VFD Vs Soft Starter

Who can i choose my drive ?

All people agrees about the necessity of using an control circuit in the starting of induction motor, due to the inrush current under the starting period (aproximately first 7Sec) that should be limited for protection and lifespan purpose .

there is various methode can be used like Star/Delta method, but when we talk about mid range and high rang power motor there is two main method take the lead which are : Soft Starter and VFD in this post we will explain each one and compare their functionality in order to distincts when it suitable to use one instead of the other.

VFD:

Variable Frequency Drive who is the beast of the electrical control field based on his flexibilty and high effecency by variying the voltage and frequency of the output with a large rang due to it's topology , is composed from Power electronics converters contorlled by PWM techniques , it composed mainly from 3 main level :

Rectifier: in the this stage the input voltage (fixed frequency 50/60 Hz) is converted to DC voltage via 3 phase full wave rectifier .
Filter: the DC voltage given by the rectifier contain fluctation du to the switching and to the sinusoidal wave form, so it need to be filtred in order to get a pure DC voltage (constant value) , lany type of filters are used in this level according to requirments , the common one is LC filter .
Inverter : at this level the cruicial operation of the VFD is taken place , the inverter is composed from 3 leg each one contain two power switch (Mosfet, IGBT , SIC,..) according to the needed switching frequency and reverse voltage these switches are controlled via pwm techniques who contol the states On/Off of the switches in order to deliver a variable voltage and frequency to drive the motor with the desired speed/torque .

various methods are used to control the inverter such as PWM (pulse width modulation) , SVPWM(space vector pulse width modulation) and other , but all are based on the concept of PWM .

Now let's explore some features of VFD that give motor control effecincy and excellence :

Frequency Control: By adjusting the frequency of the electrical supply to the motor, a VFD can ramp up or down the motor's speed smoothly. This feature is particularly useful in applications where precise speed control is required, such as in pumps or fans.
Energy Efficiency: VFDs offer energy savings by matching the motor's speed to the required load, reducing energy consumption during periods of lower demand. This feature makes VFDs an attractive option for applications with varying loads or where energy efficiency is a priority.
Advanced Control: VFDs can provide additional functionalities like acceleration and deceleration ramps, overload protection, and diagnostic capabilities, enhancing motor performance and reliability.

Soft Starter:

Unlike VFDs, which control both speed and torque, soft starters primarily control the starting torque of the motor by varying the input voltage current of the motor durnig starting up, via an power converter who is AC controller who based on SCR or Traic for high power application, the process of soft stater is varying the RMS value of the input voltage and current by controling the firing angle of switches, until the motor reach the full speed according to the firing angle α= 0, so the soft starter varying the speed just by varying the voltage without acting on the frequency of the voltages by the following equation :

T_e=(3.V_rms^2.R_s)/(ω_s.S)

with Te=Pe/Ω_s

Te : electromagnetique torque
Pe= electromagnetique power
Vrms: RMS value of voltage
Rs: resistance of stator winding

so the soft starter cannot drive the motor with speed above the nominal speed of the motor , so it present a smooth acceleration of the motor in order to avoid inrush current and mechanical stress, and when the motor reach his full speed a ByPass contacot is energized in order to eliminate the AC controller by short-circuited, and offer a circuit to the current without passing by (SCR or Triac) in order to reduce reactive energy . the rise time of speed is adjuted by the user and it take useally from (2 to 5 )sec the schematics of the soft starter is presented by the following figure:

When deciding between a VFD and a soft starter for starting induction motors, several factors should be considered:

Voltage Ramp-up: Soft starters limit the initial voltage supplied to the motor, reducing the inrush current and mechanical stress during startup. This gradual voltage ramp-up ensures a smoother start, minimizing wear and tear on mechanical components.
Current Limitation: By limiting the starting current, soft starters prevent voltage dips in the electrical system and reduce the demand on power distribution infrastructure. This feature is beneficial in applications where multiple motors start simultaneously or where there are capacity constraints.
Simple Design: Soft starters are generally simpler in design compared to VFDs, making them easier to install, operate, and maintain. They are often preferred in applications where basic motor control is sufficient, and advanced features of VFDs are unnecessary.

Comparison and Selection Criteria:

Speed Control: If precise speed control throughout the motor's operating range is required, a VFD is the better choice.
Energy Efficiency: For applications with varying loads or where energy savings are a priority, a VFD offers superior energy efficiency compared to a soft starter.
Cost Considerations: Soft starters are generally more cost-effective than VFDs, making them a preferred option for budget-conscious projects where advanced control features are not essential.
Mechanical Stress: If minimizing mechanical stress and wear on the motor and driven equipment is crucial, a soft starter's gradual voltage ramp-up provides smoother starting compared to the abrupt changes in speed and torque associated with VFDs.
Application Requirements: Consider the specific requirements of the application, including starting torque, acceleration time, and speed regulation, to determine the most suitable starting method.

At the end of this post we have learn that both VFDs and soft starters offer effective solutions for starting induction motors, each with its unique features and benefits. Choosing between them depends on factors such as speed control requirements, energy efficiency goals, budget constraints, and mechanical considerations.

VFD Vs Soft Starter Who can i choose my drive ?