FAQ
TL;DR: For ~6.5 kW at ~400 Vdc, 30×225 W resistors need ~260 cfm airflow; they “work well” with forced air. [Elektroda, DAVID CUTHBERT, post #21660071]
Why it matters: This FAQ helps power engineers choose safe, affordable high‑power DC loads for distortion and qualification tests, fast.
Quick Facts
- Rectifying 230 Vac line–neutral yields ~400 Vdc; plan for a 500 V load range. [Elektroda, Jim Mikolich, post #21660070]
- 30 Ohmite 225 W resistors with ~260 cfm handle ~6.5 kW under forced air. [Elektroda, DAVID CUTHBERT, post #21660071]
- Three 2.5 kW TE Connectivity resistors can form a ~7.5 kW load; approx. $80 each. [Elektroda, DAVID CUTHBERT, post #21660072]
- Commercial HVAC duct heaters can be rewired for DC and fan‑cooled for dissipation. [Elektroda, Peter Blackford, post #21660073]
- Active AC loads (e.g., Chroma, California Instruments) exist, but expect higher cost. [Elektroda, Kevin Parmenter, post #21660075]
What’s the most cost‑effective way to sink ~6.5 kW at ~400 Vdc?
Use a resistor bank with forced air. One proven setup uses 30×225 W Ohmite parts and ~260 cfm total airflow. This achieves the target power with controlled temperature rise. Keep wiring short and use proper guarding and fusing. “The … resistors … work well especially with some forced air.” [Elektroda, DAVID CUTHBERT, post #21660071]
Can I use an HVAC duct heater as a DC load?
Yes. Commercial duct heaters scale well at multi‑kilowatt levels and can be rewired from three‑phase to a DC (single‑phase) configuration. Add standard fans for heat removal and design for electrical and thermal safety clearances. Verify resistance and mounting per your test plan. [Elektroda, Peter Blackford, post #21660073]
Are finned strip heaters a viable option?
Finned strip heaters in the kilowatt range are convenient and reasonably priced. They need strong forced air. They are commonly run from AC; for DC use, check resistance stability and inductance against your measurement needs before committing. [Elektroda, David Erickson, post #21660080]
How many resistors and how much airflow do I need for 6.5 kW?
A practical example from the thread specifies 30 pieces of 225 W resistors with about 260 cfm airflow. That supports a 50 °C air rise at 6.5 kW total. Use baffles to distribute airflow evenly across the bank. [Elektroda, DAVID CUTHBERT, post #21660071]
I need variable R up to ~50 Ω—how do I approach switching?
A 1 Ω/1 kW element array needs roughly 50 pieces to reach 50 Ω steps, plus contactors and cabinet hardware. Switching networks add cost, complexity, and safety requirements, which can rival renting a commercial load for short projects. [Elektroda, Jim Mikolich, post #21660076]
Do light bulbs or screw‑in heating elements work as loads?
They can, but beware of high inrush current on incandescent lamps. Ceramic sockets with screw‑in heating elements are an option for resistive loading. For adjustable AC loads with power‑factor control, commercial gear exists but costs more. [Elektroda, Kevin Parmenter, post #21660075]
When should I rent a programmable electronic load instead of building?
If you need fast setup, programmability, and protection features at high voltage, rental can beat build time and safety engineering. The thread notes that DIY switching and safety hardware can approach commercial load budgets, making rental attractive. [Elektroda, Jim Mikolich, post #21660076]
What safety measures are essential for high‑power resistive loads?
Provide forced cooling, thermal fuses or sensors, guarded terminals, and clearances. Use fans rated for the temperature rise and ensure safe wiring and mounting. As one poster advises, configure for both electrical and thermal safety from the start. [Elektroda, Peter Blackford, post #21660073]
What is an electronic DC load (in this thread’s context)?
It refers to a commercial programmable load that sinks power from a DC source or a rectified AC source. Such loads provide adjustable current, voltage limits, and protections, and can replace ad‑hoc resistor banks when precision is required. [Elektroda, Kevin Parmenter, post #21660079]
How do I handle a three‑phase device when tests target each phase?
Keep a three‑phase‑capable load available if your equipment under test is three‑phase. You can still perform per‑phase distortion tests while benefiting from a 3φ load bank for broader qualification or dual‑use needs. [Elektroda, Jim Mikolich, post #21660077]
3‑step: How do I assemble a forced‑air resistor load bank?
- Size power: target ~6.5 kW and a 50 °C air rise.
- Install ~30×225 W resistors on heat‑safe mounts in a ducted plenum.
- Provide ~260 cfm airflow and add thermal protection and guards. [Elektroda, DAVID CUTHBERT, post #21660071]
Who makes turnkey air‑ or water‑cooled load banks?
Watlow manufactures air‑cooled and water‑cooled resistive loads appropriate for high‑power testing. These products can simplify mechanical and thermal integration compared with scratch‑built banks. Consult datasheets for voltage ratings and mounting. [Elektroda, DAVID CUTHBERT, post #21660082]
Which brands were used or mentioned for sources and loads in the thread?
California Instruments MX series was used as a source. Amrel was mentioned for DC loads. Chroma and California Instruments provide active AC loads with adjustable power factor and phase angle for advanced testing. [Elektroda, Kevin Parmenter, post #21660079]
What DC current am I dealing with at 6.5 kW and ~400 Vdc?
Current is power divided by voltage: 6,500 W ÷ 400 V ≈ 16.25 A. Select conductors, fuses, and shunts with adequate DC ratings and temperature rise margins for this current. Validate against your source and load ratings before energizing. [Elektroda, Jim Mikolich, post #21660070]
Any edge cases or failure modes to watch?
Incandescent lamps show large inrush, which can trip supplies or damage switches. Resistor banks without enough airflow overheat, drifting resistance and risking failure. Use staged startup and interlocks to prevent overstress events. “Watch out for the inrush currents.” [Elektroda, Kevin Parmenter, post #21660075]
