Single phase Induction Motor / Capacitor start capacitor run motor / Capacitor start induction motor

Summary

The 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

00:00

🔧 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.

05:04

🔄 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.

Keywords

💡Single Phase Induction Motor

A single phase induction motor is an electrical motor that operates on a single-phase AC power supply. It is the main subject of the video script. The script discusses why these motors require a special setup to start and run properly, as they face challenges in creating a rotating magnetic field due to the single-phase power supply. The video explains how the use of capacitors helps overcome this issue.

💡Capacitor

A capacitor is an electronic component that stores electrical energy in an electric field. In the context of the video, capacitors are crucial for starting single phase induction motors and, in some cases, for maintaining their operation. The script mentions a 'start capacitor' and a 'run capacitor,' which serve different functions in the motor's startup and running phases.

💡Magnetic Field

The magnetic field is a vector field that describes the magnetic influence of electric currents and magnetic materials. The video script explains the importance of a rotating magnetic field for the operation of an induction motor. In single-phase motors, creating this rotating field is challenging without the assistance of a capacitor, which provides the necessary phase difference in the current.

💡Rotor

The rotor is the rotating part of a motor that turns as a result of the forces exerted by the motor's magnetic field. The script discusses how the rotor of a single phase induction motor needs to be set into rotation by the magnetic field, which is facilitated by the use of a capacitor to create the required phase difference in the motor's windings.

💡Winding

Winding refers to the coils of wire wrapped around the core of an electrical device, such as a motor or transformer. In the script, the windings are connected to a single-phase supply and are crucial for generating the magnetic field needed for motor operation. The video mentions an 'auxiliary winding' or 'start winding,' which works in conjunction with a capacitor to start the motor.

💡Phase Angle

The phase angle is the angle by which one alternating quantity leads or lags behind another of the same frequency. The video script emphasizes the importance of creating a phase angle difference between the current in the motor's windings and the supply voltage to generate a rotating magnetic field. This is achieved by using a capacitor, which introduces a current that leads the supply current by 90 degrees.

💡Power Factor

Power factor is the ratio of real power to apparent power in an AC circuit and is a measure of how effectively the current is doing work. The script mentions the power factor being at one in the case of a pure resistive load, which simplifies the explanation of the motor's operation. However, in practical scenarios, the inductive load of the motor affects the power factor.

💡Centrifugal Switch

A centrifugal switch is a type of switch that operates based on centrifugal force, often used in motors to disconnect certain components once the motor reaches a certain speed. In the script, it is used to disconnect the start capacitor from the auxiliary winding after the motor has started, preventing the capacitor from operating continuously.

💡Torque

Torque is the force that can cause an object to rotate about an axis. The video script discusses the need for torque to start the rotation of the rotor in a single phase induction motor. The capacitor provides the necessary phase difference in the current to generate the torque, which is essential for the motor's startup.

💡Three-Phase Supply

A three-phase supply is an electrical power supply that is provided by three alternating currents out of phase with each other by one-third of a cycle. The script contrasts the operation of single-phase and three-phase induction motors, noting that three-phase motors do not require capacitors because their supply naturally provides the rotating magnetic field needed for operation.

💡Inductive Load

An inductive load is a type of electrical load that causes current to lag behind voltage, such as that found in motors and transformers. The script mentions that the inductive load of the motor's windings causes the current curve to shift with an angle related to the voltage curve, affecting the power factor and the motor's operation.

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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