Motor Construction in Laymans Terms 2

[HotPotato]

Hello All,

I am a chemical engineer and not familiar with the intricacies of electrical components, but do know enough to do a job. I am currently bidding a job and a skid vendor is offering me either a 25 HP or 30 HP motor. I have two general questions:

1) The power supply is 460V/3PH/60Hz. Both the 25HP and 30HP motor obviously utilize this same power source, so my question is what part of the constructed motor is altered to increase HP in the motor at identical power sources? For example, does the motor become longer? Or does the motor stay the same length but the coils are more densely packed?

2) There is 500ft between the power source at the MCC and the motors in the field, and thus there is voltage drop. Remembering that I am not a 'motor expert', what lingo or terminology is used to define the lowest voltage the motor can use before it won't work? In this question I assume that the power source voltage will not increase itself to deliver the correct voltage to the motor after voltage drop (via unknown MCC magic ).

Thanks!
Mike

 

[SwinnyGG]

1) The power supply is 460V/3PH/60Hz. Both the 25HP and 30HP motor obviously utilize this same power source, so my question is what part of the constructed motor is altered to increase HP in the motor at identical power sources? For example, does the motor become longer? Or does the motor stay the same length but the coils are more densely packed?

If a supplier is offering you two motors of differing power levels for the same piece of equipment, I would say that odds are very good that both motors will be the same frame size- meaning they will use the same mounting locations and shaft dimensions/position- so that the manufacturer does not have to re-engineer anything based on your choice. Motors in the same frame size will be, visually, identical or nearly so. The changes which alter their delivered power will be internal.

2) There is 500ft between the power source at the MCC and the motors in the field, and thus there is voltage drop. Remembering that I am not a 'motor expert', what lingo or terminology is used to define the lowest voltage the motor can use before it won't work? In this question I assume that the power source voltage will not increase itself to deliver the correct voltage to the motor after voltage drop (via unknown MCC magic bigsmile ).

The minimum voltage the motor requires will be in the spec sheet and printed right on the plate.

You should always strive to maintain nominal voltage at the motor, if possible. For a given level of power, as the voltage level drops at the motor, current draw increases, meaning temperatures go up and durability goes down- especially if the peak current limit for the motor is exceeded. Low voltage will also cause starting torque to drop precipitously, and cause other problems.

Voltage drop between supply and the motor itself is a function of correctly selected cable sizes and types- this is easy and there are plenty of online calculators to get you there.

 

[imcjoek]

I would say that odds are very good that both motors will be the same frame size- meaning they will use the same mounting locations and shaft dimensions/position

It might be worth noting that motors of the same frame size can have significantly different overall lengths.
Example:
1/4HP 11.88" OAL 56C-575
1HP 13.48" OAL 56C-706

Obviously only an issue if the mounting location of the motor is cramped.

 

[jraef]

Even if the frames are the same, there is no real reason to go over kill, unless there is some future requirement under consideration. "HP" is a shorthand term for "XX amount of torque at YY speed". So if the speed is the same, a 30HP motor puts out 20% more torque than a 25HP motor. But if you don't NEED that additional torque, it's worthless. The energy a motor uses is based on two primary components: the energy required by the LOAD to perform the desired work involved, and the energy it takes to make the motor, a collection of steel and copper parts, into a motor. That second part is the "overhead" cost of running the motor and increases with motor size. If you need 25HP and you buy a 30HP motor, the added overhead of that 30HP motor is a complete waste. Again, unless you might NEED that added power, but if not, then the concept of "more is better" can have a detrimental effect on your machine efficiency. Be very careful as to how you determine your needs. there is a lot that goes into it.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[waross]

Your motor feeders should be sized for maximum 3% voltage drop.
With a 480 Volt supply and a maximum 2% voltage drop in the service conductors the total 5% voltage drop will give you adequate voltage to run the 460 Volt motor.
A 25 HP TEFC motor at 1800 RPM will be a 284T frame.
A 30 HP TEFC motor at 1800 RPM will be a 286T frame.
The bolt hole spacing of the 284T is 9 1/2"
The bolt hole spacing on the 286T is 11"
The set of holes closest to the shaft end are the same offset from the end of the shaft.
The overall length depends on the type of fan fitted and on the manufacturer and is not specified.
However a comparison of two ODP 1800 RPM motors in the same catalogue shows the 25 HP as 25" overall and the 30 HP as 26" overall.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[HotPotato]

The changes which alter their delivered power will be internal.

