Direct current motors ,Shunt wound d.c. motor,Series wound d.c. motor & d.c. motor starter

Direct current motors

When a current is supplied to a single coil of wire in a magnetic field a force is created which rotates the coil. This is a similar situation to the generation of current by a coil moving in a magnetic field. In fact generators and motors are almost interchangeable, depending upon which two of magnetic field, current and motion are provided.

Additional coils of wire and more magnetic fields produce a more efficient motor. Interpoles are fitted to reduce sparking but now have opposite polarity to the next main pole in the direction of rotation, When rotating the armature acts as a generator and produces current in the reverse direction to the supply. This is known as back e.m.f. (electromotive force) and causes a voltage drop across the motor. This back e.m.f. controls the power used by the motor but is not present as the motor is started. As a result, to avoid high starting currents special control circuits or starters are used.

The behaviour of the d.c. motor on load is influenced by the voltage drop across the armature, the magnetic field produced between the poles and the load or torque on the motor. Some of these factors are interdependent. For example, the voltage drop across the armature depends upon the back e.m.f. which depends upon the speed of the motor and the strength of the magnetic field. Shunt, series and compound windings are used to obtain different motor characteristics by varying the above factors.



Fig: Shunt wound d.c. motor

The shunt wound motor has field windings connected in parallel with the armature windings. Thus when the motor is operating with a fixed load at constant speed all other factors are constant. An increase in load will cause a drop in speed and therefore a reduction in back e.m.f. A greater current will then flow in the armature windings and the motor power consumption will rise: the magnetic field will be unaffected since it is connected in parallel. Speed reduction is, in practice, very small, which makes the shunt motor an ideal choke for constant-speed variable-load duties.




Fig: Series wound d.c. motor

The series motor has field windings connected in series with the armature windings . With this arrangement an increase in load will cause a reduction in speed and a fall in back e.m.f. The increased load current will, however, now increase the magnetic field and therefore the back e.m.f. The motor will finally stabilise at some reduced value of speed. The series motor speed therefore changes considerably with load.

Control of d.c. motors is quite straightforward. The shunt wound motor has a variable resistance in the field circuit, as shown in Figure . This permits variation of the current in the field coils and also the back e.m.f., giving a range of constant speeds. To reverse the motor the field current supply is reversed, as shown in Figure .

One method of speed control for a series wound motor has a variable resistance in parallel with the field coils. Reverse operation is again achieved by reversing the field current supply as shown in Figure .

In operation the shunt wound motor runs at constant speed regardless of load. The series motor runs at a speed determined by the load, the greater the load the slower the speed. Compounding—the use of shunt and series field windings—provides a combination of these characteristics. Starting torque is also important. For a series wound motor the starting torque is high and it reduces as the load increases. This makes the series motor useful for winch and crane applications. It should be noted that a series motor if started on no-load has an infinite speed. Some small amount of compounding is usual to avoid this dangerous occurrence. The shunt wound motor is used where constant speed is required regardless of load; for instance, with fans or pumps. The starting of a d.c. motor requires a circuit arrangement to limit armature current. This is achieved by the use of a starter .



Fig: d.c. motor starter


A number of resistances are provided in the armature and progressively removed as the motor speeds up and back e.m.f. is developed. An arm, as part of the armature circuit, moves over resistance contacts such that a number of resistances are first put into the armature circuit and then progressively removed. The arm must be moved slowly to enable the motor speed and thus the back e.m.f. to build up. At the final contact no resistance is in the armature circuit. A 'hold on' or 'no volts' coil holds the starter arm in place while there is current in the armature circuit.

If a loss of supply occurs the arm will be released and returned to the 'off position by a spring. The motor must then be started again in the normal way. An overload trip is also provided which prevents excess current by shorting out the 'hold on* coil and releasing the starter arm. The overload coil has a soft iron core which, when magnetised sufficiently by an excess current, attracts the trip bar which shorts out the hold on coil. This type of starter is known as a 'face plate'; other types make use of contacts without the starting handle but introduce resistance into the armature circuit in much the same way.

Alternating current motors for ships machinery spaces - Arrangement and distribution methods

Supplying alternating current to a coil

Supplying alternating current to a coil which is free to rotate in a magnetic field will not produce a motor effect since the current is constantly changing direction. Use is therefore made in an induction or squirrel cage motor of a rotating magnetic field produced by three separately phased windings in the stator.

The rotor has a series of copper conductors along its axis which are joined by rings at the ends to form a cage. When the motor is started the rotating magnetic field induces an e.m.f. in the cage and thus a current flow. The current-carrying conductor in a magnetic field produces the motor effect which turns the rotor. The motor speed builds up to a value just less than the speed of rotation of the magnetic field.

Fig:Squirrel cage induction motor starting

The motor speed depends upon the e.m.f. induced in the rotor and this depends upon the difference in speed between the conductors and the magnetic field. If the load is increased the rotor slows down slightly, causing an increase in induced e.m.f. and thus a greater torque to deal with the increased load.

