Single phase Induction Motor / Capacitor start capacitor run motor / Capacitor start induction motor
Summary
TLDRThe video script explains the necessity of capacitors in single-phase induction motors for starting and, in some cases, continuous operation. It illustrates the challenge of creating a rotating magnetic field with a single-phase supply and how a start capacitor, connected to an auxiliary winding, provides the needed phase difference to initiate rotation. A centrifugal switch disconnects the start capacitor once the motor reaches speed. In situations where additional torque is needed, a running capacitor can be added for continuous operation, highlighting the contrast with three-phase motors that inherently produce a rotating field without the need for capacitors.
Takeaways
- 🌀 A single phase induction motor requires a start capacitor to create a rotating magnetic field, which is essential for the rotor to begin rotating.
- 🔌 The start capacitor is connected in series with an auxiliary winding and in parallel with the main winding, providing the necessary phase difference for a short period during start-up.
- ⚙️ A centrifugal switch is used to disconnect the start capacitor once the motor reaches approximately 75% of its full speed, ensuring the motor operates efficiently.
- 📈 The current from the start capacitor leads by 90 degrees relative to the motor winding's current, which is crucial for establishing the rotating magnetic field.
- 🔧 In some cases, a running capacitor is also used in addition to the start capacitor to provide continuous out-of-phase current and maintain higher torque throughout the motor's operation.
- 🔄 The running capacitor has a lower microfarad value compared to the start capacitor and remains connected to the auxiliary winding for the motor's entire operation.
- 📊 The motor's power factor is at its highest (one) when it's under pure resistive load, simplifying the understanding of the motor's operation without the complexity of inductive load.
- ⚡ The rate of change of the supply voltage affects the capacitor's current; it's maximum when the voltage rate of change is at its peak, and minimum when the voltage is at its peak or trough.
- 🌐 Unlike single phase motors, three-phase induction motors do not require capacitors because their three phases naturally provide the 120-degree separation needed for a rotating magnetic field.
- 🛠️ The auxiliary winding, also known as the start winding, is introduced to work in conjunction with the start capacitor to create the phase difference needed for motor start-up.
- 🔄 The motor's operation under single phase supply is fundamentally different from three-phase supply, necessitating additional components like capacitors for successful operation.
Q & A
Why does a single phase induction motor require a capacitor to start?
-A single phase induction motor requires a capacitor to start because it needs to create a rotating magnetic field. The capacitor provides an additional current with a different phase angle, which is necessary to initiate the rotation of the motor's rotor.
What is the role of the start capacitor in a single phase induction motor?
-The start capacitor is used to provide a phase angle difference between the motor's voltage supply and the current in the auxiliary winding. This phase difference helps create a rotating magnetic field, which is essential for the motor to start.
Why can't a single phase induction motor start with just its resistive load?
-A single phase induction motor cannot start with just its resistive load because the resistive load does not provide the necessary phase shift in the current to create a rotating magnetic field. The motor needs an inductive reactance, which a capacitor can provide, to achieve this.
What happens when the motor reaches around 75% of its full rotating speed after starting?
-Once the motor reaches approximately 75% of its full rotating speed, the start capacitor is disconnected from the auxiliary winding by a centrifugal switch. The motor can then continue to run on its own, as the rotor's conductors cut through the rotating magnetic field to generate torque.
How does the capacitor current behave during the start of the motor?
-During the start, the capacitor current is at its maximum when the rate of change of the supply voltage is at its maximum. This provides the necessary current to create a rotating magnetic field. The capacitor current leads the single phase line current by 90 degrees, which is the required phase difference.
What is the purpose of the centrifugal switch in a single phase induction motor?
-The centrifugal switch is used to disconnect the start capacitor from the auxiliary winding once the motor reaches its predefined speed. This is necessary because the start capacitor is not designed to run continuously and could cause damage if left connected.
Why do some single phase motors also need a running capacitor?
-Some single phase motors require a running capacitor to maintain adequate torque throughout their operation. The running capacitor provides an additional out-of-phase current, which helps the motor to run with higher torque even after the start capacitor is disconnected.
How is the run capacitor different from the start capacitor in a single phase induction motor?
