What is the condition of maximum efficiency in DC motor?
For any machine condition for maximum efficiency is- (variable losses = constant losses)For the DC machine (weather motor or generator) for maximum efficiency ,its stray losses (constant losses-which includes windage loss, bearing loss,brush loss,hysteresis losses and eddy current losses on pole shoe and assuming the effect of armature reaction is completely neutralise using interpoles and compensator windings) should be equal to its copper losses (variable losses,which includes copper losses in armature winding and field winding,applicable for all dc-shunt,series,compounded machines)And its efficiency while maximum power transfer is 50%. (Maximum power transfer theorem can be used to verify this condition)
What is the efficiency of a DC motor?
The value of the efficiency of a DC motor could typically be in the range of 70 to 85%. Larger the machine higher will be the efficiency.Efficiency of DC motor= (output- mechanical)/ (input - electrical)As it is easy to measure electrical quantities more accurately and easily, it is written as= (Input - Losses) / InputInput = VI=supply voltage x line currentLosses consist of Armature and field copper losses, core loss, friction and windage loss.
When does the efficiency of a DC shunt generator have a maximum value?
Integral-horsepower shunt motors, are used where the primary load requirements are for minimum speed variation from full-load to no-load and/or constant horsepower over an adjustable speed range at constant potential.The same applies to the Shunt Generator. As it is self exciting, it needs a constant speed and steady load in order to balance the fields. This is slightly different to the normal externally excited generator.
Explain why the efficiency of a generator is more than a motor in Swinburne's test, in terms of voltage and current?
In Swinburne’s test (a no load lest), a DC machine irrespective of being a motor or a generator is always connected as a motor to perform the test.Here there is no way to measure mechanical power. So,In case of motor:-P(mech output)= P(elec) - lossesIn case of generator:-P(mech input)= P(elec) + lossesSo if a DC machine is tested upon. Irrespective of it being a motor or a generator.If we calculate motoring efficiency:-Nm= P(out)/P(in)= { P(elec) -losses}/P(elec)And if we calculate generating efficiency:-Ng= P(out)/P(in)= P(elec)/{P(elec) +losses}As the numbers P(elec) and losses have a constant value for a given machine, you will always find from the two fractions that:-Ng>NmThis is more like a number system proof.For example let us take P(elec)= 2000W and losses= 350WThen Ng= 2000/2350 = .851And Nm= 1650/2000 = .825You can also analyse this mathematical proof:-If you try to make the denominators equal of these two fractions then you will find:-Nm= {P(elec)- losses}/P(elec) = {P(elec)^2 -losses^2}/{ P(elec)*[P(elec) + losses]}And Ng= P(elec)^2/{ P(elec)*[P(elec) + losses]}By equating the denominators you can see that Nm is always lesser than Ng by “losses^2” amount no matter what value of electric power input or losses you observe.So, irrespective of the machine size or the fraction of load, the generating efficiency will always be greater than motoring efficiency for the same machine.Thanks!
Can a DC motor be converted into a DC generator?
Yes, absolutely. In fact, in a brushed permanent magnet motor, no conversion needs to be done. Just spin the shaft and it generates DC voltage/current.For a brushless DC permanent magnet motor like a computer fan, you'll have to open it up. They use an IC and several transistors to convert DC into 3 phase AC (switched DC), you'll need to remove that and insert diodes to convert the AC coming out into DC.With the DC brushless motors used in model electric airplanes and cars, the speed controller is the electronics. So you only need to connect the three wires to a set of diodes to convert the AC 3 phase that comes out when you spin it, into DC. But you'll have to spin it VERY fast. The higher the KV rating (krpm per volt, not kilovolts), the faster you'll need to spin it to get a useable voltage.A DC brushless motor is wound as a delta or wye configuration (no centertap), so you'll need six diodes:Thanks to Instructables: Windmill Generator - why won't a small brush-less motor work as a windmill generator?
Why are thin conductors and more turns used in a shunt excited DC generator?
Resistance is directly proportional to the length of the conductor and inversely proportional to the area of cross sectionTherefore thin and lenthy conductor have more resistanceIf this winding is connected in series with the armature, large voltage drop will occur due to high resistance, the generated voltage will droppedIf this winding connected in parallel with the armature it will carry less current due to high resistancebecause of large number of turns the Shunt winding will produce more magnetic field with less currentThat's why this winding is connected in parallel with the armature hence it is called Shunt winding
What is the difference between a DC shunt and a series motor?
A shunt motor has the field windings connected in parallel with the armature and a series motor has the field windings connected in series with the armature.shunt winding are parallel and hence have low current and Series motor have series winding and thus a high armature current is same as field current.Shunt DC Motor is used in Constant Torque Application and Series DC Motor is used where High starting torque required.
What is a DC series motor and its advantage and disadvantage?
AdvantagesThe series motor has the highest starting torque for a given power rating. When the motor is overloaded, the speed drops and the torque increases. While delivering high torque during an overload, the series motor draws less current and power from the source compared to a shunt or compound motorThe main application where series motors are best suited are for traction applications. Torque of the series motor is proportional to square of the field current (T α current^2). Therefore Starting torque for the series motor is quite high. Because of this property series motors are widely used in electric traction and in crane application.DisadvantagesSpeed regulation in the series motor is quite poor. With the increase in the load speed of the machine decreases. (DC shunt motor maintains almost constant speed from no load to full load). With decrease in the speed torque of the dc series motor drops sharplyDC series motor should always require to be loaded before starting the motor. Therefore series motors are not suited for the application where load on the motor is completely removed and where motors are driving the belts. Mishap to the belt leads to motor operate on no load and attains dangerous speeds which damage the motor.
What is the purpose of a load test on a DC generator?
According to whether the output power of the tested generator can be measured directly and the regulation of the motor efficiency calculation method, the purpose of the load test is also different, but the ultimate purpose is to obtain the full load of the motor under test or to specify the efficiency of the load.The power factor, current, torque and its slip rate or rotational speed are measured directly for the required efficiency, and the purpose of the load test is to measure the input and output power which can be directly used to calculate the efficiency. In addition, there are other relevant data for calculating the full load power factor of the stator input current and drawing the working curve, for the debt equipment which can not directly display the output power or output load distance of the motor.Or whether any load equipment is used but the efficiency requires indirect measurement, and the purpose of the load test is to obtain some relevant data accurately to obtain the efficiency, power factor and slip rate of the motor, etc. It is usually the stator current, three-phase input power, slip rate or stator resistance under different output power or input power at rated voltage and rated frequency.
Why does the shunt field has more number of turns in DC shunt wound machine?
Shunt field has more number of turns which are thin. Similarly, series wounded field has less turns of thick conductors. Each of these are done to give suitable resistance as per requirement. Series windings carry more current and should allow compratively less voltage drop. That's why it is so. Shunt windings are connected between line and neutral and an effective resistance is required such that the field doesn't acts as short circuited path. Furthermore, it also reduces power loss in form of heat.