• The speeds of a PAM Motor are changed electrically, rather than mechanically. This reduces HP input to the fan, thereby eliminating the need for speed-adjusting coupling devices, which are prone to slip energy loss.
• A PAM Motor is a practical and effective means of driving a load where a change of speed can provide operating economies.
• First costs are lower than that for a comparable two-speed, two-winding motor.
• As a single-winding machine, the PAM Motor is lighter, smaller and more efficient than a two-winding motor for comparable rating and application.
• Rotor heating can be reduced up to 40 percent when starting high-inertia loads by initially using the lower speed connection.
• The PAM Motor saves energy costs because speeds can be changed as operating conditions dictate changes in rates of flow.

After considering all the factors, including cost, space, power savings, and reliability, PAM Motors are an optimum choice for two-speed motor operation.

There are over 500 Westinghouse PAM Motors above 500 hp in operation at electric utilities and industrial plants, driving a variety of machinery including:

• Induced Draft Fans
• Forced Draft Fans
• Primary Fans
• Circulating Water Pumps
• Boiler Feed Pumps
• Compressors

Although the applications and speed combinations are different, the underlying reason for choosing the PAM Motor is the same:

For applications where a change of speed can offer operating economies, the PAM Motor is less costly to install and more efficient to operate than two-winding motors, two-motor arrangements, or motors with any find of hydraulic coupling.

It is typically installed close to the motor to minimize cable requirements. There are six leads (three for each speed) from the switch to the motor.

The PAM Motor is usually started on its lower speed winding to keep the inrush current low. This prolongs motor life by keeping rotor and core temperature excursions to a minimum. Starting on the lower speed is also more desirable for driven equipment considerations.

The chart below, of a typical ID fan, Illustrates how a reduction in motor operating speed will in turn reduce horsepower. This lowers the motor's energy consumption and increases your dollar savings.

By observing the fan laws we see that:

• Volume varies with speed.
• Static pressure varies as the speed squared.
• HP varies as the speed cubed.

In this example, changing speeds results in saving 1200HP. If energy is evaluated at $1700/KW x 1200HP x .746KW/HP, the savings are $1,521,840.

In this example, to attain high unit availability, the fundamental requirement is to size the ID fan such that the boiler will not become ID fan limited. Yet, care is taken not to increase the potential for problems with draft controllability and implosion because of over-sizing. Sufficient margin is added to the gas flow rate estimated from the boiler manufacturer's predicted MCR heat input so that the ID fans can handle the gas flow at the maximum demonstrated heat input without difficulty.

Additional margins are provided for increased air leakages, boiler gas path fouling, air-heater fouling, fan performance deterioration, and other uncertainties.

To account for all these allowances, typically a 15% mass flow margin and a 32% static pressure margin is applied to the MCR values to obtain the Test Block conditions.

Some fans are specified to be designed for initial base load operation with provisions for future cyclical duty. Additional revised design conditions due to scrubber and balanced draft conversion affect the ID fan size.

These requirements are shown in the Pressure / HP vs. Volume curve.

Thus the fans and motors are designed for Test Block requirements but normal operation is a MCR. This sets the stage for two-speed conditions.