Motor starters are used to handle the high electrical current that motors draw momentarily when they are started from standstill. The starters are also used for protecting the motors from the excessive heat of overloads during regular operation. Motor starting current can be several times of what the motor draws at its operating speed. Usually a fuse or circuit breaker will be used along with the starters, for protection.
These are the type of starters widely used for compressors.
- DOL- Direct Online Starter
- Start Delta Starter
- Soft Starter
- VFD – Variable Frequency Drive
DOL Starter
These starters are used for smaller motors wherein the normal voltage is supplied through the contactor on/off mechanism. The starting current is usually high in these type of starters i.e. 3 to 5 times the full load current.
Star Delta Starter
The motor is initially started with a star-connected stator winding. Once the motor reaches a certain speed or a certain amount of time has passed, the motor will run in the normal delta-connected stator winding. Starting with a star connection reduces the voltage across each winding and also decreases torque. The switch from star to delta is about 5-7 seconds and this results in a spike during the transition. Typically start delta starters are offered for 7.5 KW motors & above which is typically application dependent.
Soft Starter
Soft starter continuously controls the motor’s voltage supply during the start-up phase. This way, the motor is adjusted to the machine’s load behavior. Mechanical operating equipment is accelerated smoothly.
Comparison of Star Delta Starter and Soft Starter
- Number of States:
Star delta starters have two states i.e. low voltage, and full voltage, that the starter transitions. Soft starters, on the other hand, start-up gradually. They can have infinite states within the limits of the controlling electronics and startup requirements. - Starting Time:
StaSoft starters allows control of the starting time, while star-delta starters do not. Switch from star to delta takes 5-7 seconds with a spike during transition. - Torque Control: Soft starters offers dynamic torque control, while star-delta starters, can’t adjust the starting torque.
- Open Transition and Power Loss: With star-delta starters, between the star and delta connection, there is an open transition that can result in current transients and high torque. With soft starters, there is no such open transition and no power loss.
Variable Frequency Drive (VFD)
A variable frequency drive controls the speed of an AC motor by varying the frequency supplied to the motor.
The drive also regulates the output voltage in proportion to the output frequency to provide a relatively constant ratio of voltage to frequency (V/Hz), as required by the characteristics of the AC motor to produce adequate torque.
Advantage of VFD
- Power savings is the principal advantage of any VFD.
- The starting current of any motor is minimal which translates to a lower load on the captive power mechanism..
- Complete control over motor speed can be achieved using VFDs.
- VFDs can be programmed to vary the speed of the motor it controls, based on load factor.
- Dynamic torque control and perfect speed control.
- Advanced overload protection.
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