DETERMINE PERFORMANCE CHARACTERISTICS AND CORE LOSS OF DC SERIES MOTOR

SAFETY PRECAUTIONS 

1. Must come prepared for the lab; study the related chapter of the experiments before coming to the lab.

2. Proper care must be observed when moving equipments and parts.

3. Lab demonstration has to be attended prior to starting your lab work.

4. Power off prior to making any wire changes.

5. Do not touch any wire while the power is on inform the lab instructors immediately if a wire come out or is found hanging.

6. All lab partners must be at the workstation at all times before the power is turned on.

7. The motor should be started and stopped with load

8. Brake drum should be cooled with water when it is under load.

THEORY

A DC series motor converts electrical energy to mechanical energy. Its principle of operation is based on a simple electromagnetic law that states that when a magnetic field is created around current carrying conductor and interacts with an external field, rotational motion is generated.

The key components of a DC series motor are the armature (rotor), stator, commutator, field windings, axle, and brushes. The stationary part of the DC Seies Motor motor, the stator is made up of two or more electromagnet pole pieces, and the rotor is comprised of the armature, with windings on the core connected to the commutator. The output power source is connected to the armature windings through a brush arrangement connected to the commutator. The rotor has a central axle about which the rotor rotates.

The field winding should be able to support high current because the greater the amount of current through the winding, the greater will be the torque generated by the motor. So the winding of the motor is made up of thick heavy gauge wire. Heavy gauge wire does not allow a large number of turns. The winding is made up of thick copper bars as it helps in easy and efficient dissipation of heat generated as a result of flow of large amount of current through winding.

Principle of Operation

An external voltage source is applied across the series configuration of field winding and armature. So one end of the voltage source is connected to the winding and the other end is connected to the armature through the brushes.

Initially at the motor start up, with the voltage source connected to the motor, it draws a huge amount of current because both the winding and the armature of the motor, both made up of large conductors, offer minimum resistance to the current path. The large current through the winding yields a strong magnetic field.

This strong magnetic field provides high torque to the armature shaft, thus invoking the spinning action of the armature. Thus the motor starts rotating at its maximum speed in the beginning. The rotating armature in the presence of the magnetic field results in counter EMF, which limits the current build up in the series combination of armature and winding.

Thus series motors once started will offer maximum speed and torque but gradually, with an increase in speed, its torque will come down because of its reduced current. Practically this is what required from the motors. Due to the high torque provided by the armature, the load on the shaft is set to rotate initially. Subsequently lesser torque will keep the load on the move. This further helps in increasing the heat dissipation of the motor. However, the amount of torque generated by motor is directly proportional to the winding current. The higher current demands a higher power supply, too.

Motor Speed

In DC series motors, a linear relationship exists between the amount of torque produced and the current flowing through the field windings. The speed of the motor can be controlled by varying the voltage across the motor, which further controls the torque of motor.

To increase the speed of the motor, decrease the field current by placing a small resistance in parallel to the winding and armature. The decrease in current will result in lowering of magnetic flux and counter EMF, which further hastens the motor’s speed.

To decrease the speed, use an external series resistance along with the field winding and armature. This will reduce the voltage across the armature with the same counter EMF, thus resulting in a lower speed of motor.

Unlike DC shunt motors, series motor does not operate at the constant speed. The speed of the motor varies with change in the shaft load, so speed control of the motor is not easy to put into practice.

Applications, Advantages and Precautions

Series motors can produce large turning effect, or torque, from a stand still. These motors have found application in small electrical appliances where high torque is necessary at start up. DC series motors are used mainly for industrial applications, e.g. elevator and pulley and winches systems for carrying heavy loads. Heavy and magnificent cranes drawing thousands of amperes are driven by this motor. An automobile engine can be started by this motor which draws around 500A of current. However, these motors are not suitable where constant speed is required as the speed of series motors is dependent (varies with load) on load unlike DC shunt motors (see link below for an article similar to this one that covers DC shunt motors) whose speed is independent of load.

The construction, designing, and maintenance of these motors is very easy. Series motors are cost effective as well. A final advantage of series motors is that they can be used by providing either an Alternating Current (AC) or Direct Current (DC) power source.

