Wednesday, 30 November 2016

Brake Test of DC Machine


DC Machines can be tested by three different methods namely Direct Method, Indirect Method and Regenerative Method. Direct Method of testing of DC Machine, also known as Brake Test (if carried out for a DC Motor) will be discussed in this post.

Direct method is suitable for small DC machines. In Direct Method, the DC machine is subjected to rated load and the entire output power is wasted. The ratio of output power to the input power gives the Efficiency of DC Machine. For a DC Generator the output power is wasted in resistor.

Direct Method of testing when conducted on a motor is also known as Brake Test. Brake Test of DC Motor is carried out as shown in figure below.



A belt around the air cooled pulley has its end attached to the spring balance S1 and S2. Using belt tightening hand wheels H1 and H2, the load of motor is adjusted to its rated value. Assuming the spring balance to be calibrated in kilogram, then rated load on the DC motor is given as

Motor Output Power = Torque x Angular Speed  
                          
                                 = (Force x Radius) x Angular Speed




As the torque because of force F1 and F2 are opposing each other, therefore net torque will be subtraction of torque because of F1 and F2.

Therefore,

Motor Output = ω (S1 – S2) x r x9.8 Watt

Now assuming the terminal voltage of DC Motor to be Vt and IL to be the load current then,
Power input to the DC Motor = VtIL

Thus the efficiency of DC Motor can be calculated as below.

Efficiency = Output / Input

                     = [ω (S1 – S2) x r x9.8 Watt] / VtIL

For conducting Brake Test on DC Series Motor, it must be ensured that belt is sufficiently tight before the motor is switched on to the sully as DC Series Motor shall not be started at no load.

Disadvantages of Brake Test:

1)    The Spring Balance Readings are not stable rather it fluctuates.
2)    Output power is wasted.

3)    The frictional torque at a particular setting of Hand wheel H1 and H2 do not remain constant.

Permanent Magnet DC (PMDC) Motor – Construction, Working and Application


There are two types of winding in a Conventional DC Motor, namely Filed Winding and Armature Winding. The purpose of Field Winding is to produce the working magnetic flux in the air gap and wound on the stator of the motor whereas armature winding is wound on the rotor. But in Permanent Magnet DC Motor or PMDC Motor, we do not use field winding rather permanent magnet is used to have the working flux in the air gap. The construction of rotor of PMDC is same as that of the Conventional DC Motor i.e. rotor of PMDC Motor consists of armature core, armature winding and commutator. Stationary Carbon Brushes are kept pressed on the commutator as in conventional DC motor.

PMDC Motors are extensively used in automobiles for windshield wipers and washers, for blowers used in air conditioner and heaters, to raise and lower windows, in personnel computers disc drives etc. As millions of automobiles are manufactured per year, PMDC motors are also produced in millions. Maximum power rating for PMDC motor used in industry has been around 150 kW.

The major advantage of PMDC motor is that they require no field winding and hence no field current rather permanent magnet is used for having working flux in the air gap. Because of this the energy required in producing the field flux is saved. Not only has this, because of absence of field winding the size of PMDC motor reduced which is a great advantage.
The major disadvantage of PMDC motor is that it has limited capacity of producing working flux in the air gap. However, due to development of some new magnetic material like Somarium Cobalt and Neodymium Iron Boron, this problem has been resolved to some extent.

The equivalent circuit of an PMDC motor is shown in figure below. Mind that, in this equivalent circuit filed circuit is shown as is the case with conventional DC Motor.



In a PMDC Motor, flux Ø is constant and therefore we can write as

Ea = Km ωm

Te = KmIa

where Ea = Back emf generated
Te = Electromagnetic Torque
Km = Ea Ø is called speed-volt constant or torque constant. The value of Km depends upon the number of filed poles and the armature conductors etc.

Thus for the equivalent circuit sown above, we can write

Vt = IaRa + Ea

Ra = Armature winding resistance

Vt = IaRa +Km ωm

ωm = (Vt - IaRa) / Km

From the above equations, it can be seen that performance of PMDC motor is same as that of DC Shunt Motor having constant field. Therefore the speed of PMDC motor can be controlled by Armature Voltage Control, Armature Rheostat Control and Chopper Control.

