1. 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

2. Originally Posted by Pneuma
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
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.

3. 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.

4. Originally Posted by kdocsr05
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.
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.

5. Originally Posted by tipsrfine
I just can't believe how something so simple can be such a mystery to people who are supposed to be HVAC professionals.
Nice comment.

The key and the only key is that these types of electrical motors use energy in, has this been stated before?, two ways: To create force in the measured form of HP/Torque and/or in the form of heat.

If the motor is not matched to the mechanical load in terms of mechanical leverage, that is what all machines are for, then excessive heat is the resultant product.

6. I have found nothing that supports the idea that a motor can run at full load amps even thought it is not doing any work, let alone that if the motor can't do mechanical work it creates a lot of heat instead.

I teach a basic electricity class and one of my favorite demonstrations is when I take a shop vac and measure the amps. I write it on the board. Then I start it up and cover the hose, it makes that real high pitched about to die sound and I ask for guesses at the amperage. nearly 100% of the time they guess higher, sometimes triple the original readings. They are shocked when it turns out to be almost half. It's a great lesson in how amperage coerlates to work being prodcued, as long as voltage remains constant amperage increases proportionally with watts.

7. Tipsrfine,

The answer is in para four, I plan on doing a test myself this weekend and will add the results with data to the post.

Regards

8. Originally Posted by DeltaT
Nice comment.

The key and the only key is that these types of electrical motors use energy in, has this been stated before?, two ways: To create force in the measured form of HP/Torque and/or in the form of heat.

If the motor is not matched to the mechanical load in terms of mechanical leverage, that is what all machines are for, then excessive heat is the resultant product.
Exactly!

The big argument kept comming from people saying an unloaded motor will draw excessive amps. Some very intelligent & educated people.

I think some people were equating excessive heat with excessive amps. And some people kept arguing the importance of design specs and avoiding the question by saying there is no point in finding out the amp draw of a motor in a bench test etc.... Crap! The op's first post was directly relating to why the hell his motor was pulling high amps in a bench test. If someone can't answer the question fine, but go start your own thread about motor education and quit cluttering up a thread posing a simple question.

9. Originally Posted by DeltaT
Nice comment.

The key and the only key is that these types of electrical motors use energy in, has this been stated before?, two ways: To create force in the measured form of HP/Torque and/or in the form of heat.

If the motor is not matched to the mechanical load in terms of mechanical leverage, that is what all machines are for, then excessive heat is the resultant product.
I think you are reading your link wrong. What it said was the percentage of energy disipated as heat is a higher percentage in an underloaded motor. It's not that excessive heat is produced. It's that the heat produced per unit of usefull work is a higher percentage in an onderloaded motor. So the most efficient motor is one that uses all of its potential to do mechancial work, keeping the heat as a percentage of total energy consumed low.

10. Originally Posted by Pneuma
I think you are reading your link wrong. What it said was the percentage of energy disipated as heat is a higher percentage in an underloaded motor. It's not that excessive heat is produced. It's that the heat produced per unit of usefull work is a higher percentage in an onderloaded motor. So the most efficient motor is one that uses all of its potential to do mechancial work, keeping the heat as a percentage of total energy consumed low.
FACT Replacement by the use of a larger horsepower may
cause the motor to be underloaded. It could eventually overheat
below the full load amp rating on the nameplate.

11. Originally Posted by kdocsr05
Tipsrfine,

The answer is in para four, I plan on doing a test myself this weekend and will add the results with data to the post.

Regards
As an afterthought of your post I said to myself "He probably did just answer the question". I love the fact that you're going to perform a real test, but could you state in plain language what you expect to find as a result of that test? Please don't say, " I plan on proving farawhoever's law of inductance".

12. Originally Posted by tipsrfine
FACT Replacement by the use of a larger horsepower may
cause the motor to be underloaded. It could eventually overheat
below the full load amp rating on the nameplate.

I was referring to another link he posted. The quote you have is from the fasco facts book and in that case it has to do with how the motor is cooled. An air over motor used in a condenser for example is cooled by the air moved by the condenser fan, so if you put a 1/2 HP motor is place of a 1/4 HP condenrser fan motor it would run at less than full load amps but it would not get enough air to cool it and it could burn up. This has nothing to do with electrical properties and everything to do with how some particular motors are cooled. The way they wrote the fact it is not clear as to how the motor overheats, but read the whole book and you will see that is what they mean.

13. Originally Posted by Pneuma
I was referring to another link he posted. The quote you have is from the fasco facts book and in that case it has to do with how the motor is cooled. An air over motor used in a condenser for example is cooled by the air moved by the condenser fan, so if you put a 1/2 HP motor is place of a 1/4 HP condenrser fan motor it would run at less than full load amps but it would not get enough air to cool it and it could burn up. This has nothing to do with electrical properties and everything to do with how some particular motors are cooled. The way they wrote the fact it is not clear as to how the motor overheats, but read the whole book and you will see that is what they mean.
This is the part of the book about a motor being self-cooled. Notice the part about it being able to go over 30% as long as it's being sufficiently self-cooled. This does not contradict anything about my last quote when you go over the extra allowable 30%. It will still overheat after the 30% is exceeded even with the "self cooled" factor.

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

But I don't really care, nor am I arguing any point about why or why don't motors overheat. My only point is it is not the result of pulling excessive amps when unloaded.
Last edited by tipsrfine; 12-30-2009 at 06:30 PM. Reason: add on

Page 13 of 29 First ... 36789101112131415161718192023 ... Last

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•

## Related Forums

The place where Electrical professionals meet.