Proper care should be taken that a series motor is not operated without any load as they are totally dependent on shaft loads. As the armature speed increases, the current through the winding decreases which further helps in reducing the counter EMF. This reduction fastens the speed of the armature. As this process continues, the motor speed increases beyond the limit thus causing devastation to the motor.

EXPERIMENT No. : 1

AIM : To conduct load test on DC Series Motor and to find efficiency.

CIRCUIT DIAGRAM :

PROCEDURE

1.         Connections are made as per the circuit diagram.

After checking the load condition, mcb is closed and starter resistance is gradually removed.

3.         For various loads, Voltmeter, Ammeter readings, speed and spring balance readings are noted.

4.         After bringing the load to initial position of 1kg , bring the two pointer to clockwise direction to start the motor .

5.        Note down the corresponding Voltage,Current and Load on the spring load.

6.        To take the

Observation:-

S. No.	Supply Voltage	Current I (Amps)	Load In Kg (From Spring Balance)

  

To see the complete working video of the above panel please visit this link

https://www.youtube.com/watch?v=VGPpUozWIjg

EXPERIMENT No. : 2

AIM: To conduct Test on DC Series Motor to identify the Core Loss

CIRCUIT DIAGRAM :

PROCEDURE

1.Connections are made as per the circuit diagram.

2.         After checking the load condition, mcb is closed and starter resistance is gradually removed.

3.         Apply the maximum load to the motor for maximum rated current.

4.         Note down the reading of Supply Voltage and Armature Current.

5.         Calculate the Armature Resistance using the Formulae

                                                V=IR

6.         Calculate the loss using the formulae

                                                CL=I*I*R

S. No.	Supply Voltage	Current I (Amps)	Armature Resistance	Core Loss =I*I*R

MODEL GRAPH

List of Accessories

1.        Patch cord  ............................................................................................................... 11 Nos.

2.        Operating Manual...................................................................................................... 01 No.

3.        Digital Tachometer...................................................................................................... 01No.

4.            DC Motor with Stand................................................................................................. 01 No.

Learn Electrical Drive Online Free | VFD's for 3-Phase Motor

THEORY

A Variable Frequency Drive (VFD) is a type of motor controller that drives an electric motor by varying the frequency and voltage supplied to the electric motor. Other names for a VFD are variable speed drive, adjustable speed drive, adjustable frequency drive, AC drive, microdrive, and inverter.

Frequency (or hertz) is directly related to the motor’s speed (RPMs).

 In other words, the faster the frequency, the faster the RPMs go. If an application does not require an electric motor to run at full speed, the VFD can be used to ramp down the frequency and voltage to meet the requirements of the electric motor’s load. As the application’s motor speed requirements change, the VFD can simply turn up or down the motor speed to meet the speed requirement.

Working of Variable Frequency Drive

The first stage of a Variable Frequency AC Drive, or VFD, is the Converter. The converter is comprised of six diodes, which are similar to check valves used in plumbing systems. They allow current to flow in only one direction; the direction shown by the arrow in the diode symbol. For example, whenever A-phase voltage (voltage is similar to pressure in plumbing systems) is more positive than B or C phase voltages, then that diode will open and allow current to flow. When B-phase becomes more positive than A-phase, then the B-phase diode will open and the A-phase diode will close. The same is true for the 3 diodes on the negative side of the bus. Thus, we get six current “pulses” as each diode opens and closes. This is called a “six-pulse VFD”, which is the standard configuration for current Variable Frequency Drives.AC Motor

The AC electric motor used in a VFD system is usually a three-phase induction motor. Some types of single-phase motors can be used, but three-phase motors are usually preferred. Various types of synchronous motors offer advantages in some situations, but three-phase induction motors are suitable for most purposes and are generally the most economical motor choice.

Let us assume that the drive is operating on a 480V power system. The 480V rating is “rms” or root-mean-squared. The peaks on a 480V system are 679V. As you can see, the VFD dc bus has a dc voltage with an AC ripple. The voltage runs between approximately 580V and 680V.