The motor is almost constant speed over all values of load. It will start against about two times full load torque but draws a starting current of about six times the normal full load current. The starting current can be reduced by having a double cage arrangement on the rotor. Two separated cages are provided, one below the other in the rotor. When starting, the outer high-resistance cage carries almost all the rotor current. As the motor accelerates the low-resistance inner winding takes more and more of the current until it carries the majority. A number of different fixed speeds are possible by pole changing. The speed of an induction motor is proportional to frequency divided by the numbers of pairs of poles. If therefore a switch is provided which can alter the numbers of pairs of poles, then various fixed speeds are possible. The number of poles affects the starting characteristics such that the more poles the less the starting torque to full load torque ratio. Only the induction type of a.c. motor has been described, since it is almost exclusively used in maritime work. Synchronous motors are another type which have been used for electrical propulsion systems but not auxiliary drives.
A number of different arrangements can be used for starting an induction motor. These include direct on-line, star delta, auto transformer and stator resistance. Direct on-line starting is usual where the distribution system can accept the starting current. Where a slow moving high inertia load is involved the starting time must be considered because of the heating effect of the starting current. The star delta starter connects the stator windings first in star and when running changes over to delta.

The star connection results in about half of the line voltage being applied to each phase with therefore a reduction in starting current. The starting torque is also reduced to about one-third of its direct on line value. A rapid change-over to delta is required at about 75% of full load speed when the motor will draw about three-and-a-half times its full load current. The auto transformer starter is used only for large motors. It uses tappings from a transformer to provide, for example, 40%, 60% and 75% of normal voltage (Figure below).

The motor is started on one of the tappings and then quickly switched to full voltage at about 75% full speed. The tapping chosen will depend upon the starting torque required with a 60% tapping giving about 70% of full load torque. A smaller percentage tapping will give a smaller starting torque and vice-versa. The stator resistance starter has a resistance in the stator circuit when the motor is started. An adjustable timing device operates to short circuit this resistance when the motor has reached a particular speed.

Modern electronic techniques enable a.c. induction motors to be used in speed-control systems. The ship's supply, which may not be as stable in voltage or frequency as that ashore, is first rectified to provide a d.c, supply. This is then used as the power supply of an oscillator using high-power electronic devices. These may be thyristors (for powers up to 1.5 M W or more) or transistors (for powers up to a few tens of kilowatts). The high-power oscillator output is controlled in frequency and voltage by a feedback system. The motor speed is varied by changing the oscillator output frequency. The motor current necessary to obtain the desired torque (at small angles of slip) is normally obtained by maintaining the voltage almost proportional to frequency.

Certain protective devices are fitted in the motor circuit to protect against faults such as single phasing, overload or undervoltage. Single phasing occurs when one phase in a three-phase circuit becomes open circuited. The result is excessive currents in ail the windings with, in the case of a delta connected stator running at full load, one winding taking three times its normal load current.

A machine which is running when single phasing occurs will continue to run but with an unbalanced distribution of current. An overload protection device may not trip if the motor is running at less than full load. One method of single phasing protection utilises a temperature-sensitive device which isolates the machine from the supply at some particular winding temperature.

Overload protection devices are also fitted and may be separate or combined with the single phase protection device. They must have a time delay fitted so that operation does not occur during the high starting current period. An undervoltage or 'no volts' protective device ensures that the motor is properly started after a supply failure.

Marine electricity & ancillary equipments

In the name of Allah who is the most beneficient the most merciful

Brushless DC Motor Back EMF

Brushless DC motors (BLDC) are used where there are limitations in the use of the brush-type DC motors. In this article we discuss how it is possible operate a DC motor with no brush arrangement and also about the back EMF in a brushless DC motor (BLDC).

Introduction

A DC motor is a one which operates on supply from a DC source. The DC source may be either DC generator or from a battery. DC motors may be classified as:

·         Series wound DC motor

·         Shunt wound DC motor

·         Compound wound DC motor

·         Separately wound DC motor

In all types of DC motors, the supply is given to both stators to make it as an electromagnet. This supply is necessary because the operation of a DC motor depends on the attraction and repulsion principles of magnetism.

In the stator, the supply voltage from a DC source is given directly, and in the rotor of DC motor it is supplied by means of a brush arrangement. But in case of brushless DC motors, this supply voltage to the rotor should be supplied without any brush arrangement. Brushless DC motors are more complicated than ordinary DC motor with brush arrangements, but certain applications needs this brushless DC motor, and hence it exists.

In a brushless DC motor (BLDC), we have an exciter rotor mounted on the same shaft of the rotor of a DC motor. This exciter stator induces an EMF when a small voltage is applied to the stator of this exciter. The voltage induced in the exciter rotor is an AC voltage and this is rectified to DC by means of a rotating rectifier diode arrangements mounted on the same shaft of the motor. The rectified DC voltage is applied to the rotor of DC motor, and there is no brush required so the DC motor with this type of complicated arrangement is called a brushless DC motor (BLDC).
 

