Replyi thought this was 3 phase motor
Professional Member*
Sounds very interesting. Can't wait to hear updates. The only thing I could add: is there any DC voltage going to the motor in addition to the expected AC? Perhaps some of the low voltage lighting has backfed or induced some sort of DC voltage that is screwing with the motor?? Just a thought. I worked on a reefer that had 24Vac and 10Vdc on the same line and it burnt out relay coils repeatedly before I discovered it. Let us know what you find.
Customer:"I have a problem with my (Non-Descript Heating Appliance). It's really old (somewhere from 3 to 120 Years?) and it used to make a (insert noise here) and now it makes a (insert new noise here). Is that common with (Non-Descript Heating Appliance)'s ? "
Professional Member
i thought this was 3 phase motor
Professional Member
Delta T,
Well said, your post puts the issue back into perspective. Early on in the thread, we talked about the importance of design condition present during equipment analysis. Personally I do not remember ever testing a motor under no load conditions, never had a reason to. Why would I want to bench test a motor under conditions not offered at the site? This serves no purpose except to demonstrate a potential to work under design conditions.
I’m not sure if I agree with the statements that a unloaded signal phase induction motor will burn up after running unloaded for a couple of hours. How many times have we arrived on a site and discovered a fan motor running with a broken belt or a condenser fan motor running with a detached blade? Did we change the motor and belt or blade?
There are conditions in which a single-phase induction unloaded will draw current higher than the nameplate data that I did not see in the link, however In the interest of keeping this post focused in the right direction, I would stay away from that for now.
Local 30 New York, New York Operating Engineer
Banned
Originally Posted by kdocsr05
.
Professional Member*
Op's first post states run caps checked out fine.
Professional Member*
A point for us to ponder.
Bench testing a motor under no load or running a motor with too big/too little a load will overheat a motor after some time period according to Fasco.
They have stated that a running motor takes about 2 hours before it reaches its normal operating temp. So if its been operating two hours under a no load condition and the motor starts to get hotter then since no energy is going into producing mechanical advantage then most of the energy is going into only producing heat, then wouldn't that motors amp draw go up from the resultant heat?
Heat creates resistance in an electrical circuit. Would it go to or above the rated FLA? Maybe, who knows and who wants to perform this type of test just to see when the results would only prove that an unloaded motors running amps would increase as it gets hotter. We all already know that. But it's possible under these circumstances that a motor without thermal overload or a failed thermal overload could heat up to the point FLA would be exceeded.
So it's quite possible in my mind and experience that it's true.
Side note: my redneck cousin didn't like the way his 1956 chevrolet motor ran and demanded a new motor for his new car under warranty. Dealer said no way. So he rigged the motor to run at over 2,000 rpm out of gear and just sitting there. By the morning the motor had frozen up causing the dealer to come get it and put a new motor in it. I know, it's cheezy but that is what he did.
Why did the motor freeze up under no load. Cause when under a load all the tolerances within the many components of the internal motor become closed since it's doing work. Under no load all of the mechanical parts tolerance are flopping around which eventually led to the destruction of the motor. Interesting huh?
"The American Republic will endure until the day Congress discovers it can bribe the public with the public's own money.
- Alexis de Toqueville, 1835
Professional Member
If you read that fasco fact book, which I have a copy. It clearly says that an air over motor will overheat if it is underpowered. This is due to the fact that it is designed to have so much air blowing over the shell. An over sized condenser fan motor is a good example, oversize it and it burns up. You'll be scratching your head about it unless you realize how it is meant to be cooled, by the condenser fan!
A fan cooled motor, with a fan on the shaft inside the end bell, is not going to overheat if run unloaded. The thing is cooled by it's own fan and if it pulls 12 amps and is rated at 12 amps the windings will take it forever.
a motor will overheat if it is overloaded, either mechanically or if the voltage is low. Or it will overheat if it is not being cooled as per design.
Simply running a motor unloaded will not be the cause of the overheat.
