TECO product manager – variable speed drives Hamish Robertson explains how to unlock the best performance from your VSDs.
Variable speed drives (VSDs), when engineered correctly, transform industrial motor control by delivering precise operation, system-level efficiency, and reliability.
This article outlines key considerations for EPC (engineering, procurement and construction) and control engineers to achieve predictable outcomes from their motor systems.
A framework for control
VSDs are a mature and widely implemented technology across modern industrial control systems.
When applied with rigorous engineering discipline, they provide precise speed and torque control, enhanced process stability, and substantial system-level efficiency gains – often far exceeding the nameplate performance of the motor alone.
For utilities, EPC contractors, design authorities, and engineering teams, the real value of a VSD is realised not in the device as a standalone component, but in the predictability, reliability, and optimisation achieved when the motor, drive, driven equipment, and electrical network are engineered to operate as a fully integrated system.
This requires a practical, system-level framework for specifying and applying low-voltage VSDs in industrial motor control applications.
Key engineering considerations such as thermal performance, mechanical reliability, electrical insulation integrity, power quality, and network integration deliver reliable, repeatable, and specification-compliant outcomes across diverse industrial environments.
Drawing on proven drive platforms, this approach transforms VSDs from standalone components into governed elements of engineered industrial solutions, with disciplined engineering elevating these devices from mere components to integral system enablers.
System efficiency enablers
A VSD does not increase the intrinsic efficiency of an electric motor. Instead, it enables the motor to operate at the speed and torque required by the process at any given operating point.
When speed/torque variations are aligned with load characteristics, particularly in variable torque applications such as centrifugal pumps and fans, VSD-controlled systems achieve significant reductions in total energy consumption.
For centrifugal pumps and fans (quadratic torque loads), power scales with the cube of speed as per the affinity laws:
Using a VSD to reduce speed by 20 per cent cuts power to about 51 per cent (0.8³ ≈ 0.512), yielding 49 per cent savings vs. throttling. These improvements arise from system behaviour, not from motor nameplate efficiency, and are achieved through controlled speed reduction rather than mechanical throttling or bypass methods (which wastes energy as heat and pressure loss).
For EPC and control system engineers, efficiency must therefore be evaluated at the driven-system level, ensuring that electrical, mechanical, and process objectives are addressed concurrently.
VSD platforms with wide operating frequency capability and defined overload margins allow engineers to implement control strategies based on process requirements rather than drive limitations.
VSD and DOL operation
The use of direct-on-line (DOL), soft starters, liquid resistance starters or VSDs all remain valid within modern industrial design practice. The selection of either approach depends on process demands, network constraints, and mechanical system characteristics.
VSD-based systems provide:
- Controlled acceleration and deceleration, often requiring multiple ramp rates to suit process requirements
- Reduced starting current. VSDs typically limit inrush current to 100–150 per cent FLA (full load amperage), where a DOL typically sees inrush currents of 600–800 per cent or more FLA
- Accurate and dynamic speed regulation
- Adjustable torque delivery across the operating range. The ability to control frequency and torque independently of each other is a feature unique to VSD control.
These characteristics are particularly beneficial where mechanical stress management, frequent starting, or variable speed operation is required.
Where controlled transient performance is critical, defined overload capability becomes a key design parameter. VSDs typically offer 110–120 per cent overload for 60 seconds in normal (variable-torque) duty and up to 150 per cent (or 200 per cent briefly) in heavy (constant-torque) duty, depending on the drive classification.
This allows transient process demands to be accommodated without the requirement for excessive motor oversizing or instability due to motor overload limitations at system level.
Thermal management
In VSD-controlled motor systems, thermal behaviour is governed by the interaction between electrical losses, cooling method, and operating speed.
Most standard industrial induction motors rely on shaft-mounted cooling fans, with airflow proportional to speed. Effective engineering practice aligns duty cycle, load profile, and speed range with the thermal capability of the motor.
Where continuous operation at reduced speed is required, cooling strategy and motor selection are treated as integral design parameters.
Drive selection also contributes to predictable thermal performance. The use of VSDs can allow engineers to consider motor designs including higher pole count and look to over-speed them to achieve short-term process requirements. This enables a higher-pole-count motor to operate at its more efficient design speed for most of the duty cycle, while over-speeding briefly for peak demands.
Insulation integrity
Modern VSDs generate controlled AC (alternating current) waveforms using high frequency switching techniques. Long-term operational integrity can only be achieved through a coordinated approach encompassing:
- The use of appropriate motor insulation systems, which allow for high dV/dt transients as seen at the motor terminals
- Appropriately designed installation practices for specialised VSD cables
- Controlled inverter switching parameters. Adjustable switching frequencies can offer improvements to the electrical network, as well as audible and heating effects at the electric motor itself.
Alignment between drive characteristics and installation layout is paramount. Cable runs exceeding 50–100m often require mitigation; thresholds vary with drive switching frequency, motor type, and voltage – drive manufacturers can provide precise guidance.
Mitigation may include dV/dt filters to reduce high-voltage peaks (often used for 100–300m+ lengths), sine-wave filters (preferred where cables cannot be replaced or runs are very long), and output reactors to mitigate long-cable issues and limit winding stress.
