Table of Contents

Seminar on Electromagnetic Clutch Free pdf Report Download

A clutch is a mechanism for transmitting rotation, which can be engaged and disengaged. Clutches are useful in devices that have two rotating shafts. In these devices, one shaft is typically driven by motor or pulley, and other shaft drives another device. The clutch connects the two shafts so that they can either be locked together and spin at the s ame speed (engaged), or be decoupled and spin at different speeds (disengaged).

The clutch disc (centre) spins with the flywheel (left). To disengage, the lever is pulled (black arrow), causing a white pressure plate (right) to disengage the green clutch disc from turning the drive shaft, which turns within the thrust-bearing ring of the lever. Never will all 3 rings connect, with any gaps.

Electromagnetic clutches operate electrically, but transmit torque mechanically. This is why they used to be referred to as electro-mechanical clutches.

CONSTRUCTION

A horseshoe magnet has a north and south pole. If a piece of carbon steel contacts both poles, a magnetic circuit is created. In an electromagnetic clutch, the north and south pole is created by a coil shell and a wound coil. In a clutch, when power is applied, a magnetic field is created in the coil. This field (flux) overcomes an air gap between the clutch ro tor and the armature. This magnetic attraction, pulls t he armature in contact with the rotor face. The frictional contact, which is being controlled by the strength of the magnetic field, is what causes the rotational motion to start. The torque comes from the magnetic attraction, of the coil and the friction between the steel of the armature and the steel of the clutch rotor. For many industrial clutches, friction material is used between the poles. The material is mainly used to help decrease the wear rate, but different types of material can also be used to change the coefficient of friction (torque for special applications). For example, if the clutch is required to have an extended time to speed or slip time, a low coefficient friction material can be used and if a clutch is required to have a slightly higher torque (mostly for low rpm applications), a high coefficient friction material can be used.
In a clutch, the electromagnetic lines of flux have to pass into the rotor, and in turn, attract and pull the armature in contact with it to complete clutch engagement. Most industrial clutches use what is called a single flux, two pole design. Mobile clutches of other specialty electromagnetic clutches can use a double or triple flux rotor. The double or trip flux refers to the number of north/south flux paths, i n the rotor and armature.

This means that, if the armature is designed properly and has similar banana slots, what occurs is a leaping of the flux path, which goes north south, north south. By having more points of contact, the torque can be greatly increased. In theory, if there were 2 sets of poles at the same diameter, the torque would double in a clutch. Obviously, that is not possible to do, so the points of contact have to be at a smaller inner diameter. Also, there are magnetic flux losses because of the bridges between the banana slots. But by using a double flux design, a 30%-50% increase in torque, can be achieved, and by using a triple flux design, a 40%-90% in torque can be achieved. This is important in applications where size and weight are critical, such as auto motive requirements. The coil shell is made with carbon steel that has a combination of good strength and good magnetic properties. Copper (sometimes aluminium) magnet wire, is used to create the coil, which is held in shell either by a bobbin or by some type of epoxy/adhesive.

OBJECTIVE

A control circuit is to be designed to control the motor and drive unit. The Design specifications are to be fully implemented. An incomplete circuit and equipments are given and once it is understood appropriate values for the different components should be decided. These values should allow the circuit to perform as specified.

Design Specification

A circuit is to be designed which is
1. Allows the angular speed of the motor to build up to a value of 20 revolutions per minute (rpm), in a time of 2 seconds (s).
2. Maintains the angular speed of 20 rpm for a time of 3s.
3. When clutch pedal is pressed it brings the machine to a halt in a time of 1s.
4. Builds up the angular speed again to 20 rpm, in a time of 2s, except this time in the opposite direction.
5. Maintains the angular speed of 20 rpm for a time of 6s.
6. Brings the machine to a halt in a time of 1s.
7. Repeats the cycle above (1-6) indefinitely when powered on.

The motor must not be in dynamic braking mode at the same time as it is being driven .

Conclusions

Having designed and constructed the circuit it was felt that it met all of the given specifications although there were still a number of improvements that could have been made. These improvements have been covered briefly in the discussion section and given more time they could have been implemented in the circuit. As already mentioned the only specifications not met were that on start-up the machine should rotate for 3 seconds in one direction before braking and reversing. Using the clutch method mentioned in the discussions could solve this but the design brief given did not extend to cover the drum so has not been included in the final design.

During the course of the project a number of other points became evident which greatly ease the process of designing an electronic circuit. Simulation using a computer package such as P spice saves a considerable amount of time by allowing the circuit to be easily laid out and tested. Any changes required can be made easily without disturbing the rest of the circuit. Another advantage of P spice is the ability to produce graphs of the outputs from the circuit, which can then be scaled, formatted and printed as required. To do this for the actual circuit requires very specialised and expensive equipment.

In conclusion, the group felt that all objectives had been met and that the final circuit was successful in fulfilling its role. A number of important lessons were learned about the problems involved in designing a circuit to meet a real-world need and ways of overcoming these problems were found.