Electric Power Plants

[Dana]

... I work in a hardware store and we sell big, 240-volt air compressors; if you try to run them on 120 the thermal breaker in the motor will trip--that's the good news. The motor is trying to draw twice times the amps resulting in four times the heat.

Uh... not exactly. Power (wattage) equals I²R or V²/R. Halve the voltage and you get 1/4 the power. What is probably happening is that the motor has a starting circuit (which has much less resistance, resulting in higher amps during starting until a centrifugal switch cuts out the start circuit), and the lower voltage can't get the motor up to speed to get it off the starting circuit, so it quickly overheats.

[shadow738]

Sorry Dana, I have to disagree. W = I^2*R, but a motor is not a resistor. When just starting or anytime running slower than its proper speed, a motor looks a lot like a short circuit. I work with industrial strength motors and have recently taken classes. A motor is also a generator and what limits the current is not so much the internal resistance but the "Counter Electromotive Force" [voltage] being generated.

The simplest case is small DC motors (eg model trains) which have an internal resistance of about 10 ohms. When you first put 12 volts on them, they draw over an amp for a fraction of a second. Once they are up to "full" speed [proportional to the voltage] they draw about 1/4 of an amp. I maintained controls for these puppies for four years as a full time industrial controls technician.

Doug.

[Dana]

Sorry Dana, I have to disagree. W = I^2*R, but a motor is not a resistor. When just starting or anytime running slower than its proper speed, a motor looks a lot like a short circuit...

Mmmm... good point. Inrush current is often much higher. Though small DC motors are often speed controlled by feeding them lower voltage.

[Ron Blum]

"I work with industrial strength motors and have recently taken classes."

Out of curiosity I am wondering where one would take a class(es) in motors/motor design. I want to learn all I can. Thanks, Ron

[steveinindy]

I was thinking the same thing. Not that I want to even try to actually build one but I'd like to sound as little like a blithering idiot when I talk shop with the guys from RR.

[shadow738]

"I work with industrial strength motors and have recently taken classes."

Out of curiosity I am wondering where one would take a class(es) in motors/motor design. I want to learn all I can. Thanks, Ron

I took industrial controls and other practical courses at my local community (2 year) college, basically to catch up on stuff that has been invented since I got my electrical engineering degree from Purdue in 1968. I'm semi-retired and trying to get a full- or part-time technical job. My strength has always been that I understand and can do the practical side as well the theory. My wife is a librarian at the college so we get an employee discount on classes. I'm hoping to get a job teaching this stuff part time at one of the local colleges--I can do that without a master's degree.

The classes were "Programable Logic Controllers, National Electrical Code [for my electrician's license], Motor Controls, Industrial & Commercial wiring [to upgrade electrician's license], and Industrial Electricity" [mostly advanced motor controls]. I also take writing classe. Sorry about getting off topic, guys.

To go beyond the tradesman/technician level, I buy really old books on the subject (back when electric motors where high tech] and check out similar books from the college and public library.

It may be possible to take classes at a local 4-year college but this might be expensive.

--- Doug

[shadow738]

Mmmm... good point. Inrush current is often much higher. Though small DC motors are often speed controlled by feeding them lower voltage.

Absolutely correct. I use Permanent Magnet DC motors as an example because the speed is almost exactly proportional to the voltage, and the amps are proportional to the load [torque]. The electricity being converted to mechanical energy is the internal voltage (Counter EMF) times the amps. The amps are the (line_voltage - Counter_EMF) divided by the internal resistance.

--- Doug

[Ron Blum]

Thanks for information on the classes that you have taken. This is really helpful. It is also cool to know that you are trying to get a teaching/technical job after "retiring". Thanks, Ron

[shadow738]

I don't think anyone has said that 1 horsepower = 746 watts. Of course nothing is 100% efficient -- electric motors are typically 85-92% efficient, so if your motor is 85% efficient, then you need 877.65 watts to produce 1.0 horsepower. Note: in the metric system, watts are used as the unit for mechanical power as well as electrical power. I am also a pilot and retired electrical engineer. I've been going to Oshkosh since 1974.

[Ron Blum]

I've been going to Oshkosh since 1974.

Shadow738: Can I meet you for a couple moments at Oshkosh this week? I would love to learn good sources of motor design. In other words, what makes a motor more (or less efficient), higher torque, etc.

If so, please call 316-295-7812 to arrange a time to meet up. It would be fun (Oshkosh since 1976).

Thanks!
Ron