VFD or Soft-Starter? What should I use and Why?

There are various ways of starting an electric motor such as ‘Direct On Line (DOL), Star-Delta, Primary Resistor or power electronic devices such as Soft-Starter or Variable Frequency Drives (VFD). Here we will elaborate on soft-starters and VFDs. We will also discuss what they mean and in what applications they can be used. Both VFDs and Soft starters have their specific applications and benefits.

“There are various ways of starting an electric motor such as ‘Direct On Line (DOL), Star-Delta, Primary Resistor or power electronic devices such as Soft-Starter or Variable Frequency Drives (VFD).”

Soft-Starters

Electric motors generally require large amounts of electricity during their startup and acceleration to the nominal speed. A soft starter is used to limit the surge of starting current or ‘in rush current’ and torque of electric motors, resulting in a safe, smooth and gradual start up. Soft starters protect the motors from damage and extend lifespan of motor by:

1. Declining the heat caused by frequent start / stops

2. Reducing the mechanical stress on the motor

3. Reducing the electrodynamic stress on power cables of the motor.

Another benefit of using soft starters is reduction in starting current which places a high demand on electrical grid and results in extra cost. General applications where soft starters include fans, pumps conveyors etc.

Control Methods of Soft Starters

The soft starter can be controlled either by a direct start/stop wiring or, it can be controlled over Ethernet. Direct start/stop signals would not require a PLC. They are less expensive. Using Ethernet control, a PLC is required. This allows feedback, which will give adjustable control and monitoring capabilities to end user.

How do Soft Starters work?

A typical three-phase soft starter uses six Thyristors or Silicon Controlled Rectifiers, oriented in an anti-parallel configuration to start the electric motors smoothly.

A Thyristor is made up of 3 parts:

– Logic gate

– Cathode

– Anode

When an internal pulse is applied to the gate, it allows current to flow from Anode to Cathode which then sends current out to our motor.

When the internal pulses do not apply to the gate, the SCRs (Silicon Controlled Rectifier) are in the OFF state and therefore they restrict the current to the motor.

These internal pulses limit the applied voltage to the motor slowing down inrush current.

The pulses are sent based on ramp time so the current will be slowly applied to the motor. The motor, attached to the soft starter, will start up at a nice smooth current and top out at the preset maximum speed.

The motor will stay at that speed until we stop the motor where the soft starter will ramp down the motor in a very similar way as the ramp up.

Variable Frequency Drives

Variable Frequency Drives or VFDs as they are known more popularly are solid state electronic devices that are used for running a motor at variable speeds. This means they can ramp up and down the motor speed by adjusting the frequency of the motor.

Mostly VFDs make sense in applications where speed control is required. Since they help in speed control, this means they curtail the motor power and limit it to application requirement. This is one of the biggest reasons that VFDs are used in variable torque applications like pump and fan where they can help large amounts of energy.

To automate the speed control based on demand, VFDs can be connected to PLCs using communication protocols like RS232 and RS485. This helps makes the process much easier and more efficient.

How d VFDs work?

VFDs have three main components:

– Rectifier

– Filter

– Inverter

The Rectifier acts like Diodes, takes the incoming AC voltage and changes it to DC voltage.

Next, the Filter uses Capacitors to clean the DC voltage making it a smoother incoming power.

Finally, the Inverter uses transistors to convert the DC voltage and sends the motor a frequency in Hertz. This frequency drives the motor to a specific RPM.

We can set the ramp up and down times just like in a soft starter.

What is the difference between Soft Starter and VFD?

Well, as we said earlier both are solid state electronic devices. While soft starters just provide smooth start and stop VFDs go way beyond and provide precise speed control in addition to all the functions of a soft starter.

What should I use?

The decision depends on few factors like speed control, price, size and energy saving requirement.

Speed Control

There is no better way to control the speed of your process other than using a VFD. However, if your goal is limited to reduce in rush current a soft starter can do the job.

Price Since VFDs allow more control and automation options, they are obviously priced higher than soft starters.

Size Soft starters are compact and smaller in size than VFDs and can fit in places with space constraints.

Energy Savings Due to their ability to maintain speed control, VFDs actually help you make energy and cost savings in variable torque applications like fan and pump. if the motor is running at nominal load, the energy saved when using a variable frequency drive (VFD) is about 3%, for 75% load is about 25%, for 50% load is about 75%.

Summary

In closing, a VFD and a Soft Starter can do similar functions when it comes to ramp up or down a motor.

The main difference between the two is that a VFD can vary the speed of a motor while a soft starter only controls the starting and stopping of that motor.

When faced with an application, price and size are in the favor of a soft starter. A VFD is the better choice if speed control is required.

The good news is that if an existing application has a soft starter in place and speed control is later determined to be a requirement, a VFD can easily replace a soft starter. The opposite is true as well, a soft starter can replace a VFD.