A Variable Frequency Drive or Soft Starter will be used in many industrial applications, and HVAC is no exception. They can both produce large cost savings by reducing wear and tear on motors, pumps and associated equipment.
Automated Control Systems (ACS) carries a range of both. But what are they and what is the difference?
Motors require large amounts of energy when starting and quickly accelerating to operational speed. Soft starters and VFDs can both be used to reduce the surge of power and limit torque extending the life of motors.
Choosing which to use depends on the application, requirements and costings.
What is a Variable Frequency Drive and how does it work?
A VFD is a motor control system that controls the speed of the motor during startup and shutdown cycles as well as being able to control motor speed during the run cycle.
VFDs have three main components:
- Rectifier
- Filter
- Inverter
The rectifier acts like diodes and takes incoming AC voltage and converts it to DC. The filter then uses capacitors to clean the DC making it smoother. Then finally the inverter uses transistors to convert DC and sends the motor a frequency in Hertz which drives the motor at a specific RPM.
What is a Soft Starter and how does it work?
A Soft Starter is a solid-state device that protects motors from power surges associated with startups and sudden voltage drop on stopping.
It provides a gentle ramp up to full speed by limiting the current inrush associated with startup and a slower, gentler stop reducing torque and motor wear and tear. They are only used on start up and stopping and cannot control motor speed.
A typical three-phase soft starter uses six thyristors or silicon-controlled rectifiers (SCRs) in anti-parallel configuration to start motors smoothly. The thyristors consist of three parts:
- Logic gate
- Cathode
- Anode
When an internal pulse is applied to the gate, it allows current to flow from the anode to the cathode which sends current to the motor. When the pulse is not applied, the SCRs are in the off state and restrict current to the motor.
These internal pulses limit voltage and slow down the inrush current. Pulses are sent based on ramp times so that the current is slowly applied to the motor so that the motor benefits from a smooth current and gets to maximum speed at a pre-set value. The motor will stay at that speed until it is ramped down in a similar fashion.
Variable Frequency Drive applications
VFDs are used in applications where the following are requirements:
- Complete speed control
- Energy saving
- Custom control
- Starting currents need to be reduced while maintaining high starting torque
Soft Starter applications
Soft starters are used in applications that require:
- Speed and torque control only during startup and shutdown
- Reducing the inrush current on startup of large motors
- A gentle start is required to reduce torque spiles and tension on normal startup
VFD or soft starter – which one to select?
As is evident, both VFDs and Soft Starters perform the same functions on stopping and starting motors and if that is all that is required, then the soft starter is the choice. Additionally, soft starters are cheaper and take up less space.
However, VFDs have the following advantages:
- Energy savings
- Reduces peak energy demand
- Reduces power when not required
- Fully adjustable speed for pumps, conveyors and fans
- Controlled starting, stopping and acceleration
- Dynamic torque control
- Provides smooth motion for applications such as elevators and escalators
- Maintains speed of equipment, making drives ideal for manufacturing equipment and industrial equipment such as mixers, grinders and crushers
VFDs can offer greater energy savings than soft starters. For example, according to the affinity laws, if a fan’s speed is reduced by half, the energy consumption is only 1/8th of that at full speed. Therefore, significant savings can be achieved.
The final choice between a VFD or soft starter boils down to three factors:
1. Speed Control
If your application requires a large inrush of current but no speed control is required then a soft starter is the best option. If speed control is required, then a VFD is a must.
2. Price
Taking budget into account, a soft starter has fewer control features therefore price is lower than a VFD.
3. Size
If space is an issue and therefore device size, soft starters are generally smaller than most of the VFDs.
ACS can offer information and advice on the best device for your application. Give us a call.
