Motor size, selection

In view of their easy adaptability, they are manufactured in standard sizes suitable for common types of load applications. They are therefore available on short deliveries, in smaller ranges, say, from I kW to 200 kW. They can be selected for the required service conditions, as a motor is selected to suit a particular load requirement from the available sizes of couplings. 

Motor voltage selection is determined by economics and by the availability of adequate system capacity to permit motor starting without excessive voltage drop. Restrictions by the utility or by the size of the plant s generating capacity may limit the maximum drop to less than 20%. In some cases, system dips as small as 5% or less may be the maxi-... 

As turbine-driven plants generally cost 2-4 million Euro more than similar plants driven by electric motors the selection of drive will reflect the cost of the plant versus capacity as indicated on Figure 17 showing the regression line for LNG plant cost development versus time (year). Likewise, Figure 18 shows the cost development versus the development of LNG train size [10]. The investment may also reflect various requirements for the liquefaction train and necessary support facilities. 

In selecting a washer design for temperature control, motor sizing, compartment size, and so on, the designer should allow the speed of the hUer to determine the rate and capacity. Based on typical washers, hllers, and periodical jams that take 2-3 min, the washer is sized for a rate that is 130-140% of hller speed to recover lost operator time. 

We typically infuse 0.1nM of Fi into an observation chamber of height SOpm. If all are attached, the surface density would be of the order of 1 molecule/pm. In fact, only on lucky days do we observe as many as tens of rotating probes in a field of view some (300 pm). Most of the motors are inactive, or fail to bind the probe which is huge compared to the motor size of lOnm, in a configuration that allows unhindered rotation. Of those that rotate, we select those few that rotate fast, smooth, and with 120° symmetry (see below). Due to this subjective selection, ensemble statistics in our work is unreliable though we make every effort to increase the number of selected to convince ourselves that we are not reporting artifacts. Our statistics relies on repeated behaviors of a selected molecule we analyze many consecutive revolutions without eliminating an event in the sequence. 

In this arrangement the compression heat from the compressor is used to heat up the helium to the desired temperature. The required compressor power was 90 MW with 45 MW generated and regained by the expansion in the gas turbine and 45 MW introduced by the electric drive motor. The selected combination of the turbine and compressor on one shaft resulted in dimensions being comparable with a helium turbine of 300 MW capacity (the reference plant size at the time). 

The start-point for electrical design is the electric motor and other consumers list. This list is typically drawn up by process and mechanical engineers involved in the selection of the driven equipment. The electrical engineer needs to probe this data and understand how it was drawn up, with special reference to the expected load factors (namely, ratio of actual kW or kVA to the value used as design basis). Rarely, drive sizes are underestimated. More usually, they are consistently overestimated as a result of the practice of putting factors on factors . There may be a factor of conservatism for increased process performance requirements, a factor for driven-machine performance tolerances and deterioration of efficiency, and then perhaps a factor to allow for uncertainty, and, then again, rounding up to the nearest standard motor size. 

Duty cycle (how much is asked from a motor how often and for how long) is the final parameter. The higher the duty cycle (i.e., horsepower load above rated and time of operation) at that load, the more care should be taken in the selecting the motor size. 

The selection of a specific motor control system also requires the consideration of a number of factors. Depending on the particular type, size, and application of the motor to be controlled and the particular characteristics of the driven load, motor control selection may be simple or complex. 

Propeller size, pitch, and rotational speed may be selected by model tests, by experience with similar operations, or, in a few cases, by published correlations of performance data such as mixing time or heat transfer. The propeller diameter and motor power should be the minimum which meet process requirements. 

Protection against overloading This can be achieved by an overcurrent relay. The basic requirement of this relay is its selectivity and ability to discriminate between normal and abnormal operating conditions. Three types of such relays are in use thermal, electromagnetic and static. Thermal relays are employed for motors of up to medium size and electromagnetic and static relays for large LT and all HT motors, as discussed in Section 12.5. 

Table 12.4 Selection table for switches, fuses, relays and cables for different sizes of LT motors...

Consider a 55 kW motor to be switched direct on-line and instalied, say, at 75 m from its controlgear. To select the most appropriate cable size refer to Table 12.4, where... 

Textile motors Crane motors Determining the size of motor Sugar centrifuge motors Motors for deep-well pumps Motors for agricultural application Surface-cooled motors Torque motors or actuator motors Vibration and noise level Service factors Motors for hazardous locations Specification of motors for Zone 0 locations Specification of motors for Zone I locations Motors for Zone 2 locations Motors for mines, collieries and quarries Intrinsically safe circuits, type Ex. f Testing and certifying authorities Additional requirements for ciritical installations Motors for thermal power station auxiliaries Selection of a special-purpose motor... 

Today, almost all large motors are designed specifically for a particular application and for a specific driven machine. In sizing the motor for the load, the hp is usually selected so that additional overload capacity is not required. Therefore, customers should not be required to... 

A 5.000 hp break point has been used rather arbitrarily, as larger motors are built, ranging in size to 30,000 hp. Generally as the size increases, the synchronous motor becomes more competitive. However, final selection is not only dictated by the driver economics above, but includes the power system as well. 

A rule of thumb that was used in the past for constant speed applications wii.s to consider the selection of a synchronous motor where the application horsepower was larger than the speed. This, of course, was only an approximation and tended to favor the selection of a synchronous motor and would be considered too severe by current standards. However, the rule can aid in the selection of the motor type by giving some insight as to when the synchronous might be chosen. For example, applications ol several hp per rpm often offer a distinct advantage of the synchronous over the induction motor. In fact, at the lowest speeds, larger sizes and highest hp/rpm ratios may be the only choice. 

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