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used in accordance with the instruction manual, may cause harmful interference to radio communications. The effects
of EMI will also depend upon a number of factors external to the inverter like proximity of the inverter to the EMI
receptors, types and quality of connecting wires and cables etc. EMI due to factors external to the inverter may be
reduced as follows:
• Ensure that the inverter is firmly grounded to the Ground System of the building or the vehicle.
• Locate the inverter as far away from the EMI receptors like radio, audio and video devices as possible.
• Keep the DC side wires between the battery and the inverter as short as possible.
• Do NOT keep the battery wires far apart. Keep them taped together to reduce their inductance and induced
voltages. This reduces ripple in the battery wires and improves performance and efficiency.
• Shield the DC side wires with metal sheathing / copper foil / braiding.
• Use coaxial shielded cable for all antenna inputs (instead of 300 ohm twin leads).
• Use high quality shielded cables to attach audio and video devices to one another.
• Limit operation of other high power loads when operating audio / video equipment.
1.3.5 Characteristics of Switch Mode Power Supplies (SMPS)
Switch Mode Power Supplies
(SMPS)
are extensively used to convert the incoming AC power into various voltages like
3.3V, 5V, 12V, 24V etc. that are used to power various devices and circuits used in electronic equipment like battery
chargers, computers, audio and video devices, radios etc. These power supplies use large capacitors in their input section
for filtration. When the power supply is first turned on, there is a very large inrush current drawn by the power supply
as the input capacitors are charged
(The capacitors act almost like a short circuit at the instant the power is turned
on)
. The inrush current at turn-on is several to tens of times larger than the rated RMS input current and lasts for a
few milliseconds. An example of the input voltage versus input current waveforms is given in Fig. 1.3.2. It will be seen
that the initial input current pulse just after turn-on is > 15 times larger than the steady state RMS current. The inrush
dissipates in around 2 or 3 cycles i.e. in around 33 to 50 milliseconds for 60 Hz sine wave.
Further, due to the presence of high value of input filter capacitors, the current drawn by an SMPS
(With no Power
Factor correction)
is not sinusoidal but non-linear as shown in Fig 1.3.3. The steady state input current of SMPS is a
train of non-linear pulses instead of a sinusoidal wave. These pulses are two to four milliseconds duration each with
a very high Crest Factor of around 3. Crest Factor is defined by the following equation:
CREST FACTOR = PEAK
VALUE ÷ RMS VALUE
Many SMPS units incorporate “Inrush Current Limiting”. The most common method is the NTC
(Negative
Temperature Coefficient)
resistor. The NTC resistor has a high resistance when cold and a low resistance when hot.
The NTC resistor is placed in series with the input to the power supply. The higher cold resistance limits the input
current as the input capacitors charge up. The input current heats up the NTC and the resistance drops during normal
operation. However, if the power supply is quickly turned OFF and back ON, the NTC resistor will be hot so its low
resistance state will not prevent an inrush current event.
The inverter should, therefore, be sized adequately to withstand the high inrush current and the high Crest Factor
of the current drawn by the SMPS. Normally, inverters have short duration Surge Power Rating of 2 times their
Maximum Continuous Power Rating.
Hence, it is recommended that for purposes of sizing the inverter, to
accommodate Crest Factor of 3, the Maximum Continuous Power Rating of the inverter should be > 2
times the Maximum Continuous Rated Power of the SMPS. For example, an SMPS rated at 100 Watts
should be powered from an inverter that has Maximum Continuous Power Rating of > 200 Watts.
SECTION 1.3 |
General Information
–
Inverter Related
Summary of Contents for Evolution EVO-4248SP
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