How
do I calculate capacitor value for single phase motor?
We normaly replace by same value
then that value is never calculated, if I were you and the old capacitor cannot
be read I will buy the maximum capacity (for a voltage higher than grid)
allowed in the compartment, the higher the capacitance the lower the voltage
accross the capacitor, and see what happens. Remember that the whole purpose of
starting winding is to get an mmf around 90 degrees out of phase with grid,
that will not be acchieved but the higher the capacitance the easier the start.
What is a motor start capacitor?
With single phase motors there is no impetus to cause the shaft
to turn when power is applied. Nor is there a preferred direction of rotation
if by chance it does start (e.g. when pushed).
It is commonly applied with a starter capacitor across one winding of the
armature to advance its phase a few degrees. This is enough to make the motor
start and to cause it to rotate in a preferred direction.
Leaving a starter cap in place makes a small amount of phase imbalance all the
time so it wastes some power. Better motors and larger motors have centrifugal
switches so that the starter cap is removed by the switch opening once the
motor reaches speed. Thus achieving maximum efficiency.
Why are 3-phase induction motors self-starting, but
3-phase synchronous motors are not?
3 Phase Induction Motor-
Expression for Torque-
T= (V^2)* (R2)/(w syn)((R eq)^2 +(Xeq)^2)s
Where V- Equivalent Thevenin resistance
R2- Rotor resistance referred to stator
w syn- Synchronous angular speed
R eq- Equivalent Thevenin Resistance plus Rotor resistance at given slip s
X eq- Equivalent Thevenin Reactance plus Rotor Reactance
3 phase Synchronous Motor/Generator
Torque T= V*Ef*sin(∆)/ Xs
Where
V- Applied Voltage
Ef- Induced Voltage
∆- Load/Torque Angle
Xs- Synchronous reactance
Thus when field excitation(Ef) is zero, then T for synchronous machine is zero
but for induction machine T isn't equal to zero!
Thus induction machines are self starting wheres Synchronous machines aren't!
:)
Tabi Mir, Electrical
Engineer, M.Tech Student, passionate about my profession
[A2A]
Short answer: because synchronous motors and induction motors operate on
different principles.
Long answer:
Consider a 3-phase induction motor: A 3- phase supply to the armature produces
a rotating magnetic field. This flux being linked to the rotor coils, induces
voltage and produces current in the rotor. The current carrying rotor being
placed in a magnetic field, experiences a torque and hence begins to rotate.
Do you observe any problem in starting? No!
Now, consider a 3-phase synchronous motor. A 3-phase supply is given to the
armature, producing a rotating magnetic field. However, the rotor has its own
field produced by a D.C current flowing through the rotor windings. This rotor
field tends to allign itself along with the rotating magnetic field produced by
the stator (armature) i.e north ( of rotor) has to lock with the south (of
stator). So what happens exactly? The north of rotor tries to chase the south
of stator. But the stator magnetic field is rapidly rotating, and before the
north of rotor could lock with south of stator, the stator field has shifted
position so that its north (stator) comes in the vicinity of north ( rotor) and
they repel. Because the rotor has certain inertia and the speed of the rotating
magnetic field is too fast for it to catch up, it ends up vibrating (caught
between north-south-north-south......so on of the stator). So it fails to
start. So what do you do?
1) give a reduced frequency supply to the stator, this will reduce the speed of
rotation of the stator magnetic field and the rotor will easily catch up...once
the rotor catches up you may increase the frequency.
2) manually rotate the rotor till it catches some speed and eventually locks
the rotor field with the stator field.
3) use amortsieur windings: the concept is to start the motor as an
induction motor. Initially no DC field excitation is given and the motor
operates as an induction motor. Once it attains some speed, d.c excitation is
given and the rotor field alligns itself with the stator field, and rotor attains
synchronous speed.
Remember , in an induction motor, torque is produced due to the interaction of
induced current and flux. On the other hand, in a synchronous motor , torque is
produced due to interaction between two independent fields!
3 Phase Induction Motor-
Expression for Torque-
T=
(V^2)* (R2)/(w syn)((R eq)^2 +(Xeq)^2)s
Where V- Equivalent Thevenin resistance
R2- Rotor resistance referred to stator
w syn- Synchronous angular speed
R eq- Equivalent Thevenin Resistance plus Rotor resistance at given slip s
X eq- Equivalent Thevenin Reactance plus Rotor Reactance
3 phase Synchronous Motor/Generator
Torque T= V*Ef*sin(∆)/ Xs
Where
V- Applied Voltage
Ef- Induced Voltage
∆- Load/Torque Angle
Xs- Synchronous reactance
Thus when field excitation(Ef) is zero, then T for synchronous machine is zero
but for induction machine T isn't equal to zero!
Thus induction machines are self starting wheres Synchronous machines aren't!
