Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

Eric, I don't want this to degenerate into a pissing match, but this characterization of what occurred cannot be allowed to stand. I was the first to respond to correct the errors in your post11. In post14, I explained that a) power and energy are not the same thing, and b) power = work/time, and c) torque translates to linear force, not linear power.

You then responded to my post with your post15, repeating the error (claiming that power and energy share the same units), and issued a challenge directed at me asking "150 kJ << without my telling you, is this power or energy?"

I then politely responded to your direct challenge, explaining that Kj is energy (or work), not power.

The only criticism I will accept is that my answers were perhaps too concise.


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Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

It's not the response to post #11 that I have a problem with. It's your responding to post #15, 7 hours after the issue had been addressed with an explanation and the correction accepted, with absolutely no explanation. That's what annoyed me.

Your answers were so concise that they may as well have been "You're wrong." with no further explanation. I would suggest not using that format the next time an issue of this sort comes up unless you happen to like making not-friends. Even a comment about "power uses unit X and energy has unit Y" would be better than "It's X not Y" or "No it's not." Something like what Coffeeism said: "Well thats energy because power would be j/s." is much more helpful and constructive.

Again, if you're going to just say no, you might want to explain why you're saying no.

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

You forgot to mention that the 7 hours was during the middle of the night, and that I responded the following morning, and that I was responding to a challenge in your post that was directed specifically at me.

There was further explanation in my response. I explained that Kj is a measure of energy or work, not power. And I explained in the prior post that power is work/time, so I can be forgiven for not repeating that.

And I suggest that if in the future you feel compelled to scold someone, especially someone you don't even know, as a matter of simple courtesy you first take them aside and do it privately and see if the issue can be resolved amicably.

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Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

This conversation has edged past where it needs to be. I am guessing that the original poster still wants information, but is being dissuaded from trying by this behavior.

Take it outside guys.

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

Eric, the OP changed motors, and posted another thread to that effect elsewhere, asking about powering a 12V motor with a standard 9V battery. (The response given there was that you really want a 12V battery.)

Outside is where it's already going.

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

Enough is enough boys. Time to cool off and get perspective of where you are. This thread will be closed for about 24 hours.

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

As I promised, the thread is again open. We will be watching.

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

I feel that aside from debates on work and power we should look more at what changing the voltage on a DC motor will actually do, and more importantly the monstrous difference between and AC and DC motor.

If I run a normal 12v DC motor at only 6v the amount of power that can be produced by the motor isn't cut in half but instead on fourth. The reason for this is that Current is determined by the voltage and resistance. The amount of electrical power being used by the motor (not relevant to the amount of mechanical power it is currently producing) is directly related to Current and Voltage. The specific equations for both of these is:

I = V/R (I = current, V = voltage, R = resistance)

P = I*V (P = power)

P = V^2/R <- can be derived from previous two equations

Since the resistance of the motor at any given rpm can't change (physical property of the motor) the power varies directly with the square of the voltage. By changing the voltage at which you run a DC motor you can't hurt it. The way in which you will hurt a DC motor will be to overheat it, this result of over-volting it and pushing too much current through the wire. Basically, for you cart that is running at half voltage, if you aren't using really special DC motors you won't hurt them by running them at half voltage.

The topic of AC motors is quite a bit more complex and as a result I believe that most people here were missing the elephant in the room when it came to using a sump pump. The problem here is that it is single phase. A single phase AC motor will run go from having plenty of torque to none in a 60hz cycle. This result because the sinusoidal wave will hit 0 volts as it goes from positive to negative. If you open up most AC motors in your house you will find that to compensate for this two things are done.

1. The motor is much larger than the application requires so that it can get whatever system it was designed for started. It will also have a fly wheel or some other form of high inertial mass to keep the motor turning when it isn't producing torque.

2. The motor actually has a second set of coils that are 90 degrees (electrically) out from the main power coils, and a capacitor that changes it into a 2 phase motor. This extra circuit is turned on when the motor is starting up and is turned off by a governor when the motor gets up to speed.

The reason that I highlight this weakness of single phase AC motors it because a standard robot drive-train will be driving with a lot of direction changes. The motor would be constantly asked for large amount of continuous output torque which a single phase AC motor is not capable of. This is why high power AC motors are always 3 phase. Since there are three sinusoidal waves there will always be a power coil. In fact, 3 phase AC motor will have a much flatter output torque curve than most DC motors since these motors are usually bushed and have a certain amount of cogging torque (take an un-powered CIM and turn it with your hands to feel this).

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

Alex,
AC motors come in many types and flavors. The capacitor in AC motors doesn't always phase shift the current in a second winding, for instance. The fact that some motors run on three phase power is due to the need for all motors to have rotating magnetic fields. Single phase motor play some interesting games to achieve rotating fields. DC motors perform the same function using the brush assy and commutators for multiple windings or in the case of brushless DC motors, produce multi phase AC current for what is essentially an AC motor. What is confusing is that some AC (synchronous) motors react such that max torque is present at their design RPM. These types cannot be made to run at different speeds. If you load them to the point that the shaft rotation is vastly reduced there is no torque and the motor will stop. (Current continues to flow in this case) Some AC motors using a starter circuit, will not start without current flowing in the startup winding. If you spin them with power applied, they will run and produce torque at their design RPM. These motors often contain a centrifugal switch that keeps current flowing until the motor reaches an RPM that opens the circuit. If the switch or capacitor fails, the motor will hum and buzz and get hot but will not start.