Banned
you just contradicted the whole paragraph you wrote
.
Professional Member
I see how it could seem that way let me try to clear it up.
If you take an air over motor and run it on a bench it will overheat, beacause it is cooled from air moving over the outside of the motor, air moved by the fan you attach to the drive shaft. But it fails because it's not being cooled, not because it is unloaded.
Professional Member*
I read that book also and my understanding is the motor is designed with the amount of power it needs to pull the designed load. Without the load the extra power is converted into heat rather than expended on the load. I understand it better than I'm explaining it and I don't have the time to go back and copy those sections of the book.
I lied:
Load Factor should not be confused with the common
motor term called Service Factor. Service Factor pertains
to self-cooled motors, such as the ones designed in accordance
to NEMA. Service Factor is the percentage over
nameplate horsepower that a particular motor can be operated
at while being sufficiently self-cooled. For example, a
1.3 rating relates to a 30% reserve in horsepower that can
be drawn on if needed. This is useful when intermittent
overloads will be encountered
FACT Replacement by the use of a larger horsepower may
cause the motor to be underloaded. It could eventually overheat
and cause overload trips. The actual ampere reading will be
below the full load amp rating on the nameplate.
So sayeth FASCO the wise.
Last edited by tipsrfine; 12-29-2009 at 07:09 PM. Reason: I lied
Banned
your changing words around, think of it like this
1+1=2
.
Professional Member
Bump.
Are you sure something is not going on with the oa and return air dampers? The amp draw on the motor could change when the inlet fan pressure changes. If it moves more air in one condition, it will pull more amps than the other. If its set up on the lower inlet condition, and then regularly sees the higher inlet condition, it will run in the service factor. It may cut on/off on the iol.
Ive never seen a motor messed up due to "power" problems. Seen lots of stuff mess up due to "set-up" problems.
Professional Member
I have no idea what you mean by that equation and when someone is not clear on what you are saying, then one trick is to change your words around and try to say it a different way.
Here is what I am trying to say. The lack of a mechanical load does not cause the current to be dispated as heat rather than work, which was put forward by another poster in this thread. Nothing in ohm's law or in electrical theory supports that. At least nothing that I know of, if you know something different please share it.
Heat is the result of current flowing through the windings, that is a fact that is hard to argue with. 12 amps running through the windings will heat them according to thier resistance and whatever is cooling them. If 12 amps exists when the motor does work and it exists when it doesn't do work, very unlikely also, then the motor will be the same temperature, if the method and means of cooling the motor remains constant.
If you disagree please share some technical reference. It would not be the first time I learned something new.
Professional Member*
An AC motor create inductive reactance due to the fact that electric fields are created using AC voltage. Along with that a mechanical function of the motor called "slip" is also factored into the engineering of the motor. These two items must work together to make the motor efficient in producing "work."
If work is not produced by the motor, then the motor produced heat in place of that work because the motor is not operating under the mechanical conditions it was designed for.
The best I can do now is to include this link. Under the efficiency formula the process is explained in that what energy is not produced resulting in mechanical advantage, or "work" then the unused energy will produce in heat within the motor.
And since the motor is not producing more heat than mechanical advantage, that heat will continue to build up of some period of time causing the motor to run at higher than design temperatures.
http://progress-energy.com/custservi...ry/MISC003.asp
Ohms law, to my memory, does not take into account inductive resistance products, only inductive and resistive products. I could be wrong about that though because it's been years since I've had to deal with this stuff.
"The American Republic will endure until the day Congress discovers it can bribe the public with the public's own money.
- Alexis de Toqueville, 1835
New Guest
I believe that much of what is being said here is accurate even the contradictory statements. The problem is that it depends on what kind of motor we are talking about.
With a basic split phase induction run motor (where a capacitor is not involved once the motor is up to speed) running at full speed, there are two basic currents: the current to magnetize the windings and the current to do the work. With a low load the motor should run low amps.