:)
The basic aim
of any electrical machine is to generate sinusoidal mmf & hence flux. The
operation principle of 3-phase induction motor is that when 3-phase balanced
supply is given to stator, a rotating magnetic field (rmf) is produced & it
cuts the stationery rotor conductors & hence emf (& current) is induced
in rotor (rotor is short circuited). Due to this rotor current, rotor mmf &
hence flux is induced, which tries to nullify the cause (Lenz’s law) & so
the rotor rotates so as to reduce the relative motion between rmf & rotor
speed. So it is a self starting machine.
When a 3-phase
synchronous motor is concerned, it is a doubly excited machine, rather than an
induction machine. The rotor is supplied with a DC supply & stator is given
a 3-phase balanced supply. The stator will generate rmf as in induction motor
case. The rotor poles wont change as DC supply is given. The alignment of rotor
poles with stator rmf gives us desired speed. If the rotor rotates initially,
it aligns with stator rmf & so always runs with synchronous speed. If rotor
is stationary, it becomes difficult for the rotor poles to catch up with the
rmf speed & fails to rotate. So, they are provided with damper windings to
generate starting torque (Induction motor principle) & in running
conditions rotor aligns with rmf.
In synchronous machines the stator winding or armature winding
is excited by alternating current,therefore a rotating magnetic field is
developed in it which is moving at synchronous speed.the field winding is
excited by a dc source.a stationary field will be produced.the rotating stator
cores will interact with the stationary rotor poles.during the positive half it
will rotate in one direction but in the next half cycle the rotating stator
cores will change their position and rotor is urged to rotate in opposite
direction.due to the rotor inertia,it fails to respond to the change in the
direction of electromagnetic torque.that's why it has no starting torque.
But in Case of Induction motor the stator winding or armature is exited by
alternating current. But the rotor conductor are short circuited. the three
phase supply of stator created a flux called stator flux. Which is produced a
rotating magnetic field in side the machine. By induction principle the
flux also created a voltage & current in the rotor. this induced current
also produce a flux called rotor flux. this also created rotating
magnetic field.with interaction with rotor and sator flux a torque is produce
in rotor. And the rotor rotates freely that's why poly phase induction motor is
self staring.
Why inductor is not used instead of capacitors in
single phase induction motors for starting?
If inductor is used, the motor would start but why will you want
to use an inductor (or increase inductance of one of the windings?) which
will not only increase the cost but will also make the motor bulkier as number
of turns of coils will increase. Also, the starting performance won't be better
either (similar to a resistance split-phase motor as one winding will now have
slightly higher resistance. You will have to take in the design consideration
also while designing the starting arrangement. The power factor during such
starting will decrease further for the motoring load.
In 1-phase
induction motors, we are unable to get starting torque, So we mostly use
permanent capacitor split phase starting method in ceiling fans. Its main aim
is to split the single phase supply into 2 phases, having time displacement of
90 degrees, also mechanically the 2 windings are placed 90 degrees in space. It
gives an rmf which produces a starting torque & makes it a self starting
motor. So, the capacitor used in the other auxiliary winding mainly provides
this 90 degrees time displacement. An inductor can also be used, but it has the
following drawbacks-
1. Bulky size of
the inductor.
2. Leakage
reactance of it affects the value of the starting torque.
3. Its delay in
current affects the transients.
hope it
was clear...
Priam Gupta, Studying
Electrical and Electronics Engineering
5
We know that the supply current
already lag the supply voltage by angle close to 90deg. because of inductance
in the transmission line.
Therefore if we use an inductor
for starting purpose the only thing that would happen is the impedence would
increase and the motor wouldn't start because of the absence of any split
phase.
Whereas in case
of capacitor the current would lead the voltage hence there will presence of
phase split in the single phase supply which would start the motor.
Can we use inductor instead of capacitor to give phase
difference for starting of a single phase induction motor?
Since in a
motor,the current lags the voltage so for that reason the motor cannot self
start.
To make it self
start,the current should lead the voltage.
For this we are
using capacitor,because capacitor has an leading power factor.
If you use an
inductor,it will make it worse for the motor to start.
Because inductor has lagging
power factor.
If single phase motors (capacitor start induction run
motors) are not self starting, then why is a direct online (D.O.L.) starter not
connected to it?
For industrial applications, all motors are fed power through
motor control centres which essentially consist of multiple direct online
starters for the various motors in the plant. As far as I know, DOL is a method
of closing the power circuit which supplies power to the motor when a power
button (remotely located) is pressed.
Please understand that a single-phase induction motor does not start even if
the DOL circuit is closed. So, voltage will exist across the circuit but the
motor won't start due to the double revolving magnetic field which cancel out
each other's effect and hence torque would be zero. So, DOL starting is the
norm in industrial applications, but a capacitor is always needed to put the
motor in motion.
First of all, single-phase induction motors are not
self-starting. For making a single-phase IM self starting, capacitor,
resistance etc are used. This is known as phase splitting. Phase Splitting
means, a single phase is split into two phases making the motor self-starting.
