Flywheel: FACTOR

Maintenance:
Service- and maintenance works of the rotating converter can be done by every trained technician. A static converter can only be maintained from a specialist from the supplier/OEM. This can influence the service cost significantly.

MTBF-value:
The MTBF-Value (meantime between failure) of rotary converter is above 50.000 hrs. The MTBF of static converters is significanly lower due to the higher number of parts and electronic components.

Spares:
Spares for rotary converter are available in almost every country of the world.

Grid loading effect:
When planning static systems, you have to consider, that because of current and voltage interferences caused by static converters the Mains transformer has to be oversized by about 20%. Further on only such consumer can be connected to the transformer which are not influenced by the interferences. (Problems with UPS or diesel gen sets).

Efficiency:
The efficiencies of rotary converter and static converters are almost the same. The additional losses caused by the inverters’s harmonics are not included.

Distortion factor:
The intensity of harmonics of rotary converters is much lower than those from static converters.

Suppression of radio interference:
Suppression acc. N and K acc. VDE 0875 or MIL-standard can be fulfilled by rotary converter. Static converter already have problems with N acc. VDE 0875.

Input power factor:
The power factor is automatically controlled within the range of 0.9 to 1.0 (reactive load supply) at 50% to 100% of nominal load. Static converter consume reactive load (cos phi 0,75 - 0,85) and therby require a capacitor bank.

Overload:
Rotary converter have better overload endurability than static converter.

Shock load response:
By fast regulating the voltage at shock load often requests an enlargement of the system of static converter. Rotary converter have the capability to endure peak values of shock loads of 20 times nominal current (one half wave) or 3 times nominal current for several seconds.

Overcurrent discrimination:
The request operating static converter systems without fuses is not possible or just under certain conditions. Current peaks up to 40x nominal current for one half wave are not seldom for static converters. To obtain a selectivity highest care is necessary when selecting fuses. The installation efforts for static converters is much higher than for rotary converters.

Indirect coupling (Galvanic separation):
An indirect coupling is provided only by rotary converter.

Voltage peaks and EMP, NEMP:
Electronic parts are more responsive against voltage peaks and high magnetic fields than windings are.

Shock:
Shock proof systems with static converters are only limited practicable.




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Important questions and answers on Design of Flywheel

  Important questions and answers on Design of Flywheel

1.      What is flywheel?

A flywheel is a heavy body rotating about its axis. It acts as a reservoir of energy which is stored in the form of kinetic energy. The extra energy is stored during the idle stroke of the driven machinery and released during the working stroke. Thus flywheel controls the fluctuations of speed during each cycle of the driven machinery.

2.      What are the functions of flywheel in a machine?

The primary function of a flywheel is:

a.       To absorb energy when demand of energy id less than the supply

b.      To give out energy when demand of energy is more than the supply.

3.      What types of stresses are set up in the flywheel rims?

a.       Tensile stress due to the centrifugal force

b.      Tensile bending stress due to restraint of the arms

c.       Shrinkage stresses due to the unequal rate of cooling of casting.

4.      What are the various types of flywheel?

a.       Solid disc type

b.      Rimmed type with either arms or solid web

Solid disc type flywheel is rarely used because they have less capacity of storing energy.

Rimmed type flywheels with arms are preferred because they can store more energy. Small rimmed type flywheels are manufactured with solid web or holes drilled in the web.

5.      Why flywheels are used in punching machines?

Use of flywheel in punching machine is due to the following reasons:

a.       It decreases the variation of speed during each cycle of punching machine.

b.      It decreases the fluctuation of speed due to difference in output and input

c.       It stores energy during idle stroke and releases during working stroke.

6.      Why flywheel is used in IC engines?

In IC engine or stem engine the energy is developed during the power stroke, no energy is developed during suction, compression and exhaust strokes in 4 stroke engine. It helps the crank shaft to run at uniform speed by performing its primary function

7.      What is the difference in the function of governor and a flywheel?

Governor regulates the mean speed of an engine when there are variations in load by changing the supply of working fluid. Flywheel does not maintain a constant speed. It reduces the fluctuations.