A capacitor run motor (where you keep a capacitor connected to the start winding at full speed) is designed to act like a 2 phase motor at a given load and RPM. At either too much or to little load the efficiency drops off and the wasted electricity is converted to heat. Generally the smaller the MFD of the capacitor, the less this is a factor. I'm no engineer, but I do understand that the relationship of things like: load, back EMF, RPM, inductive reactance and capacitive reactance, are taken into account.
Last edited by ChuckHVAC; 12-30-2009 at 07:36 AM.
New Guest
Back to the OP: I'm still thinking power factor. If you have a good industrial electrican with a PF meter you could confirm or eliminate that issue. Grossly simplified, power factor is the difference between the Watts and the Volt Amps. In some cases volt amps can be twice what the watts are (PF of .5). I assume the pool room has lots of ballasted lighting of some sort. With all the capacitors and inductive loads involved the chances are things not right.
A quick DIY experiment in this case would be to reverse the power leads to the motor and see if it changes the measured amps.
Professional Member
Anyone reading this thread might be interested in this booklet also. When I have time later I want to see what it says about underloaded motors.Attachment 66082
Professional Member*
I just went through it and found nothing specific about underloaded motors and their amp draw. I have also spent the last three days reading all I could find on the internet about underloaded motors as it relates to amp draw.
EVERY article I have read very clearly states as the load goes down so too does the current. Interestingly the higher the horsepower of the motor after 5 hp the more efficient an oversized motor can actually be.
I say forget reading and posting links. Time for anyone who has the time to stop and take some amp readings anytime they are on a pm or service to any system with a motor with a pully to take the belt off, turn in on and take a quick amp draw, put the belt on and take an amp draw and then post their individual results back to this thread. I will whenever the heck some work comes in-my home has direct drive so I can't do it without a big headache.
The proof is in the pudding. I just can't believe how something so simple can be such a mystery to people who are supposed to be HVAC professionals.
Professional Member
Ohm’s laws will not apply to induction motors. Because of the inductive component associated with the design, it is the laws of Len’s and Faraday that become relevant in understanding the physical process.
An AC (Amplitude Current) induction motor consists of two assemblies - a stator and a rotor. The interaction of currents flowing in the rotor bars and the stators' rotating magnetic field generate a torque. In an actual operation, the rotor speed always lags the magnetic field's speed, allowing the rotor bars to cut magnetic lines of force and produce useful torque.
Applying Len’s law to the process the induced current within the rotor creates a counter current (emf) opposite in direction of the current provide by the stator’s magnetic field. The emf generated is proportional to the rate at which flux is linked, (Faraday’s Law of inductance).
This speed difference is called the slip. The slip increase with load and is necessary for torque production. Slip speed is equal to the difference between rotor speed and synchronous speed. Percent slip is slip multiplied by 100. When the rotor is not turning, the slip is 100 %. The emf offered by the rotor at a speed lower than the stator’s magnetic field, becomes a current regulator within the stator, (Faraday’s Law of inductance).
Design elements within the motor offer slip. The rotor design and. internal friction loses on the bearings being dominant. The load is the major contributor to slip on a motor in operation. At low values, slip is directly proportional to the rotor resistance, stator voltage frequency and load torque, and inversely proportional to the second power of supply voltage. The traditional way to control induction-motor speed is to increase slip by adding resistance in the rotor circuit. The slip of low-hp motors is higher than that of high-hp motors because rotor-winding resistance is greater in smaller motors.
With slip being a dominant element of the motor performance, why are we bench-testing motors? To me a bench test of a motor used in our industry serves no purpose. As stated by several members including myself, When evaluating the performance of a motor, design conditions should be present.
Local 30 New York, New York Operating Engineer
Professional Member*
Man when I first started reading your thread I said to myself "Here's a guy that is finally going to answer the main question we've all been dying to have answered."
You didn't. Makes me wonder why, because obviously you could have.
Posting Permissions