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

[EDIT] This post is not responding to any particular post in this thread, just providing some information possibly relevant to the discussion.

The question whether or not running a brushed DC motor (like a CIM) with a supply voltage lower than that for which it was designed is harmful to the motor depends on what exactly you are trying to accomplish.

If your intent is to get the same output power from the motor at 9volts as at 12volts then yes, this will cause more motor heating and may damage the motor over the long haul or shorten its life.

The reason for this is that the motor is less power-efficient (output power divided by input power) at lower supply voltage. With a 12volt supply, you can get a specified output power from the motor with less input power (less current), and thus less motor heating.

For example, see attached graph. The output power is the area of the speed-torque rectangle.

At 12V, the rectangle is tall and skinny, so the motor speed is high and the torque is low. Because the torque is low, so is the current, and therefore so is the I^2R heating*. And because the speed is high, the gearbox is designed to gear down the speed to increase the torque applied to the load to the desired value.

To get the same speed and torque output from the gearbox to the load with a 9V supply voltage to the motor, you need the same output power from the motor**. To get the same output power from the motor the rectangle is short and wide. Because the torque is high, so is the current, and therefore so is the I^2R heating. And because the speed is low, the gearbox must gear up the speed (or gear it down less than with 12V) to increase the speed (and decrease the torque) to the same values applied to the load to the desired value.



* there are also some heating effects due to eddy currents in the magnetic circuit which are speed-dependent, which I have ignored in this analysis

**ignoring gearbox efficiency, which may change with different gear ratio. factoring in changes of gearbox efficiency changes the numbers a bit but does not change the overall conclusion.



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Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

some great info! and thanks for stoping the argument! another question I have is (since I have decided to go the route of a previous roboteer i knew and use 4 lawn mower starters for my main drive and steering.) I will be using 2- 12v batteries which should be sufficient. I was wondering how I would accomplish speed change? the motors should do it quite easily. and then for backing up and quick direction change. how would they function? they will be connected using chains to the slightly bigger wheels. I do want about 8 mph in speed so info?...

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

For motor control (speed and direction), you'll need a speed controller of some form. IFI's Victor speed controllers and the Jaguar speed controllers used in FRC are rather expensive, though; if you went that route, you'd want to borrow some from the nearest FRC team. There are a couple of homebuilt versions out there, but you'll REALLY want to know what you're doing if you go that route. There are R/C speed controllers out there, but you'll need to check the ratings on those before you use them--most R/C controllers are designed for less than 12V (or 24V, if you've hooked up your batteries in series).

BTW, how are your batteries hooked up: in series or in parallel?

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

And, Weird Al/Tony: Google those terms if you're unfamiliar with them before you answer.


Another answer on speed and direction control: Strange as it may sound, considering the discussions about low voltages and their effects upon motors, but changing the speed of a DC Motor is as simple as changing the voltage feeding it.

FRC uses Victor and Jaguar speed controllers. These are so-called "H-Bridge controllers" (google that, too) but I fear they may be a bit out of your desired price range. What they do is take a signal from the computer or something that's controlling the robot, and change it to a variable voltage. They even reverse the voltage polarity so the motor can run backwards. And (most importantly) they can do this while supplying 50 or more Amperes of current - that's a lot.

So, let's take your lawnmower starters. Under a bit of load - say, moving a 150 pound robot up a slight hill, on grass, at 8 MPH (just under 12 feet per second, which is easier to measure) - I'd guess that the motors would draw about 13 Amps each.

If you want really good speed control, then an H Bridge is almost your only option. But if 2 or 3 speeds (also in reverse) can be considered OK - maybe for the first prototype? - then there are cheaper ways.

One example is a "resistor pack". You get some heavy wire (that can handle 15 Amps easily) and make some low value resistors - say 1/2 and 1 Ohms - to drop some voltage. As I mentioned before, lower voltage means slower speeds, so if you used a relay* to put one, the other, or both relays in series with the motor, you'd get different speeds (slower for more Ohms). This is exactly how some older cars changed the speed of the heater blower - the driver moves a switch that puts resistors in series with the blower motor, slowing the fan down. Matter of fact, if you can get to a car junkyard and find a resistor pack, that would make a dandy speed controller piece.

Anyway, the point is, you don't have to spend much if you look carefully and get stuff that can be used for things cheaply. The hardest part of all this is having the experience to recognize when something in your hands can be used to make something else happen.

So, my suggestion to you is to learn a bit more before spending any real money. Put together what you have, get pieces working first (experiment with one DC motor, one battery, and some heavy wire) and start to get that experience in how things work. After playing around for a while, you'll be able to put little things together and make a robot.

*Make sure the relay can handle the Amps! Car junkyards are a great source of DC stuff - relays, motors, switches and more... Oh, and stuff like that is cheap, and it can be cheaper if you tell them you're a kid who is getting this stuff to learn about robotics. Most junkyard owners will share your enthusiasm, and may just let you take stuff for free. (Maybe they'll want to help!)

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

i'm probably going to use parrallel because then I can have one speed controller per motor for each of the four wheels and it will have more movement options and abilitys

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

Series/parallel shouldn't make a difference in terms of how many motors you can control at a time. FRC robots can easily run 4+ motors at a time, and the cRIO, off a single 12V battery, all the time. It does make a difference in terms of voltage supplied and how long it can be supplied.

If you have two 12V batteries in series, you get 24V. If you have those same two 12V batteries in parallel, you get 12V with (as I recall) more runtime.