Designing of Distribution System (Part - 5)

Typical loads are shown, however, for the various motors being fed out of motor control center MCC-DF3A. Each motor’s designation and full load amps are shown below the motor symbol that contains the motor’s horsepower rating. Safety switch symbols are shown between the MCC and the motor symbol. Safety switches are used to electrically isolate the motor during maintenance or to ensure it does not start unexpectedly when personnel are working on or in the equipment it is powering. The operating handles of safety switches have provisions for applying a lock-out tag-out device. They are generally provided with fuse protection to ensure adequate short-circuit ratings for the application. For those situations requiring a shortcircuit rating of 10 kA or less, a non-fused safety switch may be specified.

Motor control centers are used to group overcurrent protection and different starter types for the motors in a portion of a power system. They may also contain associated control and distribution equipment as well as connectivity interfaces to industrial control or Building Management Systems (BMS). Motor starters, and motor protective overload relays are available in both electromechanical and electronic solid-state configurations. In a motor control center application, the starter is provided with either a thermalmagnetic circuit breaker or high magnetic circuit protector (HMCP) selected to permit the high inrush current of the motor while starting. Either type of overcurrent protective device provided must be selected to coordinate with the motor overload protection relay. This combination starter is mounted in a removable “bucket”. Lower ampacity buckets are wired to stabs on the rear of the bucket and manually plugged directly onto the vertical power bus bars in the MCC.

Note: Larger hp starter sizes may be physically hardwired to the bus.

Eaton’s FlashGardE motor control center “bucket” shown in Figure 1.1-14 adds an additional level of personnel safety. The FlashGard design incorporates a RotoTract™ lead screw assembly that withdraws the stab assembly off the energized bus bars and into the bucket. A spring-loaded shutter then automatically closes off access to the bus bars.

Figure 1.1-14. Freedom FlashGard FVNR Starter

The 75 hp Circulating Water Pump motors CWP1 through CWP4 shown in Figure 1.1-13are examples of full voltage non-reversing starters (FVNR). The drawing documents these as having a full load amp (FLA) rating of 96 A. Based on rating of 96 A (75 hp), which would require a NEMA Size 4 combination starter. The starter symbol shown on the drawing includes a normally open contactor. This is followed by an overload relay symbol. The overload relay measures the current flowing through the starter contacts to the motor and calculates when an extended overload condition is present that will damage the motor. A contact from the overload relay is wired into the control circuit of the starter, which deenergizes the contactor coil in the event of an overload. Electromechanical overload relays sense an overcurrent by directing the current through a melting eutectic element or a heater pack. The heat is proportional to the amount of current flowing. When the eutectic element melts or the bimetal bends due to the heat from the heater pack, the relay opens the control circuit. The “SSOL” nomenclature next to the overload relay shows these particular starters as having solid-state overload relays. The text “W/GFP” calls for ground fault equipment protection. In the past, this would have had to be added as a separate relay, however, many of the new overload relays use microprocessors to monitor a number of variables including voltage to the motor. Eaton’s C440, C441 and C445 all include phase loss and ground fault protection.

Eaton’s solid-state overload relays also have the ability to communicate status including current per phase and other key operational variables back to a control system. Motors are available in a number of winding styles and performance characteristics. The 75 hp CT-1 through CT-4 motors shown fed from MCC-DF3A are of the two-speed, two-winding variety. Note that six-pole disconnects are required for two-speed, two-winding motors. Because the cooling towers are typically located outdoors on a roof, a NEMA 3R drip-proof safety switch would be required. Many two-speed starters are applied on motor loads such as cooling towers, where the fan needed to run at a lower speed or higher speed, to optimize the heat transfer and maintain water temperature in the return supply to the chiller. ASHRE 90.1 is recommending the use of variable frequency drives in applications where they can reduce energy consumption and improve the performance of the equipment they are powering. As an example, in lieu of two-speed motors on cooling towers, VFDs are being used to maximize efficiency of the cooling process. In these cases, a sensor is placed in the wet well of the cooling tower to monitor the temperature of the water. A set-point controller in the VFD utilizes the output signal from a sensor mounted in the return water pan as feedback to modulate the speed of the fan.

The 150 hp CHWP-1 and CHWP-2 chilled water pumps in MCC-DF3A are shown being fed from solid-state reduced voltage starters (SSRV). These SSRV starters reduce the motor inrush and ramp them up smoothly to their full running speed. SSRV Starters can be used to reduce the “water hammer” effect where the pipes in the system experience a sudden thrust of pressure. Recent declines in the cost of VFDs and their associated energy savings capability have led to their growing popularity in a number of HVAC applications. While VFDs still have a higher initial purchase cost than standard starters or solid-state reduced voltage starters, they have a relatively short payback period. A savvy building owner and design engineer will recognize that the total cost of ownership and energy savings must be considered when electing to specify VFDs.

The output voltage and frequency of this VFD can be set by a digital signal from the keypad or an external analog signal such as 4–20 mA. A set-point controller in the VFD can also be used to maintain a temperature, flow rate or pressure level by utilizing an external feedback signal from a sensor. The use of VFDs in heating, ventilating, air conditioning (HVAC) has been popularized due to the VFD’s ability to save energy. When motors on centrifugal fans and pumps are operated at reduced speeds, the energy required to produce the torque at motor’s output shaft is reduced by the cube of the speed. See Eaton Application Paper IA04003002E for details.

This type of centrifugal load is best served by a variable torque VFD that optimizes the volts per hertz relationship throughout the speed range. In addition to the dramatic energy savings that can be experienced below 80% of the motor’s base speed, VFDs ensure a soft motor start and acceleration throughout the speed range. Eaton’s CPX Clean Power (18 Pulse) VFDs are available in low voltage for operation with 208 V, 230 V, 480 V and 575 V motors.