Physics And Structure Motor Starter Engineering Essay

A motor starter, also known as starting motor and starter, is an electric motor that rotates the internal combustion engine and causes it to power up. The motor starter is made up of a powerful DC electric motor and a starter solenoid (electric switch) and requires a very high current in order to turn over the engine and start it. In the car, the motor starter begins when the key is put into the ignition and turned to the start position. At this point, the battery voltage goes through the starter control circuit and activates the starter solenoid. This process provides energy to the motor starter so that it can turn over the engine. The motor starter is made up of ground, or negative, cables that connect the battery terminal to the engine block and positive cables that connect the battery terminal to the starter solenoid. The starter solenoid acts to close the circuit and connect the starter motor to the battery.

Full voltage motor starters are sometime also referred to as direct on line motor starters. Full voltage motor starters are most widely used and result in a high current. This current can range anywhere between 400 percent to 1,000 percent of a full load current, according to schneider-electric.be. Full voltage motor starters also provide a high starting torque (about 150 percent of the full load torque). Torque (also known as moment of force) is tendency of a force to rotate an object about an axis or a pivot point. It is a very important concept in engineering because it essentially refers to the measure of the resulting moment.

Reduced voltage motor starters decrease the full load current at the motor terminals in proportion to the voltage reduction. Furthermore, the full load torque of the motor is reduced by the square of the voltage reduction. These are used on machines that require a gentle start and smooth acceleration, according to electricmotors.machinedesign.com. They are also used in compounds that are limited by voltage regulations, such as power companies.

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Multi-Speed

Multi-speed motor starters are divided into multiple categories. Starters for separate winding two speed motors consist of two standard three-pole starter units. A pole is the number of conduction positions that belong to that switch. These units are interlocked both electrically and mechanically. According to electricmotors.machinedesign.com, additional units may also be used for each speed. Starters for three-speed motors are three-pole and starters for four-speed motors have two sets of three- to five-pole starters.

MOTOR STARTER COMPONENTS

Every internal combustion engine in a car built after 1940 has a starter motor. This is simply an electric motor attached at the junction between the engine and the transmission by a special housing. The housing allows the motor to protrude into the space just enough to engage the flywheel on the back of the engine and turn it until the combustion process begins. Starters are made just like any other heavy-duty electric motor, but with a few additions.

Armature

The armature is the heart of an electric motor. It rides on the central shaft, which also contains the commutator in bearings front and rear. The armature runs in the field created by the coils and is essentially a moving magnet when the coils are energized.

Commutator

The commutator is a section of the shaft at the rear of the starter housing on which the brushes run to conduct electricity.The brushes are two pieces of carbon attached to wires. These wires are connected to the battery through the starter switch. When the switch is thrown, they conduct electricity to the coils and armature causing the starter to spin.

Bendix

The Bendix drive is a unique combination of a spring and a gear. When the starter is engaged, the gear extends into the housing on the front of the unit, engages the flywheel and spins the engine to begin the combustion process. The Bendix is sometimes activated by a solenoid and a wishbone-shaped lever, depending on manufacturer.

How Motor Starters Work

Starter motors come in either standard, high-torque or gear reduction models. The primary purpose is to turn the engine over in order for ignition to commence and the engine to start. The starter will always need to engage the flywheel on the rear of the engine in order to turn the engine. The flywheel is a large wheel with teeth around the circumference and is attached to the rear of the crankshaft. Generally the starter will be installed on either bottom-side of the engine with the gear end facing rearward toward the flywheel. Some vehicles have the starter attached to the top rear of the engine under the intake manifold. The starter is an electric, high-torque motor with a gear that rides on a spiracle shaft on the end. This spiracle shaft is called the bendix. When the starter motor is activated, the gear on the shaft spins at a high speed and the spiracle shaft causes the gear to wind its way up the shaft extending it out to engage with the flywheel. When the starter is deactivated, a spring pulls the bendix back in toward the starter, disengaging it from the flywheel. The wiring for a starter is consistent with most all vehicles with the exception of the security system intervention. The main power for the starter motor runs directly from the battery or battery source to the top post of the solenoid on the starter. In some cases the starter will have a separate solenoid located on the fender well. The solenoid is just a remotely activated switch to turn the starter on and off. If it is a remote solenoid on the fender well there will be two large terminals–one on each side of the solenoid–and two small terminals in the center of the solenoid.The large diameter power wire from the battery is attached to one of the large terminals and the other large terminal goes to the starter. The small terminals are marked ‘S’ and ‘I.’ The S terminal is for a wire from the ignition switch that is activated when the key is turned to the start position. When this terminal is activated, the solenoid closes the circuit to the starter and activates it. When the key is released, the circuit is open and disengages the starter. The same thing applies with a starter-mounted solenoid.

