Starting, Stopping Motors Softly

Good reasons not to start (or stop) ac induction motors under full input voltage include exposure to extremely high locked-rotor currents and torques up to 230% of full-load torque. Solid-state, reduced-voltage (SSRV) soft starters serve to mitigate destructive effects of such very high motor inrush currents and resulting mechanical stresses on connected equipment or system components.

By Frank J. Bartos September 1, 2005
  • Cut high motor inrush currents

  • Don’t neglect soft stopping

  • Torque-ramping control

  • Solid-state starters reduce unit size

  • Bypass contactor cuts heating

  • Diagnostics, communications

Good reasons not to start (or stop) ac induction motors under full input voltage include exposure to extremely high locked-rotor currents and torques up to 230% of full-load torque. Solid-state, reduced-voltage (SSRV) soft starters serve to mitigate destructive effects of such very high motor inrush currents and resulting mechanical stresses on connected equipment or system components.

Many newer soft starters need not be rated for continuous operation. An integrated bypass contactor actuates after the motor reaches operating speed, linking the motor to the line, at which point the starter can be turned off. Protection and diagnostic role of soft starters also is on the rise due to solid-state controls used as the logic engine. However, silicon-controlled rectifiers (SCRs) remain the workhorse power components.

At ABB Inc., emphasis is on soft starters’ wider function as motor-protection relay devices. “Soft starters now are expected to do more than just soft-start/stop induction motors,” says Paul Terry, product manager for ABB Low Voltage Products & Systems. Terry lists phase imbalance, phase reversal, phase loss, and instantaneous trips for over/underload conditions among protective functions supplied by “top-tier” soft starters—based on inputs of motor current, voltage, and temperature. In ABB’s PST Series devices, a user-programmed timing threshold and more accurate motor sensors limit nuisance tripping.

Charles Forsgard, director-Motor Control Business at Schneider Electric’s North American Operating Div., notes that SSRV devices originally had simple voltage-ramping designs, which provided a “softer” mechanical start and reduced motor inrush current, but their effect depended on the driven load’s characteristics. “If a motor was lightly loaded, there could be no ‘soft start’ effect at all,” he says.

The new approach focuses on torque-ramping rather than voltage-ramping methods. It relies on a new generation of control algorithm able to determine power and power factor, using information about motor voltage and current, from which the soft starter derives real stator power, stator losses, and as a result, real power delivered to the rotor. “Power to the rotor is used to calculate actual motor torque, and the SSRV starter will follow the torque ramp as long as motor load does not exceed the current limit setting,” continues Forsgard.

In torque-ramping, the torque controller (see diagram) uses operator-entered values of motor nominal torque, initial torque, and torque limit—input at the “torque reference/ramp” block—plus torque ramp time, to generate a desired motor torque. “The controller is then used to control thyristor firing, according to actual motor torque versus the desired value,” Fosgard states. “[Importantly], motor torque is no longer strictly dependent on an applied motor voltage or the motor’s speed-torque characteristics, but is increased according to a timed ramp.”

Polarity balancing, smaller devices

The latest feature in SSRV soft starter technologies at Siemens Energy and Automation Inc. is a patented control technique called “polarity balancing.” The two-phase control ensures smooth, quiet start-up by generating uniform motor starting characteristics (speed, torque, and current), thus avoiding direct-current components, notes Steve Koch, product manager, RVSS Starters, at Siemens E&A.

For maximum flexibility and adaptability to varying loads, Schneider Electric’s torque-ramping control of SSRV soft starters makes initial torque, torque limit, and acceleration time user-adjustable.

He explains that current resulting from overlapping in two controlled phases will flow in the third uncontrolled phase, leading to asymmetric distribution of three phase currents during motor start-up. In addition, power semiconductors switching in the two controlled phases produce dc components, which can lead to detrimental motor noise at starting voltages below 50%. Polarity balancing reportedly eliminates these effects during start-up. (Two-phase control is common in soft starters for the sake of economy.)

“Acoustic quality of the start-up process practically attains the quality of a three-phase controlled start-up. This is enabled by continuous dynamic adjustment or balancing of current half-waves of different polarity during motor start-up,” says Koch.

Among recent developments noted by Jeff Lovelace, drives product manager at Baldor Electric Co., are more compact soft starter packages for the same power output, due to considerably smaller SCR sizes. He also mentions faster control logic thanks to greater use of lower cost microprocessors and keypad programming setup of soft-starters instead of using potentiometers. “Since the microprocessor is built-in, we can tweak control output to improve power factor when running lightly loaded,” states Lovelace. He refers to this is as “optimization” that avoids over fluxing the motor.

