Motor starters are usually fitted with a trip device which deals with overcurrents from just above normal running current of the motor to the stall current. The aim should be for the device to match the characteristics of the motor so that full advantage may be taken of any overload capacity. Equally, the trip device must open the starter contactor before there is any danger of permanent damage to the motor.
Contactors are not normally designed to cope with the clearance of short-circuit conditions, and it is therefore usual for the contactor to be backed up by HRC fuses or by circuit-breaker.
The arrival on the scene of very compact motor starters and the need to provide proper back-up protection to them has posed a problem. BS EN 60947-4-1 (1992) (previously BS 4941) ‘Motor starters’, describes three types of co-ordination, the most onerous condition (type C) requiring that under fault conditions there shall be no damage to the starter or to the overload relay. The usual back-up device will be the HRC fuse. It is important that the user check with the manufacturer's catalogue to ensure that the correct fuse is used to secure this co-ordination.
The starter motor in your automobile is a DC motor. If you were to accidentally reverse the battery polarity, the DC motor would still rotate in the same direction. Reversing polarity of the battery will not cause the motor to rotate in the opposite direction.
To reverse the direction of rotation of this type of motor, either the current through the stator winding or the current through the armature must be reversed. Reversing both of them will result in the same magnetic polarities between the armature and the stator poles. This results in the same direction of rotation.
The industry's standard for reversing the direction of rotation of a DC motor is to reverse the direction of the current through the armature. When a DC motor has more than one set of windings, shunt and series, as well as interpoles, the currents through all the stator windings would need to be reversed in order to change direction of rotation. This is far more complicated than merely reversing armature current.
All engines require a toyota starter motor to turn them over before firing. In conventional vehicles, this is a straightforward, but powerful, direct-current electric motor. When the starter switch is activated by the driver, current flows to a solenoid attached to the starter motor. This current moves a lever into the solenoid that then causes a cogwheel of the motor to mesh with the teeth on the circumference of the flywheel. At the same time, an electrical contact is closed to allow a large current to flow and rotate the starter motor as well as the engaged flywheel. Typically, currents of hundreds of amperes are required to start the engine and are provided by the battery, which is generally a 12-V lead–acid module. The battery is recharged by the alternator–rectifier combination when the engine is running. Automotive batteries have improved enormously over the years and have far longer lives than formerly, even though they may be called upon to power many more functions. Although guarantees may be for two or three years, in practice batteries often operate for eight years or longer before failing. Moreover, modern car batteries no longer require periodic ‘topping-up’ with de-ionized water. Further information on the evolution of the lead–acid battery is given in Section 7.4, Chapter 7.
A starter motor is required to run the internal combustion engine up to a speed sufficient to produce satisfactory carburation.
The starter motor is mounted on the engine casing and a pinion on the end of the BMW starter motor shaft engages with the flywheel teeth. The gear ratio between pinion and flywheel is about 10:1. A machine capable of developing its maximum torque at zero speed is required. The series wound motor has speed and torque characteristics ideal for this purpose.
The engagement of the pinion with the flywheel is effected in different ways. Perhaps the two most commonly used are the inertia engaged pinion and the pre-engaged pinion methods.
In inertia engagement the drive pinion is mounted freely on a helically threaded sleeve on the armature motor shaft. When the starter switch is operated, the armature shaft revolves, causing the pinion, owing to its inertia, to revolve more slowly than the shaft. Consequently, the pinion is propelled along the shaft by the thread into mesh with the flywheel ring gear. Torque is then transmitted from the shaft to the sleeve and pinion through a heavy torsion spring, which takes up the initial shock of engagement. As soon as the engine fires, the load on the pinion teeth is reversed and the pinion tends to be thrown out of engagement. Inertia drives are usually inboard, i.e. the pinion moves inward towards the starter motor to engage with the ring gear; an inboard is lighter and cheaper than an outboard starter.
To obtain maximum lock torque (i.e. turning effort at zero speed), the flux and armature current must be at a maximum, so resistance in the starter circuit (windings, cables, switch and all connections) must be a minimum; any additional resistance will reduce the starting torque. Generally, the inertia engaged mercedes starter motor is energised via a solenoid switch, permitting the use of a shorter starter cable and assuring firm closing of the main starter-switch contacts, with consequent reduction in voltage drop. The use of graphite brushes with a high metallic content also assists in minimising loss of voltage.
While inertia drive has been the most popular method of pinion engagement for British petrol-engined vehicles, the use of outboard pre-engaged drive is increasing. The pre-engaged starter is essential on all vehicles exported to cold climates and for compression ignition engines which need a prolonged starting period.
The simplest pre-engaged type of drive is the overrunning clutch type. In this drive, the pinion is pushed into mesh by a forked lever when the starter switch is operated, the lever often being operated by the plunger of a solenoid switch mounted on the motor casing. Motor current is automatically switched on after a set distance of lever movement. The pinion is retained in mesh until the starter switch is released, when a spring returns it. To overcome edge-to-edge tooth contact and ensure meshing, spring pressure or a rotating motion is applied to the pinion. An overrunning clutch carried by the pinion prevents the motor armature from being driven by the flywheel after the engine has fired. Various refinements may be incorporated, especially in heavy-duty starters. Among these are: a slip device in the overrunning clutch to protect the motor against overload; a solenoid switch carrying a series closing coil and a shunt hold-on coil; an armature braking or other device to reduce the possibility of re-engagement while the armature and drive are still rotating; a two-stage solenoid switch to ensure full engagement of the starter pinion into the flywheel teeth before maximum torque is developed (Figure 44.15).
The engine may be started either by an electric honda starter motor or by compressed air.
An increasing used form of motor starter is known as “soft start”. Soft starters utilise sold state technology, typically thyristors, to supply the motor.
In a “soft starter” voltage and frequency of supply to the motor is varied in a controlled way in order to provide the required torque as the speed increases up to full load.
Soft starts can be arranged to provide up to 200% full load torque at starting, whilst limiting the current drawn from the supply to perhaps 350% rather than the 600% typically experienced from direct on line. Other parameters and facilities including kick start ability, ramp time to full speed and low load energy saving are available depending upon supplier.
Soft starters are available for the largest 400/600 volt motors. By specifying soft starters the specification of the associated supply system can be relaxed since large starting currents and resultant voltage drops will not occur.
Some users are specifying speed control inverters for motor starting even when full speed control facilities are not needed. So used inverters provide a soft starter capability, have good motor control, protection and diagnostic facilities as well as providing an energy saving function, if needed.
The engine starting quality is strongly influenced by the Jeep starter motor and the injection strategy. Indeed, an insufficient amount of kinetic energy initially provided to the system will not compensate for the energy loss caused by the DMF resonance. An adequate starter motor must be carefully chosen to fulfil this requirement, even under critical conditions with low battery voltage or corroded components of the starter system. Moreover, the engine should not be fired too soon during the starting phase before the starter motor reaches a stationary speed.
- Creado: 27-10-21
- Última sesión: 27-10-21