ELECTRICAL NOTES
Principles of a motor:
Faraday’s law of electromagnetic induction:
“Whenever the magnetic flux linked with a
closed circuit changes, an emf is induced in the circuit. The induced emf lasts
as long as the change in magnetic flux continues.”
Fleming’s left hand rule:
“The forefinger, middle finger
and the thumb of the right hand are held in the three mutually perpendicular
directions. If the fore finger points along the direction of the magnetic field
and the middle finger is along the direction of current, then the thumb points the
direction of the motion of the conductor”.
Types of motors used on board
· Induction motor
It is used where load changes eg. pumps
· Synchronous motor
It is used for cranes, turning gears where the speed should be constant.
1. When a 3Φ supply is given to the stator winding of the motor, it produces rotating magnetic field on it. At a speed called synchronous speed.
Ns = 120f/p
2. When the conductor (Rotor winding –Armature) is placed inside the rotating magnetic field with its ends shorted, As per law of electromagnetic induction and Fleming’s left hand rule this rotating magnetic field induces emf on the rotor. E= dΦ/dt
3. Since the ends are shorted, the induce emf sets up the root current.
4. This rotor current produces the magnetic field on rotor.
As per Lenz’s
law “The induced current produced in the circuit always flows in such a
direction that it opposes the cause that produces it.”
Since the magnetic field in stator is
rotating, the flux cut by the conductor in rotor is varying with time, which is
why emf is induced in the rotor.
5. Here the induced current is the rotor current and the cause for producing the current is the varying flux over the conductor i.e., rotating magnetic field. So the magnetic field produced by the rotor current exhibits torque and starts rotating in the direction of the rotating magnetic field and tries to stop that varying flux. So the rotor rotates to stop the varying flux and tries to attain the synchronous speed but which will never happen. This is termed as “Slip”
6. Slip is the difference between rotor and synchronous speed.
It is expressed as a percentage of synchronous speed.
Slip = (Ns-Nr)/Ns * 100 %
What happens when there is no slip?
An induction motor cannot run normally at synchronous speed. This is because the rotor conductorswill then be stationary with respect to rotating magnetic field. No emf would be induced in the rotor and thus no rotor current and torque developed. Hence slip is the important factor of induction motor.
Even at no-load the rotor speed is slightly less than synchronous speed , so that current can be induced in the rotor and torque is produced.
Why speed of the motor reduces when a load
is connected?
At no-load the rotor speed is only slightly less than synchronous speed since the only torque required is that needed to overcome the rotational losses of friction and windage.
When load is applied to the motor, shaft of the motor tends to slow down. This allows the constant speed rotating field to cut the rotor conductors at a faster rate which induces larger rotor current. The result is an increased torque output at a slightly reduced speed.
I α V , V α T2 and F α 1/T
Why starting current is high?
When rotor is at rest, Nr= 0 hence slip is 100%. More rate of change of flux results in more induced resulting in more induced current on rotor. The induced current in rotor causes the stator to draw more current form the bus bar during starting.(It is like when secondary of a transformer is short circuited it will cause primary coil to draw more current from the mains. Here stator is the primary and rotor is the secondary which short circuited.) When rotor picks up the speed the amount of induced current reduces hence current drawn by the stator decreases. Starting current will be 4-8 times the rated current.
Why we need starters for induction motors?
· To prevent restart of motor after a power failure.
· When power to the motor is interrupted started will stop the motor
· It includes
o Over current protection.
o Speed control arrangements.
o Remote controls.
o Limiting starting current.
Types of starting methods.
What is Direct on line starting (D.O.L):
In this system the mains supply is directly connected to the stator of the motor. It provides less torque with high starting current. Used for motors up to 3.7KW.
Operation:
When
the start button is pressed power is supplied to the main contactor, MC, all 4
contacts close and the motor starts. Contact MC1 also closes and this ensures
that when the start button is released power is still supplied to the
contactor. To
stop the motor the stop button is pressed, this cuts off the power to the
contactor, MC, which in turn cuts off the power to the motor
The motor is protected by fuses and by an overload device fitted into each phase of the motor, if the overload operates it opens a switch in the contactor supply circuit and cuts off the power to the motor.
Auto transformer: Auto transformers can supply more current to the
motor while keeping the voltage low, the transformer steps up the current to
make it greater than the line current input during start up.
· It uses taps on a transformer winding to control the power input to the motor. The taps are typically set up to provide 80%, 65% and 50% of the line voltage. Activating any one of the three taps on the windings allows the adjustment of current and torque to required value.