You may like to read Speed Control of DC Motor

Tuesday, 29 November 2016

Rating of Synchronous Generator or Alternator


The rating of AC Machines such as Transformer, Alternators etc. are determined by the heating and hence losses in them. These losses are made up of Ohmic (I2R loss ) and Core Loss and a small amount of friction & windage loss. The ohmic loss depends upon the current and the core loss depends upon the voltage, therefore the losses in an electrical AC machine are unaffected by the power factor of the load. In view of this, the rating of AC machinery is determined by volt-ampere of the load not by the load power alone. Therefore the Alternators or Synchronous Generators are rated in kVA or MVA.


It shall be noted here that the size of boiler or the fuel requirement is solely determined by the output power and not on the volt-ampere. For example, for 700 MW load at 0.85 power factor, the rating of Alternator or Synchronous Generator will be 823 MVA (700/0.85 = 823) while the fuel requirement and size of boiler will be decided by 700 MW. To be more accurate, as the efficiency of Carnot Cycle is around 33%, if we assume Carnot Steam Cycle for simplicity then the fuel requirement or size of boiler to generate a power of 700 MW will be around 3 times the output power i.e. 3x700 = 2100 MW. It is very surprising that only 1/3 of the total input power (2100 MW) is converted into electrical power output but it’s true. Remaining power is lost in NDCT (Natural Draft Cooling Tower and losses during the whole steam cycle).

It shall be mind here that for stating the rating of an Alternator, we must include the steady state operating power factor along with the MVA / kVA rating. The need for mentioning power factor arises because the Alternator designed to operate at 0.85 power factor will require more filed current if operated at 0.8 power factor to maintain constant terminal voltage. More filed current at lower power factor means more losses in the field winding which in not desirable. If on the name plate of an Alternator, it is not mentioned that whether the power factor is lagging or leading then it shall be assumed to be lagging because for constant terminal voltage, lagging power factor will require more field current than the leading power factor. Thus lagging power factor puts a limit on exciter output and current in the field coil.

Under rated power and voltage conditions, reactive power flow handled by an Alternator is limited by the armature heating for operating power factor near the rated value. If the operating power factor is away from the rated value, then the reactive power flow is limited by both the armature and filed heating. Figure below shows a typical Name Plate of a Generator of 120 MW. Mind the kVA rating and Power factor.

  

Normally Generator Capability Curve is provided by the manufacture from which we can get the operating point and the various limits like armature heating, filed heating and End part heating. We will be discussing the Generator Capability Curve in next post.

Open Circuit and Short Circuit Characteristics of Synchronous Machine


Open Circuit Test and Short Circuit Test are performed on a Synchronous Machine to find out the parameters of Synchronous Machine and hence to have an idea of their performance.  Open Circuit Test of Synchronous Machine is also called No Load, Saturation or Magnetizing Characteristics for the reason which will be clear after going through the post.

For getting the Open Circuit Characteristics of Synchronous Machine, the alternator is first driven at its rated speed and the open terminal voltage i.e. voltage across the armature terminal is noted by varying the field current. Thus Open Circuit Characteristic or OCC is basically the plot between the armature terminal voltage Ef versus field current If while keeping the speed of rotor at rated value. It shall be noted that for OCC, the final value of Ef shall be 125% of the rated voltage.

Figure below shows the connection diagram for performing the Open Circuit Test of Alternator.



As clear from the figure above, an Ammeter is connected in series with the field circuit to measure the field current and a Voltmeter is connected across the armature terminals to note down the voltage generated. Figure (b) shows the plot between If and Ef. It can be seen from the graph that the relationship between the field current If and no load generated voltage Ef is linear up to certain value of field current but as the the field current increases the relationship no longer remains linear. The linear part of the relationship is because, at small value of filed current the whole mmf is required by the air gap to create magnetic flux but as the value of mmf exceeds some certain value, the iron parts get saturated and hence the relationship between the flux (No load generated emf is proportional to flux) and field current no longer remain linear.

