Less power for intelligent motor controls—more power to users

Dramatic size reduction of soft starters and related motor control devices has arrived. The key to this advance is a new design that separates control and logic circuits to cut power requirements and focuses exclusively on low control voltage at 24 V dc.Emergence of 24 V dc control has been ongoing, but mainly driven by low-amperage applications.

By Frank J. Bartos, July 1, 1999

Dramatic size reduction of soft starters and related motor control devices has arrived. The key to this advance is a new design that separates control and logic circuits to cut power requirements and focuses exclusively on low control voltage at 24 V dc.

Emergence of 24 V dc control has been ongoing, but mainly driven by low-amperage applications. Power-control devices face bigger problems, since they need to regulate high currents and power draw. Cutler-Hammer (Milwaukee, Wis.) seized an opportunity to make 24-V dc motor control more practical to use. Underway for two years, the development is based on a worldwide customer survey. It’s part of an overall program, Intelligent Technologies (IT), to meet needs of OEMs and end-users for smaller, safer, more cost-effective power controls, explains Charles Kane, manager of Standard Power Control Components at Cutler-Hammer.

Fully integrated solution

The first IT introduction is a solid-state reduced-voltage (SSRV) family called IT Soft Starts. Cutler-Hammer calls this “the industry’s first 24 V dc-controlled, fully integrated soft starter”—meaning that a bypass contactor and overload relays are built into the unit. The six-bridge SCR design does not need to use isolation contactors.

Available in August 1999, IT Soft Starts comprise four frame sizes and 17 models in the range of 4-960 A max. (see photo). Physical sizes are said to be 30-80% smaller than competitors’ units. All fit into the standard MCC cabinet. As a result, these products can replace existing across-the-line starters.

IT Soft Starts are designed for industrial demands of motor-driven compressors, blowers, fans, pumps, mixers, crushers, etc. Overall benefits include:

Reduced starting torque that lowers stresses on the mechanical drive and driven load (avoiding possible product damage or personal injury, pump cavitation, and water hammer in pipes); and

Reduced stress on the electrical supply, helping to meet utility requirements for reduced-voltage starting, eliminating voltage dips and brown-out conditions.

Making 24 V dc work

The IT solution required fresh designs, among them cost-effective, low-voltage power supplies specifically for motor control devices. These replace larger, functionally challenged control power transformers used in ac control. The new power supplies differ from existing 24 V dc units. They can relax some costly precision requirements of power supplies available for sensitive electronics. For example, the new supplies work with looser voltage regulation, higher current inrush, higher input voltages (i.e., above 240 V) and most importantly, lower power requirements. Reduced power means reduced current, which alleviates a concern for voltage drop with long cable runs when using 24 V dc versus higher voltage ac control.

In earlier soft starters, bypass contactors were used to reduce size; dc coils were larger and more costly than ac counterparts. With 24-V control, redesign of the coil controller and dc coil also brought size and cost benefits.

Safety, regulatory, reliability benefits

Low control voltage also reduces safety and reliability issues, including personal safety of the users. For example, it becomes easier to separate high- and low-voltage devices in control cabinets. Costly enclosures or safety interlocks can be eliminated.

In the U.S., use of 24 V dc control simplifies meeting NEC and OSHA regulations. And from a global view, one dc voltage bypasses the need to transform a gamut of ac input voltages used worldwide.

Another advantage comes from the 24-V power supply that adds capacitance for voltage regulation and ride-through. Voltage ride-through simplifies compliance with the CE Mark’s Machinery Directive. Reliability benefits include a basically “free pass” to meet the CE Mark’s Low Voltage Directive (which starts at 75 V dc). “Low control voltage further simplifies compliance for pilot devices, relays, and I/O devices that interface with power controls,” says Cutler-Hammer’s Mr. Kane.

Operating modes, protection

For best possible motor protection, IT incorporates two forms of starting: voltage ramp and current limit. The first method brings the motor to an initial torque level (programmable up to 90% of locked-rotor torque), then motor voltage is gradually increased over the programmable ramp time. When the motor reaches 90% of rated speed, the built-in bypass contactor closes to limit power consumption and heat build-up.

Current-limit starting permits users to program the maximum starting current applied to the motor during the ramp period. This protects the motor from overload during extended start ramps, and supplies greater total torque for accelerating difficult-to-start loads.

IT Soft Starts include advanced motor protection features, such as overload, stall, jam, phase loss, phase reversal, and SCR over-temperature protection. A “kick start” feature (an adjustable pulse of up to 550% of current or 90% of torque for up to 2 sec) supplies an extra push for “stubborn” inertia loads. Also, a “soft stop” feature introduces an extended coastdown to avoid sudden stops with high-friction loads.

For flexibility in different applications, overload protection can be set for trip class 5, 10, 20 or 30 to match different motor designs. It can be set for 30% to 100% of the unit’s FLA (full-load amperes) rating.

The starter senses motor stall and jam conditions, and will shut the motor down before damage occurs. During normal running, both stall and jam protection can be disabled; a time delay prevents nuisance trips. Phase loss and phase imbalance protection—user enabled or disabled—rely on the starter’s default stop profile to avoid motor damage. A user-enabled phase-reversal feature prevents motor starting under this condition.

IT Soft Start users have easy access to motor status indication. A discrete user interface allows simple configuration of the starter via a set of potentiometers and DIP switches. Users can quickly view torque level, time settings, and which features are enabled or disabled.

LED indicators display red for stall, jam, phase loss, and phase-reversal conditions. An overload indicator flashes red to warn of an impending overload, changing to red when the device has tripped. An LCD-type user interface module is a future option.

It’s just the start

IT Soft Starts represent only Phase 1 of Cutler-Hammer’s ambitious, multiphase Intelligent Technologies program. Future phases of IT will focus on other power control elements and a holistic look at intelligent technologies.

For more information on Cutler-Hammer’s Intelligent Technologies, visit www.controleng.com/freeinfo .

Author Information

Frank J. Bartos, executive editor fbartos@cahners.com .