and DC side of this cluster is fully continuous, no ripple.
Caution: max. 260VAC are allowed on the charger input, so in a 230V/400V 3-phase system the chargers
must be connected in Wye configuration, not in Delta configuration!
In the following diagrams, examples for a cluster of 3 charger units (fully symmetrical), one of 5 charger units and
one of 2 charger units (partly symmetrical) are shown. As explained above, power flow on AC and DC side of the
cluster must be the same at any time (apart from the losses of course).
Therefore in the following diagrams, just the instantaneous output power supplied to the battery is displayed over
time for the different configurations.
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3 charger units of 3.3kW each
connected symmetrically to a 3phase AC outlet
Phase 1: 1 unit
Phase 2: 1 unit
Phase 3: 1 unit
[kW]
10.0
5.0
Ripple: 0%
0.0
0
5 charger units of 3.3kW each
connected to a 3-phase AC outlet
Phase 1: 2 units
Phase 2: 2 units
Phase 3: 1 unit
[ms]
5
10
15
20
10
15
20
10
15
20
25.0
[kW]
20.0
15.0
10.0
Ripple: 20%
5.0
0.0
0
2 charger units of 3.3kW each
connected to a 3-phase AC outlet
Phase 1: 1 unit
Phase 2: 1 unit
Phase 3: -
[ms]
5
10.0
[kW]
5.0
Ripple: 50%
0.0
0
[ms]
5
3. Conclusion
As a direct result of the single phase AC input, the DC power output has to be oscillating with double the frequency
of the AC input, thereby producing a ripple of 100%. In 3-phase configuration, the power pulsation depends on the
configuration: fully symmetrical it is zero (apart from small tolerances), for 5 units it is 33% and for 2 units it is 50%.
For the many years which this type of charger circuitry has been in use (it was already applied in the NLG4 chargers)
there never occurred problems that the battery couldn't accept that kind of current.
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