ENERGY EFFICIENT MOTOR :-

An "energy efficient" motor produces the same shaft output power (HP), but uses less input power (kW) than a standard-efficiency motor.

Energy efficient motors have the following positive features compared to standard motor:

1) Higher quality low loss laminations for magnetic circuit

2) More & better quality copper in the windings.

3) Better quality insulation

4) Optimised air gap between the rotor and stator.

5) Reduced fan losses.

6) Closer matching tolerances

7) A greater core length

EFFECTS OF HARMONICS ON MOTOR :-

Harmonics increase motor losses, and can adversely affect the operation of sensitive auxiliary equipment. The non-sinusoidal supply results in harmonic currents in the stator which increases the total current drawn. In addition, the rotor resistance (or more precisely, impedance) increases significantly at harmonic frequencies, leading to less efficient operation. Also, stray load losses can increase significantly at harmonic frequencies. Overall motor losses increase by about 20% with a six-step voltage waveform compared to operation with a sinusoidal supply. In some cases the motor may have to be de-rated as a result of the losses. Alternatively, additional circuitry and switching devices can be employed to minimize losses.

Instability can also occur due to the interaction between the motor and the converter. This is especially true of motors of low rating, which have low inertia. Harmonics can also contribute to low power factor.

MOTOR - GOOD MAINTENANCE PRACTICES :-

A checklist of good maintenance practices to help insure proper motor operation would include.

 

1) Inspecting motors regularly for wear in bearings and housings (to reduce frictional losses) and for dirt/dust in motor ventilating ducts (to ensure proper heat dissipation).

2) Checking load conditions to ensure that the motor is not over or under loaded. A change in motor load from the last test indicates a change in the driven load, the cause of which should be understood.

 3) Lubricating appropriately. Manufacturers generally give recommendations for how and when to lubricate their motors. Inadequate lubrication can cause problems, as noted above. Over-lubrication can also create problems, e.g. excess oil or grease from the motor bearings can enter the motor and saturate the motor insulation, causing premature failure or creating a fire risk.

4) Checking periodically for proper alignment of the motor and the driven equipment. Improper alignment can cause shafts and bearings to wear quickly, resulting in damage to both the motor and the driven equipment.

5) Ensuring that supply wiring and terminal box are properly sized and installed. Inspect regularly the connections at the motor and starter to be sure that they are clean and tight.

SELECTION OF MOTOR :-

A. Torque Requirement

The primary consideration defining the motor choice for any particular application is the torque required by the load. The relationship between the maximum torque generated by the motor (break-down torque) and the torque requirements for start-up (locked rotor torque) and during acceleration periods is very important. The thermal loading on the motor is determined by the duty/load cycle. One important consideration with totally enclosed fan cooled (TEFC) motors is that the cooling may be insufficient when the motor is operated at speeds lower than its rated speed.

B. Sizing to Variable Load

Industrial motors frequently operate under varying load conditions due to process requirements. A common practice in cases where such variable loads are found is to select a motor based on the highest anticipated load. In many instances, an alternative approach is typically less costly, more efficient and provides equally satisfactory operation. With this approach, the optimum rating for the motor is selected on the basis of the load duration curve for the particular application. Thus, rather than selecting a motor of high rating that would operate at full capacity for only a short period, a motor would be selected with a rating slightly lower than the peak anticipated load and would operate at overload for a short period of time. Since operating within the thermal capacity of the motor insulation is of greatest concern in a motor operating at higher than its rated load, the motor rating is selected as that which would result in the same temperature rise under continuous full-load operation as the weighted average temperature rise over the actual operating cycle.

Losses in INDUCTION MOTOR :-

Losses are the source of inefficiency in motors, i. e. energy that goes into a motor but does not produce useful work. Losses in induction motors are classified into two types:

1. No-load Losses: These losses are independent of load and incurred even when the motor is idling.

2. Load dependent Losses: Vary as function of motor loading

The losses in a motor are of two types such as fixed i.e. independent of load on the motor and the other variable i.e. dependent on the load.

Fixed losses consist of Iron loss and mechanical loss (friction and windage loss). The iron loss vary with the material and geometry and with input voltage whereas friction and windage losses are caused by friction in the bearings of the motor and aerodynamic losses associated with the ventilation fan and other rotating parts.

Variable losses consist of resistance losses in the stator and in the rotor and other stray losses. Resistance to current flow in the stator and rotor result in heat generation that is proportional to the resistance of the material and square of the current. Stray losses arise from a variety of sources and are difficult to measure directly or to calculate and are generally considered proportional to the square of the rotor current.

THUMB RULE FOR SELECTING CAPACITOR FOR P.F IMPROVEMENT ( To be connected with motor )

The size of capacitor required for a particular motor depends upon the no-load reactive kVA (kVAR) drawn by the motor, which can be determined only from no-load testing of the motor. In general, the capacitor is then selected to not exceed 90 % of the no-load kVAR of the motor. (Higher capacities could result in over-voltages and motor burn-outs). Alternatively, typical power factors of standard motors can provide the basis for conservative estimates of capacitor ratings to use for different size motors

DIGITAL MOTOR PROTECTION RELAY - CIRCUIT DIAGRAM

Current carrying capacity of ALUMINIUM BUS BAR