Variable speed drives are used throughout modern industry to control the speed and torque of electric motors. They are found in applications ranging from pumps and fans to conveyors, mixers, compressors and production machinery.
For buyers and maintenance teams, understanding what a VSD does is useful even when the final drive selection is being handled by an engineer or automation specialist. It helps you identify the information required when requesting a quotation, replacing an existing drive or comparing possible alternatives.
What is a Variable Speed Drive?
A Variable Speed Drive, commonly abbreviated as VSD, is an electronic device used to control the operation of an electric motor by varying the electrical supply delivered to the motor.
For AC motors, speed is closely related to the frequency of the electrical supply. A VSD changes the effective output frequency and voltage supplied to the motor, allowing the motor to operate at speeds other than simply full speed.
You may also encounter the terms Variable Frequency Drive or VFD. In many everyday industrial applications, VSD and VFD are used interchangeably when referring to electronic drives controlling AC motors, although VSD is the broader term.
How Does a VSD Work?
A typical AC variable frequency drive performs three broad electrical stages.
1. Rectification
The incoming AC supply is converted into DC power through a rectifier stage.
2. DC Link
The converted power is stabilised in the drive's DC link or DC bus. Capacitors and other components help smooth and store energy within this stage.
3. Inverter Output
The inverter stage electronically switches the DC power to create a controlled AC output for the motor. By controlling the output frequency and voltage, the drive can regulate motor speed and operating characteristics.
Modern drives use semiconductor switching devices and sophisticated control algorithms to perform this process rapidly and accurately.
Why Control Motor Speed?
Many industrial processes do not need a motor to operate at full speed continuously. Controlling motor speed allows the process to operate according to actual demand.
Typical reasons for using a VSD include:
- Controlling pump flow or pressure
- Adjusting fan airflow
- Changing conveyor speed
- Controlling mixers and agitators
- Matching machine speed to production requirements
- Providing controlled acceleration and deceleration
- Reducing mechanical stress during starting
- Improving process control
- Reducing energy consumption in suitable variable torque applications
VSDs and Energy Saving
VSDs are often associated with energy savings, particularly in centrifugal pump and fan applications.
When a process requires less flow or airflow, reducing motor speed can be substantially more efficient than running the motor continuously at full speed and controlling the process mechanically with valves, dampers or similar restrictions.
However, installing a VSD does not automatically guarantee an energy saving in every application. The actual benefit depends on the load, operating profile, motor, process and control strategy.
Common VSD Applications
Variable speed drives are used across many industries and infrastructure environments, including:
- Pumps and booster systems
- Borehole and water systems
- Wastewater applications
- HVAC fans and pumps
- Conveyors
- Compressors
- Mixers and agitators
- Machine tools
- Packaging machinery
- Material handling systems
- Mining and mineral processing equipment
- Manufacturing and production lines
What Information Is Needed When Selecting a VSD?
A VSD should not be selected only because its kilowatt rating appears to match the motor. Several electrical and application factors need to be considered.
Useful information includes:
- Motor rated power in kW
- Motor rated voltage
- Motor full load current
- Incoming supply voltage and phase
- Motor frequency
- Application or load type
- Required speed range
- Starting and overload requirements
- Environmental conditions
- Enclosure or IP rating requirements
- Control method
- Required digital and analogue inputs and outputs
- Communications requirements
- Braking requirements where applicable
The motor nameplate is one of the most useful sources of information when requesting a replacement or new drive.
Why Motor Current Matters
Motor power rating is useful, but rated motor current is particularly important when sizing a drive. Two motors with the same nominal kW rating can have different current requirements depending on motor design, efficiency, voltage and application.
The selected drive must be capable of supplying the required motor current and handling the application's overload demands.
Normal Duty and Heavy Duty Applications
Drive manufacturers may specify different output ratings depending on the expected duty.
Variable torque applications such as many centrifugal pumps and fans can have different overload requirements from constant torque or heavy duty applications such as conveyors, crushers, mixers or lifting equipment.
This is one reason why selecting a drive solely from a kW figure can lead to an incorrect specification.
Control Methods
Modern VSDs can offer different motor control methods depending on the drive family and application. These may include basic voltage to frequency control, sensorless vector control and more advanced closed loop control methods.
The appropriate method depends on factors such as required speed accuracy, torque performance, motor type and application.
Communications and Automation
A VSD can operate as a standalone motor controller, but it can also form part of a larger automation system.
Depending on the model, drives may communicate with PLCs, HMIs and SCADA systems through industrial protocols such as Modbus RTU, Modbus TCP, PROFINET, EtherNet/IP or other manufacturer supported networks.
Communications can allow the control system to start and stop the drive, change speed references, monitor operating values, read fault information and integrate the motor into a broader process.
Replacing an Existing VSD
If an existing drive has failed or become obsolete, do not assume that a drive with the same kW rating is automatically a direct replacement.
When requesting a replacement, provide:
- Manufacturer
- Exact model or part number
- Motor nameplate information
- Supply voltage
- Application
- Existing control wiring requirements
- Communications protocol if used
- Any option cards or accessories installed
- Clear photographs of the existing drive and nameplate where possible
Physical dimensions, terminal arrangements, communications, parameter structures and control functionality can differ significantly between drive families.
VSD or Soft Starter?
A VSD and a soft starter are not the same device.
A soft starter is primarily used to reduce electrical and mechanical stress during motor starting and stopping. Once the motor is running, it normally operates at line frequency.
A VSD provides ongoing control of motor speed by varying output frequency and voltage. If the process requires continuous speed control, a VSD is generally the relevant technology.
We will cover the differences between VSDs and soft starters in more detail in a separate Knowledge Centre guide.
If you are replacing an existing drive, send us the manufacturer, model number, motor nameplate details and application information. If you are specifying a new drive, provide the motor and supply information together with the operating requirements.
Products that form part of our active online catalogue can be viewed through the Softcore Group Industrial Solutions store. Other brands, models and project specific requirements can be quoted on request.
Technical Note
This article provides general product information. VSD selection, installation, programming and commissioning must take account of the specific motor, electrical system, application, environment, manufacturer requirements and applicable standards. Where engineering judgement is required, the equipment should be selected and implemented by an appropriately qualified technical professional.
Related catalogue examples: Variable Speed Drives. For an equipment enquiry, contact Softcore Group Industrial Solutions at info@softcoregroup.co.za.