Choosing a variable speed drive for an electric motor involves more than matching the kilowatt rating printed on the motor nameplate. A drive must be suitable for the motor, incoming electrical supply, mechanical load, operating environment and control requirements of the application.
For buyers and maintenance personnel, knowing which information to collect before requesting a quotation can prevent incorrect selections, unnecessary delays and repeated technical queries.
This guide explains the main information typically required when selecting a VSD.
Start With the Motor Nameplate
The motor nameplate is usually the best starting point. It contains the electrical information needed to understand what the drive will be expected to operate.
Useful nameplate information includes:
- Rated motor power in kW
- Rated voltage
- Full load current
- Rated frequency
- Motor speed in r/min or rpm
- Power factor
- Efficiency information where shown
- Connection information such as star or delta
- Motor insulation and duty information where relevant
When requesting a replacement drive, a clear photograph of both the motor nameplate and the existing VSD nameplate can save considerable time.
Do Not Select a VSD on kW Alone
Motor power is important, but it should not be the only selection criterion.
Two motors carrying the same nominal kW rating can have different rated currents. Drive manufacturers can also assign different current and overload ratings to drives depending on operating duty.
For this reason, the motor's full load current and the VSD's applicable output current rating should be checked together with the power rating.
1. Confirm the Incoming Supply
Before selecting a drive, confirm the electrical supply available at the installation.
Important details include:
- Supply voltage
- Single phase or three phase supply
- Supply frequency
- Any known supply limitations or unusual conditions
A drive must be designed for the relevant input voltage class. The output arrangement must also be suitable for the motor being controlled.
Do not assume that all drives with the same motor power rating accept the same input supply.
2. Confirm the Motor Rated Current
Motor current is one of the most important values in VSD selection.
The drive's applicable continuous output current should be sufficient for the motor and application. The required margin depends on the manufacturer's ratings, duty classification and load characteristics.
If the motor nameplate is available, include the rated current in every VSD enquiry rather than supplying only the motor kW.
3. Understand the Load
The mechanical application affects how much torque and overload capability the drive may need.
Typical variable torque applications include many centrifugal pumps and fans. Their torque demand generally changes significantly with speed.
Constant torque or heavier duty applications can include conveyors, mixers, extruders, positive displacement equipment and other machinery where substantial torque may be required across the operating range.
The exact duty classification depends on the application and the drive manufacturer's rating methodology, but telling the supplier what the motor actually drives is essential.
4. Check Normal Duty and Heavy Duty Ratings
Many VSD ranges publish more than one power or current rating for the same physical drive.
A drive may have a higher rating under normal or variable torque duty and a lower rating where greater overload capability is required. This is why two listings for what appears to be the same drive can show different kW values.
Always compare the applicable current and overload specification rather than assuming the largest displayed kW number applies to every application.
5. Consider Starting and Overload Requirements
Some loads accelerate easily. Others require substantial torque during starting or can experience temporary overloads during operation.
Useful information includes:
- How quickly the machine must accelerate
- Whether the load starts under mechanical load
- Whether sudden load changes occur
- Required overload duration
- Whether frequent starting and stopping is expected
Applications with demanding starting torque or overload requirements may require a different drive size or control strategy from a simple pump or fan application.
6. Determine the Required Speed Range
Identify the minimum and maximum operating speed required by the process.
Operating a motor at reduced speed can affect motor cooling, particularly where the motor relies on a shaft mounted fan. Very low speed operation, high torque at low speed and operation above base speed can require additional engineering consideration.
The motor, driven equipment and mechanical system must all be suitable for the intended speed range.
7. Identify the Control Method
How will the VSD receive commands?
A simple application may use local keypad control. Other applications may require external push buttons, selector switches, potentiometers, analogue references, digital inputs, PLC control or network communications.
Before selecting the drive, identify requirements such as:
- Start and stop inputs
- Forward and reverse operation
- Preset speeds
- Analogue speed reference
- Analogue feedback
- Relay outputs
- Digital outputs
- PID control
- External fault or interlock signals
8. Check Communications Requirements
If the drive will form part of an automation system, confirm the required communications protocol.
Depending on the VSD family, communications may be built in or may require an optional interface or communications card.
