Using a variable frequency drive (VFD) can be beneficial in many constant speed applications driven by electric motors, such as those that require controlled starting and have been historically served by a reduced voltage soft-starter (RVSS). While an RVSS and a VFD can both provide a controlled start, let’s examine the benefits of each technology and when it makes sense to use one over the other.
RVSS vs. VSD
The differences between RVSS and VFDs and when to select one or the other for an application is determined by the following factors (when using a NEMA design B three phase induction motor):
- Across-the-line starting results in inrush current between 600% to 800% (locked rotor current) and produces approximately 200% of rated torque (locked rotor torque).
- The sudden acceleration of the load from the 200% torque surge generated when across-the-line starting.
- Zero driving torque when the starter is de-energized resulting shock loading to the piping and connection components (water hammer effect).
An RVSS can be used to limit inrush current and reduce mechanical stresses on the motor and device it is powering during the starting cycle. The RVSS ramps the starting voltage from 40% (typical) to 100% over a set time (2 - 15 seconds typical). Starting torque is significantly reduced, rising to full torque at rated voltage.
By using an RVSS, locked rotor torque will be approximately:
Rated Torque x 2 x (% applied voltage)2
At a 40% start voltage, locked rotor torque will be:
Locked Rotor Torque = Rated Torque x 2 x (0.40)2 = 0.32 (32% of rated torque)
Benefits of using an RVSS
- Reducing the inrush current can lessen brownouts and help avoid peak demand surcharges. The reduced starting torque decreases stress on the motor windings and minimizes motor heating caused during across-the-line starting.
- An RVSS can, in certain cases, allow more frequent starts per hour than across the line starting.
- An RVSS can also help mitigate the water hammer effect when stopping a centrifugal pump.
Points to remember
- An RVSS produces substantially reduced starting torque.
- An RVSS does not provide controlled variable speed operation nor the ability to change rotation without a reversing contactor.
- Due to the phase angle controlled variable voltage waveform, an RVSS creates significant harmonic distortion and reduced power factor during the start cycle.
When to use a VFD
Because both the voltage and frequency are varied with a VFD, the motor will be at 100% flux at any speed resulting in the ability to produce 100% torque at 100% current at any speed below base speed. Therefore, a VFD can be used as a full torque soft starter in place of an RVSS. When used in this capacity, a VFD is capable of starting loads that require up to 200% torque such as mixers and production machinery with no inrush current.
Benefits of using a VFD in place of an RVSS
- Varying speed optimizes the process.
- A VFD can reverse rotation without an expensive reversing contactor set and wiring.
- Smooth, controlled acceleration and deceleration minimizes mechanical stresses and water hammer effect if used on a pump.
- No bypass contactor required.
- Because the VFD is 100% rated and the current does not exceed 100%, the number of starts per hour is unlimited.
- Harmonic distortion is significantly less that that created during the start cycle of a phase angle controlled RVSS.
Parker has recently introduced the AC10 series of general purpose VFDs, available at 230V to 20HP and 460V to 250HP and offer:
- Easy configuration via an onboard keypad or with Parker’s DSELite software.
- Built-in RS422/485 communications.
- High starting torque, unlimited starts per hour and rotation reversal.
Article contributed by Bill Riley, business development manager for the Drives Business Unit, Electromechanical & Drives Division North America, Parker Hannifin Corporation.