Greek Alphabet's

Metric System

Standard Connecting Wire

AC MOTORS

Most of the world's motor business is addressed by AC motors. AC motors are relatively constant speed devices. The speed of an AC motor is determined by the frequency of the voltage applied (and the number of magnetic poles). There are basically two types of AC motors: induction and synchronous.

INDUCTION MOTOR - If the induction motor is viewed as a type of transformer, it becomes easy to understand. By applying a voltage onto the primary of the transformer winding, a current flow results and induces current in the secondary winding. The primary is the stator assembly and the secondary is the rotor assembly. One magnetic field is set up in the stator and a second magnetic field is induced in the rotor. The interaction of these two magnetic fields results in motion. The speed of the magnetic field going around the stator will determine the speed of the rotor.

The rotor will try to follow the stator's magnetic field, but will "slip" when a load is attached. Therefore induction motors always rotate slower than the stator's rotating field. Typical construction of an induction motor consists of 1) a stator with laminations and turns of copper wire and 2) a rotor, constructed of steel laminations with large slots on the periphery, stacked together to form a "squirrel cage" rotor. Rotor slots are filled with conductive material (copper or aluminum) and are short-circuited upon themselves by the conductive end pieces. This "one" piece casting usually includes integral fan blades to circulate air for cooling purposes. The standard induction motor is operated at a "constant" speed from standard line frequencies. Recently, with the increasing demand for adjustable speed products, controls have been developed which adjust operating speed of induction motors. 

Microprocessor drive technology using methods such as vector or phase angle control (i.e. variable voltage, variable frequency) manipulates the magnitude of the magnetic flux of the fields and thus controls motor speed. By the addition of an appropriate feedback sensor, this becomes a viable consideration for some positioning applications. Controlling the induction motor's speed/torque becomes complex since motor torque is no longer a simple function of motor current. Motor torque affects the slip frequency, and speed is a function of both stator field frequency and slip frequency.

Induction motor advantages include: Low initial cost due to simplicity in motor design and construction; availability of many standard sizes; reliability; and quiet, vibration free operation. For very rapid start-stop positioning applications, a larger motor would be used to keep temperatures within design limits. A low torque to inertia ratio limits this motor type to less demanding incrementing (start-stop) applications.

SYNCHRONOUS MOTOR - The synchronous motor is basically the same as the induction motor but with slightly different rotor construction. The rotor construction enables this type of motor to rotate at the same speed (in synchronization) as the stator field. There are basically two types of synchronous motors: self excited ( as the induction motor) and directly excited (as with permanent magnets).

The self excited motor (may be called reluctance synchronous) includes a rotor with notches, or teeth, on the periphery. The number of notches corresponds to the number of poles in the stator. Oftentimes the notches or teeth are termed salient poles. These salient poles create an easy path for the magnetic flux field, thus allowing the rotor to "lock in" and run at the same speed as the rotating field.

A directly excited motor (may be called hysteresis synchronous, or AC permanent magnet synchronous) includes a rotor with a cylinder of a permanent magnet alloy. The permanent magnet north and south poles, in effect, are the salient teeth of this design, and therefore prevent slip. In both the self excited and directly excited types there is a "coupling" angle, i.e. the rotor lags a small distance behind the stator field. This angle will increase with load, and if the load is increased beyond the motor's capability, the rotor will pull out of synchronize.

The synchronous motor is generally operated in an "open loop" configuration and within the limitations of the coupling angle (or "pull-out" torque) it will provide absolute constant speed for a given load. Also, note that this category of motor is not self starting and employs start winding's (split-phase, capacitor start), or controls which slowly ramp up frequency/voltage in order to start rotation. A synchronous motor can be used in a speed control system even though a feedback device must be added. Vector control approaches will work quite adequately with this motor design.

However, in general, the rotor is larger than that of an equivalent servomotor and, therefore, may not provide adequate response for increment applications. Other disadvantages are: While the synchronous motor may start a high inertial load, it may not be able to accelerate the load enough to pull it into synchronize. If this occurs, the synchronous motor operates at low frequency and at very irregular speeds, resulting in audible noise. Also for a given horsepower, synchronous motors are larger and more expensive than non-synchronous motors.

Basic Definitions

Ambient Temperature - The temperature of the air, water, or surrounding earth. Conductor capacity is corrected for changes in ambient temperature including temperatures below 86°F. The cooling effect can increase the current carrying capacity of the conductor. 

Bonding Jumper - A bare or insulated conductor used to ensure the required electrical conductivity between metal parts required to be electrically connected. Frequently used from a bonding bushing to the service equipment enclosure to provide a path around concentric knockouts in an enclosure wall: also used to bond one raceway to another.

Demand Factor - For an electrical system or feeder circuit, this is a ratio of the amount of connected load (in kva or amperes) that will be operating at the same time to the total amount of connected load on the circuit. An 80% demand factor, for instance, indicates that only 80% of the connected load on a circuit will ever be operating at the same time. Conductor capacity can be based on that amount of load.

