If a rectifier diode shorts, a very high current flows through the associated exciter winding thus causing excessive heating and probable failure of the exciter.
The voltage regulator may fail due to high voltage ripple reflected back through the exciter field.
If a rectifier diode opens, the voltage regulator will substantially increase the excitation.
If the CT secondary winding is not grounded, the capacitive coupling between primary and secondary could allow the CT secondary voltage to float up to the voltage of the mains.
This is a serious safety hazard.
1. Rotational speed is a direct function of frequency.
2. Cooling fan speed is a direct function of rotational speed.
3. Back emf decreases as the motor slows down.
4. Current increases with reduced back emf.
5. The magnetic capacity of the motor's magnetic circuit is designed to the relationship: Voltage/frequency ( V/f ).
If the frequency drops, then the V/f ratio goes up. This means that the magnetic flux increases and the motor needs a larger magnetic circuit. Without it, the magnetic circuit can become overloaded. This is called saturation and it leads to a rapid increase in current draw and a corresponding large increase in temperature.
If the frequency increases, the V/f ratio drops with no issues as the magnetic circuit will remain plenty large enough for the magnetic flux. But the motor may have a worse power factor.
Electrical equipment is defined and certified as explosion proof when it is enclosed in a case, which is capable of withstanding the explosion within it of a hydrocarbon gas/air mixture or other specified flammable gas mixture.
It must also prevent the ignition of such a mixture outside the case either by spark or flame from the internal explosion or as a result of the temperature rise of the case following the internal explosion.
The equipment must operate at such an external temperature that a surrounding flammable atmosphere will not be ignited.
Types of explosionproof protections
Ex i - Intrinsically safe
Ex m - Encapsulated
Ex e - Increased safety
Ex d - Flameproof
Ex p - Pressurized
Ex v - Ventilated
Ex n - Non
sparking
Ex nC - Non
incendive
Ex nR - Restricted
breathing
Ex o - Liquid immersion
Ex q - Powder filling
Ex h - Constructional safety
Ex t - Protection by enclosures
Ex fr - Flow restricting
Flameproof - I this type of protection the enclosure which houses the electrical equipment is designed in a manner that the explosion inside the enclosure due to ingress of explosive/flammable gas or vapor will not be transmitted/communicated to outside hazardous atmosphere.
Intrinsically safe - In this type of protection the equipment is designed in such a manner that the electrical energy which can enter explosive environment is so low or restricted in a manner that it cannot ignite an explosive gas air mixture.
Zone classification
It is based on the likelihood and the duration of an explosive atmosphere.
Zone classification for flammable gases is divided into Zone 0, Zone 1, Zone 2 and for combustible dusts Zone 20, Zone 21, Zone 22.
Zone 0
Ex ia, Ex ma, Ex ta, Ex da, Ex h
Place in which an explosive atmosphere consisting of a mixture with air of flammable substance in the form of gas, vapor or mist is present continuously for long periods or frequently. Generally, it is limited to confined spaces.
Zone 1
Ex ib, Ex mb Ex tb, Ex db, Ex h, Ex eb, Ex ob, Ex p, Ex q, Ex v
Place in which an explosive atmosphere consisting of a mixture with air of flammable substance in the form of gas, vapor or mist is likely to occur in normal operation occasionally.
Zone 1 usually includes locations where volatile flammable liquid or liquified gases are transferred, gas generator rooms, pump rooms etc.
Zone 2
Ex ic, Ex mc, Ex tc, Ex dc, Ex h, Ex ec, Ex nC, Ex nR, Ex oc, Ex p, Ex fr, Ex v, Ex n
Place in which an explosive atmosphere consisting of a mixture with air of flammable substance in the form of gas, vapor or mist is not likely to occur in normal operation but if it does occur, will persist for a short period only.
Complete segregation from Zone 1 location. Well ventilated and can rapidly disperse the abnormal gas concentrations.
Gas group
Electric equipment for hazardous gas areas is grouped as:
Group I - for mines susceptible to methane
Group II - for all places with an explosive gas atmosphere except mines susceptible to methane
Group II is further divided into IIA, IIB and IIC.
I Methane
IIA Propane
IIB Ethylene
IIC Hydrogen
Temperature rating
Electrical apparatus must be selected so that the maximum surface temperature that could cause an ignition of the flammable product is not exceeded.
rating max surface temperature
T1 450 degC
T2 300 degC
T3 200 degC
T4 135 degC
T5 100 degC
T6 85 degC
Alternatively, it utilises the increased power input for carrying more load than machine 2.
This is possible because of angular advance of machine 1 with respect to machine 2.
Here E1 advances E2 by an angle α. The resultant voltage Er sets up a current Isy which is almost in phase with E1.
Hence, power per phase of machine 1 is increased by an amount =E1 Isy whereas that of machine 2 is decreased by the same amount.
Since Isy has no reactive component, the increase in fuel supply does not disturb the division of KVAR but it increases KW of machine 1 and decreases that of machine 2.
Suppose the initial operating conditions of the two identical alternators in parallel with KW and KVAR load being shared equally, thus operating in same power factor. Each machine supplies a load current I so that total output current is 2I.
Regulation for Radio battery capacity test
6. Where a reserve source of energy consists of a rechargeable accumulator battery or batteries:
.1 a means of automatically charging such batteries shall be provided which shall be capable of recharging them to minimum capacity requirements within 10 h; and
.2 the capacity of the battery or batteries shall be checked, using an appropriate method, at intervals not exceeding 12 months, when the ship is not at sea.
