Sparks Electrical News August 2019

MCCs AND MOTOR PROTECTION

15

WEIGHING UP YOUR OPTIONS FOR SIMPLE MOTOR CONTROL

A long, long, time ago, motors were started by a contactor closure on the line con- trolled by energising or de-energising a coil. Simple on-off push button control. It was great for small and large motors alike. As time passed, other methods of motor control emerged and included utilising variable frequency drives and soft starters. This added more control of the starting, run time and stopping of the motor. Doing so offered the reduction of mechanical wear and tear of the system. Of course, the energy savings when starting or reducing current during runtime were a bonus. What is the most efficient way of running a starter with the least amount of heat loss? Across the contacts of a contactor. When the contactor is closed, current passes through the now connected busbars. No electronic components generate heat. Where is the benefit in this? Bypassing a soft starter while at speed is an ex- ample. Using a bypass contactor while at speed

applications could be easily ued for the control, thereby saving valuable panel space. Using PLC control reduces the need to have push button contacts rated for large inrush cur- rent for coils of larger contactors. PLC control is typically in the mA range, while the hold-in cur- rent and associated inrush current would be seen by the control voltage source and would be much higher. What about the control voltage itself? Does it have to match the PLC input voltage? The PLC input voltage typically does not match the coil voltage, say, 24 V dc for the PLC input. A per- son could have coil voltage of 120 V ac, and have the coil controlled by the 24 V dc PLC input. This is how an interposing relay would behave, but, in this case, it would be built into the coil. Control voltage would be different from the voltage being controlled. Every application is different, so which method provides the best fit and function for your task? age (LV) VSD panels, and a second module for control and automation. An additional E-house comprising one module for a sepa- rate (LV) substation has also been supplied. “Our expertise and experience in designing appropriate mechanical and electrical solutions using 3-D modelling software makes this kind of innovation possible,” says Richards. Using a turnkey approach allows WEGAutomation Africa to efciently construct the full solution at its dedicated E-house facility in Heidelberg, saving customers the complexity of managing multiple contractors on site. At this facility, the modules are trial-fitted, assembled and tested before transportation to site. “As with our other E-house projects, we are able to provide quality assurance before units leave the facility, reducing the various risks and costs of on-site construction and commis- sioning, including specialised resources,” Richards says. Other important aspects of this E-house design include its fire rating, fire detection systems and HVAC systems for heating, cooling and ventilation. Rock wool mineral insulation between the cladding segregates the modules and provides a two hour structural fire rating. Climate control systems will ensure that the coastal temperatures and humidity of KwaZulu-Natal do not disrupt smooth operations. Enquiries: www.rockwellautomation.com

for the voltage and the current of the coil to which they are connected. A momentary button (normally open) in combination with an auxiliary contact, is used to energise the coil and make a hold-in contact to maintain the circuit. When another momentary button (normally closed) is pressed, this will open the hold in contact, remove power to the coil, and the motor will stop. This is three-wire control. Again, contacts are rated for the current of the coil. Some wiring of components will have to take place, as you can see. Is there a way with fewer push buttons and wiring to accomplish the same thing? You could use a programmable logic controller, also known as a PLC, output to control a contactor coil. Control can be accomplished by utilising the outputs and simple coding of the PLC instead of using external push buttons. Using a contactor PLC input to control the con- tactor would free up space traditionally needed for push buttons, etc. In addition, a PLC that is most likely already in the system controlling other

will enable the silicon-controlled rectifier (SCR) to be gated off, producing lower heat. There are times when a purely solid state device is desired, for example, in a high vibration or dirty environ- ment. For those situations, solid state devices are required. For the rest of the applications, a bypass contactor is perfect. What about just starting a load with simple on/ off control or in an emergency say, an applica- tion needs to be energised quickly and with little concern about wear and tear. For example, emer- gencies such as control of town flooding. In those cases, contactors are still very viable. So what are your options for closing a con- tactor? As stated earlier, energising a coil from a control power source connected to simple push buttons will perform the on/off control. A maintained button or two-position switch, used mainly in two- wire control, will hold the coil energised. The coil will be de-energised when the button is pushed again and the voltage to the coil is removed. Of course, the push button contacts have to be rated

REGULATING EFFICIENCY IN MOTORS CAN HELP STABILISE POWER SUPPLY S outh Africa could go a long way to cut the risk of future load-shedding by adopt- ing a minimum efciency performance standard (MEPS) for electric motors. According to Fanie Steyn, manager rotating machines at Zest WEG Group, an MEPS would significantly reduce the peak power demand on the national grid. Importantly, the step could be made at no cost to government and would also bring substantial savings to industry’s electrical energy costs. “The MEPS would phase out the least-efficient electric motor classes by setting a minimum standard for the efficiency of motors imported and sold in South Africa,” he says. “The essential challenge now is that about 280 000 electric motors are imported each year, many of which are low efficiency motors rated at IE1 level as standard.” Steyn highlights the great strides recently achieved in the efficiency of electric motors. Energy savings of between 2,1 % and 12,4 %, depending on the individual power rating, can be made by converting from a standard efficiency IE1 motor to a premium efficiency IE3 motor. The capital cost differential is slight and is quickly recouped by lower operating costs. “It is estimated that as much as 30% of all energy produced globally is con- sumed by electric motors,” he says. “It is therefore easy to see why improving motor efficiencies has a huge impact on national energy consumption.” It is significant that more than 42 countries already have MEPS in place. These standards apply mostly to three-phase low voltage motors from 0,75 kW to 375 kW capacity. The MEPS is applied at import stage, so the process would be handled in the conventional manner by customs agencies. “If the 150 000 low voltage motors entering the country each year were IE3 rated instead of IE1, the national grid could be relieved of about 195 million kWh in a single year,” says Steyn. “This means almost three billion kWh over the next five years.” He adds that this would also mean lower carbon emissions from power stations. South Africa committed to reduce these emissions by signing the Paris Agreement in 2016. “Implementing MEPS will have significant benefits for everyone,” Steyn concludes. The Zest WEG Group, a subsidiary of leading Brazilian motor and controls manufacturer WEG, has a strong commitment to contributing to the development of the African region, and has been servicing the continent for more than 37 years.

MULTI-LEVEL E-HOUSE FOR OIL AND GAS SECTOR

WEG AUTOMATION AFRICA, formerly known as Shaw Con- trols, has designed, engineered, manufactured and supplied a fit-for-purpose E-house solution into an oil and gas sector ap- plication at a major South African port. Using a draft concept supplied by the customer, WEG Automation Africa has taken its E-house design and manufacturing capabilities to new heights. Bevan Richards, managing director of WEG Automation Africa, explains that the company was responsible for the complete mechanical and electrical design as well as the manufacture of the E-house solution, which comprises five modules. Accommodating space constraints on site, and in what Richards says is probably in a ‘first’ for South Africa, the design is a multi-level construction which will be fitted onto the available footprint. The E-house solution comprises three modules at ground level with two above. The structure, including all electrical substation equipment, weighs approximately 90 tonnes. A section of the ground floor houses a medium voltage (MV) switch room with a dry- type transformer for enhanced safety, and a module fitted with three 450 kW MV variable speed drives (VSDs). The third module contains a 1 MVA generator set, sourced from Zest WEG Group’s dedicated genset manufacturing facility in Cape Town, for backup power. The first floor comprises one module with a low voltage motor control centre (MCC) and four freestanding low volt-

Enquiries: www.zestweg.com

Enquiries: +27 (0)11 723 6000

SPARKS ELECTRICAL NEWS

AUGUST 2019