ABBdriveX – Page 10 – AC DRIVES SOLUTION.

AQ: Motor line starting and ramp starting with VFD

Variable frequency drives are important power electronic devices. When we start an electric motor, we are increasing from 0 speed to full operating speed. A VFD ensures that the motor accelerates (increases its speed) to its full speed in a smooth manner, without causing much irregularities. In other words, VFDs make the motor accelerate uniformly.
VFDs are also easy to install and use. VFD drives are not only for starting motors (like the normal starters), but for easy speed control as well.

The difference between line starting a motor and ramp starting the motor with a variable frequency drive is that the motor/load does not pull the 6-7 times rated current of the motor, because the motor winding are not saturated with the full EMF produced to get the motor to synchronous speed it is ramped to it. If you are not trying to control the motors speed from process control then a soft start will serve the same purpose. The VFD drive main purpose is to control the V/F of the motor.

You will have to adjust the ramp time on the VFD or soft starter to over the force required to turn whatever the motor is turning, this can be accomplished with both devices. Soft starter is less expensive than variable frequency drive, thus it has limitations.

AQ: Is frequency inverter better than soft starter in motor control?

There are hundreds of applications for a frequency inverter. I use them on a pump to test pumps with voltages from 208-600VAC 3PH 50 and 60 HZ. You just have to size the frequency inverter to the largest 208 HP motor, so it can handle the current. Many people are installing them on pumps, fans and air compressors to get the energy savings of lowering the speed on the motor to maintain the pressure, temperature and flow. Frequency inverters also have the ability to ride through power dips, since the DC bus to store in a capacitor bank.

It is correct the frequency inverter will reduce the staring current of and induction Motor, but as all of you know that the motor have to drive a load the starting torque is related to starting current, also the main role of frequency inverter is to control the speed.
The starting current is related to the rotor conductor structure or classes because we can get direct starting currents within 1.5-4 times the full load depend on the squirrel cage design or construction.

The effect is, that at the reduced frequency during start, the full torque can still be developed at nominal current. As soon as the frequency hits the nominal slip frequency, the nominal torque will also be developed, at nominal full-load current. (The slip frequency is the nominal frequency multiplied by the full load slip percentage, i.e. around 2.5Hz for a 50Hz motor with a full load slip of 5%).

It really depends on the application. If you are only interested in starting current, then soft start is what you need.

AQ: Why there are different type’s conductor cables, like EPR, XLPE

As far as the cables insulation material is concerned, EPR and XLPE insulated cables to some extent are having similar properties. In this respect, there are different types of Electrical cables such as ETFE ,FP, HOFR , LSF,LSOH, MI, PILC, TRS, VR, CTS, CSP, PTFE, etc.

However, it may be necessary to conduct a rough comparison (insulation) between the PVC and XLPE cables to clear the picture.
1. PVC/SWA/PVC multicore sheathed cables are manufactured in all sizes up to 400 mm² in accordance to BS 6346, the allowable operating temperature up to 70 °C.
2. XLPE Cables are used at max. ambient temp. of 90°C and are made to BS 5467. These cables have better insulation qualities than PVC and available in sizes up to 400 mm² or 1000 mm² Single Core.

Both type of cables are easy to lay and bending and they have less bending radius up 8 times nominal diameter.

These Different types of cables are not only based on the insulation material, are also either classified as cables of Aluminum conductors or Copper Conductors. Regardless, each has it own characteristics which can be appropriate to a range of installation / application since there are many wiring systems that may be adopted. In deciding the type of wiring system for particular, many factors have to be taken into consideration e.g….

a. Whether alteration & extensions are expected or not. Also, whether is going to be executed during the construction, in a completed project or as an extension of existing system.
b. Type of Project / building, function, purposes and ambient and environmental conditions.
c. Expected duration (life time) of the Installation.
d. The required layout, safety & constraints.
e. Feasibility & Cost

Eventually, I confirm that armored PVC & XLPE Insulated cables are now being used widely for feeders, submain cables & Industrial Installations.
Such Cable consists of multi conductors insulated by PVC or XLPE, with PVC sheath and steel wire armor (SWA), and PVC sheath overall.

