Variable speed drives and soft starters are both widely used with industrial electric motors, and at first glance they can appear to solve the same problem. Both can reduce the harshness of direct on line starting, both are electronic motor control devices and both can form part of an industrial control system.
But they are not interchangeable.
The fundamental difference is that a soft starter is primarily intended to control the motor during starting and, on suitable models, stopping. A variable speed drive can control the motor throughout operation by varying the electrical frequency and voltage supplied to it.
Understanding that distinction makes it much easier to decide which technology belongs in a particular application.
What Is a Soft Starter?
A soft starter is an electronic device designed to reduce the electrical and mechanical stress associated with starting an AC motor.
Instead of applying full line voltage to the motor immediately, a typical soft starter uses semiconductor devices such as thyristors to progressively control the voltage applied during acceleration.
Once the motor reaches normal operating speed, many soft starters use an internal or external bypass arrangement so that the motor effectively operates from the normal line supply.
Depending on the model and application, a soft starter may also provide controlled deceleration or soft stop functionality.
What Is a VSD?
A Variable Speed Drive, or VSD, controls the motor by electronically varying its output frequency and voltage.
A typical AC VSD converts the incoming AC supply to DC and then uses an inverter stage to create a controlled AC output for the motor.
Because motor speed can be controlled throughout operation, a VSD can do much more than simply soften the starting process.
It can adjust motor speed according to process demand, receive speed references from a PLC or control signal, maintain pressure or flow in suitable applications and provide extensive monitoring and automation functions depending on the drive family.
The Main Difference
The easiest way to distinguish the two technologies is to ask one question:
Does the motor need to change speed during normal operation?
If continuous or process based speed control is required, a VSD is generally the relevant technology.
If the motor normally runs at full line speed and the primary requirement is simply to reduce starting stress or starting current, a soft starter may be more appropriate.
VSD vs Soft Starter at a Glance
| Requirement | VSD | Soft Starter |
|---|---|---|
| Reduced mechanical starting stress | Yes | Yes |
| Reduced starting current | Yes, subject to application and sizing | Yes, subject to application and settings |
| Continuous speed control | Yes | No, not its normal function |
| Variable process speed | Yes | No |
| Controlled acceleration | Yes | Yes |
| Controlled deceleration | Yes | Available depending on application and soft starter |
| Process control capability | Often extensive | Generally more limited |
| Industrial communications | Common on modern drives | Available on many modern soft starters |
| Normal motor operation at line frequency | Not necessarily | Typically yes after starting |
When Is a Soft Starter Commonly Used?
A soft starter is often considered where a motor is expected to run at essentially constant speed once it has started.
Typical applications can include:
- Pumps that do not require variable flow control
- Fans operating at fixed speed
- Conveyors operating at a fixed process speed
- Compressors where the application is suitable
- Crushers and other machinery where reduced starting shock is beneficial
- Industrial machinery with high starting stress
Actual suitability depends on motor characteristics, load torque, starting frequency, required start time and the soft starter manufacturer's application guidance.
When Is a VSD Commonly Used?
A VSD is typically selected when controlling motor speed provides an operational or process benefit.
Applications can include:
- Variable flow pumping systems
- Pressure controlled pumping
- HVAC fans
- Variable speed conveyors
- Mixers and agitators
- Production machinery
- Process equipment requiring adjustable speed
- Applications where a PLC or control system changes motor speed
Starting Current
Starting current is an important consideration with industrial motors.
A motor started directly from the line can draw a current substantially higher than its normal running current during acceleration. The actual value depends on the motor and system.
Both soft starters and VSDs can reduce the impact of motor starting compared with direct on line starting, but they achieve this in different ways.
A soft starter progressively controls voltage during the start. A VSD controls motor operation through its variable frequency and voltage output.
The required starting performance must still be checked against the motor, load and manufacturer's ratings.
Mechanical Stress
Electrical starting is only part of the issue. Sudden motor acceleration can also place stress on couplings, belts, gearboxes, shafts, pumps, conveyors and driven machinery.
Both technologies can provide controlled acceleration that reduces abrupt mechanical shock.
If that is the only objective and the motor then runs at fixed speed, a soft starter may provide a simpler solution. If the process also benefits from speed control, a VSD offers additional capability.
Energy Efficiency: A Common Misunderstanding
A soft starter should not generally be selected as an energy saving device simply because it reduces starting current. The starting period normally represents only a small portion of the motor's operating time.
Once bypassed, the motor generally operates from the normal supply at line frequency.
