In this context control means the human machine interface (HMI) the fancy name for the push buttons that control the motor. There can be any number of control points for a motor, the most common will be a local control station (LCS) at the motor, a local control panel CP1 for a group of motors all related to a single system or sub-system, a control room panel CP2 for area or system control, and SCADA control points. There can be as many or as few control panels as is required by the system the only limit is the total control cable length which must not cause a volt drop of more than 2.5% in the control circuit when the pilot contactors or relays are energised, or 10% when a pilot contactor or relay is energising and that the cable capacitance is low enough to prevent the pilot contactors or relays self maintaining when the circuit is opened.
Similarly the incorporation of SCADA systems is purely optional.
A sample of a MCC compartment; with ident 09/916054; is included in the pdf version of this article. The compartment houses a star delta starter for a 55kW motor located 150metres from the MCC together with it’s start alarms, local control station and it’s associated control panels. The tables in section 02.09.10.00 motor load data show a 55kW motor has a full load current of 103A.
It is a separate file entitled single line diagram.pdf with an original paper size of A1 (841mm x 597mm) printing on to an A1 sheet will give a scale of 1:1, on an A2 sheet a scale of 0.707:1 and on an A3 sheet a scale of 0.5:1. To be able to read the drawing a recommended minimum paper size of A3 should be used, at the stated scaling the general text on the drawing has a minimum height of 0.9mm, it should be noted that the push button indicators located in the control panels have 0.75mm high engraving at A1 which will reduce to 0.375mm at A3.
The local control station functions have been described in section 02.09.17.00 entitled safety of this article, the key switch, start button(s) and emergency stop should be 30.5mm IP65 operators in a metal enclosure also with an IP rating of IP65.
Every control panel section for a particular motor’s controls should be identical, they should consist of push button indicators.
An obvious observation, when you press a push button indicator you expect that button to illuminate to indicate that the action is acknowledged, what you do not expect is that when you press an illuminated push button it extinguishes to indicate that the requested action is acknowledged.
Therefore when a motor is running the appropriate green push button indicator, either the manual start or auto run enable and running in auto is/are illuminated. If it is then required to stop the motor the red push button indicator is pressed, when the relevant contactor(s) have de-energised the red push button indicator illuminates.
That is totally logical.
What is completely illogical is that when you press the red push button indicator, which is illuminated, to stop the motor and various green push button indicators illuminate, this is what the standards suggest although as they are written by ‘lawyers’ it is not possible to fully understand them as they are somewhat ambiguous (it keeps the ‘lawyers’ in work), remember many politicians start life as lawyers. It is a pity those that write these standards do not inhabit the world of normal people and they show no signs of having any practical experience of engineering or production control whatsoever and are also completely incapable of logical thought and reasoning.
The theory appears to be that green is a safe condition so green should show motor stopped and red is a dangerous condition so red should show motor running.
Think of traffic lights, with that system green would be stop as when the traffic is stopped that is the safe condition and red would be go as that is the unsafe condition!!!!
The group of push button indicators for a motor should consist of a minimum of title indicator, fault/accept indicator button, local indicator button, remote indicator button, stop indicator button, manual start indicator button, and automatic enable indicator button.
The control panel push button indicator layout for a uni-speed uni-directional motor is shown below (it shows the push button indicators for the motor featured in the drawings appended to the pdf version of this article). The picture below shows control at that control position, that is local, with the motor stopped, no faults present and the starter not in the control test position and automatic selected. The control test position is a function of the fused switch and a test switch which allows the control system to operate without the motor rotating this is to allow system testing.
The push button part of each push button indicator is considered below.
The push button sections of the push button indicators should only be active when control is at that control position and local control is selected, when remote is selected control is passed to the next higher level. Control can be taken at any time by a lower level control panel.
Question, someone is bound to ask this one, should a control panel’s stop buttons always be in circuit, that is active, the answer is an emphatic no, they should only be in circuit when control is at that panel in which the stop button is situated. The reason, since the control panel is only active when manned there is no merit in having the stop button active when the control panel is not manned. If the button is active when the control panel is unmanned then the button can be operated by unauthorised persons acting in ignorance or maliciously. Also the stop button is just that and it is not an emergency stop button and should not be considered as such.
The highest level control panel when set at remote should pass control to the SCADA system, if a SCADA system is included, where instead of physical push buttons control is by clicking on a screen button with suitable password protection as necessary.