Topic : Variable Speed Drive Name : G.S.Hegde Email : ghegde@adityabirla.com

A PRACTICAL APPROACH TO ENERGY CONSERVATION USING AC VARIABLE DRIVES

GRASIM, HARIHAR.

Motor loads contribute for 80-90% of energy consumed in most of the process industries. There are three distinct type of loads, based on the torque requirement of the driven equipment, Viz., a) Constant torque Variable Horse Power machines (Ex.: Conveyor belts.) N1 N2 N1 N2 Speed Speed

HP

Torq

ue

Fig-1 b) Constant Horse Power Variable Torque machines. (Ex.: Drilling & Tapping M/C’s) N1 N2 N1 N2 Speed Speed Fig-2 c) Variable Torque Variable Horse Power machines. (Ex.: Centrifugal Pump & Fans). N1 N2 N1 N2 Speed Speed

Torq

ue

HP

HP

Torq

ue

Fig-3 It is to be noted that for loads in category ‘a’ the power drawn is directly proportional to the speed, for loads in category ‘b’ the power drawn remains constant irrespective of the speed, whereas for loads in category ‘c’ the power drawn varies as cube of speed ie. P α N3. This third category loads are generally centrifugal pumps and fans used in almost all process industries and power plants. These loads account for about 50 to 60 % of total electrical loads in process industries. As is well known, at project stage, selection of pumps and fans is done taking into account the maximum head and maximum discharge required for the application and substantial safety margin also is provided. This is very true in old plants. In process industries many times these pumps and fans do not operate at their full capacity and the flow rates are adjusted periodically or continuously based on process requirements. Most of the time the required flow rate is obtained by throttling the valves or dampers provided in the pipe line. In early 1970’s when the cost of energy became more important in deciding the cost of production, it was well realized that throttling of valves and dampers to obtain the required flow rates really result in loss of energy, which can very well be reduced / eliminated by controlling the speed of the pump or fan to regulate the flow. For decades a variety of adjustable speed drives are being used in process industries like pulp and paper, cement, textile plants, chemical plants and power plants.

Speed variation of industrial drives, over the years was achieved by using drives such as,

• Steam Turbines • Variable cone pulleys and stepped pulleys • PIV (Positive infinite variable drives • Hydraulic couplings • Gears with stepped change over facility • Multispeed cage motors • Eddy current couplings • Slip ring motors with rotor control devices • DC motors with armature and field control devices • Commutator (Schrage) motors • AC variable drives (Inverters)

Typical pump power absorption curve and fan power absorption curve below illustrate the power drawn by centrifugal pump (Fig-4) and centrifugal fan (Fig-5) at different flow rates using different methods of flow control. 100 75 Fig-4 (Pump) 50 25 0 0 25 50 75 100

% FLOW 125 100 75 Fig-5 (Fan) 50 25 0

% S

YSTE

M KW

DEM

AND

0 25 50 75 100

%SY

STEM

KW

REQ

UIRE

MENT

% FLOW EDDY CURRENT COUPLING, AC V/F, DC DRIVE, LOAD CHARACTERISTIC,

CYCLIC, THROTTLE, DAMPER, BYPASS, In addition to the energy savings offered by the variable speed drives, other advantages that go with them are – • Increased motor life • Lesser strain on the drive equipments and lesser breakdowns • Lesser strain on power transmission devices like couplings, pulleys, gear boxes and hence improved

life • Reduced maintenance cost Out of the variety of variable speed drives being used in the industry over the years, AC variable speed drives (Inverter) are the latest to arrive in the market and they are slowly replacing the conventional drives mentioned above. Generally these drives can be used for any existing squirrel cage motor. These drives have some distinct advantages over other types of variable speed devices such as • They can be retrofitted and they won’t occupy space inside the plant (unlike Dynodrives, Conepulleys,

Geardrives, etc.) • They offer the maximum efficiency • Precise speed variation available, like in DC drives • They are less costly compared to DC drives • Soft start facility is built in and well suited for high starting torque applications • These drives are available for larger capacity motors ( upto 5000 KW) • Vector control drives are available to meet large torque requirements However there are some disadvantages, that go with the inverters such as, • Performance reliability • They introduce harmonics into power supply system • More time and more skill is required to repair the drives • Cost of repair is high • Quick obsolescense Perhaps these disadvantages discourage the process industries to go in for inverter drives in a big way even though there is wide scope for their installation. Since these drives are unreliable and attending to the problem is time consuming and costly, process industries and power plants hesitate to install them for critical applications like process pumps, ID and FD fans etc.. Since cost of failure is quite high the industries naturally prefer to ignore the energy saving aspects in such cases. The poor service available from some manufacturers further complicates the matter. While going in for AC variable speed drives for any application, it is very essential to analyse and verify whether there is justification to go in for these drives. Following points have to be considered to avoid any misjudgment. 1. Confirm whether the production process itself requires variation in speed drives irrespective of

whether energy saving potential exists or not. 2. In case of pumps and fans, verify whether the required flow rates for the process does or does not

vary much. a) If the flow rate requirement does not vary much, check whether the required flow rate is

achieved by throttling the valves or dampers permanently. In this case there is no justification for going in for variable speed drives. Any one of the following steps may save energy. i) Change the speed of fan / pump by replacing pulley / gearbox / coupling etc. to

suit the required flow rate . ii) Change the impeller or trim the impeller to suit the required flow rate. iii) Change the pump / fan itself to suit the required flow rate.