8.      Coefficient of fluctuation of speed is ------------ of maximum fluctuation of speed and the mean speed

Ratio

9.      Due to centrifugal forces acting on the rim, the flywheel arms will be subjected to ------------- stresses

Tensile

10.  Why flywheel arm are usually elliptical?

This shape helps in more section modulus for the dame weight. This results in more strength than circular section

11.  Under what consideration the shaft for a flywheel is designed?

It is designed under shear stresses produced due to the combined action of torsion and bending moment.

12.   In a flywheel, the major axis of the elliptical section of the arm is the plane of rotation. Write done the reason for this arrangement.

The arms may have to carry the full torque load due to high inertia of the flywheel when the energy input to its shaft is cut off. The arm may be assumed as a cantilever fixed at the hub and carrying the load at the rim end. This bending moment lies in the plane of rotation of the flywheel. Therefore, the major axis of the arms must be parallel to the tangential force F acting on the flywheel.

Bending moment on the arm, M = F(R-d_h/2)

                                    F = t/(nR)

Where n = number of arms

Section modulus of arm, Z = (π/32 )b₁a₁²

Bending stress σ_b = M/Z

13.   On what basis the material of flywheel is selected?

a.       High tensile strength

b.      High fatigue strength

c.       Low shrinkage

14.  What are the advantages of having elliptical section of flywheel arm?

The flywheel arms are made of elliptical with major axis twice the minor axis. The major axis lies in the plane of rotation and provides double the resistance against bending moment.

15.   Difference between flywheel and governor

The function of a governor in engine is entirely different from that of a flywheel. It regulates the mean speed of an engine when there are variations in the load, e.g. when the load on the engine increases, it becomes necessary to increase the supply of working fluid. On the other hand, when the load decreases, less working fluid is required. The governor automatically controls the supply of working fluid to the engine with the varying load condition and keeps the mean speed within certain limits.

As discussed above, the flywheel does not maintain a constant speed; it simply reduces the fluctuation of speed. In other words, a flywheel controls the speed variations caused by the fluctuation of the engine turning moment during each cycle of operation. It does not control the speed variations caused by the varying load.

16.  Define the following terms

Coefficient of Fluctuation of Speed, coefficient of steadiness, fluctuation of energy, maximum fluctuation of energy, Coefficient of Fluctuation of Energy

Coefficient of fluctuation of speed: The difference between the maximum and minimum speeds during a cycle is called the maximum fluctuation of speed. The ratio of the maximum fluctuation of speed to the mean speed is called coefficient of fluctuation of speed.

Coefficient of steadiness: The reciprocal of coefficient of fluctuation of speed is known as coefficient of steadiness and it is denoted by m.

Fluctuation of energy, maximum fluctuation of energy: 
The fluctuation of energy may be determined by the turning moment diagram for one complete cycle of operation. Consider a turning moment diagram for a single cylinder double acting steam engine as shown in Fig.  The vertical ordinate represents the turning moment and the horizontal ordinate (abscissa) represents the crank angle.

 

A little consideration will show that the turning moment is zero when the crank angle is zero. It rises to a maximum value when crank angle reaches 90º and it is again zero when crank angle is 180º. This is shown by the curve abc in Fig. and it represents the turning moment diagram for outstroke. The curve cde is the turning moment diagram for instroke and is somewhat similar to the curve abc. Since the work done is the product of the turning moment and the angle turned, therefore the area of the turning moment diagram represents the work done per revolution. In actual practice, the engine is assumed to work against the mean resisting torque, as shown by a horizontal line AF. The height of the ordinate aA represents the mean height of the turning moment diagram. Since it is assumed that the work done by the turning moment per revolution is equal to the work done against the mean resisting torque, therefore the area of the rectangle aA Fe is proportional to the work done against the mean resisting torque. We see in Fig.  that the mean resisting torque line AF cuts the turning moment diagram at points B, C, D and E. When the crank moves from ‘a’ to ‘p’ the work done by the engine is equal to the area aBp, whereas the energy required is represented by the area aABp. In other words, the engine has done less work (equal to the area aAB) than the requirement. This amount of energy is taken fromthe flywheel and hence the speed of the flywheel decreases. Now the crank moves from p to q, the work done by the engine is equal to the area pBbCq, whereas the requirement of energy is represented by the area pBCq. Therefore the engine has done more work than the requirement. This excess work (equal to the area BbC) is stored in the flywheel and hence the speed of the flywheel increases while the crank moves from p to q.