Douglas Yates, product specialist at Danfoss North America Motion Controls, mentions innovative low thermal expansion (LTE) technology that applies new materials to virtually eliminate thermal expansion effects (and other problems) experienced with typical power-chip technology used in conventional power relays. Excessive heat generated by the power chip can lead to metal fatigue due to different thermal expansion rates between chip, heat conductor, and the current carrier clip, he explains. “In addition, air pockets in the soldering process can create hot spots on the chip, which can also impair performance and cause breakdowns.”

LTE design incorporates fewer soldering points to increase heat dissipation. A new one-shot vacuum soldering process reportedly prevents formation of air pockets and hot spots. Used in Danfoss MCI soft starters, LTE technology provides solid-state relay switching speed and long device life. “This technology can outlast conventional solid-state relays by a factor of 10, which means significantly greater reliability and longevity for the customer,” Yates says.

Built-in bypass

Steve Litzau, product manager, Rockwell Automation, emphasizes compactness and higher functionality of SS starters compared to previous generation devices. Today’s designs have a built-in bypass, which opens when the motor reaches full speed to reduce power (heat) losses in the solid-state power section. This translates to much smaller enclosures without need for special cooling. Litzau sees about 75-80% of new starters being released with built-in bypass. “This percentage continues to rise,” he says, adding that “solid-state starters typically have built-in, configurable inputs/outputs; and rather than operating in a stand-alone design, they can be set-up, controlled, and used to provide monitoring feedback through networks, such as DeviceNet.”

Eaton Corp. likewise considers soft starters’ “run bypass mode” noteworthy. This feature, part of its IT (Intelligent Technologies) product line, significantly reduces heat generated by the starter, which minimizes enclosure sizes and costs. “Also, the bypass contactor is internal to the soft starter, eliminating the need for additional devices, further reducing enclosure sizes and minimizing installation time,” says Rodney Partain, product manager, power control. Eaton’s IT low-voltage soft starters (such as S752, S801, and S811) use 24-V dc pulse-width modulated (PWM) coil control for the bypass contactor, said to consume minimum power of just 5 W in steady state. “The PWM coils, in combination with an efficient power supply, work to reduce negative effects of electrical system disturbances and protect against power loss,” he adds.

Low-voltage IT soft starters (cited above) and medium-voltage MV801, come with soft starting/stopping control and flexible protective features. S811 starter adds communications capabilities via a digital interface module (DIM) that includes an easy to use operator interface, according to Partain. DIM enables users to safely configure, commission, monitor, and troubleshoot their system. S811 connects to various networks, including DeviceNet, Ethernet, and Profibus, via built-in communication capabilities of Cutler-Hammer QCPort (Quick Connect).

Stop softly, as well

All article contributors noted the importance of “soft stopping” features in SSRV starters—particularly to mitigate noisy, destructive water-hammer effects in pumping applications.

ABB’s Terry considers “soft stopping” frequently neglected by installers and users, although many soft starters offer the feature. This may be due to “fear” of changing factory default settings often via complex interfaces or just lack of awareness of soft-stopping benefits. To alter this effect, ABB has incorporated a plain-text HMI (interface) into its PST Series starters to guide an operator/installer through set-up by selecting from clearly worded application-based programming groups most closely matching the application (see photo).

Lovelace at Baldor agrees about “hidden” soft-stopping capability. “Most people consider using a ‘soft start’ for just that purpose, to start a motor. They do not realize the benefits it offers for soft stopping of the load,” he says. Advantages include preventing mechanical stresses on the machine when the motor halts abruptly.

Limitation of earlier (voltage-ramping) SSRV starters also extended to soft stopping, because of inherent lack of real deceleration control. It applied especially when dealing with lightly loaded motors, according to Schneider Electric’s Forsgard. With modern torque control, however, an SSRV starter’s linearly decreasing torque ramp can decelerate centrifugal pump loads. “Gradual reduction in speed makes it possible to coordinate check-valve closing without causing water hammer,” adds Forsgard. The soft starter continuously monitors motor load torque so that it’s ready to start the linear deceleration ramp of pump loads when it receives a stop command—even with the motor only 60-70% loaded.

Siemens offers three ways to stop motors in its Sirius starters: coast to rest, soft stop, and dc injection. New Sirius 3RW44 combines dc injection braking and closed-loop torque control (where voltage is gradually reduced using software and current feedback) to enable rapid stoppage of driving loads. Focus on soft stopping is highly application dependent. “Soft stopping is particularly important in hydraulic pumps to avoid water hammer,” says Koch.

