VFD vs DOL Selection

Introduction

This article presents the criteria for the selection of the method of starting and operation of medium voltage motors.

The various criteria were presented and discussed for assessing the basis of selection of the recommended method of starting and operation of the motors.

Induction Motor Starting and Operation Method

The following two methods (options) for starting and operation of the motors were studied and evaluated:

1. Direct Online Starting
2. Variable Frequency Drive (VFD)

Direct-on-line (DOL) starter provides a simple and cost-effective way to start and stop motors. Theses are easy to install, operate and maintain. On the other hand, a variable frequency drive (VFD) offers a precise control over motor speed, allowing for energy savings and enhanced motor performance. VFDs are ideal for applications that require speed modulation or torque control.

To determine the best choice for the motor control needs, it is important to consider several factors. Understanding the advantages and limitations of both DOL starters and VFDs is crucial in making the right decision.

So, whether the requirement is a basic and economical motor control solution like DOL starters or require advanced features such as speed control and energy efficiency offered by VFDs, this article will weigh the pros and cons, ultimately serves as a guideline towards the best choice for the motor control requirements.

Selection Criteria

Direct-on-line (DOL) or Variable Frequency Drive (VFD) methods will be analyzed on given criteria provided herein. The analysis shall be performed on each criteria listing their respective strengths and weaknesses which will then be collectively weighted. The method of starting which will emerge as more beneficial to the project requirement shall be duly selected.

Some criteria to be considered during selection are:

• Operational Flexibility
• Production Efficiency
• Physical Dimension
• Life Cycle Cost and Economics
• Troubleshooting, Maintenance and Reliability
• Design and ESP Performance Impact
• Impact on Power Supply Network
• Energy Consumption and Sustainability

Operation Flexibility

DOL starter is a high risk to the motor due to its high starting current and has a very tight operating range to adapt to operating condition changes.

VFDs have a higher reliability and safety during motor start-ups. VFDs are more flexible to adapt to the changes in operating conditions.

Production Efficiency

DOL starter have a relatively lower production efficiency as it uses mechanical throttle device to control process flow, requires shutdown for transformer tap change, and has a narrow operating range to force motor to operate outside its operating range.

VFD is highly efficient in production as it used frequency adjustment to control process flow, no transformer tap change, no shutdown required for all well condition changes, and large operating range allowing motor to operate inside its permissible range hence extending motor life.

Physical Dimension

DOL starter comprises of two major components,

1. Indoor Equipment,
2. Outdoor Equipment.

The indoor equipment of DOL occupies less space than VFDs, however, DOL requires an additional space for an outdoor transformer while for VFDs the transformer is integral part of the indoor VFD equipment.

Life Cycle Cost and Economics

Initial Cost of Equipment

DOL starters have a lower initial equipment cost compared to VFDs. The initial investment cost for DOL starters is lower because they do not require complex electronics for motor control.

VFDs are generally more expensive upfront compared to the DOL starters due to the additional electronics and control mechanisms required for speed regulation. However, the potential energy savings and improved efficiency provided by VFDs can often justify the higher initial investment over the lifespan of the equipment.

Cost of Installation

DOL starter requires a larger area for the equipment footprint due to the outdoor equipment (i.e. power transformer) while VFD only requires a smaller area for its indoor equipment. Due to the larger footprint required to locate the outdoor equipment, the total cost of installation is higher for DOL.

Operation and Maintenance Cost

As the motor speed is fixed for a DOL, the energy consumption during the motor operation will remain constant. A mechanical throttling device (i.e. choke valve) is required to limit the flow in a variable flow process. Although this is an effective means of control, mechanical and electrical energy is wasted.

Figure 1 Energy Saving -Throttling Devce vs VFD

As VFD alters the power frequency to the motor, speed, flow, and energy consumption are reduced in the system. The comparative maintenance cost for DOL and VFD systems can vary based on several factors, including the complexity of the equipment, the operating environment, and the application requirements. The operating environment and application requirements for both systems will be the same hence comparative maintenance cost will be based on the complexity of the equipment.

DOL starters are relatively simple in design and have fewer components that can wear out or require regular servicing. However, DOL starters can experience high mechanical stress during motor startup due to the abrupt application of full voltage, which may lead to more frequent maintenance or replacement of ESP over time. The frequent maintenance or replacement of the motors makes the OPEX for DOL higher than VFD.

In addition, the moving parts of the conventional switching element need frequent inspection and maintenance. Failures in the moving parts of conventional circuit breakers due to an unusually high number of switchings are a phenomenon that increases the maintenance costs of DOL and may cause downtimes in production.

VFDs are complex electronic devices consisting of power electronics, control circuitry, and cooling systems. Maintenance requirements for VFDs may include periodic inspections, firmware updates, and replacement of components such as cooling fans, capacitors, and power modules. In addition, the requirement for skilled personnel to perform VFD maintenance tasks is essential.

VFDs are more susceptible to environmental factors and temperature fluctuations, which may require additional maintenance measures to ensure reliable operation. Proper regular preventive maintenance is crucial for maximizing the lifespan and reliability of VFDs.

Given the above, the operation and maintenance cost of DOL is higher than VFD.

Loss of Revenue

As there is wide uncertainty to evaluate the ESP performances which depend on reservoir conditions / well performance, considered loss of revenue is based on the following two cases: Case 1: Production Optimization Gain assuming well with water cut of 60 to 80 %, VFD can deliver additional production up to 500 to 1000 BPD production where as DOL can deliver up to 200 to 400 BPD. Case 2: Productivity loss by 50%, assuming water cut 60 to 80 %, VFD can maintain production of 500 to 1000 BPD where as DOL could be zero production due to potential ESP failure as it will be operating in downthrust conditions. In the above production scenarios, VFD can deliver higher production volume compared to DOL.

Troubleshooting, Maintenance and Reliability

DOL has a limited flexibility for troubleshooting related to well conditions compared to VFD which a better flexibility to changing well conditions. However, DOL is easier to troubleshoot due to lesser component in contrast to the complexity of a VFD.

Despite being easier to troubleshoot, DOL requires more maintenance, in particular the outdoor transformer. Based on actual site experience, VFDs are more reliable due to less breakdowns compare to DOL.