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Fig. 29-3 Synchronous Motor-Driven Process Compressor. Source: Compressed Air and Gas Institute

Fig. 29-4 Induction Motor. Source: United States Motors

29-4) is driven by an electric current at a speed less than the synchronous speed, current flow in the stator windings induces current flow in conductors contained in the rotor. The magnetic attraction between the rotor and stator produces torque. The difference between normal operating speed and synchronous speed that induces current to flow in the rotor is called slip.

When an induction machine is driven by a prime mover above its synchronous speed, circulating currents are induced in the rotor bars by the reactive current of the power source and the magnetic field interactions between the rotor and stator are converted into electric power. Thus, induction machines can function as either motors or generators.

Induction motors are most commonly applied in capacities ranging from fractional hp (or kW) to several hundred hp (or kW), and range in efficiency from less than 70% for small motors to more than 96% for large units. One disadvantage of induction motors is that they take lagging current, producing PFs from 0.75 to 0.90 at full load, and substantially lower under part load. Induction motors also draw high starting instantaneous current, typically producing a system voltage drop. Once started, induction motors are very stable and able to withstand voltage drops of up to 25%.

Prior to the advent of solid state variable frequency drives (VFDs), dc motors were used for variable speed applications. Dc motors provide ease of control and comparatively high operating efficiency. Basic types of dc motors include shunt-wound, series-wound, and compound-wound, with each selected based on speed and torque characteristic of the driven load.

Shunt (or parallel) motors are best applied to controlled speed operations, since speed varies only slightly with load. In a series motor, the armature and field are in series. Speed is almost inversely proportional to the current and torque varies as the square of the current. Therefore, an increase in current produces a larger proportionate increase in torque, making series design well adapted for applications that require large starting torque such as cranes, hoists, and traction work. If the load is removed from a series motor, it can accelerate to a destructive speed. It is, therefore, common for series motors to have additional parallel windings that serve to limit the maximum speed.

The mixing of parallel and series windings results in a compound motor design. Compound motors feature higher starting torque than shunt motors, but poorer speed regulation. They are well suited for applications in which large and intermittent increases in torque occur as with punches and rolling mills.

In shunt-wound motors, speed is controlled by adjusting the armature voltage from zero to rated nameplate voltage (while applying full rated motor field voltage). The motors can provide a constant torque capability through nearly the entire controllable speed range, with power increasing from 0 to 100%. With some dc shunt motor designs, by weakening the motor field, speed can be further increased up to four or five times base speed. Over this speed range, the dc motor can demonstrate a constant power characteristic where torque decreases and is inversely proportional to speed. In a series-wound motor, speed can be controlled for any load by varying the voltage. Figure 295 shows a dc motor-driven process compressor.

Fig. 29-5 DC Motor-Driven Process Compressor. Source: Compressed Air and Gas Institute

Motor Ratings and Efficiency

The most common design for motors of up to a few hundred hp is 460 volt, 3 phase Delta. For larger motors, 2.3 kV or 4.16 kV are more common. For very large motors, 7,500 hp (5,600 kW) or more, 13.2 kV is typical. The National Electrical Manufacturers Association

(NEMA) provides both manufacturers and motor users with guidelines for the standardization of motor dimensions and performance. Internationally, the commonly used rating system for motors is provided by the International Electro-technical Commission (IEC), a European standards organization. There are numerous differences in the design philosophies between the European and United States rating standards and care must be taken in application of IEC-rated devices to NEMA rated systems and vice versa.

There is a standard NEMA table for motor manufacturers that appears in the National Electric Code (NEC), providing the minimum and maximum kVA/hp for a motor on starting. Motor starting torque may be less than 100% or greater than 400% of the full-load torque, depending on the motor design. When voltage is applied to a motor at rest, the motor appears to be in the stalled rotor condition and will typically draw from four to six times its full load running current with a PF in the range of 0.15-0.20 and starting kW of 0.9-1.2 times the full load.

When operating on a utility derived power system, motor starting is typically not problematic except when many motors are started simultaneously. However, when operated on an on-site generator derived system, the starting inrush could stall the generator set and will at least tend to cause voltage and frequency transients. Overcurrent protective devices must be selected with the ability to avoid unnecessary tripping during normal starting inrush while providing adequate protection.

Where applicable, staggered starting or reduced voltage starting are commonly employed to minimize the effects of inrush currents. Three basic types of reduced voltage starting are: Star-Delta starting, auto-transformer starting, and soft starting. Star-Delta starting arranges the motor stator windings in a Star (Wye) configuration upon initial application of terminal voltage and then, at a given motor speed, reconnects the stator in the Delta configuration. Auto-transformer starting is similar, except that voltage is controlled by the auto-transformer. Soft starting uses gated SCR switching to control starting inrush, and is a feature of ac adjustable speed drives.

Full-load motor rating is based on maximum winding temperature, which may be determined from NEMA StandardMG-1, based on rated ambient temperature and the insulation rating (i.e., NEMA designation A, B, F, and H). Service factor is the maximum overload that can be applied without exceeding the temperature limitation of the winding insulation.

Electric service quality is always an important consideration for motor operation. NEMA specifies that motors must provide satisfactory performance when the motor voltage supply is within 10% of the motor nameplate rating. At the end of this limit, motors will generally experience lower efficiency and potential failure. As voltage decreases, motor current must increase, resulting in increased heating of the motor winding. Motors are highly susceptible to damage from voltage imbalances. A voltage imbalance of 3%, for example, may cause a rise in motor winding temperatures by as much as 25%. Motor protection includes phase current-sensing thermal devices that heat at the same rate as the motor and can take the motor off-line before unstable operating conditions can cause damage.

Input power requirement of a motor is determined by dividing the power output by motor efficiency. When power output is expressed in hp, conversion to kW is achieved by multiplying the hp by 0.746 kW/hp as follows:

hp x 0.746kW/hp

'Hmotor

Where n mot0r is exPressed as a decimal. The relation is expressed in SI units as follows:

kWm motor

Where subscripts e and m refer to electric power input and mechanical power output, respectively.

Premium-efficiency motors typically cost 20 to 30% more than standard-efficiency motors, depending on capacity and speed. The cost premium is largely the result of the use of more and better materials. Lamination material is a higher grade, higher cost steel. Typical standard-efficiency motors use low-carbon steel laminations, while premium-efficiency motors typically use high-grade silicon steel laminations that are thinner but have lower electrical losses. There are also more laminations and the rotor and stator core are lengthened. In addition, the laminations slots are larger in premium-efficiency motors, so more larger-diameter copper can be used in the windings. Friction and windage losses are reduced in premium-efficiency designs. Premium-efficiency motors tend to run at lower operating temperatures, resulting in longer life for lubricants, bearings, and motor insulation. They also generally operate at higher PFs.

Figure 29-6 provides a representative efficiency comparison for standard- and premium-efficiency motors

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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