Troubleshooting, Maintenance and Protection of AC Electrical Motors and Drives

Technology Training that worksTechnology Training that Workswww.idc-online.com/slideshare

Troubleshooting, Maintenance & Protection of AC Electrical

Motors and Drives


Industrial electricity is all about Single-phase and Three-phase transformers, AC and DCmachines - their construction, design, testing, operation and maintenance. For troubleshooting Electrical equipment and control circuits, it is important to know the basic principals on which the electrical equipment works.

Industrial Electricity


A transformer works on the principle of electromagnetic induction - an e.m.f. is induced in a coil if it links a changing flux. Transformers are designed to change the voltage of electrical power supply from one value to another. They are widely used in power systems.

Principle of a transformer


e.m.f. Induced

f = fmax sinwt

fmax –the maximum value of the flux in webers,

w = 2pf

f – frequency in cycles per secs.

The e.m.f. induced in the winding is given by Faraday’s Law and the direction of induced e.m.f. is given by Lenz’s Law.


Faraday’s Law

When the flux linking with the circuit changes, an e.m.f. is induced in the circuit proportional to the rate of change of flux. The instantaneous value of the e.m.f. induced in the primary winding is given by :

e1 = – N1 df

dt

The maximum e.m.f. is given by :E1max = – 2 p fmax f N1


Lenz’s Law

The polarity of the induced e.m.f. is given by the Lenz’s Law. The direction of the induced e.m.f. is such that the current induced by it tends to stop the change producing it.


Ideal Transformer • Winding resistances are negligible

• All flux produced is confined to core of transformer and links fully both windings

• The permeability of the core is high enough so magnetizing current required to produce flux and establish it in the core is negligible.

• Eddy current and hysteresis losses are negligible


V = -E 1

E 2

E 1

I0

E 1

E = V2 2I 2

I (N eg lig ib le )0

I 1

V = -E1 1

(a ) N o L o a d (b ) O n L o a d


I1

I1 I1

N 1 N 1N 2 N 2Z 2

Z 2

Z 2

V 1 V 1V 1

I2 I2(N /N )1 2

2

(N /N )1 2

2

(A ) (B ) ( C )


• Measurement of winding resistance

• Measurement of Voltage ratio

• Test Phasor voltage relationship

• Measurement of impedance voltage, short circuit• impedance and load loss.

• Measurement of no load loss and no load current.

• Measurement of insulation resistance.

• Dielectric test

• Temperature rise

Testing Transformers


•The electromechanical energy conversion device is a linkbetween electrical and mechanical systems. •When the mechanical system delivers energy through the device to the electrical system, the device is called a generator. •When electrical system delivers energy through device tothe mechanical system, the device is called a motor.

Electromechanical Energy Conversion


Tm = Te + Tf

and, e and i are in same direction.

where, Tm – is Mechanical Torque

Te – is Electrical Torque

Tf – is Torque lost due to friction

For a Generator


Hold the conductor in right hand with fingers closed around conductor and thumb pointing towardsthe direction of the current. The fingers will point towards the direction of the magnetic lines of the flux produced around the conductor.

Right-hand Rule


The direction and the travel in which it has to be rotated are related to each other the same way as the direction of the current in the conductor and the direction of the field that would be produced due to the current. The magnetic field exists in the plane perpendicular to the conductor.

Cork Screw Rule


Flux can be produced by causing the current to flow through a coil instead of a conductor.

The direction of the magnetic flux in the coil is given by the Right-hand rule.

Flux produced by Current Carrying Coil


The e.m.f. induced due to rotation of a conductor or a coilin a magnetic field is given as:e.m.f. (e) = B L v

where: B – is flux density of magnetic field (wb/m2)L – length of the conductor perpendicular to magneticfield (meters)v – velocity of conductor (m/sec)

E.m.f induced due to Rotation of a Conductor in a Magnetic Field


Fleming’s right hand rule states that if the forefinger of the right hand points in the direction of the field and the thumb towards the motion, then the middle finger points towards the direction of e.m.f. induced.

