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What is the work way of Motor run Capacitor ?

Capacitor is a device for storing an electrical charge.In a way, a capacitor is a little like a battery. Although they work in completely different ways, capacitors and batteries both store electrical energy. If you have read how batteries work, then you know that a battery has two terminals. Inside the battery, chemical reactions produce electrons on one terminal and absorb electrons at the other terminal. A capacitor is a much simpler device, and it cannot produce new electrons — it only stores them. A capacitor consists two metal plates insulated from each other. When opened, the inside looks like two sheets of foil with wax paper in between them and rolled tight, similar to a roll of paper towel. Years ago the oil filled type used PCB's as a coolant. Today most capacitors are the dry type.

Capacitors are charged with electricity, then releases its stored energy at a rate of sixty times per second in a 60 cycle alternating current system.The sizing is critical to motor efficiency just as sizing of batteries is critical to a radio. For example, a radio that requires a 9V battery will not work with a 1.5V size battery. Thus, as the battery becomes weaker the radio will not play properly. A motor that requires a 7.5 mfd capacitor will not work with a 4.0 mfd capacitor. Much the same way, a motor will not run properly with a weak capacitor. This is not to imply bigger is better, because a capacitor that is too large can cause energy consumption to rise. In both instances, be it too large or too small, the life of the motor will be shortened due to overheated motor windings. Motor manufacturers spend many hours testing motor and capacitor combinations to arrive at the most efficient combination. There is a maximum of +10% tolerance in microfarad rating on replacement start capacitors, but exact run capacitors must be replaced. Voltage rating must always be the same or greater than original capacitor whether it is a start or run capacitor. Always consult manufacturers to verify correct capacitor size for the particular application.

There are two basic types which are used in electric motor :
1)Run capacitorsare rated in a range of 3-70 microfarad (mfd).Run capacitorsare also rated by voltage classification. The voltage classifications are 370V and 440V. Capacitors with ratings above 70 microfarad (mfd) are starting capacitors.Run capacitorsare designed for continuous duty, and are energized the entire time the motor is running. Single phase electric motors need a capacitor to energize a second phase winding. This is why sizing is so critical. If the wrong run capacitor is installed, the motor will not have an even magnetic field. This will cause the rotor to hesitate at those spots that are uneven. This hesitation will cause the motor to become noisy, increase energy consumption, cause performance to drop, and cause the motor to overheat.

Themotor run capacitorsusually have two styles. one is metallized polypropylene AC / motor-run capacitors which are used in most of the older equipment. Large round metal cans. The other ismetallized polypropylene film capacitorsthat are much smaller and come with a mounting tab that can be screwed anywhere on the frame. of the unit. Both types offer the exact same qualities and electrical characteristics. Most important is to get the exact electrical ratings for the capacitor and then the case style.

2) Starting capacitors are housed in a black plastic case and have a mfd range as opposed to a specific mfd rating on run capacitors. Start capacitors (ratings of 70 microfared or higher) have three voltage classifications: 125V, 250V, and 330V. Examples would be a 35 mfd at 370V run capacitor and an 88-108 mfd at 250V start capacitor. Start capacitors increase motor starting torque and allow a motor to be cycled on and off rapidly. Start capacitors are designed for momentary use. Start capacitors stay energized long enough to rapidly bring the motor to 3/4 of full speed and are then taken out of the circuit.

Introduction of Motor

An electric motor is basically composed of windings around a magnet. Motors are either multi-phase or single-phase. Multi-phase motors generate starting torque along the various windings by applying out of phase voltages to each winding in a pattern that generates a torque force in the desired direction. Single-phase motors must generate the same starting torque however they have only one phase to work from. This means they have to have a method to generate a shifted version of the single phase voltage to send to one of their windings.

There are three common methods of creating single-phase electric motors: capacitor start, split-phase, and shaded pole. Each other these motors has some method to provide starting torque to the motor by shifting the voltage given to one of the windings on the motor by some angle. This phase shift corresponds to one winding of the motor having a voltage before another coil. The difference in time between when one coil has a voltage and when a second coil has a voltage causes the torque force and begins the movement of the motor.

To start to solve why capacitive start motors work we can generalize Ohm's Law, V = IR, and say that V = IZ where Z is a generalized impedance. The impedance is composed of an the inductance, capacitance, and resistance. Inductance will cause the current to lag the voltage, capacitance will cause the current to lead the voltage, and resistance has no effect on the timing between the current and voltage.

The Function of The Start and Run Capacitor used for Motors

Most smaller, single phase motors usually have a permanent magnet armature that is pushed or pulled around by the rotating inductive field produced by the stator (outside) windings. The inductive field rotates simply as a result of the positive or negative alternations of the 60HZ AC current flowing through the windings. The problem is that when the voltage is applied, the 60HZ is applied immediately, the rotation of the field through the windings begins immediately, and the armature has no chance to react (or catch up, as it were) to the field.

The start Capacitor provides that electrical "push" to get the motor rotation started. It does this by creating a current to voltage lag in the seperate start windings of the motor. Since this current builds up slower, the armature has time to react to the rotating field as it builds up, and to begin rotating with the field. Once the motor is very close to it's rated speed, a centrifugal switch disconnects the start cap and start windings from the circuit. Watching a single phase motor starting you can see that this all happens very quickly.

Without a start Capacitor(such as when one burns up) when the voltage is applied, the motor will just sit and hum. But if you were to grab the shaft and give it a spin, the motor would start and run normally.The run capacitor and aux. windings never drop out of the circuit in a cap-start, cap-run motor. The current to voltage lag is always present, which makes the motor act like a two phase motor. The advantage of a cap-start, cap-run motor over a cap-start, induction-run motor is that cap-run motors operate at a higher power factor than induction-run. Single cap-start / run motors use only one capacitor and are for lower torque applications. Two value cap-start / run motors use two capacitors, a higer value capacitor for starting, and lower value for running. The centrifugal switch switches from the high capacitor to the low capacitor, but the aux windings never leave the circuit. Two-cap motors are for higher torque applications.

In a purely capacitive circuit the current will lead the voltage by 90 degrees. In a purely inductive circuit the current will lag the voltage by 90 degrees. Of course, there is always some resistance in any circuit, so whether inductive or capacitive, the lead / lag relationship between current and voltage will be somewhere between 0 and 90 degrees. I say all that to say this; the net effect of the difference in the capacitor is that it will change the angle of lead / lag between voltage and current in your motor.

In summary, the capacitor provides a delay in the energy given to one of the windings. This delay causes the forces of the motor to be unbalanced and the motor then starts. Economically, capacitor start motors are often more costly due to the inclusion of the capacitor however they have the most starting torque This means that you probably have one in your refrigerator, washer, dryer, or other application where you may need a lot of starting force but you won't find them in your electric fan.

In addition,potential relays are also important. Potential relays are used to electronically connect and disconnect to starting capacitors from the motor circuit . Each relay has a specific voltage rating to place the start capacitor in series with the start winding and a specific voltage to take it out of the circuit. Each rating is based on the electromagnetic field generated by the rotation of the motor. The motor manufacturer studies the effect of placing in and taking out the capacitor to increase starting torque with as little winding flex as possible. Potential relays have four ratings:
(1) continuous coil voltage,
(2) minimum pick-up voltage,
(3) maximum pick-up voltage,
(4) drop out voltage.
A potential relay is difficult to check and should always be replaced when a start capacitor is replaced. The exact size designed for that particular motor must be reinstalled. The potential relay must also be replaced if contacts are found to be open.

Souce from :

http://www.capacitors-china.com

About the Author

Manufacturer of Motor Capacitor in China