TechArchive

Original article date: June 1999

Emotron launched the MSF range of soft starters to coincide with the industry fair at Hanover. A new starting method means that mechanical and electrical stresses can be significantly reduced

Soft starters are used for gentle starting and stopping of induction motors. Pumps compressors mills fans and saws are applications where they are used. If a three-phase AC motor is not adequately protected the sudden change in rotation torque and speed which occurs on starting and stopping will jolt the equipment linked to it. Over the long-term this leads to increased mechanical wear of gearboxes clutches transmission and conveyor systems.

Abrupt starting and stopping can also damage goods being handled by mechanical equipment. The filling and distribution of glass bottles and containers loaded onto a conveyor holds the potential for a minor disaster. One sudden jerk during starting and stopping and the whole process line could be turned into a mass of broken glass and dripping liquid or sticky product.

With pumps soft starters eliminate pressure shocks in pipes and valves when the pump starts or stops. This poses a particular safety hazard when the transfer of volatile and/or inflammable liquids is involved.

Soft starters control the voltage supplied to a motor during starting and stopping to ensure smooth acceleration and deceleration. The disruptive effects of sudden starts/stops are thereby eliminated. The gradual supply of current to a motor also eliminates unwanted tripping erratic current supply and motor overheating.

Usually soft starters are only active during the starting and stopping procedure; the rest of the time the motor is supplied directly from the mains. However it is useful to be able to keep control of the motor and the load at all times so Emotron’s new MSF soft starters are designed to continuously control or monitor the running of the motor instead of controlling it only during starting and stopping.

As well as smooth starting and stopping of induction motors the MSF range features low speed jogging. Emotron’s soft starters also incorporate a new control method are smaller than their predecessors and offer new functions for load monitoring and protection.

Emotron has also developed a new starting method based on torque control instead of the voltage or current control previously used in soft starters. Advantages of the new method include reduced starting current and more accurate control during starting and stopping. Pumps and fans are applications where the mechanical and electrical stresses can be significantly reduced with the new starting method. Other applications are centrifuges compressors escalators belt conveyors stirrers stone crushers and saws.

Low-speed jogging is especially useful in applications where a motor has to run at low speed before starting or after stopping. One example is centrifuges where the opening needs to be in a particular position after stopping. Also in big conveyor systems low speed is useful for maintenance purposes or for simple positioning on stopping.

Based on the same approach as used in the company’s EL-FI motor load monitors Emotron has included functions to monitor and protect the motor and the driven equipment. In what could be described as an “electronic shear pin” the soft starter continuously measures the load on the motor and can trigger an alarm or a shutdown as soon as an unacceptable strain is detected.

The Emotron MSF range includes units for motors with rated currents between 17A and 1400A. In terms of mechanical design they are available to either IP20 or IP00.

• Emotron Drives
• Trevor Stonehewer
• 01283 500808

June 1999