Power generating system . . . Gate resistor value . . . Direct drive motor . . .
Dear Search Engineer: I am trying to design and manufacture a power generating system. Spec output is 100V ac, 10A. Do you know where I can obtain information on coupling the generator to the motor? And any information on a manufacturer of the generator and motor (5-10 hp)? —J.C., DN reader
Dear J.C.: One reader suggests that you use a "standard" ac generator, 115V, 60 Hz (which may be purchased, coupled to any power source you want, such as wind generators, gasoline engines, etc.), and connect a "standard" off-the-shelf ac power supply. If you select a "modern" switched power supply, you will end up with a low risk and simple piece of equipment to manufacture.
Dear Search Engineer: I am looking for an instrument that will allow me to measure the amount of water vapor in a low pressure (0.2-2 torr) nitrogen stream. I do not want to pay for a residual gas analyzer system. Any suggestions?—J.F., DN reader
Dear J.F.: One reader suggested that a possible solution is to use a chilled mirror hygrometer of the type available from Yankee Environmental Systems. Check it out at http://rbi.ims.ca/4394-519.
Dear Search Engineer: Can you tell me the formula for calculating the ideal gate resistor value (RG) for a P channel enhanced FET switching circuit driven from a CMOS microprocessor output. I understand that you could drive the FET directly from the CMOS output, but if you use a series resistor for the gate, you can reduce potential long-term damage to the CMOS output. Do you know how to calculate this ideal value? —P.L., DN reader
Dear P.L.: It would depend on the circuit. Presumably this is a power-FET, so it has a high gate capacitance. If you put a resistance in series with it, you've created a low-pass filter, so make sure the time constant is acceptable for the load you are driving. You might do better by putting a cheap CMOS buffer between the µP output and the pFET gate; if that goes, it's a lot cheaper to replace than a µP; in addition, you reduce the load on the µP, as a buffer would probably offer more drive capability.
Dear Search Engineer: I am looking for a budget way to control the speed of a 220V ac 3-phase delta 3-hp direct drive motor used for a polishing table (yes, I know pulleys would be nice). Exact constant speed is not required, just a lot slower than the 3,450 rpm that the table spins at now. Any suggestions? —T.G., DN reader
Dear T.G.: You haven't said if you want constant or variable speed. If the latter, remember that if you change ac motor speeds by whatever electronic means, it affects the available torque, dramatically! If you can't use pulleys, try a double cone system, or a variable speed gears-based reducer, or change the motor to dc—the speed can be modulated electronically without much loss in power. If none of these work for you, check the torque results of using an ac power conversion system, then price it out and compare to the other options.
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