The use of insulated bearings or bearing housings and shaft grounding brushes eliminate the potential of EDM (electrical discharge machining), also known as bearing fluting or EDM pitting, which can lead to premature bearing failure. When these elements are engineered together, inverter-fed motor control systems deliver consistent electrical performance over their service life, supporting predictable operation in industrial duty environments.
Mechanical reliability and component life expectancy
VSD-controlled systems allow torque and speed to be applied smoothly and predictably, significantly reducing mechanical shock during starting and stopping events, as well as varying loads.
This controlled behaviour contributes to the extended service life of shafts, couplings, gearboxes, pipework and driven equipment – factors that aren’t easily quantifiable, but can substantially extend service life (often by a factor of 2–5× or more in high-cycle or shock-prone applications), though exact gains depend on duty profile and baseline conditions.
At the system level, mechanical reliability is achieved through coordinated motor and drive selection, grounding practices, and mechanical alignment. Drive platform selection may also be influenced by the installation environment and enclosure requirements.
Power quality and network integration
As power-electronic devices, VSDs introduce harmonics, typically reducing THDi (total harmonic distortion) from 70–100 per cent unmitigated to 30–40 per cent with basic line reactors or DC (direct current) chokes, or less than 5–8 per cent with active harmonic filters or similar advanced mitigation. This ensures motors comply with IEEE 519 limits at the point of common coupling (widely applied in Australia alongside AS/NZS 61000.3.6 for voltage distortion).
Power factor can also be improved with appropriate mitigation. When designing modern electrical systems, managing harmonics efficiently is critical to meeting distribution requirements, controlling costs, and delivering robust long-term performance.
While low harmonic or active front end (AFE) VSDs have their place, they’re often not the most practical or economical solution for multi-drive installations in industrial and infrastructure projects.
AFE VSDs are well-suited to single, high-power dynamic loads, particularly where regeneration is required or where strict point-of-connection limits apply.
However, they come with trade-offs. These drives are typically large and heavy, increasing switchroom size, structural loading, transport complexities and costs. As an inline solution, they operate permanently regardless of load conditions, resulting in higher electrical losses, increased heat output, and greater air-conditioning demand.
In many cases, AFEs are over-specified relative to local distribution limits, driving up capital cost without delivering proportional system-wide benefit.
By contrast, active harmonic filters (AHFs) offer a system-level alternative. Installed as a shunt-connected solution, AHFs dynamically mitigate harmonics to predefined targets aligned with network and distribution requirements.
A single AHF, which can manage harmonics across multiple VSDs, making it the most cost-effective option for complex plants and facilities, deliver higher overall efficiency, lower heat loads, and reduced operating costs. This approach yields secondary benefits, such as smaller switchrooms, simpler logistics, and the ability to specify standard, compact, lower-cost VSDs, reducing both upfront capital and spare parts inventory. They also mitigate plant-wide neutral harmonic currents and can be easily scaled as loads change.
For EPCs and engineers, balancing compliance, constructability, and whole-of-life cost, AHFs offer a flexible and scalable harmonic mitigation solution engineered for the infrastructure and project requirements, not just the drive.
Effective system integration considers distribution design, coordination with upstream equipment, and communication with plant automation systems.
Specification discipline
Predictable project outcomes depend on clear definition at the specification stage. For VSD-controlled motor systems, this includes:
- Operating speed range and duty profile
- Continuous and intermittent load characteristics
- Environmental and installation conditions
- Cooling and thermal performance expectations
- Integration with electrical and mechanical systems.
Where space or simplicity is prioritised, select a VSD supporting both networked (e.g. Modbus, Profibus) and hardwired control on a common platform, providing defined control behaviour aligned with standard industrial duty profiles, while remaining consistent with system-level specification requirements.
A unified drive portfolio
Across industrial projects, a consistent engineering approach benefits from access to a complete range of VSDs designed for defined operating conditions and application requirements.
TECO’s low-voltage VSD offering includes models such as the TECO F510 (optimised for pump and fan variable-torque loads), TECO A510 (heavy-duty vector control for demanding applications), TECO E510 (general-purpose with advanced features), alongside the TECO E710 and compact TECO L510s, selected based on installation requirements, environmental conditions, and application-specific constraints.
This portfolio-based approach allows EPC contractors and control system engineers to apply a consistent design philosophy while selecting the most appropriate VSD for each project application.
The successful application of variable speed drives is defined by engineering control rather than component selection.
When motor behaviour, thermal performance, electrical characteristics, mechanical integration, and network interaction are addressed as a unified system, TECO VSD-controlled installations deliver reliable, efficient, and predictable performance throughout their operating life.
A system-level approach, supported by TECO VSD platforms engineered with defined operating characteristics, transforms the variable speed drive from a standalone device into a governed element of an engineered industrial solution.
TECO’s unified portfolio enables EPC teams to standardise on reliable, application-matched platforms while maintaining flexibility across project requirements.
To learn more, visit teco.com.au
This feature appeared in the Autumn edition of Pump Industry.
