Choosing the correct Variable Speed Drive (VSD) size is critical for reliable motor operation.
An undersized drive can cause trips and overheating, while an oversized drive increases cost unnecessarily.
This guide explains how engineers and technicians can size a Variable Speed Drive correctly for industrial applications.
Selecting the correct drive is critical for performance and reliability.
If you’re unsure which unit fits your application, you can view our range of variable speed drives.
Motor Nameplate information required
Never select a drive smaller than the motor current rating.
Not sure which Drive size you need ?
View available VSDs matched to your application or speak to a Technician for assistance.
You need:
- Rated Power (kW)
- Rated Current (Amps)
- Voltage
- Frequency
- RPM
The most important value when sizing a Variable Speed Drive is the motor rated current, not only the motor power.
Drives are selected primarily by current rating, not kW.
- Always size drives by motor current
- Never by kW alone
Select Drive Based on Current
Always choose:
Drive Current ≥ Motor Rated Current
If a motor rated current is 18A, the drive must be rated 18A or higher.
Never select a drive smaller than the motor current rating.
Not sure if you’re selecting the correct drive ?
Incorrect sizing is one of the most common causes of VSD faults and motor issues. Our technicians can help you select the right drive based on your motor current and application.
Consider Application Type
| Process application | Duty/OverLoad | VSD type | VSD type | Torque Requirements |
|---|---|---|---|---|
| Pumps, Fans & Blowers | 110 –120% | General | V/F | Variable |
| Conveyors | 150% | General | V/F | Constant |
| Crushers, Mixers & Compressors | 150% | High Starting Torque – Open Loop | V/F | Constant |
Pumps, fans and blowers typically require normal duty drives. In these cases, a general purpose variable speed drive is suitable.
Conveyors, hoists and crushers require heavy duty drives designed for higher starting torque.
Normal Duty
- 110–120% overload
Heavy Duty
- 150% overload
Heavy-duty drives typically allow overload ratings of approximately 150% for short periods, while normal-duty drives allow approximately 110–120% overload.
Check Supply Voltage
- 230V single phase → small drives
- 230V Three Phase → Asian & American markets
- 400V three phase → industrial drives (380V plus mining, steel and heavy)
- 525V three phase → industrial drives
- 600V three phase → industrial drives
Drive and motor must match.
Always ensure the drive voltage matches both the supply and the motor rating.
- Incorrect voltage selection is a common cause of Drive faults – see common VSD Faults section on how to avoid them
- Incorrect voltage selection can damage the drive or motor
- Always ensure the drive voltage matches both the supply and the motor rating
Common Sizing Mistakes
Common mistakes include:
- Selecting by KW only
- Ignoring current rating
- Ignoring OverLoad
- Ignoring ambient temperature
- Ignoring cable length
- Troubleshoot – drive faults by incorrect sizing
Correct sizing improves reliability and reduces downtime however if incorrectly sized, you may need to replace it with a higher rated unit.
Experiencing Trips or OverHeating ?
Incorrect drive sizing is often the cause.
Simple Sizing Example
Electric motor:
- 7.5 Kw
- 400VAC
- Rated current = 15.2A
Drive Selection:
Choose drive ≥ 15.2A
Example: 18A drive
When to Oversize a Drive
- Heavy loads
- Frequent starts
- High temperature
- Long motor cables
Selecting the correct drive size improves system reliability and drive lifespan.
In these cases, selecting a higher-rated drive from our VSD range is recommended
Correct Variable Speed Drive sizing improves reliability, reduces downtime, and prevents unnecessary costs.
What happens when a drive is incorrectly sized ?
- Frequent tripping during startup
- Overheating and reducing lifespan
- Poor motor performance
- Increased maintenance and downtime
If you’re unsure whether your current drive is correctly sized for your application, it’s best to verify before installation or replacement.
Typical Drives for This Application
Based on the above sizing principles, the following solutions are typically used:
Incorrect sizing is one of the most common causes of drive trips, overheating and reduced equipment life.
Not sure if your Drive is correctly sized ?
Incorrect sizing is one of the most common causes of drive trips, overheating and reduced equipment life.
Our technicians can help you confirm your sizing or recommend the correct replacement.
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Supporting industrial clients across South Africa with drive systems, motor control solutions, and technical support.
