Selecting the correct variable frequency drive (VFD) for a three-phase motor is not simply a matter of matching the motor’s kW rating to the drive’s kW rating. A properly sized drive must accommodate the motor’s full load current, supply voltage, application duty, overload requirements, acceleration profile, ambient conditions, and installation environment.
For UK industrial applications, choosing the right Invertek drives solution can improve motor control, energy efficiency, and equipment reliability while reducing the risk of nuisance trips and premature component failure.
This guide explains how to size a VFD correctly, with particular attention to the Invertek Optidrive E3 range and the practical requirements of a variable frequency drive UK installation.
Why Correct VFD Sizing Matters
A VFD controls the motor speed by varying the output frequency and voltage. Unlike a direct-on-line starter, it can regulate acceleration, deceleration, speed, and torque throughout the operating range.
However, the drive must be capable of supplying the current demanded by the motor and driven load.
An incorrectly sized VFD can result in:
- Overcurrent or thermal trips
- Insufficient starting torque
- Motor overheating
- Reduced low-speed performance
- Excessive drive temperature
- Unreliable acceleration
- Premature drive or motor failure
The most important principle is, therefore: Size the VFD according to motor current and application duty first, and use kW as a secondary selection reference.
1. Start With the Motor Nameplate
Before selecting an Invertek VFD, record the motor’s complete nameplate information.
For a typical UK three-phase motor, check:
| Motor Parameter | Why It Matters |
|---|---|
| Rated power (kW) | Establishes the approximate drive size |
| Rated voltage | Must match the VFD output voltage |
| Full-load current (A) | Critical for drive selection |
| Frequency (Hz) | Normally, 50 Hz in the UK |
| Speed (RPM) | Establishes operating characteristics |
| Power factor (cos φ) | Used for motor configuration |
| Efficiency | Helps determine operating performance |
| Connection (Δ/Y) | Must be configured correctly |
| Duty rating | Determines thermal and overload requirements |
2. Match the VFD Output Current to the Motor
The motor’s full-load current should be compared with the VFD’s continuous output current rating.
For example:
- Motor rated current: 13.8 A
- Required VFD continuous output current: ≥13.8 A
The selected drive must provide at least the motor’s required operating current under the intended duty.
The drive’s overload capability also needs to be considered. The Invertek Optidrive E3 provides overload capacity depending on the applicable model and operating conditions.
3. Check the Motor and Supply Voltage
UK industrial installations commonly use approximately 400 V three-phase supplies. However, the motor nameplate must be checked before selecting the drive.
A motor may, for example, be rated:
230/400 V Δ/Y
On a 400 V three-phase VFD supply, the motor would normally be connected in star according to the manufacturer’s specified configuration.
A mismatch between the VFD output voltage and motor connection can cause incorrect flux levels, excessive current, or poor torque performance.
4. Consider the Application, Not Just the Motor
Different machines impose variable loads on a motor.
Variable-torque applications
Fans and centrifugal pumps generally follow variable-torque characteristics. As speed decreases, the required torque and power can fall significantly.
Constant-torque applications
Conveyors, mixers, compressors, positive-displacement pumps, and many machine tools can require relatively high torque across their operating range.
High-inertia applications
Large fans, centrifuges, flywheels, and other rotating systems can place substantial demands on the drive during acceleration and deceleration.
- Longer acceleration ramps
- Increased braking capability
- A braking resistor
- Appropriate overload capacity
- Careful control of deceleration energy
5. Compare Light-Duty and Heavy-Duty Requirements
| Factor | Light/Variable Torque | Constant/Heavy Duty |
|---|---|---|
| Typical applications | Fans, HVAC, centrifugal pumps | Conveyors, mixers, compressors |
| Torque demand | Reduces with speed | Relatively constant |
| Overload requirement | Usually lower | Often higher |
| Acceleration demand | Generally moderate | Can be demanding |
| Braking | Often limited | May be significant |
| Drive selection | Closely matched rating | May need a larger/current-rated drive |
6. Understand the Invertek Optidrive E3 Range
The Invertek Optidrive E3 is designed for a broad range of motor-control applications. The range offers multiple configurations for different industrial requirements.
7. Account for Ambient Temperature and Installation
VFD ratings are affected by environmental conditions.
- Panel temperature
- Ventilation
- Enclosure IP rating
- Altitude
- Dust accumulation
- Humidity
- Nearby heat-producing equipment
- Required switching frequency
8. Consider Cable Length and EMC
The motor cable is another important part of the VFD installation.
Long motor cables can increase capacitive current and place additional electrical stress on the drive and motor insulation. EMC requirements should also be considered, particularly in industrial environments containing sensitive instrumentation or control equipment.
9. Do Not Forget Braking Requirements
A motor running under VFD control can become a generator during rapid deceleration or when the load drives the motor.
- Cranes
- Hoists
- Centrifuges
- High-inertia fans
- Conveyors with descending loads
These applications may need dynamic braking equipment.
10. Example: Sizing a VFD for a 7.5 kW Motor
Consider a UK industrial conveyor with:
- Motor rating: 7.5 kW
- Supply: 400 V three-phase
- Frequency: 50 Hz
- Motor rated current: 15 A
- Constant-torque operation
- Frequent starting and stopping
A basic selection would begin with a 7.5 kW-class VFD. However, the engineer should then verify:
- The VFD output voltage matches the motor.
- Continuous output current is at least 15 A.
- Overload capacity is appropriate for the conveyor.
- Acceleration time is achievable without excessive current.
- Braking requirements are satisfied.
- Ambient temperature does not require derating.
- Motor cable length is within the drive's specified limits.
- The enclosure rating suits the installation environment.
Common VFD Sizing Mistakes
Selecting by kW alone
A motor’s kW rating does not tell the whole story. Always check the rated current.
Ignoring Duty Cycle
A drive operating a lightly loaded fan is very different from one powering a heavily loaded conveyor.
Oversizing unnecessarily
A significantly oversized VFD can increase capital cost and may complicate protection, cabling, and system coordination.
Forgetting derating
High enclosure temperatures, altitude, and other environmental conditions can reduce usable drive capacity.
VFD Sizing: A Practical UK Engineering Approach
For a reliable variable frequency drive UK installation, use this sequence:
Motor nameplate → voltage → full-load current → application duty → overload → speed range → acceleration/deceleration → braking → environment → cable/EMC → final VFD model.
Conclusion
Correct VFD sizing is fundamentally an engineering exercise rather than a simple product-matching task. Motor current, load characteristics, overload requirements, braking, environmental conditions, and installation constraints all influence the appropriate drive specification.
Invertek drives, including the Invertek Optidrive E3, provide a range of ratings and configurations for industrial motor-control applications. The key is to select the exact drive model against the motor’s nameplate data and the machine’s real operating requirements.
For UK engineers, maintenance teams, and OEMs, Industrial Motor Warehouse can help identify suitable motor and drive solutions for new installations, replacements, and upgrades. Always consult the manufacturer’s technical documentation and have electrical design, protection, and installation work carried out by a suitably competent person.
