Medium Voltage (MV) Soft Starter for motor start & stop

Medium Voltage (MV) Soft Starter

A Medium Voltage (MV) Soft Starter is a solid-state motor starting device designed to start and stop medium-voltage AC motors smoothly by gradually increasing and decreasing the applied voltage. It reduces the high inrush current and mechanical stress associated with direct-on-line (DOL) starting. Most MV soft starters use thyristors (SCRs) for voltage control and a bypass contactor to eliminate power losses after the motor reaches full speed.

Typical Voltage Range
·2.3 kV
·3.3 kV
·4.16 kV
·6.6 kV
·11 kV
·13.8 kV (some models)

Main Components
·SCR (Thyristor) power modules
·Vacuum bypass contactor
·Vacuum isolation contactor
·Current transformers (CTs)
·Voltage transformers (VTs)
·Microprocessor-based controller
·Protection relays
·Cooling system (air or liquid, depending on rating)

Working Principle
·Start command is issued.
·SCRs initially apply reduced voltage to the motor.
·Output voltage gradually ramps up according to the programmed acceleration time.
·Motor current and starting torque increase smoothly.
·Once rated speed is reached, the bypass contactor closes.
·SCRs are bypassed, and the motor operates directly from the supply with minimal losses.
·During stopping, controlled voltage reduction provides a smooth deceleration if soft-stop is enabled.

Key Features
·Adjustable starting current limit
·Adjustable voltage ramp-up/ramp-down
·Soft start and soft stop
·Kick-start function for high-inertia loads
·Built-in motor thermal protection
·Phase loss and phase imbalance protection
·Overcurrent and overload protection
·Undervoltage and overvoltage protection
·Communication protocols such as Modbus, Profibus, Profinet, and Ethernet/IP (model dependent)

Advantages
·Reduces starting current to approximately 2–4 × rated current (compared with 6–8 × rated current for DOL starting).
·Minimizes mechanical stress on shafts, couplings, gears, and belts.
·Reduces voltage dips on the power network.
·Extends motor and driven equipment life.
·Lower installation and operating cost than a Variable Frequency Drive (VFD) when variable speed is not required.
·Improves pump operation by reducing water hammer.

Typical Applications
·Centrifugal pumps
·Water treatment plants
·HVAC fans
·Compressors
·Conveyors
·Crushers
·Ball mills
·Blowers
·Industrial process motors
·Mining, cement, steel, oil & gas, and power plants

Typical Technical Specifications
·Rated Voltage: 2.3–13.8 kV
·Rated Power: 200 kW to 15 MW+
·Frequency: 50/60 Hz
·Starting Current: Adjustable (typically 2–4 × FLA)
·Starting Time: 1–60 s (adjustable)
·Protection Degree: IP31, IP42, IP54 (depending on enclosure)
·Cooling: Natural air or forced air

Leading Manufacturers
·ABB
·Siemens
·Schneider Electric
·Rockwell Automation (Allen-Bradley)
·WEG
·Motortronics
·AuCom
·Toshiba
·Eaton

MV soft starters are widely used where smooth starting and stopping are required but continuous speed control is unnecessary, making them a cost-effective solution for large fixed-speed medium-voltage motors.