(SOLAS
1974, as amended, regulation IV/13)
7.4.6 The capacity of the radio batteries should be checked at intervals not exceeding 12 months when the ship is not at sea. One method of checking the capacity is to fully discharge and recharge the batteries using normal operation current over a period of 10 hours. Assessment of the charge condition can be made at any time, but it should be done without significant discharge of the battery when the ship is at sea. Another method could be to check the capacity by means of a battery tester, e.g., in connection with a radio survey. (SOLAS 1974, as amended, regulation IV/13, IMO resolution A.702(17) and COMSAR/Circ. 16)
Note: - When determining the battery capacity, the following should also be taken into consideration:
- the battery is normally not fully charged.
- reduction of capacity due to ageing.
- reduction of capacity due to high
or low temperatures; and
- reduction of capacity due to rapid
discharge.
(COMSAR/Circ.32, ANNEX, Page 33)
Radio battery capacity requirement
7.4.4 If the capacity requirement of radio batteries is to be maintained over their normal life cycle, an extra 40% capacity should be added to the minimum calculated capacity.
(COMSAR/Circ.32, Annex, page 32)
2.2 For guidance, the nominal battery capacity to comply with the minimum capacity requirements at all times is 1.4 times the load determined in paragraph 2.4 multiplied by the intended period of operation (1 hour or 6 hours in accordance with SOLAS IV/13.2).
(COMSAR/Circ.16, Annex, page 4)
Key terminologies to understand:
minimum capacity requirement = load determined for the radio system x 1hr or 6hr
Most of the ships are required to have only 1hr intended period of operation.
hence, minimum capacity requirement = load determined for the radio system.
nominal battery capacity = 1.4 x minimum required capacity
The installed battery capacity will be equal to or greater than the nominal battery capacity, whichever is available in the market.
The minimum required capacity can be safely taken as 72% of the installed battery capacity.
The radio battery capacity should not be less than the minimum required capacity at any time. The annual battery capacity test is being carried out to ascertain whether the present battery capacity is higher than the minimum required capacity or not. During charging cycle after the capacity test, the battery should be charged to minimum required capacity in 10hrs. However, the battery requires more hours of charging to get it fully charged.
The capacity of a lead acid battery is normally quoted at 20 hours of discharge at an operational temperature of 20°C. But in general, most of the lead acid batteries having capacity of 100Ah or more can deliver the battery capacity in 10 hours of discharge. Battery capacity will decrease over the period of time due to various factors. Hence the battery capacity needs to be checked.
Consider if a radio battery of 200Ah, 12V, 02nos are in use:
for 10 hours discharge, we need 20A constant discharge (200 / 10 = 20A).
But the current will be decreasing rather being constant due to decreasing battery voltage and increasing internal resistance. The rate of current decrement is related the health of the battery.
For a 20A current discharge we need a 24V lamp load of 480W (20A x 24V) which can be made with 8nos 60W,24V lamps.
So, we can prepare a lamp load bank of 24V, 60W, 8nos for capacity test with each lamps having its own switch.
Disconnect the battery from the system and the charger.
Connect the load bank to the battery through a switch or breaker.
Connect a multimeter across the battery terminal to measure the battery voltage.
Connect a DC clamp meter to the load bank.
Switch on all 8nos individual lamp switches and then switch on the main breaker of load bank.
Immediately note down the voltage and current.
Continue to note down the voltage and current every minute for next five minute. In this five minute, the battery voltage will dip and rise which will indicate the general health of the battery. A good battery will have a shallow dip and will rise soon.
Continue to note down the voltage and current every half an hour until the battery voltage drops to 21.12V, 88% (1.76V x 12cells). The safe discharge voltage of a cell according to most of the makers is 1.75V (end cell voltage). SOLAS chapter IV does not say anything about how much should be the maximum voltage drop while discharging. However, following can be considered for any battery; as per SOLAS Reg II-1/13-4, a battery "which shall operate without recharging while maintaining the voltage of the battery throughout the discharge period within 12% above or below its nominal voltage". The 12% drop of a cell voltage resulted to be 1.76V and it will be 21.12V (88%) for the battery.
Calculate the Ah discharged (current x time) for every half an hour and add all to get the total discharged Ah.
Calculate the battery % capacity with respect to nominal battery capacity.
Disconnect the load bank and connect the battery to the charger.
Connect the multimeter and DC clamp meter accordingly.
Switch on the charger and note down the voltage and current.
Continue to note down the voltage and current every hour for next 10hrs.
Connect the battery to the system and keep charger on. The battery will continue charging. The automatic battery chargers will not charge the battery to its nominal value in 10hrs. The charger will control the charging current throughout to avoid any battery damage. However, the charger can easily charge the battery more than the minimum required capacity in 10hrs which is required by SOLAS if the battery is good.
Calculate the Ah charged which should be more than the minimum required capacity.
Plot the discharging and charging parameters on graph for record keeping.
If the battery capacity is less than 80% or the Ah charged in 10hrs is less than minimum required capacity, then initiate the battery replacement procedure.
Capacity test can be carried out at PORT or at anchorage inside port limit while awaiting berth & not proceeding/waiting for sea passage as per Singapore flag state. The 10hr charging has to be completed before ship leaves the port.
