100 Most Important MCQ on Electric Drive-2021 | Industrial Drive MCQ

Answer 4. Landing the Load

Explanation:-

When the boom is lowered, the load will both travel away from the crane and become lower. The reverse. The mechanical load brake is a key component of crane safety. It serves as a secondary braking mechanism of the crane hoist, controlling the lowering speeds of rated loads and preventing loads from free falling. All of the torque of a hoist motor is applied to lifting loads. In the lowering phase, however, gravity takes over, and the mechanical load brake is in place to counteract the gravitational pull to ensure a safe, controlled descent of the load. Hence low speed is preferred for lowering the load because precise and smooth control is required for placing the load in its given location or position.

Answer 4. 100 HP to 500 HP

Explanation:-

Rolling mills are generally classified according to their products, since, very often, a particular mill is capable of manufacturing a specific product, and the design of millstand, millhead, auxiliaries and drives changes with the type of product.

A blooming mill produces blooms of any length of rolled metal having almost a square cross-section.
If the rolled metal is of rectangular cross-section, it is called a slab and the mill that produces it is a slabbing mill.

Blooms of reduced cross-section are known as billets. A billet can be further rolled into bars, squares, angles, rods etc. and the same mill can produce these varieties of different sizes and shapes. Such mills are termed as merchant mills.

A plate rolling mills reduce slabs into plates, whose thickness is small compared to the width. Plates are changed to strips in a strip mill and to thin sheets in a sheet mill.

In all the above-mentioned types of steel mills, a range of sizes having similar shapes can be manufactured by changing the number of passes or by using stand rolls having the appropriate pattern.

The Rolling mill is mainly dependent on electrical energy and about 60 to 70% of electrical energy is spent on rolling operations and balance is consumed on an auxiliary operation like cutting of roll stock, furnace blower, crane operation, etc. The increasing cost of the electrical energy made it essential for steel rolling mills in India to consider the electrical power requirement seriously. In a rolling mill, electrical power contributes a major part in the cost of rolling.

The motor of the rolling mill should possess the following characteristics

• Superior efficiency
• Robust frame and shaft construction to withstand the severest duty
• Less demand for maintenance
• High degree of control and responsiveness
• Great capacity for acceleration and deceleration.
• Greater flexibility for extreme operating conditions.
• it should be capable of developing short time torque to the extent of 2 or 3 times of rated torque.
• It should be capable of having wide speed variation.
• It should be in a position to maintain preselected speed within tolerances from no load to full load. Therefore it should have the quick response to change.
• Rolling mill requires high power motor generally dc motor and slip ring induction motor of multi-kilowatt (above 100 H.P motor) is preferred.
• According to the survey of ROLLING MILLS IN RAIPUR. The normal sizes of electric motors range from 400 H.P. to 1000 H.P. the

Answer 3. Any of the above

Explanation:-

Depending upon the type of mill selection of motor has to satisfy the following conditions.

• Superior efficiency
• Robust frame and shaft construction to withstand the severest duty
• Less demand for maintenance
• High degree of control and responsiveness
• Great capacity for acceleration and deceleration.
• Greater flexibility for extreme operating conditions.
• it should be capable of developing short time torque to the extent of 2 or 3 times of rated torque.
• It should be capable of having wide speed variation.
• It should be in a position to maintain preselected speed within tolerances from no load to full load. Therefore it should have the quick response to change.

Various motors used for Rolling Mills

1. Induction Motor
2. DC Motor

Induction Motors:

AC slip ring motors are suitable for roughing and re-rolling mills where very precise speed control is not required. We can change the speed of slip induction motor by introducing resistance in the rotor circuit. This method is wasteful as the reduction in speed is proportional to copper losses taking place in the rotor circuit. As such slip motors are used for small re-rolling mills where wide speed range is not required. Besides wastage of energy taking place in the rotor, the introduction of resistance in rotor circuit converts the speed-torque characteristic of the induction motor from shunt to series. Therefore speed can not e maintained constant from no load to full load. This will make the motor to fly off to no load as soon as stock passes out of the stand.

As such slip ring motors are suitable for roughing stands of the rolling mill. •Wastage of energy in the rotor of slip motor which is proportional to the reduction of speed can be avoided by employing cascaded induction motor.

DC Motor

The DC motors, because, of their inherent characteristics (smooth and precise speed control over the wide range), are best suited for the rolling mills. Speed control is effected either through Ward Leonard system or by grid controlled mercury arc rectifiers. D.C. motor is the only type of motor which can satisfy the stringent requirements of modern rolling mills.  By varying the applied voltage to the armature and keeping field constant it is possible to change the speed of the motor at constant torque up to rated speed. In this way requirement of higher constant torque at the low speed needed for effecting the heavy reduction in first passes is achieved.

On the other hand, low torque and higher speed are required for rolling operation in final passes when for the section of the stock has become smaller, the torque needed is low and elongated material requires higher speed to clear off in the shortest possible time. This achieved by weakening field after rated speed is achieved by the increase of applied voltage to the armature.

