A failed or obsolete variable speed drive can bring a machine, pump, conveyor or process to a stop, and replacement enquiries are often urgent. The temptation is to send only the old drive's power rating and ask for the nearest equivalent.
Unfortunately, a matching kilowatt rating does not necessarily make a new VSD a suitable replacement.
Drive families change over time. Physical dimensions, current ratings, terminal arrangements, control functions, communications protocols, option cards, braking arrangements and parameter structures can all differ between an older drive and a current model.
Collecting the right information at the start makes it much easier to investigate a technically appropriate replacement and reduces unnecessary delays between purchasing, maintenance and technical teams.
Start With the Existing VSD Nameplate
If the existing drive is still accessible, photograph its nameplate before removing or disposing of it.
The most useful information usually includes:
- Manufacturer
- Exact model or type code
- Part number where shown
- Input voltage and phase
- Input current
- Output voltage
- Output current
- Applicable power rating
- Frequency information
- Serial number where useful for manufacturer identification
The exact model code is particularly important. A long drive code may contain information about voltage class, enclosure, current rating, options or product generation. Do not shorten it unless you are certain the omitted characters are irrelevant.
Photograph the Entire Drive
A nameplate photograph is important, but additional photographs can reveal information that the model number alone does not make obvious.
Where safe and practical, useful photographs include:
- The front of the VSD
- The nameplate
- The control terminals
- The power terminals
- Installed option cards
- Communications modules
- External keypad or operator panel
- Braking resistor or braking unit
- The electrical panel showing available installation space
Do not remove covers or access live electrical equipment merely to obtain photographs. Electrical equipment should only be accessed in accordance with site procedures by appropriately authorised personnel.
Get the Motor Nameplate Information
The replacement drive ultimately has to operate the motor, so the motor nameplate is just as important as the old VSD.
Record or photograph:
- Motor rated power in kW
- Rated voltage
- Full load current
- Rated frequency
- Rated speed
- Connection information such as star or delta
- Duty and insulation information where relevant
Motor current deserves particular attention. A replacement should not be selected solely because its nominal kW rating resembles that of the old drive.
Confirm the Incoming Electrical Supply
State the actual electrical supply available at the installation.
Useful information includes:
- Supply voltage
- Single phase or three phase
- Supply frequency
- Any transformer or unusual supply arrangement relevant to the drive
Do not assume that the old drive's voltage class can be inferred correctly from the motor kW rating.
Tell Us What the Motor Actually Drives
The application matters because different loads place different demands on a VSD.
Examples include:
- Centrifugal pump
- Positive displacement pump
- Fan
- Conveyor
- Mixer or agitator
- Compressor
- Crusher
- Extruder
- Machine tool
- Packaging equipment
- Hoist or lifting equipment
- Other production machinery
A pump and a heavily loaded conveyor with the same motor power can have very different torque and overload requirements.
Check the Existing Control Method
The replacement drive must be able to receive the commands the existing system expects.
Determine whether the VSD is controlled by:
- Its local keypad
- External start and stop push buttons
- Selector switches
- A potentiometer
- A 0 to 10 V analogue signal
- A 4 to 20 mA analogue signal
- Digital inputs
- A PLC
- An HMI or SCADA system
- Industrial network communications
If the existing drive uses several digital inputs, analogue signals or relay outputs, record how those signals are used.
Communications Can Determine Whether a Replacement Works
A replacement may be electrically capable of operating the motor but still be unsuitable for the automation system if it cannot communicate with the existing PLC or network.
Identify the communications protocol where applicable. Examples can include:
- Modbus RTU
- Modbus TCP
- PROFIBUS
- PROFINET
- EtherNet/IP
- CANopen
- Other manufacturer specific or industrial networks
Also check whether communications are built into the existing drive or provided by an option card or external module.
A current replacement model may require a different communications accessory even when it supports the same protocol.
Record Any Installed Option Cards
Older drives may have optional hardware installed for functions that are not part of the standard drive.
Examples include:
- Fieldbus communications cards
- Additional digital or analogue I/O
- Encoder or feedback cards
- Relay expansion modules
- Safety option modules
Photograph option cards and their labels wherever safely accessible. Replacing the base VSD without accounting for required options can leave the machine unable to operate as intended.
Check the Braking Arrangement
Determine whether the existing system uses:
- A braking resistor
- An external braking unit
- A regenerative arrangement
- No additional braking hardware
Applications with rapid deceleration, high inertia or overhauling loads can require specific braking capability. A new drive should not be assumed to work with existing braking hardware without verification.
Measure the Available Space
Current drive models are not necessarily the same physical size as the equipment they replace.
Record the available:
- Panel height
- Panel width
- Panel depth
- Mounting space
- Ventilation clearances
A technically suitable replacement may require panel modifications if its dimensions or clearance requirements differ from the original.
Check the Enclosure and Environment
Describe where the VSD is installed.