We can get rid of the AC ripple on the DC bus by adding a capacitor. A capacitor operates in a similar fashion to a reservoir or accumulator in a plumbing system. This capacitor absorbs the ac ripple and delivers a smooth dc voltage. The AC ripple on the DC bus is typically less than 3 Volts. Thus, the voltage on the DC bus becomes “approximately” 650VDC. The actual voltage will depend on the voltage level of the AC line feeding the drive, the level of voltage unbalance on the power system, the motor load, the impedance of the power system, and any reactors or harmonic filters on the drive.

The diode bridge converter that converts AC-to-DC, is sometimes just referred to as a converter. The converter that converts the dc back to ac is also a converter, but to distinguish it from the diode converter, it is usually referred to as an “inverter”. It has become common in the industry to refer to any DC-to-AC converter as an inverter.

When we close one of the top switches in the inverter, that phase of the motor is connected to the positive dc bus and the voltage on that phase becomes positive. When we close one of the bottom switches in the converter, that phase is connected to the negative dc bus and becomes negative. Thus, we can make any phase on the motor become positive or negative at will and can thus generate any frequency that we want. So, we can make any phase be positive, negative, or zero.

Notice that the output from the VFD is a “rectangular” wave form. VFD’s do not produce a sinusoidal output. This rectangular waveform would not be a good choice for a general purpose distribution system, but is perfectly adequate for a motor.

If we want to reduce the motor frequency to 30 Hz, then we simply switch the inverter output transistors more slowly. But, if we reduce the frequency to 30Hz, then we must also reduce the voltage to 240V in order to maintain the V/Hz ratio (see the VFD Motor Theory presentation for more on this). How are we going to reduce the voltage if the only voltage we have is 650VDC?

This is called Pulse Width Modulation or PWM. Imagine that we could control the pressure in a water line by turning the valve on and off at a high rate of speed. While this would not be practical for plumbing systems, it works very well for VFD’s. Notice that during the first half cycle, the voltage is ON half the time and OFF half the time. Thus, the average voltage is half of 480V or 240V. By pulsing the output, we can achieve any average voltage on the output of the VFD.

With the above theory a basic understanding of the Voltage Frequency Drive.

To get more into detail ,please see the youtube video of the actual trainer

Hope you like my vieos,dont forget to like and subscribe there are more than 19 videos on the different technologies of electrical drives

Three Phase Power Measurement by Two Wattmeter Method (theory and practical)

Two Wattmeter Method of Power Measurement

Two Wattmeter Method can be employed to measure the power in a 3 phase, 3 wire star or delta connected balanced or unbalanced load. In Two wattmeter method the current coils of the wattmeter are connected with any two lines, say R and Y and the potential coil of each wattmeter is joined across the same line, the third line i.e. B as shown below in the figure A.

The total instantaneous power absorbed by the three loads Z1, Z2 and Z3, are equal to the sum of the powers measured by the Two wattmeters, W1 and W2.

To explain the above concept practically,the youtube video shows a working panel in which there are Analog Wattmeters (lagging and leading ),resistive load with required ammeter and voltmeter.

The readings are taken by applying the load using switches and the reading are noted down in a table and results are compared theoretically and practically.

Happy Learning!

I hope you like my post ,please share and subscribe.

Designing a AC Motor Controller

In this blog i will be talking about the iasues related with designing a Single Phase Induction Motor Controller for a 220v AC Motor.

Let me start by stating that designing with microcontroller is bkth fun and effective.

So let me give you some headsup on the flowchart of the program.

1. Keep your hardware ready,work out the circuit on paper and understand the limits of your circuit i.e. voltage range and current range depending up9n the size of motor you are going to use.

2. Test out your circuit by always keeping a lamp load in series with your load,it helps in case of short circuit.

3. Now your aim is to replace your working circuit firing mechanism with the pulse width modulated waveform from your microcontroller.

4. Most important thing is to read the datasheet of the power electronic device you are using to understand the minimum pulse width required to trigger the device i.e 20ns

5. Always generate the pwm with reference to the interrupt signal activated by the external 50hz zero crossing detector output.

Today I have completed my panel for the same topic discussed above and attached the picture of it in the blog.Dont forget to see my youtube videos.

Have a great day designing new projects with these inputs.