AQ: Transformer Magnetic Design

AQ: Read control wiring diagram of relays in substation

There is a ANSI/IEEE standard that defines the standard number identification for electrical devices. You will find that some of the more common ones are 50 over current, 51 short terms over current, 27 under voltage, 59 over voltage, and 50G ground over current detection relay.

Older installations may have the older electro-mechanical relays. Mst installations have converted to using sensing devices that transmit to PLC, DCS or protective relays. Today’s protective relays are essentially PCs that monitor a number of power system parameters for metering or protection purposes. They also have programmable outputs and settings.

As stated previously, search the web for some examples or guidelines on electrical schematics. There is also a reference standard develop by IEEE and ISA to define the symbols used to represent the hardware or software functions that input to PLCs or are the functions within the PLC (or DCS). The older form of schematics was drawn horizontally, but the same ladder logic used today is drawn vertically with each line numbered in sequential order. The line numbers are used in the device identification and as a reference in the PLC programming.

It can be confusing, but think of items in series as AND logic versus items in parallel being OR logic. Understanding the And / OR logic will enable you understand how the respective logic components function on an electronic card used in a PLC.
I recommend that you join IEEE and ISA. They usually have local chapters that meet to network and share information. It is also a good way to network and meet persons at other companies and of course meet vendors who are eager to meet persons who work for engineering firms that they market to.

AQ: Starter of SAG Mills with rotor resistance

Q: For now I am working on a mining project which involves starting two SAG mills, the method of starting these mills is by rotor resistance and likewise we are using an energy recovery system (SER), could someone tell me how this system works SER? Each mills have two motors of 8000 kW at 13.8 kV.

A: For large mills requiring variable speed, the wound rotor motor and SER drive are economical for a total rating of approximately 2MW to 16MW. Above 16MW, the gearless drive (cyclo-converter) is typically used because gearboxes and pinion gears reach their present limit in size. Around 2MW and below, the squirrel cage/VVVF drive is simple and cost effective.

Advantages of the wound rotor/SER drive are:
1. If the SER converter drive fails, the drive can be switched to fixed speed bypass – starting the usual way with the LRS.
2. The converter only needs to be sized for 15-20% of the total motor rating with associated reduction in floor space, air-conditioning etc. The converter is only sized for the feedback energy which is proportional to the speed difference from synchronous speed. The drives are typically set up to run between about 85% to 110% of synchronous speed for an optimized arrangement.
3. Relatively low capital cost when all things considered – including spare motor cost etc.

Brush/slip ring maintenance is one issue. However, when the brushes are specified correctly for the load, the wear is manageable. Once the maintenance program is set up for shutdowns, it is not a major issue.
I expect that this type of drive would be the most common large mill variable speed drive in the world’s minerals processing industry for the range mentioned above for the last 15 years (approximately).

The SER drive converter controls the voltage in the rotor. Motor speed is proportional to rotor voltage. Resistance in the rotor indirectly achieves the same thing (with a different torque curve shape), but energy is lost in the resistors which is very inefficient. The SER drive via a feedback transformer feeds energy back into the power supply. This returned energy is proportional to the speed difference from synchronous speed. So at say 85% speed, 15% of the motor rated power is returned from the rotor to the supply. At a hypersynchronous speed, the SER drive feeds power into the motor rotor allowing it to run faster than synchronous speed. So for a fixed torque and higher speed, the power obtained from the motor is higher than the motor nameplate rating.

Gearbox ratio is best set up to allow the speed range to be covered using the SER drive’s hyper-synchronous capability.

AQ: AC motor maximum torque

As per Torque/Slip characteristic for AC Motor, the value of the Max. Torque can be developed is constant while the Starting Torque occurs @ S=.1, (T proportional to r2 and S also proportional to r2 where r2 is the rotor resistance, the ratio r2/x2 when equal to 1 gives the max. Torque w.r.t Slip at Starting. Wound rotor motors are suitable and recommended for application for MV drive where it is required to be started on load such as ID. Fans, S.D Fans, Drill, etc.

