Technical Notes

Variable Frequency Drives vs. Fluid Couplings

1. VFD’s allow for improved process control vs. fluid coupling (0.1% to 0.5% speed regulation throughout the operating speed range).

2. VFD’s communication capabilities allow for a variety of remote control and monitoring of the drive and process parameters (4-20 mA analog control, REMOTE I/O, Data Highway Plus, DeviceNet, Modbus, Modbus Plus, etc.).

3. VFD’s do not require maintenance under normal use.

4. VFD’s control the motor directly (a complicated mechanical drivetrain in not required). This means a fewer torsional resonances that can cause mechanical problems with the driven load, motor and mechanical coupling.

5. VFD’s not only can reach synchronous speed but can also overspeed the motor if required to increase capacity (motor must be sized for this capability). Fluid couplings always have a coupling slip making even attaining synchronous motor speed impossible.

6. VFD’s (Allen-Bradley CSI-PWM technology) have an SCR rectifier that allows the drive to regenerate power back to the line (for example - downhill conveyor applications). This regeneration capability can also be used to brake the load (in conjunction with a mechanical brake to hold in the stopped position). This isn’t possible with a fluid coupling.

7. VFD’s have the capability to be bypassed by an electromechanical starter or electronic soft-starter, if required. The fluid coupling is in line between the motor and the load – it cannot be bypassed. When the fluid coupling fails the process must be shut down until repairs are made.

8. VFD’s are much more efficient than fluid couplings. The efficiency advantage is approximately 5% at full speed and full load and can be more than 25% more efficient as the speed and loading decreases (see attached graph). This means that, if fluid couplings are used, the customer must purchase motors that are significantly oversized (more $$) for the application because of the HP losses in the fluid coupling.

9. VFD’s control the motor starting current. The starting current is usually very close to or equal the motor rated full load current. The inrush current for the motor used with a fluid coupling will be approximately the standard 600% of

full load current (NEMA B squirrel cage motor). This may cause voltage dip problems on weaker power systems.

Fig.1 – Fluid coupling mechanical cross section diagram

Fig.2 – Fluid coupling construction

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Variable Speed

Advanced Fluid Drive

Standard fluid Drive

Speed

Efficiency