· Used for smooth start up but not more than two consecutive starts. Requires cooling time after consecutive starts.
Star
delta starter.
The motor stator windings are connected in star to start the motor and then are connected in delta for normal running of the motor, this will reduce the voltage applied to the motor.
Star connected winding. Delta connected winding.
The value of phase current drawn depends on the impedance of each winding of the field and this does not vary as it is inbuilt into the mot As the line voltage is the same in both star and delta it can be seen that line current in star is one third of the value in delta.
A star delta starter has three contactors to operate. When starting the main and the star contactor are closed and the delta contactor is open, the motor will start up. After a set time interval the star contactor is opened and the delta contactor is closed, this will then apply full line voltage across each phase of the motor.
The start contactor and the delta contactor must never be made at the same time as this would cause a complete short circuit across the three phase supply. The control circuitry for the starter must include interlocks to prevent short circuiting of the mains.
A motor soft starter is a device used with AC electric motors to temporarily reduce the load and torquein the powertrain and electrical current surge of the motor during startup. This reduces the mechanical stress on the motor and shaft, as well as the electrodynamic stresses on the attached power cables andelectrical distribution network, extending the lifespan of the system.Motor soft starters can consist of mechanical or electrical devices, or a combination of both. Mechanical soft starters include clutches and several types of couplings using a fluid, magnetic forces, or steel shot to transmit torque, similar to other forms of torque limiter. Electrical soft starters can be any control system that reduces the torque by temporarily reducing the voltage or current input, or a device that temporarily alters how the motor is connected in the electric circuit.
Electrical soft
starters can use solid state devices
to control the current flow and therefore the voltage applied
to the motor. They can be connected in series with the line voltage applied to the
motor, or can be connected inside the delta (Δ) loop of a delta-connected
motor, controlling the voltage applied to
each winding. Solid state soft starters can control one or more phases of the
voltage applied to the induction motor with the best results achieved by
three-phase control. Typically, the voltage is controlled by reverse-parallel-connected silicon-controlled rectifiers (thyristors), but in some circumstances with three-phase control, the control
elements can be a reverse-parallel-connected SCR and diode.
Working of synchronous motor.
Like induction motor we are giving 3Φ supply to the stator which produces rotating magnetic field at synchronous speed.
Rotor is made of electromagnets. We need external D.C supply for excitation.
Synchronous motors are not self-starting, we need analternative method to rotate it from rest to synchronous speed.Which is done by connecting a D.C motor with the rotor.
When the rotor attains the synchronous speed. The rotor’s north pole is attracted by the stator’s south pole and rotor’s south pole by stator’s north pole. Once their poles are magnetically locked external starting device can be removed.
When load is given , rotor tends to slow down but the strong magnetic attractive force will pull back the rotor to synchronous speed. But stator runs ahead of rotor at same speed. This is called Load angle.
What is load angle in synchronous motor?
The angular difference between the position of stator and rotor is known as load angle. As the load on the motor increases the load angle increases.
Comparison between synchronous motors and
induction motors.
1. For a given frequency, Synchronous motors run at same speed irrespective of load while the speed of the induction motor falls with increase in load.
2. Synchronous motors can be operated under wide range of power factors, both lagging and leading, but induction motor always runs with lagging power factor.
3. Synchronous motors are not self-starting.
4. Variation of torque with respect to voltage is more in induction motor than in synchronous motor.
5. A d.c excitation is required for synchronous motors.
6. Synchronous motors are usually more costly and complicated than induction motors.
How to control speed of induction motor?
· Frequency is directly proportional to speed of the motor. Using frequency converter we can alter the speed of the motor.
· Increase in voltage causes reduction in slip and thereby acceleration of the motor. Voltage is directly proportional to the speed of the motor.
What is single phasing of motors?
1) Singlephasingoccurswhenoneofthe3phases isopen,hencetheremaining circuitscarryexcesscurrent.
2) One phase of the circuits becomes open, due toblown fuse, faulty contactor,
or brokenwire
3) It prevents amotor
fromstarting, but a running motormay
continueto run with this fault
4) For a runningmotor, it can be detected
byoverloaded device insupply line, or overheating
5) For an idle motor, it cannotbe started.
What are all the Effects of single phasing?
1)
Due to single phasing the current in the remaining two phase increases and it
is approximately 2.4 times the normal current value.
2)
Single Phasing reduces the speed of the motor.
3)
The motor becomes noisy and starts vibrating due to uneven torque produced in
the motor.
4)
If the motor is arranged for standby and automatic starting then the motor will
not start, and if the overload relay provided fails to function then the motor
may burn.
