Best Electronic DC Loads for 6.5kW 400V Applications—Recommendations and Experiences

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#1 21660070 04 Nov 2011 13:12

Jim Mikolich Jim Mikolich

Anonymous

Post #1
21660070 04 Nov 2011 13:12

Setting up for a distorted voltage test, need to have a variable resistor to dial in the THD. the resistor is on the rectified side of a bridge so seeing only DC. but the power dissipated is on the order of 6.5kW... accounting for derating I need around 10kW or more for this load.. I have seen a few electronic loads.

Also, rectifying 230Vac l-n so Voltage into load is around 400Vdc so 500V range is necessary.

Good Experiences? bad experiences?

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#2 21660071 04 Nov 2011 13:17

DAVID CUTHBERT DAVID CUTHBERT

Anonymous

Post #2
21660071 04 Nov 2011 13:17

The very common and inexpensive Ohmite 225 W resistors (see Digikey) work well especially with some forced air for this sort of load.

At 6.5 kW and allowing for a 50 degree C rise in the air it will need 260 cfm of air on 30 resistors. For a simple test load I do not derate the resistors
#3 21660072 04 Nov 2011 13:23

DAVID CUTHBERT DAVID CUTHBERT

Anonymous

Post #3
21660072 04 Nov 2011 13:23

Digikey also stocks the 2500 watt TE Connectivity resistors. You will need only three and at $80 you have an inexpensive load. These work well and look impressive.
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#4 21660073 04 Nov 2011 13:26

Peter Blackford Peter Blackford

Anonymous

Post #4
21660073 04 Nov 2011 13:26

For this range of power, I have found that using a commercial (HVAC) duct heater works well. They are available in many sizes, so you will have a wide choice to select the best one for your app. Although configured for 3P, they can easily be rewired for DC (1P) application. Use standard fans for helping heat dissipation, and as always, be sure to configure for electrical and thermal safety.
#5 21660074 04 Nov 2011 13:29

DAVID CUTHBERT DAVID CUTHBERT

Anonymous

Post #5
21660074 04 Nov 2011 13:29

Digikey has the Huntington FSE1000-1.0 resistors in stock. These are 1 ohm, 1000 watts and $42 each.
#6 21660075 04 Nov 2011 13:49

Kevin Parmenter Kevin Parmenter

Anonymous

Post #6
21660075 04 Nov 2011 13:49

those are all great ideas - I have used resistor load banks as well as light bulbs (watch out for the inrush currents) also you can screw in heating elements in place of the light bulbs in the ceramic sockets you can buy from home depot or lowes. if you need an active electronic load for AC that you can adjust PF and phase angle loads for various reactive conditions Chroma and california instruments makes them I cannot remember how much power they offer -but be prepared to spend some money for these. We used them for UPS - Inverter and motor drive testing in one of my prior assignments.
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#7 21660076 04 Nov 2011 13:54

Jim Mikolich Jim Mikolich

Anonymous

Post #7
21660076 04 Nov 2011 13:54

at one ohm and needing to achieve a load of 50ohms plus some flexibility I would need... well 50 of those.. so 2,000 dollars there.. plus switches contactors etc... labor to build, a cabinet to put in, thermal fuses safety and fans. after all that is said and done, I would be nearing the 30,000 dollar price tag of the electric load I was eyeballing (We would rent and thus not spend 30k)
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#8 21660077 04 Nov 2011 13:55

Jim Mikolich Jim Mikolich

Anonymous

Post #8
21660077 04 Nov 2011 13:55

This intrigues me, as the device is actually a 3ph device, however the test required is a voltage distortion onto each input phase, but having a 3ph load around also helps our cause in buying/renting something suitable. plus its getting to be winter in the midwest hah
#9 21660078 04 Nov 2011 13:57

Jim Mikolich Jim Mikolich

Anonymous

Post #9
21660078 04 Nov 2011 13:57

The full setup looks something like this..

California Instruments MX30 power supply to our device to a lightbulb and resistive load (we are using the lightbulb and ceramic sockets as loads for low inductance starting) now I need to parallel in this new load for a qualification test. the actual EUT is 3phs 8700kVA device so having dual use for the load would be nice. we know it can drive a bridge to a DC load if necessary too.
#10 21660079 04 Nov 2011 14:06

Kevin Parmenter Kevin Parmenter

Anonymous

Post #10
21660079 04 Nov 2011 14:06

OK - that makes sense now. I saw the title of DC load then saw all the reference to AC specs - THD etc.. i have also used Amrel DC loads I do not believe they make AC loads that may work for you if you can do the bridge in front of it.
best of luck
#11 21660080 10 Nov 2011 06:13

David Erickson David Erickson

Anonymous

Post #11
21660080 10 Nov 2011 06:13

Along these lines, Hotwatt in Danvers MA makes finned strip heaters in the KW range. They are designed for 120 or 240V at various power ratings. Convenient to apply, all they need is lots of forced air. Reasonable prices too.

I have used them only as heaters running off AC so can't speak for their resistance stability, inductance, etc.

http://www.hotwatt.com/finned.htm
#12 21660081 10 Nov 2011 12:20

Per Zackrisson Per Zackrisson

Anonymous

Post #12
21660081 10 Nov 2011 12:20

Just a reflektion. How would a common water boiler work? cheap and you might get refreshments at the same time. You might be needing a couple but the price is right....
And reuseable.
#13 21660082 10 Nov 2011 12:57

DAVID CUTHBERT DAVID CUTHBERT

Anonymous

Post #13
21660082 10 Nov 2011 12:57

WATLOW makes air cooled and water cooled loads.
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Topic summary

✨ The discussion addresses the need for a high-power variable resistive load capable of handling approximately 6.5 kW to 10 kW at around 400 V DC for distorted voltage testing and THD adjustment on the rectified side of a bridge. Several solutions are proposed, including using multiple high-wattage resistors such as Ohmite 225 W resistors with forced air cooling, TE Connectivity 2500 W resistors requiring fewer units, and Huntington FSE1000-1.0 1000 W resistors. Commercial HVAC duct heaters and finned strip heaters from Hotwatt are suggested as cost-effective, high-power resistive loads that can be rewired for DC use with adequate forced air cooling. Light bulbs and heating elements in ceramic sockets are also mentioned for low inductance loads, though with caution regarding inrush currents. For more advanced and adjustable electronic loads capable of AC testing with power factor and phase angle control, brands like Chroma and California Instruments are referenced, though these are significantly more expensive. Amrel DC loads are noted for DC applications but may require a bridge for AC testing. WATLOW is mentioned as a manufacturer of air-cooled and water-cooled loads. The discussion highlights considerations such as derating, thermal management, safety features, and cost comparisons between building resistor banks and renting commercial electronic loads.
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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]

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