Similarly when the crank moves from q to r, more work is taken from the engine than is developed. This loss of work is represented by the area CcD. To supply this loss, the flywheel gives up some of its energy and thus the speed decreases while the crank moves from q to r. As the crank moves from r to s, excess energy is again developed given by the area DdE and the speed again increases. As the piston moves from s to e, again there is a loss of work and the speed decreases. The variations of energy above and below the mean resisting torque line are called fluctuation of energy. The areas BbC, CcD, DdE etc. represent fluctuations of energy. A little consideration will show that the engine has a maximum speed either at q or at s. This is due to the fact that the flywheel absorbs energy while the crank moves from p to q and from r to s. On the other hand, the engine has a minimum speed either at p or at r. The reason is that the flywheel gives out some of its energy when the crank moves from a to p and from q to r. The difference between the maximum and the minimum energies is known as maximum fluctuation of energy 

Coefficient of Fluctuation of Energy 

It is defined as the ratio of the maximum fluctuation of energy to the work done per cycle. It is usually denoted by CE

FlyWheel:Rotational Speed

Rotational Speed

The second factor determining the energy a flywheel stores is the rotational speed. 

 

The energy a flywheel stores is proportional to the square of the rotational speed. Therefore, if a flywheel’s speed doubles, the amount of energy it stores will quadruple.

 

Rotational speed is a factor that must be measured using a tachometer

 

 

 

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FlyWheel: Moment of Inertia

Moment of Inertia

Newton’s Second Law of Motion states that force is equivalent to the mass of the object multiplied by its acceleration. However, this equation is different for an object that rotates. In rotation, force is replaced with torque, acceleration is replaced with rotational acceleration, and mass is replaced with the moment of inertia. Newton’s Second Law of Rotation states that torque is equivalent to the moment of inertia multiplied by rotational acceleration. The object’s mass, radius,and inertia constant

determine the moment of inertia. 

The mass and radius of a flywheel can easily be determined by weighing and measuring. However, the inertia constant of a flywheel is dependent on the shape 

.

Flywheels are usually one of two shapes: a ring with spokes or a solid disk. A ring with spokes most clearly resembles a wheel, while a solid disk looks like a CD with out a hole in the middle. 

Since flywheels usually only come in these two shapes we will only be concerned with two values of inertia constant

.

The inertia constant for a ring is 1.0 and the inertia constant for a disk is 0.5. The difference in the inertia constants is due to the fact that all of the mass in a ring is concentrated at its circumference, while the mass of a disk is evenly distributed from the center to the outside.

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FlyWheel: Energy

Energy

When more kinetic energy can be stored in a fly wheel, the less energy needs to come from the main power source. This increases the efficiency of the mechanism’s energy output. Because a flywheel can be used in such a variety of mechanisms, the amount of energy stored in the flywheel varies. There are two factors that control the amount of energy the flywheel stores: the moment of inertia

and the rotational speed of the flywheel

 

 

 

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FlyWheel Benefits

Benefits

Energy moving in and out of the flywheel can be used to provide temporary and constant power. A flywheel’s greatest benefit is in mechanisms where the main power source is provided in unsteady bursts. By using conservation of energy, the flywheel stores energy as it is being released from the main power source in a surge or burst. As the main source of energy decreases, the

energy stored in the flywheel is released. Mechanisms will receive an uninterrupted sup

ply of energy. Currently, flywheels are used in electrical grids to level out power surges and in cars to smooth the rapid explosions from the engine that provide power.

 

 

 

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