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0 20 50 75 100 125 150 175 200 225 250 275 TORQUE,%

FIGURE 19 Typical speed-torque characteristic curves for a centrifugal pump and a squirrel-cage induction, NEMA design B motor. Excess torque accelerates pump.

0 20 50 75 100 125 150 175 200 225 250 275 TORQUE,%

FIGURE 19 Typical speed-torque characteristic curves for a centrifugal pump and a squirrel-cage induction, NEMA design B motor. Excess torque accelerates pump.

Ta = Jtotal X a: Acceleration torque = Inertia X Angular acceleration a = X 2p: Angular acceleration = Max RPM/acceleration time ta

TC = Torque from all other constant forces, friction, windage, preload, and so on.

NOTE: Use consistent units depending on whether U.S. Customary or SI. The time to accelerate the pump is equal to in USCS units t in SI units t

WK2 X Arpm

308T MK2 X Arpm 9.55T

WK2 = total weight (force) moment of inertia, lb • ft2 Arpm = change in speed

T = torque, lb • ft (N • m) MK2 = total mass moment of inertia, kg • m

Because the difference in torque is not uniform over the speed range, the curve (Figure 19) can be analyzed by assuming discrete changes in speed and an average torque over this change in speed. A time period can be calculated for each discrete speed change, and all time values can be totaled to obtain the complete acceleration time.

Increasing the inertia of the pump, the operating speed of the pump and motor, or the torque required by the pump at any speed will result in a longer acceleration time, which may not be possible for the motor. Each motor can operate at a reduced speed and at a torque in excess of rated torque for a given time. Beyond this time, the windings and/or rotor can be damaged. The use of an inverter-driven ac induction SR or PM brushless motor eliminates all of these problems associated with the use of line-fed ac induction motors. This is due to the torque control capability of the inverter/motor drive system using current control. The limit on the ability of the adjustable speed drive to control acceleration is a function of the current capability of the inverter (as long as the thermal limit of the motor has not been exceeded).

When an application is analyzed and found to present an acceleration-time problem, consideration should be given to

1. Unloading the pump during acceleration

2. Reducing the inertia of the rotating parts

3. Selecting a larger motor

4. Considering an adjustable speed motor and drive

When very large pumps are started, the line voltage will sometimes drop because of the high starting current, which causes the motor torque to be reduced by the square of the voltage ratio. Naturally, the acceleration time is greater because of the reduced torque produced by the motor. This will not hamper the ability of the motor to accelerate the pump as long as the motor develops more torque than is required to drive the pump at any speed over the accelerating range. The use of SR or PM brushless induction motors eliminates this problem because these motors do not draw high inrush currents. In fact, the starting current is controlled very precisely.

A similar analysis can be made for synchronous motors after the accelerating speed-torque curve for the motor is known. It should be recognized that a synchronous motor operates as a squirrel-cage induction motor up to the moment of synchronization. At that time, the synchronous motor must have an additional capability of synchronizing torque, tTOTAL = Ta + TC: Total torque = Acceleration torque + constant torque frequently called pull-in torque, to accelerate the motor from subsynchronous to synchronous speed. If the motor cannot develop enough torque at its synchronous speed, it will pull out or stall. When this type of motor is used for a pump, it is advisable to unload the pump during acceleration operation to reduce the torque required from the motor for acceleration.

When using a synchronous motor, attention should always be given to the breakaway torque that is required to start the pump from zero speed. This is particularly important with constant displacement pumps, where the pump will operate at a constant torque over the entire accelerating speed range. Again, a better selection might be an inverter-driven ac induction motor, SR or a PM brushless motor.

Service Factor Motors are available with service factor ratings that range from 1.0 to as high as 1.5. A service factor implies that a motor has a built-in thermal capacity to operate at the nameplate power times the service factor stamped on the nameplate. It should be noted, however, that when the motor is operated at the service factor power, the motor will operate at what is termed a safe temperature. This means that the motor will operate at a total temperature that is greater than the temperature for a motor designed for the same power with a 1.0 service factor. Consequently, it is not advisable to apply a motor with a service factor larger than 1.0 where the continuous power requirements will be greater than the normal power.

A service factor rating on a motor is to provide an increased power capacity beyond nominal nameplate capacity for occasional overload conditions. Also, the speed-torque characteristics are related to the nominal power rating and not the service factor power. When adjustable speed motor/drive systems are selected, the old service factor ratings are not useful. The supplier of the drive system will assist in selecting a thermally rated system that will satisfy the pumping operation. All inverters are equipped with thermal safety protection for both the motor and the drive so the system cannot be overloaded to minimize overheating. This is a very useful feature for many pump applications.