Sirius 3RW44 starter uses closed-loop torque control to prevent sudden water pressure change when a pump is switched off or to mitigate mechanical stresses on a conveyor belt being stopped. Siemens also mentions milling machines as requiring optimum braking. For example, when a milling head with a 15-kW motor that machines bores in an aluminium automobile engine block is powered down, long stopping times occur due to the milling head’s high moment of inertia. Resulting high idle times for tool changes or machine set up can’t be tolerated. “Closed-loop torque control and dynamic dc braking are used in Siemens’ 3RW44 starter to reduce the machine’s long stopping times,” adds Koch.

Opposite of starting

Soft stop is the direct opposite of soft start, notes Yates, from Danfoss. Line voltage applied to the motor is gradually ramped down to zero (or a preset low point)—to extend motor stopping time. Danfoss’ Ci-tronic family is among motor control devices that offer soft stop and soft start with easily adjusted precision. Ramp-up/down times can be set from 0.5-10 seconds and starting torque is adjustable over 0-85% of nominal torque. Moreover, a kick-start feature (full torque applied for 200 ms) is available for high-breakaway torque applications, such as loaded conveyors and packaging machines.

Ci-tronic controllers handle applications with frequent starts and stops, according to Yates. “Many such controllers incorporate a zero cross switching technique (contactor always switches when voltage is zero) to ensure optimum speed and accuracy,” he adds.

Rockwell Automation also points to soft starters’ ability to shorten as well as lengthen motor stopping time. While many applications seek to avoid sudden stops, others rely on braking for quicker stopping to raise throughput or operation efficiency. Litzau offers the example of a bandsaw, where faster than “coast to rest” braking can help maintenance and minimize total downtime.

“Too many products focus entirely on soft starting and don’t discuss the benefits of soft stopping,” says Eaton’s Partain. Cutler-Hammer IT soft starters offer various stopping options. S801 and MV801 include a pump control option, where sophisticated algorithms reportedly minimize pressure surges that cause water hammer when pumps start or stop. Also, S701 soft-start controller’s dc braking option enables quick stopping of high-inertia loads and can eliminate a mechanical brake, explains Partain.

Safe, 24-V dc control

Use of low-voltage dc for actual control of soft starters brings benefits of operator protection and compliance to safety standards. DC voltage is more often converted inside the device, however some manufacturers offer an external (direct) 24-V dc design.

Eaton has been a low-voltage dc control proponent for some time, having introduced its IT Soft Starts family of SSRV devices exclusively with 24-V dc control in 1999 ( CE , July ’99, pp. 9-10). This type of control eliminates personnel hazards while working on control systems. “Further, use of 24-V dc control simplifies compliance with NEC and OSHA regulations,” says Partain. “From a global perspective, one dc voltage bypasses the need to transform the variety of ac input voltages used worldwide.” Eaton’s S752, S801, and S811 low-voltage starters use 24-V dc control.

ABB began using 24-V dc control about the same time. Its soft starters derive low dc voltage typically via an internal ac-to-dc power supply, stepped down on the control board. In new PST Series, the internal power supply has a wide 100-250 V ac input suitable for global markets. Operator safety remains intact as all output contacts are at 24 V dc, explains Terry. “This gives the best of both methods—ease of 110 V ac (or 220 V) and safety of 24 V dc,” he adds.

Baldor’s Lovelace sees adoption of 24 V dc control marketplace dependent. U.S. users have high acceptance of 120 V ac control, while European users, spurred by specific regulations, look for 24 V dc control. “Making your control work in both markets may require some additional hardware,” he says, adding that newer U.S. system integrators are paying attention to low-voltage dc control.

Litzau at Rockwell Automation sees low-voltage dc control, much like communication capabilities, catching on in solid-state starters. “Low voltage provides a higher degree of safety within the controller from inadvertent contact,” he says. “It also allows users to employ the same power supply as the communication architecture for common control.” Rockwell offers 24 V dc and 120-240 V ac control for its soft starters.

Shrinking cost of variable-frequency drive (VFD) technology—and its reach to “micro” sizes—has for some time been cited as a potential threat to motor starter viability. With the addition of torque control to SSRV soft starters, Schneider Electric still sees a continuum existing for these devices, driven by the same cost and size reductions as in VFDs. Attractiveness of SSRV soft starters will remain, “particularly in large horsepower applications where process speed control is not required,” adds Forsgard.

Litzau agrees about SS starters’ continued viability as a motor control alternative to VFD starting or direct on line/full-voltage starters. “They’re a great solution when motor speed control is not required, but where users and OEMs still wish to control motor starting or stopping on their equipment. In addition, solid-state starters produce fewer harmonics and have a lower installed cost than variable-frequency controllers,” he concludes.