Resistive load from PowerFlex 40

Hoping for a sanity check from some of our drives gurus.

I have an application where I need to run a 500 watt resistive heater at 115V, 400 Hz. The load is an ordinary immersion heater.

It's a ground-based test stand; the load is not aboard a vessel, so I don't need to meet any emissions or distortion requirements. I'll be doing the insulation testing and ground bond testing separately, with different instruments.

Could I take a PowerFlex 40 with 480V 3-phase input and program it to restrict the output to 115V and ignore the phase imbalance ?

I'm going to go put on my flameproof pants and try anyway, but I figured I'd solicit any experience from the PLCTalk community first.

Wow! Never tried it, but it sounds feasible. Is it a single phase heater?

Thanks, I realize I didn't specify.

Yes, it's an ordinary 115V single phase resistive immersion heater for water.

Not sure you will be able to inhibit the phase loss fault.

The PF75x range have an Application for Adjustable voltage which may be able to be used.

Alan

ps let us know how you went, did you need your flameproof pants?

Frequency is irrelevant for a resistive heater. Are you required to test it at that frequency because of the possible skin effect changes in current capacity? Looking at a resistor element as a conductor means more of the current will flow on the surface areas, effectively increasing the resistance. but if the purpose is to have it be a heater anyway, I fail to see the need for testing it at 400Hz. It will eventually get to the same heat, but may take a little longer.

Anyway, there is no phase imbalance protection to disable, the PF40 is only concerned with DC bus ripple (as is the PF52x). Having a load on only one set of output transistors will not really affect that. Shouldn't know or care really, other than for the total output current flowing through that one set of transistors. So yes, you would just set P031 to be 115V, P032 and 35 to be 400Hz, A084 to a value of "5" (No V/Hz boost), then make sure you are in V/Hz mode with A125 set to "0". If it were me, I would ramp it somewhat slowly just because depending on the type of heater element, the cold resistance may be much lower than the hot resistance, so the VFD may trip on HW Over Current if you are too aggressive.

Thanks for the input ! Your expert perspective on how these drives work is always appreciated.

The customer requires that we do the test with the same voltage and frequency that will be used onboard the vessel, so we salute and say yessir.

Ken Roach said:

..., so we salute and say yessir.

Click to expand...

Always a prudent policy when someone is paying you.

By the way, it's common practice to perform simple load current tests on VFDs by using immersion heaters in water. We used to call it a "crab boil" when I worked up there in Seattle building drive panels. A 250HP drive connected to heaters makes a LOT of heat very fast in a 55 gal drum of water. No need to let all that heat go to waste, so toss in a few fresh crabs and have a lunch party!

I know that ab used to make single phase units, do they go to 400 hz?

james

Ken, one caution. The resistance elements will see pulses from the drive that are 650V high on a VFD fed by 480VAC. The "115V" output is only a limit on the pulse width where an inductive load would integrate the pulses to the 115V equivalent current.

If the 650V pulse height is a problem, place a 480V/208V transformer in front of a 230V/208V VFD with sufficient rated output current. That way the output pulses to the resistors will only be around 310V high. That would surely require less insulation on the output side of the VFD.

Otherwise, either system should work but, as jraef suggests, start low and ease your way up in output power. You may find that the resistors reach their desired heat value at a voltage near but not exactly at 115V on the VFD display.

James Mcquade said:

I know that ab used to make single phase units, do they go to 400 hz?

james

Click to expand...

The drives sold as "single phase" refer to the INPUT power, not the output. The output is always 3 phase on a VFD (from A-B anyway). Some drives monitor for output phase loss, A-B drives do not. The 400Hz output is now very common on LV drives, in fact most are capable of higher frequencies, some up to 1000Hz (although now limited to >600Hz by law in this country).

Side note: Theoretically, any VFD can be fed single phase power, all it is doing is rectifying to DC anyway. But in doing so from a single phase source, all of the motor 3 phase power is going to come from 4 diodes instead of 6, and the DC bus ripple increases significantly. So a "single phase input VFD" is just one in which the front end components and bus capacitance is increased to handle the effects of single phase input. In many cases, including A-B drives, you can feed single phase into ANY drive, so long as you de-rate it and different designs require different levels of de-rate. Small cheap "component class" drives typically need a 65% de-rate if used at rated ambient, or a 50% de-rate if the maximum ambient is also de-rated. That's because the caps work harder and the parasitic resistance results in more heat, so the maximum operating environment must be typically de-rated to about a 25C (which is often unrealistic for industrial control panels). "Premium" drives, meaning those with DC bus chokes, can typically use the 50% de-rate without the need for additional temperature de-rating. YMMV.

I always appreciate especially the input from users jraef and DickDV, because you guys are such experts you don't treat the drive like a magic box the way many of us do. Boiling the issues down to the essential methods and components really helps the rest of us understand.

The customer took a look at the cost adder to build a UL control panel with a drive in it and conceded that VRMS is the same at 60 Hz as it is at 400 Hz, and agreed that we could test the heater with normal 120V/60 Hz power.

I work for a very excellent employer who decided that for Pi Day, there would be a dozen pies in the lunchroom. Combine that with a win for Ohm's Law, and I'm having a good day.

jaref,

thanks for the clarification.

james

Wow, thanks for the excellent explanation, Jraef! Would you be so kind to explain why the US limits VFD frequency to 600Hz? I've not heard of that before!

Dept of Defense directive that took effect in 2015 to reduce the likelihood of "bad hombres" being able to run centrifuges for uranium enrichment. We don't know what the speed is they need to operate, but it's something above 600Hz.

It's buried in here, middle of page 15 of the PDF.
https://www.bis.doc.gov/index.php/f...13-nsg-plenary-rule-79-fr-46316-8-7-2014/file

Well, I've learned something today. Not sure if I should be perturbed at all by how quickly you were able to dig up that document and point to it