Timers are expensive!

[Galaxie]

HP is a common rating used for the more powerful aquatic air pumps I believe. 1/3 hp may run at 250 watts, but its startup power will be about triple. So, they are simply suggesting to not use motors that operate over 1/3 hp. Their startup current will fry the timer. Most typical electronic devices don't produce so much startup current, so they are making a special note for those people using pumps with HP ratings.

My garbage disposal is 1/3hp & blew a circuit once when I had a toaster on at the same time. It must be running near 1200 watts at startup.

 

[joephys]

This is the kind of post that I was getting irritated at. That is simply wrong. The 1/3 hp IS NOT WHAT IT RECOMMENDS! IT IS THE RATING OF THE MOTER IN THE TIMER!!! This is why I quit reading this post, I wish I never made it. If its start up was triple it would be 750 W, the unit is rated to use 1800 W. Inductors don't have a huge start up power, they resist changes in current, so they start very low and work their way up! Since they resist a change of current, they produce a huge voltage when shut down, but the current remains the same. (The voltage produced is dependent on the resistance consistant with ohms law.)

There is an interal resistor in timer. The motor in the timer is a 1/3 hp motor, but because of the high resistance, the current to the motor is limited, so much so that it causes it to turn so slowly (some gear's help with this) that it takes a day to make a complete turn. The power limiting resister causes the timer to use something like half a watt.

I hope this is done now.

 

[Galaxie]

Sorry to irritate you, but I'm trying to understand this. As the picture above says, its "Rating" is the maximum power by which you can plug things in to it. They show you the rating in amps (as with common household applicances) and HP (as with inductive motors). I would say the worst design ever made would be a timer that needs 1/3 hp to turn a wheel that probably only requires 1/256th horsepower.

Wibber was correct, but I think you misunderstood him. An inductive motor "does" take a lot of power to spin it up to speed. There are no gears in an inductive motor, so the motor struggles and struggles as it consumes gobs of power....until it gets up to its ideal RPM, where the power levels off. I said "triple" as a guess, because all inductive motors are different. If it runs nominally drawing 250 watts, its inrush could be 4, 5, or 10 times that. Also, inductive motors don't have nearly as much power consumption when they are shutting down as when they are powering up. There could be some some additional current, but inrush is where you things can really burn up.

I mean, think about it. A timer is a $7 clock. Have you ever seen a clock with a horsepower rating? Only inductive motors are rated by horsepower and there is no inductive motor in a timer.

 

[Wibber]

If its start up was triple it would be 750 W, the unit is rated to use 1800 W.

First there is the power factor. Motors do not have power factors of 1. If you take a power factor of .75, while the 'true' power may be 248.6W (746W/3 or one third horsepower), the apparent power would be 331.6 VA.

To find the constant circuit current of a 248.6 W load, using P=VI at 120V, 331.6 VA/120V finds a constant circuit current of 2.76A.

Sounds moderate, a piddly 2.76 A on a 15 A circuit. Three times that is a measly 8.28A. All within 15A. A lower power factor will make things worse, but house and home is likely safe.

Take the 1800W and make it an inductive load at a power factor of .75. That's a 2400 VA load, of a constant load of 20A at 120V. That's an inrush current of 60A. Things are hot, fires are starting, fishies are dead.

1800W as a purely resistive load has a power factor of 1. 1800W corelates to 1800 VA, which at 120V, is 15A.

 

[Native American]

Wibber was correct, but I think you misunderstood him. An inductive motor "does" take a lot of power to spin it up to speed. There are no gears in an inductive motor, so the motor struggles and struggles as it consumes gobs of power....until it gets up to its ideal RPM, where the power levels off.[/QUOTE]This is better than I could ever explain it. Keeps your house safe, allows an underwriting lab to give it a seal of approval, keeps the manufacturer's liability insurance lower. :)

I actually took two apart recently: one, to fix the switching pawl on a clock-style timer, and I should've shot a digi pic of it while I had it apart (that clock motor is tiny). The second was a digital timer where the switching gearing was not doing its job...just reassembled it, and all was fine...the tiny motor in that one is about the size of an 18ga wire nut!

v/r, N-A

 

[justintoxicated]

Dang, So I guess I can't plug my 2 HP air compressor into my timer to wake up my roomates while I'm at work