Electric motors are the dominant form of motive power on Earth for a reason: they’re efficient, scalable and versatile. That versatility has driven the technology to multiple uses in industrial settings, in both intermittent duty and continuous RPM service. Three phase induction motors are robust, low cost and self-starting, but engineers and technicians have long understood the differences between the types of service in critical factors such as inrush current, overload and undervoltage, heat rise and efficiency.

For constant speed applications such as pumps or blowers, the typical starting method uses a reduced voltage soft starter (RVSS). It’s a simple, proven technology, but even in steady state, low duty cycle applications like these, variable frequency drives show definite advantages.

Advantages and Disadvantages of VFD vs. Soft Starters

Let’s assume a typical NEMA design B three phase induction motor. Here are the starting factors:

Across-the-line starting applies full line voltage to the stator windings, resulting in an inrush current (locked rotor current) between 600% and 800% of steady state and produces approximately 200% of rated torque. This causes mechanical stress and significant heating to the motor and motor windings. If not dissipated, the heat rise may reduce service life and also may limit the number of starts per hour.

In larger motors, only as few as two or three starts per hour may be allowed. In pump applications in the process industries, for example, this limitation can affect the choice of motor, pump design or even force the overrating of a system for a given application. 

This 200% torque surge and sudden acceleration has implications for reduction gearing, couplers, clutches and other mechanical components in the drive system as well as for downstream machinery as well as the products handled by the machinery. A mixer impeller, for example, may cavitate or aerate a batch liquid or semisolid product in a hard start.  

Across-the-line starting also means zero driving torque the instant the starter is de-energized. Sudden torque on is a factor in many applications, but sudden torque off can also have serious consequences. Consider the case of centrifugal pumps with significant head—when the starter is de-energized, the falling fluid volume carries a great deal of energy, shock loading piping and other components due to the “water hammer” effect. Another example is geared drive systems, which can experience unwanted backlash at shutdown, accelerating wear.

RVSS addresses these issues by ramping starting voltage from, for example, 40% to 100% over a set time, typically 2-15 seconds. This limits inrush current and starting torque.

Locked rotor torque using an RVSS will be approximately equal to:

Rated Torque x 2 x (% applied voltage)²

For example, for 40% start voltage

Locked Rotor Torque = Rated Torque x 2 x (0.40)² = 0.32 (32% of rated torque)

Inrush current is reduced from the 600% - 800% to approximately 200% - 300%. This can have beneficial effects plantwide, with fewer issues with voltage regulation and a reduced need to protect sensitive equipment like PLCs from a transient undervoltage condition. Mechanical stress and heating are reduced during starting, which may allow for more starts per hour for the motor. The same ramping can be used during stopping.

Important Points to Remember When Choosing RVSS

As explained above, an RVSS produces substantially reduced starting torque. In many applications, this offers lower shock loads, less wear and better control of process cold starts. Some applications, however, need high starting torques. An example is batch mixing of thixotropic compounds that need high initial shear forces to thin the semi-solid into an easily mixed state. Applications with a quadratic torque curve, where load torque is zero at startup and increases with speed, will not be affected by the reduced starting torque.

A bypass contact set or an external bypass contactor may be used to remove the load from the RVSS after the start cycle is completed. This permits the RVSS to be designed for intermittent duty and reduces size and cost. Limiting starts per hour will help avoid overheating.

Simplicity and low cost are benefits, but there are disadvantages to RVSS:

• Unlike a VFD, an RVSS does not provide controlled variable speed operation, nor the ability to change rotation without a reversing contactor.
• RVSSs may include additional motor monitoring functions for connection to a SCADA, PLC or other IIoT system.
• An RVSS creates significant harmonic distortion and reduced power factor during start cycle. This is due to the phase angle controlled variable voltage waveform.
• During normal running following the start cycle the power factor will be that of the motor, typically 0.85.

While VFDs were first developed to achieve variable speed operation, the falling cost of VFDs now allows system designers to look at the other benefits of the technology.

The VFD applies both a variable voltage and a variable frequency to the motor. Starting at near zero voltage and frequency, it ramps both up in synchronicity until the desired speed is achieved. Keeping the voltage and frequency synchronized keeps the motor at 100% flux at any speed, and therefore capable of 100% torque at 100% current at any speed below base speed. It is possible to use a VFD as a full-torque soft starter, providing torque proportional to current (100% continuous; up to 200% intermittent) from standstill up to rated speed.

This is a major advantage for many applications. Optimum rotational speed and optimal torque are frequently in conflict, and machinery designers compromise to achieve the best performance possible—or overrate motors to get the best of both worlds. VFD technology can free equipment designers from the tyranny of the torque curve, and reduce or eliminate the need for reduction gearing, torque converters or flywheel/accumulators in many types of service.

The VFD option is highly desirable over RVSS in most cases that require high starting torque, such as mixers and other production machinery with minimal inrush current. No bypass contactor is required at the start of the start cycle, because unlike the RVSS, a VFD is rated for continuous operation. In addition, some VFDs can synchronize the output waveform to the main supply. This allows for closing a bypass contactor across the VFD at rated speed, then shutting off the VFD. This is often called an overlapping bypass and it allows the same VFD to be used to start other motors, much like a jet engine “start cart” on the ramp in an airport.

Benefits of Using a VFD in Place of an RVSS

For many, perhaps most users, the primary benefit of VFD is 100% starting torque at 100% current (no inrush) versus 32% starting torque and 200% inrush typical of an RVSS at 40% starting voltage. However, the benefits of VFD go beyond motor starting.  A VFD allows reverse rotation without an expensive reversing contactor set and wiring, and no bypass contactor is required. VFD allows the ability to vary speed, as well as allows smooth, controlled acceleration and deceleration, which reduces stresses on equipment, motor and drivetrain components.  Unlike RVSS, starts per hour are not limited. VFD’s create some harmonic distortion, but it is typically much less than that created during the start cycle of a phase-angle controlled RVSS. If necessary, VFD-generated distortion can be mitigated with standard choke and line reactor techniques.

Today, efficiency is paramount for both cost and environmental reasons. Reactive power does no work, and the drive for unity on power factor is more relevant than ever. VFD offers a power factor of 0.96 or better.

To make the switch to VFD in industrial applications, consult an experienced vendor and think about brands that have extensive experience in power and motor control systems. Parker’s North American Electromechanical and Drives division is a broad base manufacturer of industrial drives and related equipment.

Variable Frequency Drives offer so much more than resistive motor control devices that they should be considered for any industrial equipment design or retrofit application. Anywhere high starting torque, unlimited starts per hour and rotation reversal are needed, VFDs are a cost effective, reliable and simple solution.

To learn more about Parker’s VFD products, download the Parker Hannifin whitepaper.