Thus, the motor can then produce 2.75 875 = 2406 ft-lb of torque to accelerate the load. Note the vertical red line in the chart indicating the motor can supply this torque between zero and full load speed.
Thus, the new AC VFD equivalent must be able to supply the same, or slightly more starting torque than the existing application.
Next, we have to calculate the motor required hp from the required torque. Please note that a motor with the same base rpm should be chosen:
The new calculated motor horsepower is 549 hp. This may present a problem because the new motor is 275% larger than the original application! Most likely, the new motor will be smaller than the existing wound rotor motor and can ft into the existing motor’s footprint.
Keep in mind that we are intending to replace the wound rotor motor and resistor bank with a new motor and VFD. As a result, we have the capability of sizing the new motor and drive to accommodate a 200% overload for 60 seconds to accelerate the load. The new nameplate of the AC system will have to indicate that it is sized for 200% overload for 60 seconds. The VFD is also required to be sized for the 200% overload.
The next step is dividing the new calculated motor horsepower by the overload rating (which is 200%)
The new horsepower in this example is 549/2 = 274 hp.
Rounding up to the next standard hp, we will choose a 300-hp motor. The nameplate rpm will remain the same.
The end result is a new application that will be 300 hp and can provide 200% overload. Comparing system torques, the existing wound rotor motor system could produce a maximum of 2406 ft-lb; the new system can produce a maximum of 2626 ft-lb.
Retrofitting wound rotor motor-based control systems to a VFD will not only provide speed and torque control that is more precise, it will also eliminate a list of potential issues: •;Susceptibility;of;slip;ring;commutator;filming;issues •;Slip;ring;brush;maintenance •;Slip;ring;brush;rigging;burning;and;melting •;Slip;ring;contactor;pitting;and;maintenance •;Energy;losses;in;the;resistor;bank.
For anyone with this older technology, converting to a variable frequency drive offers many advantages. AC variable frequency drives offer maintenance free reliable performance. The variable frequency motors are also more robust than slip ring motors. In addition, active front end VFD can maintain a power factor close to unity and can be configured as regenerative, thus providing energy savings and smoother control. PPI
Mike Kozlowski is paper and forest products industry engineer, Baldor Electric, Greenville, SC
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