VFD's and Reciprocating Loads 3

[Thealanator]

Anyone have experience with VFD's on motors that drive reciprocating pumps or compressors? My gut tells me the pulsating currents would not be good for electronic components.

 

[jraef]

It's OK as long as you know what you are doing. The problem isn't the pulsation, it's regeneration. On the "down stroke" of reciprocating loads, the motor can over speed slightly relative to the VFD output frequency and regenerate, then the VFD absorbs that. But in a lot of cases the regen may end up being more than the VFD can absorb and it charges up the DC bus to the point where it eventually trips on Over Voltage. There are several mitigation strategies now for that.

One is to use a drive with a feature designed for this application, which basically monitors the shaft load on the fly very quickly and when it sees the load drop, it backs off on the excitation to help avoid the regen. This is essentially a "freebie" feature in that it takes no added hardware, but not all drives have it, and those that do will not be the low end versions.

Another way is to install dynamic braking, where a resistor bank is attached to the DC bus with a braking transistor. When the DC bus voltage climbs to an unacceptable level, the transistor fires and dumps energy into the resistor. This however requires that dynamic braking package hardware to be added to the VFD, plus you have to deal with the heat from the resistor bank.

A third way is to use a Line Regenerative Drive, a special version that is essentially like two drives in inverse parallel. So instead of a simple rectifier front end, you have a transistor based front end that is used to pump the regen energy from the motor back into the source. There are several versions of this to consider, but they all involve more hardware and is the most expensive option.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
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[zdas04]

Jref,
As usual an excellent explanation of the electric side. If I can butt in and talk about the driven side ...

Not all recip pumps and compressors are the same. What jref described is a single-acting device with a small number of cylinders (1-2). By single acting I mean that only one side of the piston does work. Most (not all) recip compressors are double acting so the load is required on both the upstroke and the downstroke and there is not "coasting" step. Most recip pumps (again not all) are single-acting and you need to be careful about places in the crank revolution where the device is not doing much work. All double acting recips and any single acting recip with three or more cylinders will look to the VFD much like a rotary load. These are positive displacement devices so the benefit of the changing the speed on the economics isn't huge (but there are many process reasons where a VFD is a great idea).

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[Thealanator]

Great information from both of you! Thanks!

 

[jraef]

Yes, I was not actually considering reciprocating compressors as much as things like jack pumps, vibrating machines, things like that where the load is not mechancially balanced or continuous.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[waross]

Many reciprocating machines depend on the flywheel to smooth out the torque pulsations and as a result the current pulsations. Beware of running your machine so slow that you lose too much flywheel affect.

Bill
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Jimmy Carter