Electric Motor Variable Load

For VFD testing I have found that a pump and water load seems to work by far the best (and is cheap too).

http://cr4.globalspec.com/comment/214539

http://cr4.globalspec.com/comment/482133

This is what we use to test VFDs. The eddy current or other similar loads or mechanical brakes do not work as well (for various reasons), especially at low speeds or with large speed differences.

looks like a good solution, thanks.

But as it's my first crack at VFDs bench testing I have some more (newbie) questions.

The range of the loads will be anything between 0.5 to 7hp, the test will be functional so I'm guessing the tank doesn't have to be that big, but how small/big, 50l, 100l?

As to the choice of the motor I'm not quite sure the way to go, motor-pump combo for higher loads, resistive for smaller?

I just got my hands on this motor-pum:

motor-pump

MTH4 40/4 that looks to me ideal for the application, when comes to assembling just have to find the right tank.

It's a bit small, just 2hp but it's something I could start with.

If I get another one like this around 7-10hp mount it on the same tank..what do you think?

Thanks.

For testing VFDs I prefer a motor load because it is more representative of a motor (inductance, resistance, etc) and dissipates far less heat than a resistive load. resistive loads such as heaters work fairly well as inexpensive resistive loads but I would not personally recommend them for VFD testing.

The water tank needs to be large enough to allow the aerated outflow water introduced into the top of the tank to not interfere with the pump in flow (otherwise the motor load won't remain constant). The motor pump combo works well at both high and low loads (the motor load is directly controlled by the mechanical water outlet valve), the only disadvantages are that it is a bit bulky and it is full of water. One other thing to note is that the internal friction in the motor pump will actually heat the water up over a long period of time (I was quite surprised when I saw that the first time).

If I get another one like this around 7-10hp mount it on the same tank..what do you think?

As long as the tank is large enough you could.

Attached are some pictures of our working 1.5hp motor pump water load. Dimensions are 540x540x490mm, constructed out of fibreglass with an effective volume of about 100L.

The black gunk in the tank is accumulated motor pump grease (I haven't cleaned the tank in 4 years).

As a really rough estimate I would guess a tank with a volume of twice to three times this should be ok for a 7-10hp motor for long duration tests (eg- 8 hour operation at full load).

One final note, make sure the tank seals are solid as you don't want water leaks. also, make sure you have a drain valve included so you can empty the tank.

very helpful, thanks.

Will let you know the results.

here is my 'contraption':

Groundfos motor-centrifugalpump combo (1.5kw), Danfoss VF3 valve driven by AMV35 control I control remotely from my bench, oh, I've almost forgotten a 180l hot water tank that I got from recycling center for free.

Some welding, pipe & wiring work & voila!

The 2nd pic is of American Rotary Rotary Phase Converter (10HP), that I can recommend to anybody that would need 3-phases in 1-phase (240V) wired environment.

Phase unbalance between .8 to 2.5% with the above load.

Thanks again for pointing me in a right direction.

Centrifugal pump has that advantage that doesn't require any over pressure valve, max load is when there is maximum flow.

Thanks again for pointing me in a right direction.

Your welcome.