Consulting-Specifying Engineer - July 2008 - (Page 42)

constant-torque has a slightly larger kVA rating. The selection depends upon the load driven. Centrifugal pumps and blowers are variable-torque loads: The torque demanded by the load is reduced as the speed is reduced. In fact, the torque demanded is proportional to the square of the speed. The following proportional relations hold: Horsepower (hp) α (rpm)3 Torque (ft-lb) α (rpm)2 Flow (gpm) α (rpm) Therefore, if the pump speed is reduced to 50% of the rated speed, the required torque is reduced to 25% of the rated torque. The torque produced by the motor that drives the pump or fan is proportional to the product of the airgap flux and the stator winding current. The VFD that powers the motor keeps the airgap flux constant at all speeds (constant V/f ratio). Therefore, the motor current also reduces to 25% of the rated current. The motor I2R losses reduce to 6.25% of the losses at the rated current. Motor and VFD would have no problem dissipating the heat. Motor current 180º 0º 360º 540º wt for extended periods of time. In such cases, an independently driven blower would be necessary to provide adequate airflow. Flux-vector drive Many applications demand unusual operating conditions of the electric motor. For example, an elevator or a crane drive requires operation in both directions. In addition, the motor is required to produce a positive torque while rotating in the negative direction and a negative torque while rotating in the positive direction, required for braking when the load is being raised or lowered. Such drives are said to operate in all four quadrants of the speedtorque plane (see Figure 3). In some other applications the motor is required to produce the rated torque at zero speed, such as a marine winch motor. An ac induction motor cannot meet these requirements without the help of VFDs, but dc motors can. But dc motors are expensive and require constant maintenance because of the commutator and brushes. A flux-vector drive is used in such applications. Figure 2: Pulse-width-modulated drives produce a near sinusoidal current. Source: Syed Peeran It’s a different story with constant-torque loads, such as rotary and screw compressors, elevator drives, hoists and cranes, and reciprocating pumps. The motor torque, which must be the same as the load torque during steadystate operation, is the same at all speeds, and the motor current remains the same at all speeds. Motor efficiency is lower and heat dissipation is a problem because of the reduced airflow from the shaft-mounted fan. The problem is more severe in totally enclosed, fan-cooled motors used in Class 1, Division 2 hazardous locations, particularly when the motor operates at reduced speed Input #218 at csemag.com/quickResponse 42 Consulting-Specifying Engineer • JULY 2008 http://www.selindustrial.com/cse7 http://www.selindustrial.com/cse7 http://csemag.com/quickResponse

Table of Contents Feed for the Digital Edition of Consulting-Specifying Engineer - July 2008

Consulting-Specifying Engineer - July 2008 Contents Viewpoint Letters News M/E Roundtable 40 Under 40 Using Demand-Based Reset Strategies VFDs and Motors: Making the Right Match Grounding and Bonding Practices for Hazardous Areas Codes & Standards Case Study New Products Equipment Lifecycles Advertiser Index Green Space

Consulting-Specifying Engineer - July 2008

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