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