I am mostly interested in the internal changes here. Are there MORE coils installed, or are the coils thicker? I have trouble finding information on motor construction because a lot of material regarding car motors comes up. This video is a great overview of motor construction Link. Internally, what is actually changed to give 30HP instead of 25HP?

The minimum voltage the motor requires will be in the spec sheet and printed right on the plate.

Below is the nameplate of the customer's existing 25HP motor. I do not see the minimum voltage required listed on the plate, but I may be missing it.

Your motor feeders should be sized for maximum 3% voltage drop. With a 480 Volt supply and a maximum 2% voltage drop in the service conductors the total 5% voltage drop will give you adequate voltage to run the 460 Volt motor.

By "motor feeder", are you inferring that there is a voltage drop through the MCC bucket? If so, are you saying that the voltage drop through the MCC bucket should not exceed 3%? Likewise, are you saying that the maximum voltage drop through power cable should not exceed 2%?

 

[jraef]

I am mostly interested in the internal changes here. Are there MORE coils installed, or are the coils thicker?

It would not be "more coils" because that means more POLES, and poles affect the speed. It is either more turns inside of the coils or larger / longer coils or heavier magnet wire so that more current is drawn or a larger diameter of the rotor and/or all of the above. It also relates to the geometry of the slots and the types of overlapping done on the coils, the number and depth of the rotor bars etc. etc. etc.. As a general rule though, given that you are looking for something in a standardized off-the-shelf mounting arrangement, it will mean a larger diameter, so longer wires inside of the coils, more suface area for creating magnetic force.

I have trouble finding information on motor construction because a lot of material regarding car motors comes up.

Try searching for "AC induction motor construction". Still, the average info you are going to find will be generic in nature, not specific to how a 25HP differs from a 30HP, because that is only relevant to someone wanting to manufacture motors.

Below is the nameplate of the customer's existing 25HP motor. I do not see the minimum voltage required listed on the plate, but I may be missing it.

Not all nameplates are actually required to state the voltage tolerance, unless they are NOT designed to an industry standard specification. On that nameplate, it says "NEMA Premium", so that means it is a motor designed to NEMA MG-1 specifications (NEMA is National Electrical Manufacturers Assoc., with "national" meaning USA). NEMA design specifications for motors are that they perform at nominal torque with a voltage supply of +-10% of nameplate. So that motor is rated for 460V on the nameplate. 460V is used BECAUSE they expect there will be some voltage drop from a 480V system before it gets to the motor terminals. That motor therefore is designed to perform within specs with a voltage of between 414V and 506VAC at the peckerhead.

By "motor feeder", are you inferring that there is a voltage drop through the MCC bucket? If so, are you saying that the voltage drop through the MCC bucket should not exceed 3%? Likewise, are you saying that the maximum voltage drop through power cable should not exceed 2%?

What me meant (if I may, Bill) was that you will be guaranteed 480V +-5% at your service entrance terminals by the power company. From there to the MCC bus, you should have less than 2% VD, and from the MCC bucket to the load you should strive to have less than 3% additional VD. So that way if the utility drops to 456V (480 - 5%) and you get 433V at the MCC bus (456 - 2%) and drop to 420V at the motor terminals (433 - 3%) you are still well within the motor's low end tolerance of 414V.



"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[itsmoked]

And, keep in mind that the idea is not to aim for the lowest voltage you can manage (414V) because that means the motor will be drawing more than the nominal current and hence I[sup]2[/sup]R losses are higher and the motor will run hotter which equates to a shorter life.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Comparing the specs for a 30 HP motor with a 25 HP motor, the major difference is the length.
If you draw 30 HP from a 25 HP rated motor it will overheat. Winding the motor with a slightly heavier wire will reduce the heat.
The frame size specs do not specify the length of the motor, and it is the length of the stator iron that is significant, not the overall length.
I suspect that the first and easiest solution will be to increase the length of the stator.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[HotPotato]

EMA design specifications for motors are that they perform at nominal torque with a voltage supply of +-10% of nameplate. So that motor is rated for 460V on the nameplate. 460V is used BECAUSE they expect there will be some voltage drop from a 480V system before it gets to the motor terminals. That motor therefore is designed to perform within specs with a voltage of between 414V and 506VAC at the peckerhead.

jraef, thank you once again for your very thorough answers. It has just dawned on me that the motor nameplate states 460V - this may sound crazy but I did not recognize that the nameplate was listed 20V below the MCC supply voltage. Also, understanding that "NEMA MG-1" means +/- 10% at nominal torque really helps me understand the acceptable tolerances as well as how the industry identifies such things. If it exists, where could I find concise information on these types of NEMA specifications (without reading entire standard docs)?

