Surface Mount - NOT!

[Ham4CW]

I work with components of all shapes and sizes, most are run of the mill through hole or SMD components.

While walking past the 'scrap' area at work a couple of resistors caught my eye, and I just couldn't resist taking a few snaps of them.

If you look closely you will see my red/burgundy coloured pen resting on one of the resistors (to give you an idea of the scale of them).

The resistors are 26" long by 6 1/2" diameter. Their component value? 10 Ohms @ 15,000 Watts!

They are wire wound types, or rather ribbon wound (three metal ribbons run in parallel around the ceramic formers, and each resistor has two steel end plates/mounting brackets).

Wow!

73, Mark...

 

[GM0WEZ]

Five would be ideal for a QRO dummy load, were it not for the inductance (which I imagine would be huge).

 

[CT2GXW]

What a beast! Do you know what they were used for?

 

[Ham4CW]

Five would be ideal for a QRO dummy load, were it not for the inductance (which I imagine would be huge).

Maybe that could be cancelled out with a suitable amount of capacitance. ;D

I never thought to measure the inductance at the time, I would guess at a few hundred micro Henries perhaps?

Could be a good competition, "Roll up roll up, guess the inductance of the 10 Ohm 15kw resistor!"

I'll have to drop the LCR meter across one when I'm back at work after the weekend (assuming they are not blown of course).

73, Mark...

 

[Ham4CW]

What a beast! Do you know what they were used for?

I didn't see what item they came out of, though generally any high wattage resistors of that size/type would form part of the brake circuit for an industrial drive unit.

When you try to stop a large industrial electric motor rapidly it becomes a generator, and the more of that energy you can 'absorb' so the more quickly you can stop it. Some industrial drives do this by regenerating the energy, dumping back in to the mains supply, others simply drop a resistive load across the motor.

You could stop the motor by simply shorting it out, but the windings do not last very long if you do that! But, if you add a resistor in series then the energy will be dissipated as heat (in the resistor rather than in the motor windings).

If you only need to stop the motor once in a while then low wattage resistors can be used since the duty cycle is very low. But if you have a large, fast moving machine with lots of mass, and you want to keep stopping it frequently, then the duty cycle will be a lot higher so high wattage resistors are required to cope with the heat involved.

Some drive systems use a combinations of both ideas. If you have a bank of say four or five drives all sharing the same power supply then it is possible to dump the surplus energy back in to the power supply reservoir capacitors for the other drives to use. But, if there is more energy recovered than can be used at any moment in time then the excess is dumped in to a resistor or resistor bank. It all depends on how large the system is and what company designed it.

Older drive systems tend to use brake resistors only. Newer more modern drives use regeneration primarily so as to recover the energy rather than waste it.

If you need help getting to sleep there is some info here >> https://control.com/textbook/variable-speed-motor-controls/ac-motor-braking/

Hope that lot makes sense! ;D

73, Mark...

 

[CT2GXW]

Thanks for the explanation Mark, it makes sense. I know the concept of regenerative braking, and to use a resistor instead makes sense.

From a mechanical point of view, It's confusing to think why dissipating the electric energy available at the terminals makes a free spinning motor come to a stop. Some mechanical force seems to be missing. But in terms of energy conservation it does make sense. In the end, it's an electric generator, and if we dissipate the electric energy at the terminals, then of course more energy will be necessary on the rotary part to keep the voltage potential, or else it comes to a stop.