150

NEMA 1 or NEMA 12

Floor

Infinite

"Drive may be designed for constant torque or for torque varying as the square of the speed.

"Drive may be designed for constant torque or for torque varying as the square of the speed.

Figure 10 displays some representative speed-torque characteristics of the drive utilizing a squirrel-cage induction motor and a representative centrifugal pump. Note that each characteristic intersects the zero-torque point at a different speed value. This differs from those of other drives, illustrated in Figures 2 and 5. The steepness of the slope of each characteristic as it rises from its zero-torque value indicates its low-speed regulation and provides a clue as to its ability to maintain speed with little fluctuation as load torque varies slightly. Motor speed is a function of frequency adjustment; the voltage is adjusted only to accommodate moderate changes in motor impedance. Table 5 lists significant application data.

Solid State Adjustable Frequency Inverters The opportunity for adjustable speed for ac induction motors has been realized by the development of the adjustable frequency control as well as the more recent vector drives. Similar adjustable speed drives are now available for the control of switched reluctance (SR) and permanent magnet (PM) brush-less motors. These three technologies are well suited for pump applications that can benefit from adjustable speed.

There are many manufacturers that offer variable frequency with constant or variable voltage IGBT-driven PWM inverters for reliable and performance programmable adjustable speed control of ac induction motors. These drives are available in power up to the hundreds of horsepower (kilowatts). They offer many standard features that can be useful for driving pumps. Some of these are summarized as follows:

• Wide input voltage range allowing standardization of motors.

• Dual frequency operation (50 or 60 Hz)

• PWM output for sine wave voltage at selected output frequency

• Constant or variable torque

• High starting torque at programmable linear acceleration/deceleration

• Over-current protection

• Phase to phase and phase to ground short circuit protection

• Maximum output frequency of 120 Hz for double motor speed

• External contacts for other uses

• Analog speed output proportional to frequency

The use of these types of adjustable speed drives provides pump speed ranges up to nearly twice the base speed of the ac motor. The details of an actual selection of a system should be discussed with an application from a supplier in order to select the right product and size for the pump. An example of a low-power ac inverter is shown in Figure 11.

These classes of solid state adjustable ac motor drives take the 50 or 60 Hz ac line voltage from the grid (wide voltage tolerance range OK) and first rectify it into a dc voltage. Some filter capacitors are sometimes used to attain a smooth dc voltage. Then the dc is converted into sine waves, one for each phase, at a frequency that is selectable to achieve the speed of the motor and pump according to the number of poles in the motor. The drive

FIGURE 11 Low-power AC inverter (motor controller) for three phase motors with sine wave outputs and an output frequency range from 1 to 120 Hz (courtesy of Leeson Electric Corp.)
FIGURE 12 AC inverter (6) step simulated sine wave shown for one phase

is designed to maintain a constant volts/Hz relationship. Speed can be controlled to about 10% by inputting the frequency setting. The sine waves can be approximated, as shown in Figure 12, by the use of a simple six-step transistor switching method. Alternatively, they can be very precisely generated using a PWM switching topology at a high carrier frequency to achieve a very low-distortion sine wave. The dynamic response of these systems is sufficient for many pump applications. However, if there are sudden load changes and speed must be accurately maintained, such as for metering pumps, a higher dynamic response will probably be needed. For applications that require high acceleration, the constant volts/Hz algorithm will not allow a fast enough dynamic response because the system bandwidth will likely be too low.

Another problem with these drives is that they usually need a voltage boost at low speeds because of the low frequency. This would be a problem only for metering pumps run over a wide speed range, which is very rare.

Flux Vector Drives The highest performance class of solid-state adjustable speed drives for ac induction motors are known as flux vector drives. The rectification requirement of the incoming ac power is the same as is the regeneration of sinusoidal currents for each of the three phases to power the motor. The very important difference between them is that the phases are controlled in a closed loop fashion. The control block of the flux vector drive must receive rotor pole angular position feedback information. With this information, the inverter driven ac induction motor can be made to operate like a servomotor. Most flux vector drives utilize a digital signal processor (DSP) for all of the control functions using the software commands programmed by the supplier (user or supplier can modify). These include the various control loops such as speed (within 0.5% if required), acceleration (linear or to a function), current, torque, power, and several more over a very wide speed range. Drives of this type are available for high power with output frequencies of 800 Hz from several suppliers and up to 3 kHz from a few suppliers.

Flux vector drives can achieve a dynamic response in the 10-millisecond range and very smooth speed regulation, down to zero with ordinary induction motors. The concept of the vector control is to observe the present position of the rotor poles and formulate the control to achieve a dc servomotor performance. The control scheme synthesizes the two currents in the motor. If the calculation is done correctly, one of the synthesized currents controls the flux in the motor and the other controls the torque. In other words, one current vector in the stator phases lies along the vector of the rotor flux and the other current vector lies in quadrature or at 90° out of phase. The DSP constantly receives rotor pole position information and constantly recalculates this relationship at all speeds in spite of load changes.

An important feature of flux vector drives is that with simple and minor software modifications they can easily be used to drive PM brushless motors. The drive has transistor gate drivers controlled by the commands from the DSP that receives shaft angle position information from a shaft sensor such as an encoder, resolver, or hall sensors. The poles on the rotor do not move with respect to the shaft and rotor current (true synchronous operation without "slip") as do the poles in the induction motor. It is actually much easier to use this vector flux vector inverter for a PM brushless than for an ac induction motor. This is because there are seldom any calculations to required to run the brushless motor unless phase advance is needed. However, for pump drives, this requirement seems unlikely.

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