5)
The windings will melt due to overheating and can give a fatal shock to the
personnel.
6) It may cause overloading
of the generator
What is the cause for single phasing?
Single Phasing is usually
caused when:-
1) One of the three back up fuses blows (or fuse wire melts).
2) One of the contactor for motor is open circuited.
3) Single phasing might also be caused due to wrong setting of the protection devices provided on the motor.
4)
Contactors are coated due to oxidation hence not conducting.
5) Relay contacts damaged or
broken.
Q. How to Protect Motor from Damage
Due to Single Phasing?
All
motors above 500 KW are to be provided with protection devices or equipments to
prevent any damage due to single phasing.
The
rule stated above does not apply to motors of the steering gear system installed
on the ship. Only on the detection of the single phasing an alarm will be
sounded. However the motor will not stop as it is essential for safety or
propulsion of the ship.
The most commonly used
protection devices for single phasing are
·
Electromagnetic Overload
Device.
·
Thermistors.
·
Bi-metal strip.
Q. Overhauling of an Electric Motor
o
Disconnect
the electric circuit.
o
Isolate
the system and tag out.
o
Remove
the fuse & check the electric supply is not coming.
o
Remove
the coupling bolts after marking.
o
Lift the
motor with the help of lifting gear.
o
Open the
end cover & remove the fan and clean the fan.
o
Remove
the bearing and renew it.
o
Check the
condition of winding visually & check the insulation resistance by
using megger. If low, then apply
varnish on it & dry it.
o
Check the
continuity.
o
Check the
air gap between stator and rotor.
o
Assemble
the motor.
o
Before
putting it into operation, check the load being taken & any noise coming
from it.
What is the importance of air gap?
If
the air gap around a rotor is not uniform the motor may not start in certain
positions. Because the rotor is not centered, probably due to worn bearings,
there is an out of balance magnetic pull.
In the induction motor the air gap should be
as small as possible if the motor is to act with a high power factor. An
increase in air gap increases the reactance of the motor and lowers its power
factor. Small motors are accurately machined and centring of the rotor is very
important so ball or roller bearings are fitted.
|
Air gap |
Motor size |
|
0.25mm |
1kW |
|
0.75mm |
10kW |
|
2.0mm |
100kW |
Q. How to make sure motor
is in good condition.
While Running:
·
No noise
·
No vibrations
·
Motor frame temperature normal.
·
Current drawn by the motor is normal.
·
No dirt or dust or no corrosion on fins.
·
Drawing rated current.
During Stop condition.
·
Isolate the system
·
Dismantle the parts
·
Check condition of internal parts like bearing,
fan.
·
No damage to rotor, stator.
·
Insulation resistance to check.
·
No signs of overheating should be there.
While starting:
·
Developing correct torque & high
starting current.
·
Coming back to its normal rated current in
designated time.
Q. Why
motor insulation deteriorates?
a. Moisture – results in decrease in the measured value of insulation
resistance.
b. Dust & dirt deposits – reduces insulation resistance & failure of
insulation due to oxidation.
c. Oil & Grease – prevents heat dissipation & causes dirt and
dust to settle on the insulation.
d. Ageing – over the period of time the insulation deteriorates due to it
getting affected by temperature variation, mechanical stresses, vibrations,
moisture and some varnish tend to become hard with age & crack during the
operation.
e. Temperature – Excessive temperature dehydrates and oxidizes the insulation,
making it brittle & disintegrate under vibration & shock.
Q.what
should we do when motor is flooded with water.
·
Disconnect the electrical
supply.
·
Put MEN AT WORK tag.
·
Remove the coupling bolts.
·
Now, lift the motor using
lifting gear & put it on a clean place.
·
Remove the cover & fan.
·
Take out bearing & the
shaft.
·
The salt contamination must
be removed by thoroughly washing with clean fresh water (preferably warm)
or if possible , distilled water.
·
De-greasants must be used
if it is found that there was ingress of oil.
·
Spirits or alcohol may be
used to clean contacts.
·
Dry the motor with dry air
& then switch on its heaters or use powerful lamps. During this process,
keep its inspection covers open to permits moisture to escape.
·
Put the varnish on the
winding & let it dry.
·
Check the insulation
resistance. If it is ok then assemble it back & put it for use.
·
The motor must be started
with less load and the current is monitored for few hours to confirm its
satisfactory operation.
Q. what checks to be
made when motor is running hot.
a. Single-phasing
b. Fan is working or not.
c. Overload is occurring.
d. Check condition of bearing
How can we find the type of connection in the terminal box?
Star
Delta
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