Starting large induction and synchronous motors on weak power systems creates high inrush currents that depress system voltage and threaten grid stability. VarStec’s MotorVAR™ is a system-fit, medium-voltage capacitor bank designed to provide precise, transient-free reactive power locally at the motor bus during the starting cycle. By supplying the necessary magnetizing VArs at the point of use, MotorVAR™ preserves available starting torque and reduces voltage sag without the cost, harmonic distortion, or maintenance complexity of VFD or RVSS systems. This page answers frequently asked technical questions about MotorVAR™ sizing, motor torque support during starting, and the engineering calculations used to confirm a successful motor start.

Technical Q&A

A fundamental challenge of motor starting is that starting torque is proportional to the square of the motor terminal voltage (Torque ∝ V2). Traditional reduced-voltage methods, such as line reactors or RVSS, limit inrush current by intentionally dropping the voltage at the motor terminals, which significantly constrains available torque.

In contrast, the MotorVAR™ system acts as a local reactive power source. By connecting a precisely sized capacitor bank in parallel with the motor during the start, the system neutralizes a portion of the motor’s inductive inrush. This supports the bus voltage, ensuring that the motor terminal voltage remains high enough to develop the torque necessary to overcome the process load.

Determining the optimum capacitor size for a MotorVAR™ application is not a simple one-size-fits-all kvar selection. It requires evaluation of the motor, the connected load, and system and source impedance data. Using the MotorVAR Sizing Tool, VarStec engineers evaluate three primary criteria:

  1. Voltage Sag
    The tool calculates the expected voltage at the motor bus, main plant bus, and point of common coupling (PCC) to confirm acceptable system performance during motor starting.
    • Motor Bus Voltage: Confirms that the motor maintains adequate voltage during starting to develop the required torque. This value is typically evaluated at 85% to 90% of nominal system voltage.
    • Main Plant Bus Voltage: Confirms that plant voltage remains within acceptable power quality limits during starting. This value is typically 90% of nominal or better.
    • PCC Voltage: Confirms that voltage sag at the utility interconnection meets applicable interconnect requirements. This value is typically 96% of nominal.
  1. System Inrush Current
    The tool calculates the net current drawn from the upstream source so the MotorVAR can be sized to reduce inrush current and help keep the starting event within utility or system interconnect requirements.
  2. Motor Starting Torque
    The tool calculates the electrical torque available during motor starting. This value must then be compared against the connected load torque requirement to confirm a successful start. VarStec typically recommends maintaining a torque margin of at least 50%.

Yes. In some severe applications where the system is exceptionally weak, the VarStec MotorVAR + Line Reactor configuration is utilized. In this scheme, the MotorVAR™ capacitor is connected to the secondary bus while a line reactor is inserted in series with the motor.

The sizing tool models this combined impedance of the motor and the reactor (ZT=ZM+ZR) in parallel with the capacitor’s capacitive reactance (ZC). This “Hybrid” approach allows for maximum inrush current limitation at the source while using the capacitor to “boost” the voltage at the reactor’s line-side, ensuring the motor still receives enough voltage to accelerate the load successfully.

MotorVAR™ is a viable alternative to a Variable Frequency Drive (VFD) for motor starting when the motor is intended to run at full speed and only requires reactive power support during the starting event. In these applications, the engineering problem is not variable speed control, but maintaining sufficient motor terminal voltage and starting torque while limiting upstream voltage sag and source current. MotorVAR™ addresses that problem directly by supplying local reactive power at the motor bus during starting, allowing the motor to start successfully across the line without the added complexity of a medium-voltage drive system.

Compared with a VFD-based starting solution, MotorVAR™ offers several practical advantages in fixed-speed applications:

  • Lower installed complexity: MotorVAR™ avoids the bypass switchgear, synchronization logic, and associated control complexity often required with VFD-based motor starting systems.
  • No steady-state harmonic source: MotorVAR™ does not introduce the characteristic harmonics associated with variable speed drives.
  • No power-electronic dependence: The system is based on conventional medium-voltage capacitor bank technology rather than large proprietary drive assemblies.
  • No dedicated drive enclosure requirement: MotorVAR™ does not require the conditioned E-House space often associated with large medium-voltage drives.
  • Standard maintainable components: The system uses conventional capacitors, fuses, relays, switching devices, and controls that plant personnel are generally familiar with and can maintain without factory drive specialists.
  • Plant-wide starting coverage: A single MotorVAR™ system can be designed to support the starting of all large motors in a plant. This improves the economic case because one shared MotorVAR™ installation may replace multiple dedicated motor starting devices.

From an engineering standpoint, MotorVAR™ is most attractive where variable speed operation is not required and the real objective is to start a large motor successfully on a weak system with the simplest practical medium-voltage solution.

Related Technical Topics

VarStec’s Engineering-Led Approach to MotorVAR™ Motor Starting

VarStec applies practical engineering experience to the design, sizing, and manufacture of MotorVAR™ medium-voltage motor starting systems for large fixed-speed motor applications. MotorVAR™ is a cost-effective alternative to VFD-based starting solutions when the engineering objective is to maintain motor terminal voltage, preserve starting torque, and limit upstream voltage sag without the added cost and complexity of medium-voltage drive systems. To support proper application, VarStec provides the MotorVAR Sizing Tool to help EPCs, consultants, and end users evaluate voltage sag, inrush current, and starting torque requirements for a given motor and power system. VarStec can also assist directly with the engineering review, sizing, and final design of the MotorVAR™ solution, delivering a System-Fit™ motor starting package engineered to meet the performance, reliability, and economic objectives of the project.