What Drilling Contractors Need to Know About Motors
At the heart of every well operation is a pump drive system that you may know very little about. But you should.
- Powering Your Pumping System
- Drive and Motor Compatibility
- Drive Panels: Closed vs. Open-Loop Systems
- The Bottom Line
What you don’t know about heavy-duty pumps, the electric motors that drive them and the pump control panels that protect them CAN hurt you. Many drillers don’t realize, for example, that these components will not necessarily work properly together as a system unless they are appropriately specified and installed. Some components may have been chosen for their low purchase price, without necessarily factoring in the features required to deliver trouble-free service in the years after the product warranty expires. Even when problems are covered under warranty, you must still often deal with an inoperative system requiring urgent repair, as well as the time and money spent resolving the issue.
The more you know about the features on these systems, in other words, the better able you will be to ensure optimal performance at the lowest overall cost.
Here is a quick primer to get you started.
A good place to start is with the pump control panel. There are three basic kinds: across-the-line, soft-start and variable-speed.
With an across-the-line pump panel, the motor gets full voltage and immediately comes up to speed the moment the operator turns on the switch. These simple control panels are comprised of a disconnect, a set of fuses and a contactor that connects the motor directly to power line. The pump itself runs at “synchronous speed,” that is, the speed at which a motor spins when 60 Hz power is applied and it has no load, minus a little bit called “slip.”
If you are operating a large motor, however, you may not want to bring it up to full speed immediately. Large motors typically draw more current at startup, placing a heavy load on the electrical grid, which can lead to power disturbances. That is why some pumping systems use a soft-start pump panel. The contactor on these panels is replaced with a soft starter that brings a motor to full synchronous speed gradually. A soft-start can also minimize well water draw down — which is important, given that pumps pull a significant amount of water from a well at startup. If water is drawn from a well faster than it is replenished, the water level inside the well drops, putting the well walls at risk of collapse.
Many well operations today opt for the third option: variable-speed pump panels. Instead of a soft starter, these panels include a variable frequency drive (VFD) that makes it possible to slow down the motor and reduce the flow. VFD panels, in short, provide an efficient way to control the flow. VFDs also consume less energy than other approaches. A well-designed pump panel makes it possible to slow the pump so that it operates at peak efficiency regardless of flow, minimizing the amount of power it consumes.
A well-designed VFD panel can be invaluable in a well operation. But it can also be hard on the motors it drives, as well as local power lines, due to something called pulse width modulation (PWM).
The PWM voltage waveforms created by drive panels are shaped differently and greatly exceed the line voltage supplied by the electrical utility. Because PWM can damage motor windings and bearings, it is important to use inverter duty motors that are built to withstand the abuse of these waveforms.
When selecting a vertical motor intended to run on an inverter, it’s important to choose one that exceeds NEMA MG-1 Part 31 specifications. Keep in mind, however, that this specification was created before the full effects of PWM voltages on vertical motors were understood by industry, and only addresses motor insulation protection, not thermal and bearing protection. Additional inverter duty vertical motor features, such as motor winding thermostats, insulated bearings and shaft grounding rings, also help extend the life of vertical motors run on inverters.
PWM can also introduce disturbances (called harmonics) on power lines that interfere with the operation of the equipment attached to the lines. Electrical utilities have guidelines and policies for limiting disturbances introduced by VFDs. If your operation causes disturbances, you may be required to make adjustments that help to minimize any harmonics introduced onto the grid.
Installing a line reactor, either as part of the drive or as a separate component in the drive panel, usually accomplishes this objective. In a minority of instances, it may be necessary to install a more costly harmonic filter between the drive and incoming power.
Choosing the right drive panel is also important. Its job is to make a motor run at the required speed and torque. Line voltage always enters the drive running at 60 Hz, which it transfers into DC voltage and then back to AC at the frequency and voltage needed to control the speed of the pump set, or torque. A tiny computer inside the VFD coordinates these changes.
While the drive can make these changes automatically, it needs sensors to serve as its “eyes and ears,” regularly measuring and feeding it information on flow, pressure and motor temperature. Drive panels can also be designed to react to warnings sent by these sensors, such as reports of overheated or inoperable motors.
In a closed-loop system, sensors constantly feed this information back to the drive, which it uses to make the adjustments automatically. Today’s “smart” systems also simultaneously alert end users about the adjustments taking place via text message or other communication.
Not every well operation, however, requires that level of automation, which is why some prefer an open-loop control system. With these systems, the user simply sets a speed on a panel, and the controller inside the VFD listens to and acts upon that command. Some drive manufacturers also provide apps that drillers can use to control the system from their phones.
A well-designed pumping system should also deliver years of efficient operation. The best systems have similar features, including:
- Features that protect against lightning and power line disturbances, both of which can cripple a pump panel’s operation.
- Control schemes that assess power coming into the motor and alarms that alert you to system problems.
- Enclosures that protect the panels from the elements, as well as vents, fans and other features to keep the panel cool in extreme heat.
- Communication capabilities that meet the end user’s requirements, whether that means setting speed and flows, or reporting information in the manner the user is equipped to receive it.
- Inverter duty vertical motors that have features designed to protect them against the effects of PWM waveforms for extended motor life.
What does all this mean?
It means that when tasked with selecting or recommending components to power a pump, look beyond purchase price. Balance your customer’s budget with the pump system needs. Tell your motor supplier how you plan to power the motor. Discuss with your pump panel builder how the pump system is physically laid out and powered. A manufacturer that offers both the motor and pump panel can guide you through this process.
It’s ultimately your job to make sure the components selected for the system will actually perform well together and include the features that help assure the system will have a long and productive life.