Common industrial protocols include:
- Modbus RTU
- Modbus TCP
- PROFINET
- EtherNet/IP
- CANopen
- PROFIBUS
Never assume that a replacement drive supports the same network as the existing unit simply because the basic electrical rating is similar.
9. Consider the Installation Environment
Environmental conditions can materially affect drive selection and installation.
Consider:
- Ambient temperature
- Altitude
- Dust
- Moisture
- Corrosive atmosphere
- Ventilation
- Indoor or outdoor installation
- Enclosure requirements
Many standard VSDs are designed to be installed inside a suitable electrical enclosure rather than exposed directly to harsh plant conditions.
Temperature and altitude can also affect the drive's permissible output and may require derating according to the manufacturer's instructions.
10. Confirm the Required IP Rating
An IP rating describes the degree of protection an enclosure provides against access, solid objects and water.
A standard panel mounted drive and a drive intended for exposed washdown or outdoor conditions can require very different enclosure arrangements.
The required protection should be determined from the installation environment rather than selected solely for convenience.
11. Check Braking Requirements
Some applications need the motor to decelerate quickly or control energy returned from the mechanical load.
Depending on the application, this may require a braking resistor, built in braking transistor, external braking unit or another regenerative solution.
High inertia loads, rapid deceleration and overhauling loads require particular attention. Braking requirements should be identified before the drive is ordered.
12. Consider Motor Cable Length and EMC
Long motor cables can affect the electrical stress seen by the motor and drive. Certain installations may require output reactors, filters or other measures according to the drive manufacturer's recommendations.
Electromagnetic compatibility, cable screening, earthing and routing are also important in industrial installations, particularly where sensitive instrumentation and communications equipment are present.
13. Check Whether Additional Functions Are Required
Depending on the application, useful or required drive features may include:
- Safe Torque Off
- Built in PID control
- Multiple motor profiles
- Encoder feedback
- Sleep and wake functions for pumping
- Fire mode for relevant HVAC applications
- Motor temperature monitoring
- Application macros
- Special communications interfaces
Feature availability differs between drive ranges, so these requirements should be identified before choosing a model.
Replacing an Existing VSD
Replacement projects need additional information because the new drive must interface with an existing system.
Provide the following where possible:
- Existing VSD manufacturer
- Exact model or part number
- Photograph of the VSD nameplate
- Motor nameplate photograph
- Supply voltage
- Application description
- Control wiring information
- Communications protocol
- Installed option cards or accessories
- Any known braking equipment
- Available installation space
A modern replacement can differ in physical size, terminal arrangement, parameter structure and communications capability from the original unit. A technically suitable replacement is therefore not always a drop in replacement.
A Practical VSD Enquiry Checklist
When requesting a quotation, providing the following information gives the Softcore Group Industrial Solutions team a much better starting point:
- Motor: ___ kW
- Motor rated current: ___ A
- Motor voltage: ___ V
- Incoming supply: ___ V, single phase or three phase
- Application: pump, fan, conveyor, mixer, machine or other
- Required speed range: ___
- Control method: keypad, terminals, analogue signal, PLC or other
- Communications protocol: ___
- Environment and enclosure requirement: ___
- Braking requirement: yes, no or unknown
- Existing drive model if replacing: ___
What If You Do Not Have All the Information?
That is common, particularly when buyers are replacing equipment already installed in a plant.
Start with photographs of the motor and existing drive nameplates, the application and the available electrical supply. From there, additional information can be identified where necessary.
Catalogue products can be purchased or quoted through the Softcore Group Industrial Solutions online store. Brands, models and configurations outside the active online catalogue can be quoted on request, with pricing, availability and lead time confirmed separately.
For a replacement drive, include the existing drive model and motor nameplate information with your enquiry. For a new application, include the motor details, electrical supply, load type and control requirements.
Technical Note
This guide provides general product selection information and is not a substitute for application specific engineering. Final VSD sizing, electrical design, protection, installation, programming and commissioning should comply with the drive and motor manufacturers' instructions, applicable standards and the requirements of the specific installation. Where engineering judgement is required, selection and implementation should be completed or verified 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.