As you aware the torque is directly proportional to the rotor resistance “r2” & varies with slip “S”, hence injection of resistance into the rotor via Slip Rings, High Starting Torque can be got while the Speed, efficiency and Starting current will be reduced. Therefore resistance is the most practical method of changing the torque (i.e. wound rotor Slip ring Motors). Moreover, the Max torque can be achieved at starting when rotor Resistance “r2” = The Stator impedance, at starting S=1.

On the other hand, the slip of the Induction Motor (speed) can be changed by “extracting” electrical power from rotor circuit, more extraction increases the slip. By using thyristorized Slip-Recovery Scheme “ i.e Kramer Scheme” feedback of Power from rotor circuit to supply circuit which also known as “the slip Power recovery scheme”. The scheme is simply consists of rectifier and an inverter connected between slip-rings and the A.C Supply circuit. The Slip Rings voltage is rectified by the rectifier and again inverted to AC by the inverter and feedback to supply via a suitable Transformer. Such arrangement gives good efficiency with high cost due to Rectifier and Invertor.

AQ: What is the surge impedance load

The surge impedance loading (SIL) of a line is the power load at which the net reactive power is zero. So, if your transmission line wants to “absorb” reactive power, the SIL is the amount of reactive power you would have to produce to balance it out to zero. You can calculate it by dividing the square of the line-to-line voltage by the line’s characteristic impedance.

Transmission lines can be considered as, a small inductance in series and a small capacitance to earth, – a very large number of this combinations, in series. Whatever voltage drop occurs due to inductance gets compensated by capacitance. If this compensation is exact, you have surge impedance loading and no voltage drop occurs for an infinite length or, a finite length terminated by impedance of this value (SIL load). (Loss-less line assumed!). Impedance of this line can be proved to be sqrt (L/C). If capacitive compensation is more than required, which may happen on an unloaded EHV line, then you have voltage rise at the other end, the ferranti effect. Although given in many books, it continues to remain an interesting discussion always.

The capacitive reactive power associated with a transmission line increases directly as the square of the voltage and is proportional to line capacitance and length.

Capacitance has two effects:

1 Ferranti effect
2 rise in the voltage resulting from capacitive current of the line flowing through the source impedances at the terminations of the line.

SIL is Surge Impedance Loading and is calculated as (KV x KV) / Zs their units are megawatts.

Where Zs is the surge impedance….be aware…one thing is the surge impedance and other very different is the surge impedance loading.

AQ: Transmission line low voltages and overload situations

Q: I want to know just what the surge impedance loading (SIL) is but its relevance towards the improvement of stability and reliability of a power network especially an already existing one with various degrees of low voltages and overload situations?

A: The surge impedance loading will provide you with an easy way of determining if your transmission line is operated as a net reactor (above SIL, so external sources of (2) line-voltage-drop limitation
(3) steady-state-stability limitation

In contrast with the line voltage drop limitation, the steady state stability limitation has been discussed quite extensively in the technical literature.

However, one important point is rarely made or given proper emphasis; that is, the stability limitation should take the complete system into account, not just the line alone. This has been a common oversight which, for the lower voltage lines generally considered in the past, has not led to significant misinterpretations concerning line loadability

At higher voltage classes such as 765 kV and above, the typical levels of equivalent system reactance at the sending and receiving end of a line become a significant factor which cannot be ignored in determining line loadability as limited by stability considerations, so surge impedance loading plays a fundamental role in reliability and stability.

AQ: Difference between PLC and DDC system

PLC is defined as Programmable Logic Controller. It is a hardware, Includes processor, I/P & O/P Modules, Counters, Function Blocks, Timers,,, etc. The I/Os are either Analogue or Digitals or both. PLC can be configured to suit the application and to programmed in a logic manner by using one of the programing language such as Statement List, Ladder Diagram,, etc Interaction in real time between inputs and the resultant of the outputs through the program logic – PID – gives the entire Control System. While the Digital Control System I believe it is Software/ System that uses only Digital Signals for control and PLC/PC/Server/Central Unit may constitutes an Integral part of this system.