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Difference between Soft Starters, VFDs and Servo drives

The various ways of starting your electric motor show technological advancement. They are: Direct-On-Line (DOL), Star-Delta and Solid-State electronic devices.

  • The DOL starter pulls current equivalent to the motor’s full load. The starting current is about 5 to 6 times the running current of the induction motor causing large voltage drop making the motor run hotter and eventually burn out. This voltage drop not only shortens the motor’s lifespan but also causes malfunction of the other devices connected.
  • The Star-Delta starter however, draws much less current compared to DOL starter (about one-third of the DOL current). It starts the motor on star topology – because this topology offers reduced starting current – until full load before it switches to delta. However, a requires a lot of wiring.
  • The Solid-State Electronic Devices include: Soft Starter, Variable Frequency Drive (VFD), and Servo Motor.
  • 1. A Soft Starter limits the surge of the inrush current thereby resulting in a smooth and gradual start-up while avoiding the complexity of Star-Delta wiring.

    2. A VFD however goes an extra step by allowing the frequency of the motor to be varied and can get feedback on its state but without taking any action as a result of the feedback. A soft starter however only soft starts and stops the motor.

    3. With a Servo Drive, physical properties like speed, torque and position of a motor are what can be varied. The feedback system in a servo drive compares the real and desired variables and automatically adjusts the value until the desired value is achieved.

    As a PLC programmer, knowing how each of these drives work and their peculiarities is not first of all to prefer one over the other but to increase expertise on the scenarios where each drive can be used in the industry.