Fleming’s Right Hand Rule for Determining the Direction of e.m.f. Induced


Gives the relationship between the direction of the current, the direction of field and the direction of the motion. If the forefinger of the left hand points towards the field, the middle finger points towards the direction of the current and the thumb points towards the direction of motion.

Fleming’s Left Hand Rule


If a conductor carries current in a magnetic field, then a mechanical force is exerted on it. The force exerted on the conductor is given as :

F = B L I

Force on a Conductor in a Magnetic Field


If the conductor on a rotor has a radius of r, then the torque produced on the conductor is given as :

T = F r or

T = B L I r

For a coil with two sides, the electromagnetic torque will be double. The Power is given as :

Power = T wr

Torque on a Conductor in a Magnetic Field


e.m.f. is induced enough to overcome the drop of volts in resistance of the winding and supply the load at the required voltage.

The electromagnetic torque produced by current-carrying conductors is counter torque, opposing the rotation.

A Generator or a Motor Action


The rotational or speed e.m.f is produced in opposition to the applied voltage. This is known as counter or back e.m.f.

Mechanical torque is produced as required by the load driven by the motor. For more torque and mechanical power output, there must be more input to the motor from the mains. The motor draws current according to the requirement of the load.

Motor Armature


• The voltages are induced in the windings, the load currents and the terminals voltages under different loading conditions.• The speed at which the machine works under different loading conditions, frequency.• The power input or the output received from the machine.• The torque produced under different loading conditions.

Basic Characteristics of Electrical Machines


W = V I cosfhere,

W – is the power (watts)V – is R.M.S. VoltageI – is R.M.S. Currentcosf – is Power Factor [ = W / V I ]

Single Phase Power System


For Delta-connected system :Line Voltage = Phase VoltageLine Current = 1.732 * Phase Current

For Star-connected system :

Line Voltage = 1.732 * Phase VoltageLine Current = Phase Current

Three-Phase Power Systems


When an AC circuit is to be tested, a Clip-around or clamp-on meter is quite useful. The clamp-on meter has a probe in the form of iron or ferrite core which can be opened to allow it to be clamped-on or clipped over the current carrying conductor.

When the clip is closed, it forms the magnetic core of a transformer.

Clamp-on Meter


High resistance measurements of the order of megaohms are required in electrical equipment testing and troubleshooting.

Megohmmeter based on megaohm Bridge method is used for high resistance measurements.

Megohmmeter


Harmonics• Fundamental frequency of AC power distribution system

is 50Hz or 60 Hz • Harmonic frequency - Any sinusoidal frequency, which is

a multiple of the fundamental frequency• Can be even or odd multiples of the sinusoidal

fundamental frequency


Linear and Non Linear Loads

• Linear electric load - Draws purely sinusoidal current when connected to sinusoidal voltage source – Resistors, capacitors, inductors

• Non Linear electric load – Draws non-sinusoidal current when connected to sinusoidal voltage source – diode bridge, thyristor bridge, variable speed drives, rectifiers


Fundamental wave with 3rd Harmonic


Harmonic Order• The Multiple, that the harmonic frequency is of the

fundamental frequency

Harmonic frequencies of 50 Hz fundamental are:

Even Harmonics Odd Harmonics

2nd harmonic -100 Hz 3rd harmonic - 150 Hz

4th harmonic - 200 Hz 5th harmonic - 250 Hz

6th harmonic -300 Hz 7th harmonic - 350 Hz


DO YOU WANT TO KNOW MORE?

If you are interested in further training or information, please visit:

http://idc-online.com/slideshare

Troubleshooting, Maintenance and Protection of AC Electrical Motors and Drives

Engineering

Transcript of Troubleshooting, Maintenance and Protection of AC Electrical Motors and Drives