DRIVES & MOTORS FEATURE Today’s soft starts use solid-state


devices to control the current and voltage applied to the motor. The solid state device is usually a reverse- parallel-connected silicon-controlled rectifier or thyristor, but other options are also available.


A SOFT START for driven machinery


Starting up an electric motor can cause sudden stresses in the driven machinery, electrical surges through the motor windings, and reflect shock waves of distortion back into the mains supply. And, the bigger the motor, the bigger the potential problem! Stuart Harvey of Softstart UK looks at possible solutions


M


otors are fundamental to modern life – manufacturing and


processing; water, sewage and flood defences; ventilation and air-conditioning; materials handling; passenger lifts, escalators and travelators; white goods; and gym equipment. It must, in fact, be nearly impossible to go a whole day in the developed world without the assistance of several electric motors. Once running, a motor is a


magnificent piece of technology, efficiently converting easily-delivered electrical energy into motive power. Its start-up phase, however, can be less impressive. It is not unusual for the driven machinery to shudder and screech as it ramps quickly up to its running speed, the motor itself may whine and spark, water hammer may be induced into pipework, etc. Perhaps less obvious, but entirely comparable to water hammer, harmonic corruption may be reflected back into the electrical mains supply. It is easy to appreciate that stressing


machinery and motors in this way is detrimental to efficiency and long-term reliability. Harmonic corruption can have equally damaging results within the supply system – and it may disrupt nearby machinery or incur penalty charges from the electricity provider. These problems can be alleviated by


fitting a soft start between each motor and the mains. Before we look at this option, however, we will review the traditional ways of motor starting.


MOTOR STARTING Direct on line starting (DOL) is the simplest method of starting a motor. It is cheap and simple, and can be appropriate in many cases, even with relatively large motors. However, large starting currents and voltage drops may be experienced, and mechanical inertias may cause problems temporarily. Three-phase motors


(which are very common in industrial applications) can use star-delta starting. Here, switching contactors initially configure the mains’ three phases as a star that, after a set time, converts back to the more usual delta setting. This limits inrush currents to about one-third of their natural maximum, giving a smoother, gentler start over a slightly longer time. The modern soft start is a


development of star-delta starting, providing tunable parameter settings and other advantages. It is possible to use a clutch or fluid coupling as a mechanical equivalent, but this is often an expensive and maintenance-intensive option. An electrical soft start reduces the


mechanical load and torque in the power train during start up while reducing the electrical current surge in the motor. This reduces mechanical strain in the machinery and electrical stresses in the power cables and supply network.


/ DESIGNSOLUTIONS A range of


problems can be alleviated by fitting a soft start


SOFT START BENEFITS The effect of a soft start may best be appreciated by looking at some practical examples. For instance, a soft start could prevent slippage on a belt drive, such as those used with ventilation fans, or could stop a drive chain from lashing about and jumping off its sprocket. A heavily loaded conveyor that may


not be able to start with a DOL set-up, could ramp up to speed over an extended time period with a soft start. An alternative conveyor scenario is that a soft start would allow start-up without items on the belt being upset. Soft starts are often used on


large motors, such as those found at major water pumping stations. Often, vast amounts of water have to be transported at busy times – such as at 6–8am when many people are having a shower – or at certain points in an industrial process. To achieve this, pumping stations usually have a bank of large pumps, all of which run at peak times, but which cascade down to just one or two running at quiet times. In addition, the site engineers like to spread the wear and tear by regularly switching between pumps so that they all get used (approximately evenly.) Therefore, large motors are switching on and off fairly frequently, which could cause major harmonic problems in the mains if soft


starts were not fitted. Smaller motors also benefit from being


fitted with a soft start. While they will not cause the same mains-borne corruption problems as big


motors, they experience the same


internal electrical and mechanical stresses. They also tend to drive smaller, more delicate machinery, which has to be protected from shock loads. Therefore, a rule of thumb for plant


and machinery design engineers is that if a motor is not used with a variable speed drive, it is worth considering fitting a soft start.


Softstart UK www.softstartuk.com Enter 205


DESIGN SOLUTIONS | APRIL 2014 11


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44