It depends on how well you split two phases that the motor will have better
starting torque.
Now, D-O-L starting is directly putting the motor across the mains using a
switching arrangement and a overload relay. This can be good for three-phase
self starting IM. But if a capacitor start single-phase IM is used, there is no
need to use another starter for it. However it can be used as a safety
arrangement during starting.
First of all please bear in mind
that single phase motors are not self starting.. And it is made to start only
through phase splitting, simply making another phase to counter act the effect
of double revolving magnet field. This can be achieved through providing
another winding of resistance or inductance or capacitance. So in practical
sense single phase motors are having two phase winding. On the other hand DOL
starters are devices having some switching mechanism and overload protecting
devices .These are not supposed to provide the required starting torque for
single phase motors. DOL starters are only used with lower HP motors for making
its starting process smooth and safe..So there is no use at all to connect DOL
starters for starting the single phase motor..
Will a phase 3 electric motor work on single phase?
Phil Karn, Electrical
engineer, radio ham, computer networking engineer
Yes,
but poorly and with caveats.
The
first caveat is that the motor won't start by itself. If you don't do anything
it will simply sit there and burn up.
If you
give it a twist it will start and run in that direction but with considerably
less efficiency and output power. It will probably vibrate more than usual.
There's an interesting
application for a 3-phase motor running on single phase, because the third
(unused) terminal develops a voltage that reproduces 3-phase AC power. The
voltage isn't balanced so it's not very good 3-phase, but many
people use this phenomenon to make a simple rotary phase converter. You take a
relatively large 3-phase AC motor, connect it to single phase AC and provide a
means to start it, and then you run it continuously without load just to
generate the other two phases to run other, smaller 3-phase motors in a machine
shop. They won't run as well as on true 3-phase AC, but the method is very
simple and often sufficient for many users unable to afford a true phase
converter.
What is single phasing in 3 Phase Motor?
Sudden
disappearance of a phase (any one of three phases) while the induction motor is
running is know as single phasing. Actually the loss of current (no
supply voltage in that phase means no supply current) in a phase is
called single phasing.
The causes of
Single Phasing is
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
may be damage or broken.
The effects of
Single Phasing
1) Due to
single phasing the current in the remaining two phases 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 winding may melt due to
overheating and can give a fatal shock to the personnel.
The 3 phase
motors must be connected to rated voltage and load for proper working. If due
to some reasons, one phase of the motor gets disconnected, the motor will
continue to run from the active 2 phase supply. This is called Single phasing.
If the 3 phase
motor is running and then one of the phase gets disconnected, the motor will
continue run with vibrations and reduced speed. However depending on the
loading conditions, the motors may/may not start on two phases.
Single phasing
is not desirable for the proper operation of the induction motors and
appropriate measures should be taken to protect the machine.
Causes:
1. One of the
three back up fuses blows (or fuse wire melts).
2. One of the
contactor for motor is open circuited.
3. Wrong setting
of the protection devices provided on the motor.
4. Relay contacts
may be damage or broken.
Effects of
single phasing:
1. The motor runs
with a reduced speed.
2. It operates
with an uneven torque and produces a humming noise.
3. Due to the loss
of current from one phase the current flowing through the remaining 2 phases
increases. The winding insulation, at times, may not be designed to withstand
the increased current and heat thereby damaging the insulation and causing a
short circuit between the windings and the motor burns out.
4. It may cause
overloading of the generator.
5. 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.
Protection
devices:
Overload relay: All the three
phases of the motor are fitted with an overload relay. If there is any increase
in the value of the current then this relay activates automatically and the
motor trips.
Thermistors: They are used
along with the electromagnetic relay and inserted in the 3 windings of the
motor. They sense the increase in the temperature and a signal is sent to the
amplifier which in turn amplifies the current signal and operates the relay
coil resulting in tripping.
For working of three phase motor, we need a three phase supply.
The supply is given to each of the phases. Once the motor starts rotating, if
one of the phases gets disconnected, the motor will continue to run with the
rest of the two running phases. This effect of loss of Current through one of
the phases is called as '' single phasing '' in motors.
Single phasing may occur due to any reason that results in opening of one of
the phases. The reasons can be,
1.
Blowing or melting of one of the fuses in the phase
2. Any
malfunctioning in the contactor or relay connected to a phase resulting loss of
supply to that phase etc.
Due to single phasing, the current in one phase becomes zero. So, the rest of
the healthy phases carry more current to meet the same load demand,
approximately 2.5times the previous value or rated value. Due to this,
overloading occurs in the other two phases. If the load is small, then the
motor will rotate normally without any damage to itself. But if the load is
high, then the motor will burn due to excessive current flowing in the phases,
in case if the overload relay fails to work. So, protective devices should be
employed to avoid the effects of single phasing in case of three phase motors.