Consider:
- Inside an electrical panel
- Wall mounted
- Indoor or outdoor installation
- Dust levels
- Moisture
- Ambient temperature
- Altitude
- Corrosive atmosphere
- Required IP protection
Environmental conditions can affect enclosure selection, cooling and drive derating.
Do You Need the Existing Parameters?
Where possible, retaining a record of the old drive's parameters can be extremely useful, particularly on machines with more complex control.
However, parameters cannot always be copied directly from one drive family to another. Manufacturers use different parameter structures, numbering systems and control algorithms.
The old settings should therefore be treated as useful application information rather than automatically entered into a new drive without technical review.
Important Settings Worth Recording
Depending on the application, useful settings can include:
- Motor nameplate parameters
- Minimum and maximum frequency
- Acceleration time
- Deceleration time
- Control source
- Speed reference source
- Digital input assignments
- Analogue input configuration
- Relay output functions
- PID settings
- Communications address and network settings
- Motor control mode
- Current limits
- Fault handling or restart behaviour
Is the Old VSD Truly Obsolete?
An old drive may fall into several different categories:
- Still manufactured and available
- Manufactured but difficult to source locally
- Discontinued with a manufacturer recommended successor
- Discontinued without a direct successor
- Repairable but no longer commercially sensible to repair
- Part of a legacy machine where the wider control system also needs consideration
The correct purchasing strategy depends on which situation applies.
Where a manufacturer has an official successor, that is an important starting point, but the application still needs to be checked. A successor product is not necessarily a drop in replacement for every installation.
Should You Replace It With the Same Brand?
Not always, but changing brands can increase the amount of engineering work required.
A replacement from the same manufacturer may provide a clearer migration path, familiar programming tools or documented successor information. In other cases, another established drive family may meet the electrical and application requirements.
Changing drive families can affect:
- Physical dimensions
- Terminal locations
- Control wiring
- Communications
- Parameter configuration
- Keypad operation
- Programming software
- Spare parts strategy
The objective should be a technically suitable and supportable replacement, not simply finding a device with a similar number on the front.
Can a Larger kW Drive Be Used?
Oversizing is not automatically a solution to uncertainty.
A larger drive may be appropriate in some applications, but selection must still consider motor current, control performance, protection, overload duty, installation requirements and manufacturer recommendations.
Drive size should be determined from the actual application rather than increased arbitrarily.
What If the Existing VSD Has Completely Failed?
Even if the VSD will no longer power up, useful information can usually still be collected from the equipment.
Start with:
- VSD nameplate photographs
- Motor nameplate photographs
- Application description
- Incoming supply
- Control wiring information
- Communications modules
- Panel photographs
- Any existing electrical drawings
- Previous parameter backups or commissioning records if available
If the original drive is physically damaged, do not energise or dismantle it merely to retrieve information unless this is being handled safely by appropriately qualified personnel.
What If You Only Have a Part Number?
Send it.
A complete manufacturer model or part number is often enough to begin researching the original drive specifications and determine what additional information is required.
If possible, also send photographs. Product labels can contain revisions, option codes or voltage information that may not appear in a shortened part number recorded on a maintenance list.
A Practical VSD Replacement Checklist
Before submitting a replacement enquiry, collect as much of the following as possible:
- Existing VSD manufacturer: ___
- Exact model or part number: ___
- Existing drive rated output current: ___ A
- Incoming supply: ___ V, single phase or three phase
- Motor power: ___ kW
- Motor rated current: ___ A
- Motor voltage: ___ V
- Application: ___
- Control method: ___
- Communications protocol: ___
- Installed option cards: ___
- Braking equipment: ___
- Required IP rating or environment: ___
- Available panel space: ___
- Photographs attached: VSD, motor, panel and option cards
- Electrical drawings or parameter backup available: yes or no
You do not need every answer before making an enquiry. The checklist is intended to reduce uncertainty and help identify which technical questions still need to be resolved.
Replacing More Than One Drive?
If a plant or machine has several legacy VSDs, it can be useful to provide a complete equipment schedule rather than requesting each unit independently.
Include quantities, model numbers, motor information and application details for each drive. This can make it easier to investigate replacement families, common accessories and purchasing requirements across the project.
Send the manufacturer and exact model number, motor nameplate details, application information and clear photographs wherever possible.
Where a replacement requires application specific engineering, control modifications, programming or commissioning, the final selection and implementation should be completed or verified by an appropriately qualified technical professional.
Technical Note
This article provides general product identification guidance. It does not establish that any particular VSD is a direct replacement for another. Motor control equipment must be verified against the motor, load, electrical system, control architecture, environment, manufacturer requirements and applicable standards before installation or commissioning.
Related catalogue examples: Variable Speed Drives. For an equipment enquiry, contact Softcore Group Industrial Solutions at info@softcoregroup.co.za.