Next assume that if there were no saturation (assuming no iron part is present rather only air gap is present), the relationship between the field current and no load voltage would have been a straight line and that is why the straight line ob in the figure is called Air Gap Line.  

Thus we observe that because of saturation in iron parts of machine, the no load generated voltage Ef does not increase in the same proportion as the increase in field current.

Short Circuit Test of Synchronous Machine:


For performing Short Circuit Test on an Alternator, the machine is driven at rated synchronous speed and the armature terminals are short circuited through an Ammeter as shown in figure below.



Now the field current If is gradually increased from zero until the armature short circuit current reaches its maximum safe value i.e. 125 to 150% of its rated current value. Readings of field current If and short circuit current are noted and plotted.



If you see the above plot of Short Circuit Test, you notice that the short circuit characteristics of a synchronous machine is a straight line.

Why Short Circuit Characteristics of Synchronous Machine is Straight Line?

For short circuit test, as the armature terminals are shorted, therefore terminal voltage Vt = 0. Therefore the air gap emf Er shall only be enough to provide the leakage impedance drop in the armature i.e.

Er = Ia(Ra + jXal) where Xal = Armature Leakage Reactance

As we know that, for a Synchronous machine the value of Xal is of the order of 0.1 to 0.2 per unit and Ra (Armature Resistance) is negligible thus we can write as

Xal = 0.15 (Taking average value of 0.1 and 0.2)

Ra = 0

then Er = Ia (Ra +jXal) = 0.15Ia

Taking rated current of armature, Ia = 1 pu

Therefore, Er = 0.15 pu


Thus we observe that during short circuit test, the air gap generated emf Er is only 0.15 pu which mean that air gap flux must also be 0.15 pu. As the resultant air gap flux is only 0.15 of its rated value under normal voltage condition, such a low value of air gap flux does not saturate the iron parts of synchronous machine and hence the short circuit characteristics is a straight line. It shall also be noted here that, in case of short circuit test the armature mmf is almost entirely demagnetizing in nature which results in very low value of air gap flux.

Saturday, 26 November 2016

Pulse Transformer – An Overview


The transformer which handles voltages and currents in the form of pulse are called Pulse Transformer. Pulse Transformer is mostly used in Power Electronic circuits as an Isolating Transformer to isolate source and load. It is also used in television, radar, digital computers etc. The main functions of the Pulse Transformer are as below:

1)      For changing the amplitude of voltage pulse
2)      For inverting the polarity of the pulse
3)      For coupling different stages of pulse amplifier
4)      As an Isolation Transformer
It shall be noted here that the turn ratio of an Isolation Transformer is 1:1. The Pulse Transformer core is made of Ferrite.

The input voltage to a Pulse Transformer is of discontinued type as shown in figure below. The most and basic requirement of such transformer is that it should reproduce input voltage at its primary to secondary as accurately as possible.



Figure above shows the square input voltage pulse at the input terminals. The pulse with varies from fraction of microsecond to 25 microseconds. Normally enough time elapses before next pulse arrive at the input terminals. The waveform of output voltage can be determined by using the equivalent circuit of the pulse transformer. A typical output voltage waveform is shown in figure below. In the figure below, the pulse parameters are also shown and it must be noted.



Rise time here is the time interval required for the output to rise from 0.1 to 0.9 of its final value. The transformer response for to the flat top portion of the input voltage pulse can be determined using low frequency equivalent circuit. The output voltage seems to have downward tilt or drop off during pulse duration time. The output voltage cannot be flat as this would mean that DC is flowing through Transformer which is not possible. The drop off of the output voltage pulse can be minimized by using high magnetizing inductance for the transformer.

Again, when the input pulse is zero, the output pulse do not reduces to zero instantaneously because of magnetic energy stored in the inductance of the transformer rather there is some definite fall or decay time to reach the output voltage pulse to zero value.

You may like to read Concept of Transformer Action

Pulse Transformers are quite small in size. To minimize the leakage inductance, there are few truns of primary and secondary but to have high magnetizing reactance, its core is either made of Ferrite or high permeability alloys like permalloy.