Electric starter

The electric starter has wide range applications. The main components of Electric starter is:

Main Housing (yoke)

Overrunning clutch

Armature

Field coils

Brushes

Solenoid

The modern starter motor is either a permanent-magnet or a series-parallel wound direct current electric motor with a solenoid switch (similar to a relay) mounted on it. When current from the starting battery is applied to the solenoid, usually through a key-operated switch, it pushes out the drive pinion on the starter driveshaft and meshes the pinion with the ring gear on the flywheel of the engine. Before the advent of key-driven starters, most electric starters were actuated by foot-pressing a pedestal located on the floor, generally above the accelerator pedal.

The solenoid also closes high-current contacts for the starter motor, which begins to turn. Once the engine starts, the key-operated switch is opened, a spring in the solenoid assembly pulls the pinion gear away from the ring gear, and the starter motor stops. The starter’s pinion is clutched to its driveshaft through an overrunning sprag clutch which permits the pinion to transmit drive in only one direction. In this manner, drive is transmitted through the pinion to the flywheel ring gear, but if the pinion remains engaged (as for example because the operator fails to release the key as soon as the engine starts), the pinion will spin independently of its driveshaft. This prevents the engine driving the starter, for such backdrive would cause the starter to spin so fast as to fly apart. However, this sprag clutch arrangement would preclude the use of the starter as a generator if employed in hybrid scheme mentioned above; unless modifications are made. Also, a standard starter motor is only designed for intermittent use which would preclude its use as a generator.

This overrunning-clutch pinion arrangement was phased into use beginning in the early 1960s; before that time, a Bendix drive was used. The Bendix system places the starter drive pinion on a helically-cut driveshaft. When the starter motor begins turning, the inertia of the drive pinion assembly causes it to ride forward on the helix and thus engage with the ring gear. When the engine starts, backdrive from the ring gear causes the drive pinion to exceed the rotative speed of the starter, at which point the drive pinion is forced back down the helical shaft and thus out of mesh with the ring gear.

Manual Motor Starters

Manual motor starters are simply manual switches designed to control larger current loads typical of motor control. They may be small and similar to the light switches in your home, or they may be much larger dedicated switches designed for control of high amperage circuits. These motor starters may be either Single Pole (switch one line only) or Double/Triple Pole devices (switch 2/3 lines). When a Double/Triple Pole manual motor starter is turned off, the power from the power cable is completely disconnected from the motor. Manual motor starters may also be equipped with matched heaters, which are overload protectors designed to open when the current load is too high. These heaters must be properly sized to the motor they are protecting or else they will either open too soon, or will not protect the motor. The disadvantage to manual motor controls is that they cannot have remotely located On and Off controls.

Magnetic Motor Starters

Magnetic Motor starters are essentially heavy duty relays, often equipped with heater/thermal overloads matched to the motor they start. They are then controlled using a lighter duty (low or high voltage) circuit, auxillary relay contacts and a control station (or several stations) utilizing lighter duty switches (usually momentary sometimes latching). These switches would not be capable of switching the large loads required by the motors. Because the control circuitry is separate from the Load circuit, the On/Off controls can be mounted remotely and can even be duplicated if desired. This type of motor starter will usually have an auxiliary contact switch: a smaller set of contacts that opens or closes along with the motion of the main contactors. These contacts will be used to latch the system in an on condition. Latching means that the auxiliary contact bypasses the ON button so the solenoid remains energized, until a separate OFF button cuts the power. Additional contacts (NO & NC) may also be provided and may be used for auxiliary circuits or to provide feedback to the rest of the system that the starter is engaged and the motor has power.