Hence in this way we get high speed at reduced torque i.e. H.P. remains same. • In case of reversible rolling mills, in order to ensure the high rate of acceleration, it becomes necessary to reduce the moment of inertia of motor to the minimum. This is achieved by increasing the length and diameter of the armature.

Answer. 1 Starting torque is more than double of the running torque

Explanation:-

Kiln drives: The rotary kiln is an indispensable part of a cement plant. The rotary kiln drives depend upon the type of cement making process (wet or dry). These are, in general, tubular tilted from the horizontal position with a ring fitted around them. This ring gear engages with one or two pinions. A variable speed motor drives the pinion.

The requirements of a kiln motor are the following:

1. Power requirement is very high
2. Speed control ratio is 1:10. Very low creeping speeds of 1 rpm may be required.
3. Starting torque should be in the range 200 to 250% of full load torque.
4. The acceleration of the drive should be completed in about 15 s.
5. For small periods an overload capacity of 200-250% may be required.
6. The motor must have suitable control for inching and spotting during maintenance.

The motors that meet the above requirements are ac commutator motors and Ward Leonard controlled dc motors. These have the disadvantage that the highest rating is limited by the commutator. Speed range is 1:2 in ac commutator motors whereas a speed range of 1:10 with crawling speeds is possible with dc motors. Capital outlay, lower efficiencies and limitations due to the commutator, either in the Ward Leonard or AC commutator motors, may be overcome by the use of thyristorized drives. These have a wide range of speed control as against ac commutator motors.

When two motors are used to deliver the power they must be designed to have equal load sharing without overloading any one of them. They must maintain the same speed. They may be series connected or parallel connected with closed-loop speed control.

Converter fed dc motors in static Ward Leonard control have limitations on maximum operating speed and power rating due to the presence of a me­chanical commutator. The ripple content in the armature current and possi­ble discontinuous conduction further affect the commutating capability of the motor. Therefore for power ratings beyond this value, ac drives are suitable. Tubular mills for Cement Mill Process are very slow speed high power loads.

A cyclocon­verter fed synchronous motor meets the requirements of a drive motor. The salient features of this system are:

1. The drive can be controlled to have an excellent dynamic behavior with the fast regenerative reversal.
2. The converter can be used for synchronous starting by varying the fre­quency with smooth acceleration up to desired speed. The disadvantages, such as peak starting current and consequent voltage dips in the supply can be completely eliminated.
3. A continuous gearless drive is possible at crawling speeds.
4. A motor with self-control has the characteristics of a dc motor with respect to both steady-state and dynamic behavior. It is free from hunting. It is mechanically strong, requiring little maintenance and can be built to have a rating several times that of converter-fed dc motors. The low inertia of the motor is also responsible for the fast response.
5. A four quadrant operation is simple and straightforward.
6. Poor line power factor similar to that of converter-fed dc motor.
7. Field weakening is possible above base speed.
8. The smooth speed control with minimal torque pulsations particularly at low speed is an added attraction, mainly when compared to synchronous motors or dc link converters.

Answer 4. Any of the above

Explanation:-

The requirements of a kiln motor are the following:

1. Power requirement is very high
2. Speed control ratio is 1:10. Very low creeping speeds of 1 rpm may be required.
3. Starting torque should be in the range 200 to 250% of full load torque.
4. The acceleration of the drive should be completed in about 15 s.
5. For small periods an overload capacity of 200-250% may be required.
6. The motor must have suitable control for inching and spotting during maintenance.

The motors that meet the above requirements are ac commutator motors and Ward Leonard controlled dc motors. These have the disadvantage that the highest rating is limited by the commutator. Speed range is 1:2 in ac commutator motors whereas a speed range of 1:10 with crawling speeds is possible with dc motors. Capital outlay, lower efficiencies and limitations due to the commutator, either in the Ward Leonard or AC commutator motors, may be overcome by the use of thyristorized drives. These have a wide range of speed control as against ac commutator motors.

When two motors are used to deliver the power they must be designed to have equal load sharing without overloading any one of them. They must maintain the same speed. They may be series connected or parallel connected with closed-loop speed control.

Converter fed dc motors in static Ward Leonard control have limitations on maximum operating speed and power rating due to the presence of a me­chanical commutator. The ripple content in the armature current and possi­ble discontinuous conduction further affect the commutating capability of the motor. Therefore for power ratings beyond this value, ac drives are suitable. Tubular mills for Cement Mill Process are very slow speed high power loads.

A cyclocon­verter fed synchronous motor meets the requirements of a drive motor. The salient features of this system are:

1. The drive can be controlled to have an excellent dynamic behavior with the fast regenerative reversal.
2. The converter can be used for synchronous starting by varying the fre­quency with smooth acceleration up to desired speed. The disadvantages, such as peak starting current and consequent voltage dips in the supply can be completely eliminated.
3. A continuous gearless drive is possible at crawling speeds.
4. A motor with self-control has the characteristics of a dc motor with respect to both steady-state and dynamic behavior. It is free from hunting. It is mechanically strong, requiring little maintenance and can be built to have a rating several times that of converter-fed dc motors. The low inertia of the motor is also responsible for the fast response.
5. A four quadrant operation is simple and straightforward.
6. Poor line power factor similar to that of converter-fed dc motor.
7. Field weakening is possible above base speed.
8. The smooth speed control with minimal torque pulsations particularly at low speed is an added attraction, mainly when compared to synchronous motors or dc link converters.

