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MagnaLOAD Standard Control Modes and Introducing Shunt Regulator and Rheostat

To provide control flexibility for a variety of DC sources, MagnaLOAD DC electronic loads have several user selectable control modes:

  • Voltage mode
  • Current mode
  • Power mode
  • Resistance mode
  • Shunt Regulator mode
  • Rheostat (ARx Series, WRx Series)

Depending on the control mode selected, programmed set-points and the voltage and current being driven by the connected DC source, the MagnaLOAD will automatically select the appropriate regulation state of: constant voltage, constant current, constant power, or constant resistance.

Generally, when connecting the MagnaLOAD to a regulated DC source, the MagnaLOAD should not be configured to operate in the same regulation state as the DC source as this may produce control instabilities between the two products. Instead, for example, if the DC source is regulating voltage, the MagnaLOAD should be set to any control mode besides voltage mode.

Simple MagnaLOAD applications require driving the product into a desired regulation state and tripping it off with a fault if a set value is exceeded. For these simple applications, set points other than that of the desired regulation state should be set to maximum, while the trip points should be set to the desired level in order to shut down the product. Advanced MagnaLOAD applications require crossover set point limits to ensure the product does not trip, allowing further validation of their DC source’s behavior. For these advanced applications, the available set points should be programmed to the desired crossover points; trip points are still available to be programmed for safety limits.

Voltage Control Mode

In voltage control mode, the MagnaLOAD will auto-crossover between voltage and power regulation, but will preference constant voltage regulation over all other states. Figure 1 shows a simplified voltage mode operating diagram.

Voltage control mode is popular for applications where the DC source is either constant current or constant power, such as: DC-DC converters and DC power supplies.

Current Control Mode

In current control mode, the MagnaLOAD will auto-crossover between current and power regulation, but will preference constant current regulation over all other states. Figure 2 shows a simplified voltage mode operating diagram.

In current mode, the MagnaLOAD will allow the input voltage to fluctuate while trying to maintain the current set-point in a constant current regulation state. Current regulation is typically the most common requirement for an electronic load, and likewise, current control mode is the commonly used mode for MagnaLOADs. Current control mode is most commonly used in applications such as: battery or capacitor discharge, DC-DC converters testing, and DC power supply testing.

Power Control Mode

In power control mode, the MagnaLOAD will auto-crossover between current and power regulation, but will preference constant power regulation over all other states. Figure 3 shows a simplified voltage mode operating diagram.

In power mode, the MagnaLOAD will allow the input voltage and current fluctuate while trying to maintain the power set-point in a constant power regulation state. Power control mode is popular for testing applications where the DC source has a wide full power operating range, such as: auto-ranging DC power supplies or fuel cells.

Resistance Control Mode

In resistance control mode, the MagnaLOAD will preference the constant resistance regulation state over all other regulation states. Figure 4 shows a simplified voltage mode operating diagram.

With up to 16-bit programming resolution, resistance control mode allows precise programming of a desired resistance value as the set point. The MagnaLOAD will operate in constant resistance regulation within the set-point boundaries indicated in grey in Figure 4. If the connected DC source drives the DC bus to one of the bounding set-point limits, the MagnaLOAD will auto-crossover to the appropriate regulation state. Resistance control mode is most commonly used in applications that would otherwise use just a passive resistor.

Shunt Regulator Control Mode

Shunt regulator mode is a new control mode Magna-Power developed for its MagnaLOAD product line, which has quickly become one of its most indispensable features. In shunt regulator mode, the MagnaLOAD does not begin processing power until the programmed voltage threshold is exceeded. The user sets their desired voltage threshold and desired dissipation current. Once the MagnaLOAD is enabled, it remains idle and monitors the DC bus voltage. When the DC voltage rises above the programmed voltage threshold, the MagnaLOAD engages to the programmed current setting within 1 ms to dissipate energy from the DC bus. In this mode, the MagnaLOAD acts much like a protection device, ensuring the DC bus does not exceed the programmed voltage threshold. Shunt regulator mode is particular useful in applications with brushless DC drives, which can cause the bus voltage to rise as the drive brakes. Figure 5 shows a simplified operating diagram for shunt regulator mode.

Shunt regulator mode features built-in hysteresis, to prevent oscillations and rapid toggling on-and-off. The voltage threshold which the MagnaLOAD engages in shunt regulator mode is the programmed voltage set point plus 1% of the unit’s full scale voltage rating. For example, if an ARx6.75-1000-14 (6.75 kW, 0-1000 Vdc, 0-14 Adc) MagnaLOAD was programmed to 500 Vdc in shunt regulator mode, the MagnaLOAD would begin dissipating energy at the voltage threshold calculated as:

`"500 Vdc + (1% of 1000 Vdc) = 510 Vdc"`

The MagnaLOAD would continue dissipating energy until the DC bus voltage drops below 500 Vdc. If the bus voltage again rises above 510 Vdc, the MagnaLOAD would reengage. Some applications may require a modification to this 1% adder, which can be modified through editing an EPROM value in software. Contact Magna-Power support for more details.

When sizing a MagnaLOAD for use in shunt regulator mode, it’s important to consider the current capability of the MagnaLOAD when it engages at the voltage threshold. Using the example above, for the ARx6.75-1000-14 and a programmed voltage set point of 500 Vdc, the voltage threshold for this MagnaLOAD will be 510 Vdc; therefore, the maximum current set will be:

`"6750 W / 510 Vdc = 13.2 Adc"`

Rheostat Control Mode (ARx Series and WRx Series only)

The power dissipation stage in the ARx Series and WRx Series MagnaLOAD consists of a switched matrix of resistors series coupled with linear elements. The rheostat control mode bypasses the linear elements to provide direct control of the MagnaLOAD’s switched resistor matrix. While most electronic loads need to specify slew rate for their various control modes as they ramp the internal MOSFETs up and down, rheostat control mode provides true step response without the constraints of ramp time for MOSFETs, offering an instant switch from one resistance to another.

In rheostat control mode, a total of 31 different resistor states are available. Each resistor state has an associated power limit, less than the MagnaLOAD model’s full scale rated power, which cannot be exceeded. Resistor states can be switched on-the-fly, with the DC input enabled, at the resistor state’s maximum power rating. The model’s maximum output voltage rating or maximum output current rating can be achieved at each resistor state, as long as that resistor state’s power limit is not exceeded.

The 31 available rheostat resistance values vary by model. The rheostat resistor values and power limits for each state is provided in the respective product’s User Manual in the section titled “Rheostat Mode.”

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