-The run capacitor has a much lower microfarad value compared to the start capacitor. It is designed to operate continuously with the motor, providing the necessary out-of-phase current for maintaining torque, whereas the start capacitor is used only for a short period during startup.
Why don't three-phase induction motors require a capacitor like single phase motors do?
-Three-phase induction motors do not require a capacitor because their three-phase supply inherently provides the necessary 120-degree phase differences between the phases. This naturally creates a rotating magnetic field that can drive the motor's rotor without the need for additional components like capacitors.
What is the significance of the phase angle difference provided by the capacitor in a single phase induction motor?
-The phase angle difference provided by the capacitor is crucial for the motor's operation. It allows the motor to generate a rotating magnetic field, which is essential for starting and, in some cases, for maintaining the motor's torque during operation.
Outlines
🔧 Understanding Single Phase Induction Motors and Start Capacitors
This paragraph explains the necessity of capacitors in single phase induction motors. It begins by describing the basic setup of a single phase motor and its connection to a 230-volt supply. The script discusses the challenge of creating a rotating magnetic field with a single phase supply, which normally results in a non-rotating magnetic field due to the lack of phase difference. To overcome this, an auxiliary winding with a start capacitor is introduced to provide the necessary phase shift for the magnetic field to rotate and initiate the motor's rotation. The paragraph also explains the role of a centrifugal switch in disconnecting the start capacitor once the motor reaches approximately 75% of its full speed, ensuring the motor operates efficiently.
🔄 The Role of Running Capacitors in Single Phase Induction Motors
The second paragraph delves into the use of running capacitors in single phase induction motors. It addresses situations where the motor's torque requirement exceeds what the start capacitor can provide, necessitating the introduction of a running capacitor. This capacitor, of a smaller value than the start capacitor, is connected in parallel to the auxiliary winding and remains active throughout the motor's operation to provide continuous torque support. The paragraph also clarifies the difference between start and running capacitors, noting that start capacitors cannot operate continuously and must be disconnected by a centrifugal switch, while running capacitors are designed for continuous use. The script concludes by emphasizing the importance of capacitors in ensuring the smooth and efficient operation of single phase induction motors.
Mindmap
Keywords
💡Single Phase Induction Motor
💡Capacitor
💡Magnetic Field
💡Rotor
💡Winding
💡Phase Angle
💡Power Factor
💡Centrifugal Switch
💡Torque
💡Three-Phase Supply
💡Inductive Load
Highlights
A single phase induction motor needs a capacitor at the start to provide the necessary phase angle difference.
Some single phase motors need a running capacitor that operates throughout the motor's running time.
A single phase supply with 230 volts is used for the winding of an induction motor.
Inductive load in the motor causes the current curve to shift with an angle related to the voltage curve.
In a pure resistive load condition, the power factor is one, and voltage and current curves start and end at the same time.
A single phase connection does not create a rotating magnetic field; instead, north and south poles change rapidly.
To create a rotating magnetic field, a winding with a different phase angle related to the present single phase voltage supply is needed.
Three-phase supply motors do not face this issue because the three phases are 120 degrees apart, providing a rotating magnetic field.
For single phase supply motors, an additional current with a different phase angle is needed for a short period to start the rotor.
A capacitor's current normally leads 90 degrees related to the current of a pure resistive load, providing the necessary phase angle difference.
An additional winding, called an auxiliary or start winding, is introduced and fitted with a start capacitor.
A centrifugal switch isolates the start capacitor from the auxiliary winding once the motor reaches a predefined speed.
Capacitor current leads the single phase current by 90 degrees, creating a two-phase supply for the motor with a 90 degrees phase angle difference.
Run capacitors provide additional out of phase current for continuous running and relatively higher torque.
Start capacitors are disconnected after a few seconds by the centrifugal switch, while run capacitors can run continuously.