In all these cases if the loading on the motor reduces to less than 50 % (in case of energy efficient motors), replace the motor also with smaller one. Normally the cost benefit in all these cases will be substantial. However this can be calculated before taking a decision. b) If the flow rate required varies substantially because of the normal process variations, then it becomes necessary to think of variable speed drives, as there will be high potential for saving energy. The amount of saving can be calculated by finding out the difference in energy consumed by throttling / re-circulating / venting etc. and energy consumed by varying the speed of the equipment using variable speed drive. 3. Check whether a closed loop control can be adopted to regulate the flow rates

continuously and adopt the same along with installation of drives. In fact control strategy for temperature and level can also be utilised to save energy.

4. Check the effects of harmonics created by the installation of inverter drives.It is essential to

remember that nonlinear loads like inverters, power factor correction capacitors, thyristor drives etc., which are commonly used in present day process industries, contribute for introducing harmonics in the supply system. It means a 50Hz voltage source can also produce current flows at frequencies other than 50Hz and these currents are real. These nonlinear loads introduce harmonic voltages which effect the entire power supply system. The permitted amount of harmonic voltages is described in IEEE (ANSI) 519-1992. When the harmonic content exceeds these limits, we may encounter power quality problems such as low / high rms voltages, voltage sags, voltage swells, voltage impulses (transients) etc., which can effect the remaining loads on the power system. To counter these problems installing harmonic filters or isolation transformers may become necessary.

Where large amount of nonlinear loads like AC drives and capacitors are installed, it is necessary to conduct site analysis, so that harmonic distortions are kept within limits by installing suitable compensating devices. Simple hand held instruments are available at reasonable price, to check harmonic contents or Power line analysers may be used for conducting detailed study. 5. The existence of higher harmonics produces additional losses in the motors driven by the

inverters, which results in additional heating. The effect of these losses vary over the speed range of the motor. Below 50 Hz, the combined effect of reduced fan cooling and increased motor losses may cause a motor temperature to rise above the insulation rating. As the operating speed is reduced, this situation becomes worse and therefore it becomes necessary to derate the motors for inverter use.

What all this means is that, if the inverter driven motors are heavily loaded at low speeds for long duration, the motor must be over sized. But in the case of centrifugal pumps and fans much derating may not be required as the torque and power drawn at low speed is also small. However it would be better to consult the manufacturer of the inverter before deciding the sizing of the motors.

6. Most damaging environments for electronic equipments are excessive heat, excessive

moisture and presence of corrosive vapour in the atmosphere. Experience shows that the failure of AC variable drives installed in clean well ventilated (preferably air conditioned) rooms is much less compared to those installed in other places.

Energy Conservation by reduced supply frequency

There are a number of process industries, which run their plants on their captive generator sets partially or fully. It is a normal practice to maintain the supply frequency at 50 Hz at rated voltage of the sets. It is a common knowledge that these very factories run their plant at full capacities on power supplies received from electricity boards, whose frequency goes down to as low as 48 Hz and the voltage also drops by about 10%. The plant does not face any problem, because all machinery manufacturers in India design and manufacture their equipments to give rated output at a frequency variation of + 5% and voltage variation of + 10%. If the centrifugal pumps and fans can deliver rated output at low speed( ie. frequency), it means when we are running them at higher speed (frequency) either valves / dampers are throttled or venting is taking place or re-circulation is resorted to. Therefore it should be possible to fix an optimum frequency for captive generating sets (above the lower limit recommended by the manufacturers) such that energy wasted by throttling or by re-circulation or by venting is reduced. A small reduction in operating frequency can result in a big saving in energy as the following calculation reveals. ➔ Imagine a process plant having variety of loads, and having a captive generator set, ➔ Assume daily electricity consumption = 1,00,000 KWH, ➔ Say about 50% load is of centrifugal type, ➔ Present frequency of generation = 50 Hz. ➔ Now reduce the power supply frequency just by 0.5 Hz ( ie. to 49.5 Hz ) Now, we know that Pα N3 or Pα f3 ∴ Power drawn with 50 Hz frequency P1 α 503 or P1 = K 503 & Power drawn with 49.5 Hz frequency P2 α 49.53 or P2 = K 49.53 ∴ Percentage reduction in power consumption = (P1 - P2) X 100 P1 = K( 503 – 49.53 ) X 100 503 = 3 %. This means for this particular plant, we can expect a saving of 1500 units per day just by reducing the frequency from 50 to 49.5 Hz. This comes to about Rs.18.0 lacs per annum at Rs 3.50 per unit. It should be possible to fix a lower frequency (above the limiting value of 47.5 Hz) based on the sensitivity of the loads in each plant, and we can expect substantial saving. We can simultaneously reduce the generation voltage, such that V/f remains constant. As explained earlier all the drives can deliver the required output since they are designed and manufactured after taking into account, the tolerance limits of voltage & frequency, and production process will not suffer.

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