Back EMF in Brushless DC Motor (BLDC):

According to Faradays law of electromagnetic induction, when a current carrying conductor is placed in a magnetic field that is if the conductor cuts the magnetic field), an EMF is induced or produced in a conductor and if a closed path is provided current flows through it.

When the same thing happens in a brushless DC motor (BLDC) as a result of motor torque, the EMF produced is known as “back EMF.” It is so called because this EMF that is induced in the motor opposes the EMF of the generator.

This back EMF that is induced in the brushless DC motor (BLDC) is directly proportional to the speed of the armature (rotor) and field strength of the motor, which means that if the speed of the motor or field strength is increased, the back EMF will be increased and if the speed of the motor or field strength is decreased, the back EMF is decreased.

This back EMF created acts as a resistance and we all know that any resistance in a line reduces and opposes the current flow so if the speed of the DC motor or field strength is increases, the back EMF increases which it turn increases the resistance to the current flow in windings and hence only less amount of current is delivered to the armature of DC motor. Also if the speed of Dc motor armature or field strength decreases, the back EMF decreases, which in turn reduces the resistance and hence more amount of current flow to the armature of DC motor.

When the DC motor is first started, there is no back EMF induced and as discussed above there is maximum current flow from the DC generator or distribution lines to the motor armature and as a result the motor toque will be maximum. In this case there is no resistance offered by back EMF. The only resistance available is the motor winding resistance.

During normal operation (rated speed) of DC motor, the back EMF induced will be maximum which will reduces the motor armature current to its minimum level and as a result the motor torque will also be reduced.

When the load on the motor is increased, the motor speed (RPM) is decreased and this will reducs the back EMF. This decreases in back EMF will automatically increase the motor torque thereby bringing the motor to its rated speed.

Marine Generators – Starting Checks & Procedure

Starting of Generator Engine

Starting of an engine from “stop” state is something which needs to be done with care, especially if the interval of starting is sufficiently long. The following is a checklist of all the checks which ideally need to be carried out before starting the generator. In actual practice sometimes the engineers might take some of these for granted and skip, but it is advisable not to indulge in such a practice. In fact these checks are generic for any four stroke engine starting process

1.    Check the turbocharger sump oil level, governor, alternator, forward and aft lube oil levels, and diesel oil level in service tank

2.    Open the indicator cock

3.    Prime the lube oil to all parts by hand pump or by motor driven priming pump

4.    Ensure that all jacket cooler valves, lube oil cooler valves, air cooler valves should be in open position

5.    With use of the Turning bar turn the fly wheel and check for any resistance on the bottom end bearing and check any water / fuel coming out through indicator cocks

6.    While turning engine, check all visible lube oil points are lubricated

7.    Remove the turning bar from fly wheel and put in the place

8.    Drain the auxiliary air bottle

Blow through engine (i.e.: by turning engine with air). In order to ensure that no water is inside combustion chamber if it is present it may cause water hammering

9.    Close the indicator cocks and pull lever from stop to start

10.  When the needle in RPM indicator deflects to some value of (0-25 rpm) put the lever in run condition

11.  The engine will run on fuel oil once the generator picks up the rated speed

12.  Put generator on load by closing air circuit breaker

13.  For checking the alternator fore and aft bearing lube oil level by opening oil plug in the alternator and the ring bearing while rotating splash lube oil from the sump can be seen

14.  In order to synchronize the incoming generator with running generator syncroscope method/dark lamp method is used

Starting of generator

Checks to be made while running

Once the generator has actually started to run, there are several checks which must be performed before it is left on its own to continue running. These checks pertain to verifying various parameters related to lube oil levels, temperatures and so forth. Given below is a brief checklist related to the same.

Lube oil checks

1.    Sump lube oil level

2.    Governor lube oil level

3.    Rocker arm lube oil level

4.    Alternator forward and aft bearing lube oil level

5.    Lube oil in turbine & blower side of turbo charger

Temperature checks

1.    Exhaust gas temperature

2.    Turbocharger (inlet-outlet) temperature

3.    Booster air inlet temperature

Cooler temperatures

1.    Cooling sea water (inlet – out let) temperature in cooler

2.    Jacket cooling water (inlet – outlet) temperature

3.    Air cooler (inlet -outlet) temperature

Safety Devices

Once the above mentioned parameters have been checked and found within normal range, it is safe to continue running the generator. Yet a fault can develop even at a later stage, so for this very purpose various trips and alarms are situated on the generators. An alarm gives the signal of an impeding danger and requires quick action while a trip actually trips the generator immediately because of the nature of the fault.

The various trips and alarms are mentioned as follows

1.    Alternator bearing low oil level alarm & trip

2.    Alternator bearing high temperature lube oil alarm &trip

3.    Low sump oil level alarm and trip

4.    Lube low oil pressure alarm and trip

5.    Reverse current trip

6.    Over speed trip

7.    Over load trip

8.    High and low frequency trip

9.    Jacket cooling water low pressure alarm