Efficiency Motors are designed to operate with an efficiency expressed in percent at rated voltage, frequency, and power. Efficiency is defined as shaft output power X 100

electrical input power

The efficiency of a given motor design will vary slightly from unit to unit because of manufacturing tolerances and variations in materials. For this reason, guaranteed efficiencies are usually lower than actual efficiencies. When motors are operated at reduced powers, the tendency is for the efficiency to decrease because the losses tend to be fixed.

Several other factors have an effect on motor efficiency. Increasing the applied voltage and operating a motor at its rated power will increase efficiency very slightly, whereas decreasing applied voltage will decrease the efficiency noticeably. Also, increasing the frequency will cause a very slight increase in efficiency, and decreasing the applied frequency will cause a slight decrease in efficiency.

Inverter-fed ac induction and PM or SR brushless motors are not subject to changes in their efficiencies due to changes in line voltage or frequency because the ac line voltage is rectified into a dc voltage before the regeneration of the power to the motor. The dc voltage is always reasonably constant and unaffected by changes in the line power. This of course assumes that there is sufficient voltage and power headroom in the design.

Dynamometers are used to determine efficiency for small motor ratings—up to approximately 500 hp (373 kW)—and standard methods and formulas are used for calculating efficiency for large motors. For the latter, efficiency values can vary, depending upon which "standard" procedure is used. Several suppliers market torque and power analyzers for induction motors that base their output on these formulas using current data measured from the motor. These methods are accurate enough to be acceptable. They have been reasonably verified by applying the method to smaller motors that have been tested on a dynamometer. NEMA has established one method, but some suppliers use different methods, depending upon their national standards or established practices.

Higher efficiencies have been designed into most new motors by selecting materials and proportions that reduce losses, but such design choices usually make the motor more costly. This also includes filling the stator slots with more copper of a larger wire gage to reduce ohmic losses. These new motors designed for higher efficiencies offer improvements from 2% to as high as 7% depending upon the size and manufacturer.

Motor operating efficiency in a typical application often may not be that described by the manufacturer. Quoted efficiencies are always at rated power output, rated frequency, and rated voltage. However, it is almost universal in application that a motor is oversized for its applied load and frequently operates at other-than-normal voltage. Both of these variables can greatly reduce the efficiency of a motor. However, when a motor is inverter-driven, the efficiency of the system can be much better controlled.

Power Factor The power factor of a motor is expressed as

PF = 100 cos e where e is the angle between voltage and current at motor terminals (leading or lagging).

The power factor at which a motor operates is dependent on the design of the motor and is established at rated voltage, frequency, and power output.

For induction motors, the power factor can never be 100% leading. A number of factors will influence the power factor of an induction motor:

Condition

Effect on power factor

Increase applied voltage

Decrease

Decrease applied voltage

Increase

Increase load

Increase

Decrease load

Decrease

Increase applied frequency

Slight increase

Decrease applied frequency

Slight increase

In synchronous motors, it is usual to use two varieties of motors, the 100 (unity) and 80% leading motors. The power factor of a synchronous motor operated at rated voltage and frequency is fixed by its field excitation and its power output. At a given power output, the power factor can be adjusted over a range by adjusting the field excitation. Increasing the field excitation will cause the motor to operate at a more leading power factor, and, conversely, reducing the field excitation will make the power factor lag.

Varying the power output of a synchronous motor with a constant field excitation will vary the operating power factor. A decrease in power output will cause a more leading power factor; conversely, increasing the power output will induce operation at a lagging power factor. Consequently, to operate the motor at rated power factor with a varying output power, it is necessary to adjust the field excitation. However, this is not normally done because a synchronous motor is frequently used for improving the power factor and the more leading power factor is used to accommodate power factor improvement. When a synchronous motor is overloaded and operates at a more lagging power factor, it is not usual to increase the excitation beyond its rated excitation because of the extra heating this will develop in the motor. In this case, the more lagging power factor is simply accepted.

The power factor is not of much concern for SR and PM brushless motors driven by inverters because of the control capabilities for these systems. The PM machine exhibits the best power factor of any known motor under nearly all operating speeds and power levels except if excessive field weakening or phase advance is used.

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