Megger - A test instrument for measuring the insulation resistance of conductors and other electrical equipment; specifically, a mega ohm (million ohms) meter; this is a registered trade mark of the James Biddle Co.

Difference Between MCB, MCCB, RCCB and ELCB

MCB (Miniature Circuit Breaker)

Characteristics

* Rated current not more than 100 A.

* Trip characteristics normally not adjustable.

* Thermal or thermal-magnetic operation.

MCCB (Moulded Case Circuit Breaker)

Characteristics

*Rated current up to 1000 A.

*Trip current may be adjustable.

*Thermal or thermal-magnetic operation.

Air Circuit Breaker

Characteristics

*Rated current up to 10,000 A.

*Trip characteristics often fully adjustable including configurable

trip thresholds and delays.

*Usually electronically controlled some models are microprocessor

controlled.

*Often used for main power distribution in large industrial plant, where the breakers are arranged in draw-out enclosures for ease of maintenance.

Vacuum Circuit Breaker

Characteristics

*With rated current up to 3000 A, These breakers interrupt the arc in a vacuum bottle.

*These can also be applied at up to 35,000 V. Vacuum circuit breakers tend to have longer life expectancies between overhaul than do air circuit breakers.

RCD (Residual Current Device / RCCB(Residual Current Circuit Breaker)

Characteristics

*Phase (line) and Neutral both wires connected through RCD.

*It trips the circuit when there is earth fault current.

*The amount of current flows through the phase (line) should return through neutral .

*It detects by RCD. any mismatch between two currents flowing through phase and neutral detect by -RCD and trip the circuit within 30 Miliseconed.

*If a house has an earth system connected to an earth rod and not the main incoming cable, then it must have all circuits protected by an RCD (because mite not be able to get enough fault current to trip a MCB).

*RCDs are an extremely effective form of shock protection The most widely used are 30 mA (milliamp) and 100 mA devices. A current flow of 30 mA (or 0.03 amps) is sufficiently small that it makes it very difficult to receive a dangerous shock. 

*Even 100 mA is a relatively small figure when compared to the current that may flow in an earth fault without such protection (hundred of amps) A 300/500 mA RCCB may be used where only fire protection is required. eg., on lighting circuits, where the risk of electric shock is small.

Limitation of RCCB

*Standard electromechanical RCCBs are designed to operate on normal supply waveforms and cannot be guaranteed to operate where none standard waveforms are generated by loads. 

*The most common is the half wave rectified waveform sometimes called pulsating dc generated by speed control devices, semi conductors, computers and even dimmers. Specially modified RCCBs are available which will operate on normal ac and pulsating dc. 

*RCDs don’t offer protection against current overloads: RCDs detect an imbalance in the live and neutral currents. A current overload, however large, cannot be detected. It is a frequent cause of problems with novices to replace an MCB in a fuse box with an RCD. This may be done in an attempt to increase shock protection. If a live-neutral fault occurs (a short circuit, or an overload), the RCD won’t trip, and may be damaged. In practice, the main MCB for the premises will probably trip, or the service fuse, so the situation is unlikely to lead to catastrophe; but it may be inconvenient. It is now possible to get an MCB and and RCD in a single unit, called an RCBO (see below). Replacing an MCB with an RCBO of the same rating is generally safe. 

*Nuisance tripping of RCCB: Sudden changes in electrical load can cause a small, brief current flow to earth, especially in old appliances. RCDs are very sensitive and operate very quickly; they may well trip when the motor of an old freezer switches off. Some equipment is notoriously `leaky’, that is, generate a small, constant current flow to earth. Some types of computer equipment, and large television sets, are widely reported to cause problems RCD will not protect against a socket outlet being wired with its live and neutral terminals the wrong way round. RCD will not protect against the overheating that results when conductors are not properly screwed into their terminals. RCD will not protect against live-neutral shocks, because the current in the live and neutral is balanced. So if you touch live and neutral conductors at the same time (e.g., both terminals of a light fitting), you may still get a nasty shock.

ELCB (Earth Leakage Circuit Breaker)

Characteristics

*Phase (line), Neutral and Earth wire connected through ELCB.

*ELCB is working based on Earth leakage current.

*Operating Time of ELCB:

~The safest limit of Current which Human Body can withstand is 30ma sec. Suppose Human Body Resistance is 500Ω and Voltage to ground is 230 Volt. The Body current will be 500/230=460mA.

~Hence ELCB must be operated in 30maSec/460mA = 0.65msec

RCBO (Residual Circuit Breaker with OverLoad)

*It is possible to get a combined MCB and RCCB in one device (Residual Current Breaker with Overload RCBO), the principals are the same, but more styles of disconnection are fitted into one package.