Variable Frequency Drives
In the days before variable frequency drives, it was impractical to use large motors on 480V systems because of the cost and the disruption that inrush would cause to the grid. Before variable frequency drives became more widespread, it was common to use a medium-voltage starter to control large motors. It was the only option, really.
Low-voltage drives flipped the math and completely eliminated inrush. However, some specifications have been stubbornly hanging on to using medium-voltage controllers because of limitations that date back to the 1960s. Lets think about a technology invented in 1960 that we still use today? Can you think of any. We would say not many, so why use old technology in your modern oilfield?
Times — and more importantly, the technology — have changed. Today, low-voltage drives make more sense from almost any perspective for motors of 1,000HP or less.
The math is simple and undeniable.
Consider, first the cost of a medium-voltage motor compared to a low-voltage motor:
Savings on the motor:
Consider a typical situation in a typical oilfield. What we have here are two drive solutions, one 480V, one 4160V. Both are used in upstream and midstream applications on motors with 1,000HP or less. But when you study the comparison, the only question you should ask is, “Why?”
4160V 101A maximum NEMA 1 VFD inside of A/C structure
18.4kW (assumed 95% motor efficiency)
A/C Power Consumption
25.4kW per pump, 96% system efficiency
480V 920A VFD NEMA 3R with harmonic filter
11.8kW (assumed 95% motor efficiency)
Harmonic Filter losses
14.5kW per pump, 97.7% VFD efficiency
Medium-voltage drives cost more than low-voltage drives. Significantly more. The drive costs more, and you need more infrastructure to house and protect the drive. True, it costs less to install a medium-voltage drive, but that cost is a fraction of the overall cost differences between the two options.
There are also start-up costs. Low-voltage techs are readily available in most markets. Medium-voltage techs, who require much more training, are far less common and cost more. You typically have to schedule them well in advance for start-up work.
Not only are 4160V drives more expensive initially, for applications of 1,000HP or less, they are more expensive to operate. Considering that 800-1,000HP motors are the most common in the oilfield, a medium-voltage drive is simply overkill. Compare energy usage of our two options on one pump.
4160V = 25.4kW
480V = 14.5kW
Energy savings = 25.4kW – 14.5kW = 10.9kW
At 10 cents per kWh, that is
$1.09 per hour
$26.16 per day and
Total Savings on the Motor: $8,890.00
Total Savings on the Drive + Installation: $95,500.00
Total Operational Savings (x 5): $47,742.00
Total Savings for one pump: $152,132.00
Beyond the simple comparison of cost, there are a number of reasons why 480V drives are a much better option.
Fear. 4160V drives are inherently more dangerous. They require special skills to install, maintain and repair — skills that aren’t readily available. In most markets or in a crisis, there are far fewer professionals trained in this kind of work, so you can be left waiting for weeks.
Finding professionals to work on a 480V drive is easy. They are readily available in every market. They are less expensive. In a crisis they can be on site in days, if not hours.
Uncertainty. At SPOC, we stock every part for every drive we manufacture — for both new production and service calls. If something breaks, we can have a replacement part to you within 24 hours. More importantly, our suppliers stock parts all over the country. In many cases, we can have a part on site in an hour.
If you need a critical replacement part for a 4160V drive, you’ll wait weeks.
Fear and uncertainty breed distrust. The simple truth is that the harsh conditions of the oilfield wreak havoc on all technology at some point. What happens when a drive is down and your production grinds to a halt? How quickly, and safely, can your supplier get you back up and running?
And one last thing: Innovation. There are literally billions of low-voltage drives in use today. So guess where manufacturers are putting all of their innovation efforts? Almost all of the biggest advances in drives are happening in that arena. It’s simple economics. In the oilfield alone, 480V drives are far more common because they are an easier, smarter, more efficient method for controlling lower-horsepower motors. They just make sense.
And if these three arguments aren’t convincing, we also offer DriveShield, the industry’s most comprehensive extended warranty.