Induction motor starting methods star delta starter dol starter vfd

 when we use a soft starter instead of a VFD ?

Starting Methods of induction motor 

Comparisons between Soft Starter and VFD  direct online starter dol starter
Comparisons between Soft Starter and VFD  direct online starter dol starter


How does a Direct On Line (DOL) starter work?

direct online starter dol  Full-voltage Starting Torque Speed Curve
direct online starter Full-voltage Starting Torque Speed Curve

As a basic starting method, a DOL starter applies full voltage, current, and torque immediately to the motor once a start command is provided. Normally, power is immediately removed once the stop signal is given. On and Off are the only two states of this method.
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How does a soft starter work?

Different Silicon-controlled rectifier  (SCR )Firing Angles soft starter
Different Silicon-controlled rectifier  (SCR )Firing Angles soft starter

An algorithm controlling three pairs of back to back SCRs are used to start and stop the motor. The back to back orientation of the SCRs allows the AC voltage to be controlled by changing the firing angle every half cycle (Figure ).

Voltage is either ramped up to full voltage, or is limited to provide current limit starts.

A soft starter uses voltage to control the current and torque. The motor torque is approximately proportional to the square of the applied voltage.

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How does a VFD work?

Essentially, a VFD takes AC line voltage, converts it to a DC voltage, filters the DC voltage, and then inverts the signal back. That RMS value of this inversion simulates an AC voltage. The output frequency of the drive is usually from 0 to AC input line frequency. Higher frequencies than the nominal AC are also possible when required for certain applications.

Most AC drives use a full wave diode-bridge or SCR rectifier bridge in the converter section to convert the AC source to DC voltage. Active components, such as insulated-gate bipolar transistor (IGBT) can also be used in this section. 

The filter section, primarily a capacitor bank, is used to smooth out the DC voltage that is produced from the converter section. 

A link choke or inductor can be added to improve power factor and reduce harmonics. The smoothed out DC voltage is then used by the IGBT inverter. The fast-acting switching from the inverter section generates the proper RMS simulated AC voltage levels.

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Comparisons between Soft Starter and VFD

1) Efficiency

Soft Starter

Soft starters can achieve up to 99.5…99.9% efficiency. Typically, less than 1V is dropped across an SCR. Efficiency is dependent upon the size of the soft starter and the 3-phase voltage applied.

 After the starting process is complete, a soft start with integrated bypass, as with the SMC-3 and SMC Flex, pulls in an internal bypass contactor. The SCRs are no longer firing and all running current is across the contacts maintaining or improving efficiency.

 When operating at full speed and properly loaded, soft starters are more efficient than VFDs

The SMC-50 has a parameter setting to allow energy consumption efficiency when a motor is in an unloaded condition; this setting could potentially save energy costs. The soft starter has the CE mark and should not need extra filtering for harmonics. Other soft starters may not have the CE mark and may need filtering.



Drives are typically 95 …98 % efficient. During start, run, and stop (unless set to coast stop), active components such as IGBTs are on. However, certain drives are better able to adjust power consumption during running mode. 

Select the drive based on different load characteristics, and you could potentially save energy costs. The higher the pulses in the drive, the higher the efficiency. For example, a 6-pulse drive is 96.5…97.5 % efficient. An 18-pulse drive is 97.5…98 % efficient.

2) Heat from Soft Starter or Drive

Soft Starter

In a soft starter with integrated bypass, current is carried across the contactor, therefore no active solid-state components are on to generate more heat.



When running, a VFD is inherently hotter than the soft starter due to active components constantly controlling frequency and voltage


3) Starting and stopping time accuracy


Soft Starter

Soft starters are load dependent and based on programmed start and stop settings. An algorithm adjusts voltage to increase  the current and torque to start the motor.

 Based on the back electromotive force (EMF) of the motor, the soft starter determines whether the motor is up to speed. If the soft starter detects that the motor is up to speed before the selected
timed start, the soft starter applies full voltage and indicates running status.

If the motor does not come up to speed in a set time frame, the soft starter applies full voltage (SMC Flex, SMC-50) or a percentage of the full voltage (SMC-3),depending on load.

 The exception is the SMC-50 in linear start and linear stop control, where a special algorithm, without the use of an external tachometer, allows accurate starting and stopping times with a few parameter selections, regardless of the load.


Speed control is precisely provided by drives including start and stop times, depending on the drive that is selected and the loading and overload capability of the drive.


4) Speed control


Soft Starter

Some soft starters have limited slow speed control between starting and stopping,



Drives offer continuous and fully adjustable speed any time from starting to stopping for possibly hours, due to the ability of adjusting the frequency.

Although both the drive and the soft starter can run at slow speeds, the duration of each is dependent on the motor and the load. 

Heat from running a motor at slow speeds depends on time. In order to protect the SCRs and the motor, the soft starter will reach thermal capacity if left in slow speed for too long. Continuous operation of a drive below 5 Hz requires de-rating.


5) Full torque at 0 speed


Soft Starter

Soft starters operate on a fixed frequency, and full torque is available only at full voltage. Initial torque is programmed into the soft starter. 

The associated voltage for the torque setting is the starting point of the ramp. Full torque is not available at zero speed.



In drive applications, 100% torque is available up to line frequency at base speed. Above base motor speed, horsepower is 100% and torque decreases. 

Holding torque is an advantage that a drive provides on applications like an incline conveyor
that holds the belt with the load from moving backward when stopped. 

The application will determine whether other safety features are needed in addition to full torque at zero speed with a drive. A soft starter would need to use a mechanical brake to achieve the same function


6) Maintenance


Soft Starter

Little maintenance is required for the soft starter other than keeping the fan vents clear and clean.



Depending on the drive, annual maintenance or even less frequently, parts need to be inspected, cleaned and or replaced.
For example, on a drive operating 24 hours per day, in year 3, you should replace the following (if applicable):
• cooling fan motor
• small cooling fans
• de-ionizing filter cartridge
• coolant
• electrolytic bus capacitors
• rectifier bus capacitors
• inverter snubber capacitors
• integrated gate driver power supply
• AC/DC and DC/DC power supplies
• UPS batteries

Here are some common applications of soft starter and variable frequency drive:

Soft Starters

• Applications with low or medium starting torque
• Lightly loaded applications
• Little or no speed control during run mode
• Reduce mechanical wear and damage to system
• Controlling inrush
• Power monitoring



• Single-phase applications on certain drives
• Speed control and system efficiencies operating at reduced speeds during the run mode
• Applications with high starting torque
• Continuous feedback for critical position control
• Holding rotor at zero speed
• Reduce mechanical wear and damage to system
Reference : Rockwell Automation Publication 150-WP007A-EN-P - October 2014

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