A VSD can provide meaningful energy savings in suitable applications because it can reduce motor speed during periods of lower process demand.
This is particularly relevant to many centrifugal pump and fan applications, where power demand can reduce substantially as speed is reduced.
However, a VSD does not automatically save energy in every installation. The actual benefit depends on the process, load profile, control strategy and operating conditions.
Pump Applications
Pumps are a good example of why the application must be understood before choosing between the two technologies.
If a pump simply needs a gentler start and then operates at full speed, a soft starter may be appropriate.
If the system needs to maintain pressure, vary flow according to demand or reduce pump speed during low demand periods, a VSD may provide the required process control.
The hydraulic system, pump curve, minimum operating requirements and motor characteristics must still be considered.
Conveyor Applications
A conveyor that always operates at one fixed production speed may use a soft starter to reduce starting shock, provided the starter can meet the load's starting torque requirements.
A conveyor requiring adjustable production speed, controlled low speed operation or speed commands from an automation system is more likely to require a VSD.
High breakaway torque and heavily loaded starts require careful equipment sizing regardless of the technology selected.
Motor Torque Matters
Reducing voltage during soft starting also affects the torque the motor can produce. This is important for loads that require significant torque during acceleration.
A soft starter therefore cannot be selected solely by matching the motor kW rating. Starting torque, motor current, acceleration time and load characteristics need to be considered.
VSD selection also requires attention to torque and overload duty, particularly for constant torque and heavy duty applications.
Stopping the Motor
Some soft starters provide a soft stop function, which can be useful in certain pumping and mechanical applications. A soft stop is not the same as active braking and its behaviour depends on the load.
A VSD provides controlled deceleration through frequency control and may support additional braking arrangements such as braking resistors or regenerative solutions where required.
Applications involving high inertia, rapid stopping or overhauling loads require specific engineering consideration.
Bypass Operation
Many soft starters are bypassed after the motor reaches operating speed. Bypass reduces losses and heat generation in the semiconductor power stage during normal running.
Some soft starters include an internal bypass contactor, while other installations use an external bypass arrangement. The exact configuration depends on the product and system design.
A VSD, by contrast, remains electrically involved in controlling the motor during normal variable speed operation.
Harmonics and Electrical Considerations
Because a VSD continuously uses power electronics to create a variable frequency output, harmonic distortion, electromagnetic compatibility, cable installation, motor insulation and other electrical considerations can form part of the design.
A soft starter's semiconductor devices are primarily active during starting and stopping when a bypass arrangement is used, so its normal running characteristics differ significantly from those of a VSD.
Electrical design requirements should always follow the relevant manufacturer's guidance and applicable standards.
Communications and Automation
Modern VSDs often provide extensive integration with PLC, HMI and SCADA systems. Depending on the drive, this can include speed commands, operating data, fault information, diagnostics and industrial network communications.
Modern soft starters can also provide monitoring, diagnostics and communications, but their primary role remains motor starting and stopping rather than continuous speed regulation.
Is a VSD Always Better Because It Does More?
No.
More functionality does not automatically make a product the better engineering or commercial choice.
If the application requires only controlled starting and stopping at fixed operating speed, installing a VSD may add functionality, complexity and cost that the process does not require.
Conversely, using a soft starter where continuous speed control is required will not provide the necessary functionality.
The correct choice is the one that meets the actual motor and process requirements.
Information to Provide When Requesting a Quote
Whether you require a VSD or soft starter, useful information includes:
- Motor rated power in kW
- Motor rated voltage
- Motor full load current
- Incoming supply voltage and phase
- Application or driven load
- Whether speed must vary during normal operation
- Starting frequency
- Starting and overload requirements
- Required acceleration and deceleration behaviour
- Control requirements
- Communications requirements
- Environmental and enclosure requirements
If you are replacing existing equipment, also provide the manufacturer and exact model number together with clear photographs of the equipment and motor nameplates.
If you are unsure whether your requirement calls for a VSD or soft starter, send us the motor details and a description of the application. Where application specific engineering is required, the selection should be reviewed by an appropriately qualified technical professional.
Technical Note
This article provides general technical information. Motor control equipment must be selected according to the motor, load, electrical supply, operating environment, control requirements, manufacturer instructions and applicable standards. It is not a substitute for application specific electrical or mechanical engineering.
Related catalogue examples: Variable Speed Drives. For an equipment enquiry, contact Softcore Group Industrial Solutions at info@softcoregroup.co.za.