Testing with VFD

A motor is nowhere near an inductive load.

Depends on the design of the VFD, as to whether you can use a purely resistive load. Modern VFDs monitor the current in the phases (usually two, and derive the third), then do some quite sophisticated DSP math, to arrive at motor excitation. They expect to see a real motor.

Any brake (dynamometer) has the problem of dissipating the energy it absorbs, usually as heat. A frictional brake will get very hot.

--
"Design is the reverse of analysis" 
                   (R.D. Middlebrook)

Yes, but the testing would only be for short periods of time.

We used to load a motor by driving a generator and then power a radiant room heater with that, or that type of load, you know the giant ceramic resistors that can take 100W each.

By loading the generator it was easy to vary the load on the motor.

I'd use light bulbs and switches, if I didn't need terrifically fine accuracy. If can find a high enough power VFD that puts out 120V, and if you can find them in this degenerate age, then just use lots and lots of ordinary incandescents.

Any sort of dynamometer setup is going to turn whatever power goes into it into heat. If you're talking much power (and it sounds like you are), you're going to need to deal with that heat one way or another.

I'd consider a pump that can absorb as much power as you want to test for, pumping water around. I'd be thinking mostly 55 gallon drums, and a car radiator as necessary to shed heat.

You'll probably come pretty close to a good-enough load with a motor that's just idling and a bank of resistors or lightbulbs, though.

--
Tim Wescott 
Control system and signal processing consulting 
www.wescottdesign.com

These sound like much better ideas than mine, frankly. I was forgetting that a VFD might have more to it than just outputting a fixed sinusiod. Why I should forget that when a bit of thought on how an inductance machine works and what it needs as far as voltage vs. frequency vs. load would cough up a different answer, I can't say (well, I can't say without making myself look bad :( ).

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com

Each to their own. Just the same i would buy, assemble, or rent a motor / brake assembly. That way you could test its performance over the range of loads from no load to locked rotor.

?-)

About 20 years ago I worked at a company who produced AC and DC motor speed controllers (polyspede.com). Some of our DC drives were regenerative. To test VFDs, we had DC motors mechanically coupled to the AC motors and used the DC regenerative drives to return the energy from the DC motor to the power line. It was possible to vary the amount of regeneration so as to control the load on the AC motor. BTW, just because you return the energy to the line doesn't mean that the power factor is good.

It is also possible to do the same with two coupled AC induction motors, but, unless times have changed quite a bit, you will probably not find many regenerative AC drives that return the energy to the line. Most of them burn up the energy with some sort of load on the bus (called dynamic braking, back then). Ours had a regenerative bridge.

For our customer, the VFD had to drive an induction motor which was coupled to a hydraulic motor. Then, when the hydraulic motor drove the induction motor, it had to load the hydraulic motor. The amount of energy meant that it was prohibitive to just dissipate (I think it was about 100Hp or so). That's the reason we put it back into the line even if the power factor was not good; it gave it a place to go.

Just thought for you. Good luck.

That's basically what I'd do, except I'd use a shunt wound DC motor as a generator. Generated voltage into a fixed resistive load, control by varying DC field current. The hot load can go somewhere outside, where it won't cook the operator. There must be lots of redundant DC shunt motors lying around, off retrofitted machine tools, cranes, etc.

--
"Design is the reverse of analysis" 
                   (R.D. Middlebrook)

The energy, or most of it, goes into the *load*, which can be fan- or water-cooled. A resistor bank, external to the generator, and even the building, if you want.

The armature will generate no more internal heat than it did running as a motor at full load. The current is the same, motoring, or generating. The armature resistance is the same. If it didn't need cooling as a motor, it won't as a generator. Those really big ones that did had a snail fan built on.

The rotor of an induction motor is solid, too. Sometimes having cast-in aluminum bars.

*All* the absorbed energy has to be dissipated in the rotor bars.

--
"Design is the reverse of analysis" 
                   (R.D. Middlebrook)

In the past I have seeen tanks of water (outside of course) useed as dummy loads just with plates hung in them and salt in the water (added carefully) One installation the alternator was 250 KVA so it was quite big tank although it may never have taken the full load. l-)

--
John G

I guess if had to do something like that all day you could get a grid tied inverter and stuff it back into the grid

I know a guy that has a "rolling road" in the garage for tuning cars, most of the load is inertia from the big rotating drum but it also has a brake afaict it is a rotating electro magnet between two water cooled iron discs

The cooling water is actually plumbed in to the house water heating

-Lasse

"Enigma Paul = PITA "

** Is that a VFD that operates from single phase power? ** Only if there is no filter electro after the rectifier - only possible with VFDs that take in 3 phase power. ** So it IS a *single phase input* VFD with 3 phase output.

The input current wave will be just like with any rectifier & capacitor oad - ie 120Hz half sine shaped pulses.

The problems you anticipate only happen when monitoring on the MOTOR side.

... Phil

They used to use salt water rheostats to control the starting rotor current of big 3-phase slip-ring induction motors. Steaming by the time the motor was up to speed and the rheostats were shorted out with the blades at the bottom of the tank.

--
"Design is the reverse of analysis" 
                   (R.D. Middlebrook)

Um. that is not the case when being forcibly turning way slower than rated speed. Especially not during "locked rotor" testing.

Well at least somewhere in the motor. Much can be dissipated in the stator windings as well.

?-)

What I was describing is a DC motor, used as a generator, into a fixed resistive load, and it's torque reaction controlled by controlling the field excitation. With the right choice of load resistance, and a limit on the maximum field current, it should be impossible to exceed either armature rated current or voltage.

If you want to try to lock it with a dead short, you're on your own.

--
"Design is the reverse of analysis" 
                   (R.D. Middlebrook)

Not so much the DC motor but the induction motor fed by the VFD (which must be able to cope for at least a little while). Meanwhile it is feeding tons of hash back into the mains which need to be measured. Lots of higher than mains frequency stuff there. Measuring all useful values while this is occuring is part of OPs question the way i see it.

?-)

[snip]

Squirrel cage fan with a damper to control air flow.

--
Paul Hovnanian     mailto:Paul@Hovnanian.com 
------------------------------------------------------------------ 
Incorrigible punster -- Do not incorrige.

Water pump, valve, bucket.

--

John Larkin         Highland Technology, Inc 

jlarkin at highlandtechnology dot com 
http://www.highlandtechnology.com 

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom laser drivers and controllers 
Photonics and fiberoptic TTL data links 
VME thermocouple, LVDT, synchro   acquisition and simulation