It would not be "more coils" because that means more POLES, and poles affect the speed. It is either more turns inside of the coils or larger / longer coils or heavier magnet wire so that more current is drawn or a larger diameter of the rotor and/or all of the above. It also relates to the geometry of the slots and the types of overlapping done on the coils, the number and depth of the rotor bars etc. etc. etc.. As a general rule though, given that you are looking for something in a standardized off-the-shelf mounting arrangement, it will mean a larger diameter, so longer wires inside of the coils, more suface area for creating magnetic force.

Great answer as it makes it clear that motor construction has many variables and is not bound by only a few changing parameters. This is what I was trying to understand when I started this thread.

Stepping into motor failure, what is the failure mechanism caused by higher temperatures in a motor? Do the higher temperatures melt/deform the coils? Does the higher temperature permanently change the properties of the metal winding? What irreversible changes take place?

 

[LionelHutz]

I just wanted to point out that over sizing a motor does not necessarily mean it uses more energy. It has been proven here in the past that it's possible to use a larger more efficient motor and save energy. If you double the motor size and the larger motor is more efficient at 50% load then it would actually save energy.

Now, you do have to install heavier protection and wiring and such that costs more up front.

 

[waross]

Motor failure as a result of heat;
Often the first failure is a breakdown of the insulation. That is quickly followed by turn to turn shorts or turn to ground faults.
Turn to turn shorts often act as a shorted transformer winding and draw more current leading to more heat and more insulation breakdown.
If there is not a turn to ground fault initially, a turn to ground fault may develop from the progressive breakdowns caused by a turn to turn short.
If the damage is caught in time by the protection system the motor may be rewound and put back in service.
A turn to ground fault may develop into an arcing fault. The arc often causes irreparable damage to the stator iron and the motor may now be scrap iron.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

It would not be "more coils" because that means more POLES,

Not true, coils and poles are two different things. There can be multiple coils per pole.

Great answer as it makes it clear that motor construction has many variables and is not bound by only a few changing parameters

Yup.





=====================================
(2B)+(2B)' ?

 

[waross]

We haven't started to talk about rotor design yet.
A typical squirrel cage motor has a laminated rotor with punchings through the laminations. Bars may be inserted into the holes formed by the punchings or aluminum may be cast into the passages. The cross sectional shape of the bars or castings has a lot to do with the motor characteristics. The bars may be round, oval or hourglass shaped. The depth of the bars below the surface of the rotor also affects performance.
Take a look at "THE GLOSSARY OF FREQUENTLY OCCURRING MOTOR TERMS" in the Cowern Papers. .pdf page 7

http://www.baldor.com/Shared/manuals/PR2525.pdf

The different torque curves of the various design motors are mostly the result of squirrel cage bar shape and depth in the rotor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

Back to the original post.
HP is torque times speed. Synchronous speed for an induction motor is limited by the frequency. For 60 Hz the synchronous speeds are 3600 RPM, 1800 RPM, 1200 RPM, 900 RPM and other integer fractions of 3600 RPM.
Torque is determined by the length of the active portion of the rotor, the diameter of the rotor and the strength of the magnetic field.
The magnetic field strength is generally limited by magnetic saturation and cannot be increased much in a well designed motor.
As for the choice between 25 HP and 30 HP;
The 30 HP motor can be expected to run cooler and a cooler motor may be expected to last longer.
The efficiency sweet spot for many industrial induction motors is around 2/3 to 4/5 of full oad.
The 30 HP motor may be cheaper to run than the 25 HP motor with the same load.
But check the performance specs of the motors under consideration before making a decision based on operating costs. Efficiency may vary.
Finally, when two HP motors share the same frame size, the lower HP motor may need a larger frame than the next smaller size and the frame may be slightly underutilized compared to the 30 HP motor. The windings are most likely a smaller gage and will overheat the motor is asked to produce more than the rated HP.
Speed, length, and diameter. Some frame sizes may be underutilized.

Bill
--------------------
"Why not the best?"
Jimmy Carter