Why ELRs offer the optimum solution for power test systems

Load banks are commonly utilized in product design and in
production testing of power supplies and other power conversion devices,
including motor drives and inverters. Load banks come in two forms: basic (essentially
an array of power resistors) and electronic systems that employ active circuity
to dynamically simulate changing load profiles. Load banks are often used to
perform product burn-in to root out early failures (infant mortality) as part
of the product’s final production testing.  Electronic loads are frequently integrated
into automated test systems. Test profiles are either stored in the unit or
uploaded via computer interface so that the test data can be compiled for
reporting or archiving purposes.  


What most load banks have in common is that they take up
space, consume a lot of energy, and create heat and noise. This article
describes the electronic load with regenerative output, or ELR, approach to
solving these issues. It also presents the typical questions engineers ask when
it comes to the remarkable advantages provided by ELRs, and the answers from a
manufacturer with ELRs operating in the field.


Load bank issues

In general, load banks have a number of common characteristics
that can negatively impact efficiency, ease of use, the quality of the work
environment and expense. The most obvious issue is energy consumption.  A typical 10 kW load bank, for instance, will
consume well over 10 kW of power in order to provide that function.


High power consumption leads to cooling concerns since the heat
from the load must be dealt with. Small load banks might simply require the
need for added air conditioning, Most loads are fan cooled, which further increases
energy consumption and which can also add significantly to the ambient noise level.
And for load banks that require water cooling, energy costs and significant
installation expenses can be incurred.  


Because of the power consumption and cooling requirements of
many load banks, the units can be large, bulky and sometimes immobile.


Electronic load with
regenerative output

All of the problems described above are related to the
wasted energy that load banks create. But what if most of that energy could be
captured? The answer is yes. How? By redirecting the power back to the utility
by using an inverter stage, synchronized to the power line input.


illustrates how such a closed-loop system
operates. Power is applied from the main to the device under test.  An electronic load with regenerative output
(ELR) provides the prescribed load profile and an internal micro-inverter that
is used to return the power to the mains.




Fig. 1: Closed-loop regenerative load test



The results of the ELR approach are quite remarkable – with
total energy consumption being reduced by up to 93%. It also has a significant
effect of unit’s size, cooling requirements and audible noise. At 10kW the ELR
dissipates just 700W of heat (that’s about the same as a typical hair dryer). A
simple plug connection to standard line voltage is all that’s required.


A couple other benefits of dissipating just 700W of heat
means smaller cooling fans, which reduce audible noise to whisper-quiet
operation. Regenerative loads are also 2-3 times higher power density compared
to typical air-cooled loads, which results in less rack or bench space.


The secret to the implementation of a regenerative load bank
is a back-end conversion system. As shown in Fig. 2, DC energy flows
into a DC-DC converter, which is tied into a DC-AC inverter (current source),
and which then synchronizes with the distribution grid to recycle the
energy.  This technology is similar to
grid-tied photovoltaic inverters (PVs). 




Fig. 2: Electronic load DC output is
applied to a grid-tied inverter stage.  



Q&As about ELRs


Questions often arise when test engineers investigate ELR
technology, since its benefits almost seem too good to be true.  Intepro Systems has several models of ELR
systems in production today, so we have compiled the following list of typical
questions, and our responses:


Question:  Is my energy meter going to spin backwards?


Answer: The ELR recovers up to 95% of the load energy
and recycles it back to the facility’s AC mains network. In most cases the
power recycled from the ELR is much less than the power consumed by the
building’s local distribution network (Fig.
); so, no, the meter won’t run backwards – but it will slow down




3: The ELR is typically a small part of the total utility load.


Question: Can I connect
an ELR to an independent power source?


Answer: Yes, it can be connected if the facility has
an independent source, such as a generator, PV system or battery, where the
unit under test isn’t drawing power from the utility. In this case, the ELR is a
net contributor to the local network.


Question: What happens if the utility grid drops out
and the DC energy is being generated from devices that are not connected to the
grid? In that case, will the ELR continue to delivery power and potentially
shock someone?


Answer: No. An ELR must include an automated grid
monitoring system that detects the phase voltage and frequency that is used for
grid synchronization. If the grid drops out so does the ELR. This is what is
referred to as “anti-islanding,” and operates under the same principal as
grid-tied solar inverters.  In the case
of an ELR, the unit simply shuts down and waits for the operator to turn it
back on.


Question: An ELR is more
expensive than a standard load.  Aside
from the obvious advantages, how do I justify the added cost?


Answer: Field experience with ELR installations has
shown that the return on investment through reduced energy costs is about three



Electronic loads with regenerative output provide a green solution for
load testing of power supplies and other power conversion systems. They recycle
upwards of 90% of the load energy.


Reliability of an ELR is high because the load dissipates very little
heat, per kW, applied to the device under test. Intepro’s ELR9000 (Fig. 4) and ELR5000 Series units, for example, don’t require multiple MOSFETs or IGBTs in parallel
to handle heat dissipation. 




Fig. 4: The Intepro ELR9000 rack-mounted unit is
rated at up to 10.5 kW.  Up to 10 units can
be operated in parallel to simulate higher power loads.