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Bob's Mailbox

Bob: An induction motor pulls nameplate current at namplate rpm. If it is being used as a generator, the slip is opposite what is experienced when the device is used as a motor, but rated power is produced at about the same absolute slip. If connection is made between the mains and motor/generator at any speed between rated motor full-load rpm and synchronous speed plus rated slip, the current must be equal to or less than nameplate current. The service factor rating even provides a little additional slack for speed matching. There will be no huge surge of current. There may be issues with the control system if it comes up in the wrong quadrant, or the gear box may not like the torque reversal under the same conditions. A motor used for this service should certainly be as efficient as can be afforded (But that is always true—a tautology. /rap) and that feature will decrease the full-load slip. Engagement at near synchronous speed is pretty easy for an induction machine. I often start the engines of small induction generators from a dead stop by plugging the contactor to turn the engine over. Sometimes the engines do not even have starters installed. (Check. I often start my Beetle by popping the clutch at a low speed (6 mph in second or 10 mph in third). I’ve been told this does less harm/damage/cost than turning the key and exercising the Bendix. I tend to believe it. But I have only replaced a Bendix once in 45 years/1,500,000 miles. /rap ) Remember that induction motors are often started across the line while attached to full load. Lucky motors get reduced voltage, partial winding, soft-start, or Y delta starters. Most oil-well-pumping units and gravel crushers just get oversize magnetic contactors. Starting current is limited by long lines or the service transformers. (My father had a circular table saw, and I remember when it started properly by turning on its switch. But when it refused to start (bad cap or bad winding?), he just blocked up the motor, spun the pulley, hit the switch, and lowered the motor down against its pulleys after it started— not a big deal! Just a normal trick! I’ve learned how to do it. /rap ) The control system in most wind turbines is far less sophisticated than you would expect. (I have been told that some have synchronous converters—what, 2%, 4%, 8%, 16%? /rap ) Those guys have come a long way, but they have a long way to go. Don’t give them undeserved credit, and don’t give them my address. Sorry to hear about the frostbite.

John Carroll

Pease: Not a big deal. I’m gaining on it.

Hi Bob: I just read Bob’s Mailbox from August 9, and I must make a point about total harmonic distortion (THD) in audio systems that many people miss. What really matters are which harmonics and non-harmonically related overtones are generated during distortion. For example, crossover distortion (caused by an underbiased class AB amp) is much more objectionable than clipping. That is to say that if the distortion is caused only by clipping, a higher percentage of THD is permissible than if the distortion is caused by underbiasing. (We agree, of course, that some kinds of distortion are more important than others. But when the distortion gets down to 3 or 1 ppm (not to mention 0.3 ppm), I don’t think the distortion is important anymore. Take a look at the datasheets on the LM4562 and LM4702. /rap) One attempt to weight the harmonics to make a distortion measurement scheme that reflects subjective evaluation is the GedLee metric ( From what I understand, it correlates very well with subjective ratings.

• Stephen L. Martin

Pease: Thanks. Best regards.

Hi Bob: I just got back from a 500- mile hiking trip on the Appalachian Trail and saw your piece on the cold-toe detector (“What’s All This Cold Toes Stuff, Anyhow?” 21 June, p. 24). I would think this would be an ideal application for a small micro-power microprocessor. (No, it would not be “ideal,” because I do not do microprocessors. I do human interface. /rap) Using one of these, the parts count would be reduced to four: the temperature sensor (My scheme can accommodate four or more temperature sensors. /rap), processor, signaling device, and small button cell for power. It could be made small enough to mount on the shoe, say, where the tongue of the boot starts. (When the tongue of the boot is covered with a foot of snow, I’m not going to peek to see if the LED is blinking. I want it to beep in my ear. Besides, my boot is usually covered up with leggings. /rap) (I’m not too keen on the idea of running wires up my legs!) A piezo speaker or flashing LED could warn you the temperature at the toes is below freezing. (No way! I want to know when the temperature is dropping! Your mentality that a computer can tell me that my feet are already too cold is exactly what’s wrong with all the digital guys. Sorry, but go away. Get thee behind me, Satan! /rap) The advantage of this approach is the processor can be programmed to take a temperature reading at given intervals and then to take readings at shorter intervals if the temperature approaches a dangerous level. When not active, the circuit would draw microamps, making a small button cell practical. The only downside of this approach is the need to write a simple program and programming the processor with it.

• Steve Weber

Pease: I’ve never made any mistakes writing software. Because I never write software. And I’m not going to start now

TAGS: Components
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