Some older style motor starters have built-in latching. These starters have four terminals labeled 3, P, E, and C (historical leftovers from older models). The E and C terminals are for the hot and neutral wires, respectively, and remain powered. When power is applied to 3 and P at the same time, the starter will engage until power is removed from P. Power can be removed from 3 at any time without affecting operation.

How Do AC Motor Starters Work?

Electric Motors

AC (alternating current) motor starters are used on electric motors that utilize a start and stop button or switch for the operation. Safety switches can also be employed in the low-voltage circuit that controls the power to the AC motor starter. AC motor starters are also used on large motors in which the electrical power requirements are so large that it would be unsafe to operate a single switch to turn the motor on. The motor starter can also be located at a great distance from the electric motor, so remote or automatic operation of the motor is made possible. The AC motor starter generally has three main components, the pull-in coil, the electrical contacts and the overcurrent protection.

The Pull-In Coil

All motor starters have an electrically wound coil made up of many strands of insulated wire. These wires are insulated from each other by a thin layer of varnish. The varnish keeps the electrical power from shorting against the individual wires that make up the pull-in coil. The coil is wound around a plastic form that allows a metal plunger to be pulled “in” or “out” as electrical power is applied to the coil. The metal plunger fits just inside the plastic form. When power is applied to the coil, the plunger is electrically engaged. When power is shut off from the coil, the plunger is disengaged. During the engagement of the coil and plunger, the electrical contacts touch each other.

Electrical Contacts

Attached directly or through a lever, the electrical contacts move in accordance with the plunger. These contacts are electrically connected to the motor and the power feed of the motor circuit. The contacts work in such a way that, regardless of the number of contact points, they all come together in the same moment of time. On the other hand, when power is released from the coil/plunger arrangement, the electrical power is withdrawn from all the contacts at the same moment. This ensures that no damage can occur to the electric motor or device that is being controlled by the motor starter. The electrical contacts can come in many sizes that range from a pencil eraser end (3/16 inch) to one inch in diameter. Generally, the more power that needs to be conducted, the larger the physical contact is.

Overcurrent Protection

Generally, built into all AC motor starters is an overcurrent protection device. This device monitors the overall amount of power that the motor is using while under operation. Usually a bi-metallic strip that will bend when overheated, the overcurrent protection will disrupt power to the coil and shut down the AC motor starter. Without the overcurrent protection, the AC motor starter could continually run if the motor becomes damaged and destroy the equipment that the motor is driving.

PHYSICS BEHIND MOTOR STARTERS

Electric motors operate on electromagnetic induction principle. It takes a few fractions of seconds for the windings of the electric motor to get energized and produce the electromagnetic induction. Till such time, the load to the electric supply is just the resistance of the windings and hence the initial rush of current will be high. In such circumstances it is not advisable to mechanically connect the supply to the motor. Further, for any defects in the electric motor circuit, more than the rated current might flow through the windings and thereby damage them.

Description of a Electric Motor Starter:

The above diagram shows a electric motor starter and its connections. The electric motor starter consists of a relay type contactor C, a thermal over load O and a set of on/off switch  buttons. The ‘on’ switch is normally open and the ‘off’ switch is normally closed. The above arrangement is for a single phase operation but the concept is same for 3 phase operation as well.

Function of a Electric Motor Starter:

Refer to the same diagram. When the ‘on’ switch button is pressed, the supply is given to the relay coil of the contactor and the coil gets energized. The coil, due to electromagnetic effect operates the spring loaded plunger to make the contacts with the terminals on both ends of the contactor so that the supply is given to the motor. The motor starts running. At this condition, even if the ‘on’ switch button is released, the coil continues to get the supply from the load side of the contactor through the ‘off’ switch and thus the relay continues to hold the contacts. The supply to the relay coil is given in series with a thermal overload relay. This relay opens out and breaks the supply to the motor in case the current drawn by the motor exceeds the rated current. If the ‘off’ button is pressed, the supply to the coil is cut off and the plunger opens out to break the supply. Since the load side terminals have no supply now, the relay does not get the supply even if the ‘off’ switch is released. To restart the motor you need to once again press the ‘on’ switch button.

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