Answer 4. High starting Torque

Explanation:-

Belt Conveyors are Designed to carry a wide variety of bulk materials and packed bags, boxes, pouches etc for various industries. Belt conveyors are economical and efficient conveying of materials over long distances, either in horizontal or limited inclines. It conveys high capacity of materials for lumpy ore stone, coal and wood logs. 

The coal or other material is transported using conveyors. In the event the conveyors stop with coal (or other material on them), it will require a substantial amount of torque to start the conveyor belt when it is loaded with coal. The design of the conveyor motor should look at the instance when the conveyor starts loaded to ensure it can successfully start the conveyor in this condition due to the higher mechanical starting torque that the conveyor requires when loaded with material.

The belt conveyors have the ability to provide nearly continuous-flow transportation which enables the modern continuous mining machines to operate at maximum efficiency. Also, because the amount of material transported on a conveyor system depends on the belt speed, belt width, and weight of the material transported, belt conveyors can operate over a wide range of capacities. Another advantage of belt conveyor systems is the fact that they can be utilized in level or pitching terrain either underground or on the surface.

Answer. 1. Double Squirrel cage motors with direct-on-line starters are used

Explanation:-

Generally, a belt-type conveyor cannot be started in the same way as the most electric motor driven loads because the belt constitutes a large, unwieldy mass resting along its length on a large number of rollers, all of which tends to resist starting. In this particular system, a Double squirrel cage induction motor drives the input element of the coupling, and the output element of the coupling drives the conveyor. Hence, the motor can be brought up to full speed where it can develop full load torque before it is required to start the load. 

A Double Squirrel cage motors is powerful enough to drive a loaded conveyor belt in the normal way but if the inertia is large and the power required considerable the motor may well be distressed when starting from rest. The starting current of a motor can be as high as 600% full load current, and if stalled there is no cooling air for its fan. This effect can be reduced by direct online starting.

In DOL starting method the motor will draw a very high inrush current for a very short time. As the motor accelerates, the current will begin to drop, but will not drop significantly until the motor is at a high speed, typically about 80 – 85% of synchronous speed.

The torque developed by the motor exceeds the load torque at all speeds during the start cycle, the motor will reach full speed. If the torque delivered by the motor is less than the torque of the load at any speed during the start cycle, the motor will stop accelerating.

DOL starting have a maximum start current and maximum start torque.

Another advantage is better speed control can be done in the induction motor, so it is used for conveyor belt

Answer.4. Both 2 and 3

Explanation:-

A centrifugal fan is a mechanical device for moving air or other gases. The terms “blower” and “squirrel cage fan“, (because it looks like a hamster wheel), are frequently used as synonyms. Centrifugal fans are constant displacement with normal starting torque devices or constant volume devices, meaning that, at a constant fan speed, a centrifugal fan moves a relatively constant volume of air rather than a constant mass. This means that the air velocity in a system is fixed even though the mass flow rate through the fan is not.

For blower applications, the most common motors are 2-pole (3600 rpm), 4 poles (1800 rpm), and 6-pole (1200 rpm).

The most preferred motor for the blower is squirrel cage induction motor, synchronous motor, DC shunt Motor, Split-phase motor.

Answer 4. Less than running torque.

Explanation:-

Since in case of the centrifugal pump the most preferred motor is squirrel cage induction motor. In squirrel cage induction motor. This rotor has low starting torque per ampere of starting current. The starting current is high, and these motors exhibit good running torque. Motors containing rotors of this type generally have very good speed regulation and low percent slip. It should be noted that although motors with rotors that fall into this range exhibit poor starting torque per ampere of starting current, the starting torque is still greater than the amount of running torque. These motors do not generally exhibit difficulty starting unless the load requires some time to reach normal speed. The extended time will cause the high starting current to overheat the windings.

Answer.3. Double

Explanation:-

Centrifugal pumps are the prime fluid movers in membrane plants for liquid separations. In general, centrifugal pumps are used in moderate-to-high flow under low-head conditions and operate within the rated ranges of head and velocity.

Pump specifications depend on process requirements, for example, fluid flow rate determines the capacity of the pump whereas pump head depends on fluid density and viscosity. A pump head is commonly used as a measure of energy.

Pump selection criteria

Material and energy balances are used to calculate the required flow rate while the maximum pump pressure or ‘shutoff pressure is the maximum pressure a pump will develop under no-flow conditions, e.g, when the pump outlet line is fully blocked The amount of work needed to drive the pump, increases with increase in fluid density and viscosity. The maximum values of these properties usually occur during start-up, shut down, and process upsets.

Centrifugal Pump Water horse power= Q x H x S.G./3960,

where Q = flow rate, H = head, S.G. = Specific gravity.

Hence from the above equation, it is clear that the flow rate is directly proportional to the water horsepower hence if we double the flow rate then the horse-power will also get doubled.