Transcripts
good day to everyone today we will see why a single phase induction motor
needs a capacitor at the start further some single phase motors need a running capacitor too
which will operate throughout its running time to start with you will see a single
phase supply with 230 volts and a winding of an induction motor connected to it
wave pattern of the supply voltage and the single phase current shown on the graph here
now what we need is to get the rotor of the motor to rotate like this when power supply is connected
normally an induction motor has some induction load also acting on it in addition to the
resistive load on its windings this inductive load makes its current curve to shift with an angle
related to the voltage curve but here we have taken the condition where the motor is with pure
resistive load only it is for easy understanding and this condition is when power factor is at one
so you will see both voltage and current curves start at the same time with zero value and ends to
a zero value also at the same time actually with a single phase connection magnetic field doesn't
rotate instead north and south poles are changing rapidly with a speed of 50 times per second if the
frequency of the supply is 50 hertz pulling power or the torque to start the rotation of the rotor
is not provided here with single phase connection here we have shown only one
pair of poles with windings and one would think here that increasing of pole pairs
may provide the much needed rotation of the magnetic field but it will not happen because all
the additional pull pairs provided here are with the same phase angle and what we need to do is
to provide a winding with a different phase angle related to the present single phase voltage supply
on a three-phase supply this issue doesn't arise since all three phases are with 120 degrees apart
providing the necessary rotating magnetic field for the rotor of the motor to rotate
but for single phase supply we need to provide an additional current with a different phase angle
only for a short period of time around three to five seconds till the rotor of the motor gets
around 75 percent of its full rotating speed you know the conductors on the rotor need to cut the
rotating magnetic field to provide the necessary torque required to rotate the rotor of the motor
if you consider a capacitor its current normally leads 90 degrees related to the current of a pure
resistive load so if you use a capacitor at the start of this motor it will definitely provide
the necessary phase angle difference required to provide the rotation of the magnetic field
for this we need to introduce an additional winding to the motor
like this and will be fitted with a capacitor called start capacitor
this additional winding is normally called auxiliary winding or start winding you will
see now this starting capacitor is fitted series to the start winding but is parallel
to the main winding further we need to introduce a special switch to isolate this capacitor from the
auxiliary winding when needed rises this can be done by providing a centrifugal switch and after
reaching the predefined speed start capacitor will be disconnected from the auxiliary winding
although current will not directly flow through a capacitor when connected to an alternating
voltage there will be electrons or so called negative charge flow in
and out of the capacitor continuously at this point there will be an instant current flow
out of the capacitor and into the capacitor but in the opposite direction of the negative charge
here on the graph it shows the current produced by the capacitor and is leading 90 degrees to
the single phase current of the motor winding like in the three phase supply motor is getting
a two phase supply now with a 90 degrees phase angle difference this will last only for a few
seconds but this period is more than enough to make the magnetic field to rotate making single
phase induction motor to run on its own without any starting issue further you will see that
the capacitor current is at its maximum when the rate of change of supply voltage is at its maximum
although voltage value is zero now rate of change of voltage shown in black line is at
its maximum here and is climbing upwards so you get the maximum capacitor current at this point
when voltage value is at its maximum you will see the rate of change of voltage is
at zero and is about to turn downwards now since rate of change of voltage is at zero
capacitor doesn't deliver any current at this point and the value of current is at zero here
when the rate of change of voltage is at its minimum to voltage value shows zero
at this point capacitor delivers its minimum current in fact it is the same value as its
maximum but flowing on the opposite direction when motor is started capacitor delivers its current
first starting from zero after 90 degrees past and when capacitor current reaches its maximum value
single phase line current starts with zero value so it is clear capacitor current is
leading here by 90 degrees with the required phase difference for rotating magnetic field
within few seconds after start centrifugal switch disconnects start capacitor
but induction motor will run with its rotor conductors cutting the rotating magnetic field
continuously there will be instances where the torque requirement of the single phase induction
motor is not adequate with start capacitor boosting it at the start only on these occasions
we can introduce another capacitor to the auxiliary winding for continuous running
like this and it will be with much less value related to the start capacitor microfarad value
now you will see even after the start capacitor is disconnected motor runs with run capacitor
intact with the auxiliary winding providing the additional out of phase current needs to run this
motor with relatively higher torque run capacitors are capable of running continuously with the motor
but start capacitors cannot run continuously and need to be isolated with the centrifugal switch
within few seconds hope you got fair idea now why capacitors are provided to run single phase
induction motors and why three phase induction motors just don't need any capacitor on it
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