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Induction motor noise level

The noise level created by the motor at any speed is in a fixed environment, take two motors same HP, Speed, Enclosure, and the applied voltage could be a factor of the noise, the installed conditions of a 1000 motors could vary from alignment to load, to piping connected to load, to actual load.

What are you or the customer looking for? One 5 HP motor versus another 5HP motor, one in a 50,000 sq foot plant, the other in a 500 square foot plant, while the motor under ideal isolated test conditions might be X, the noise generated from the motor in different conditions could be blamed on the motor.

Would be interested in the question broken down to specific reasons/needs. I know very few who shop based on noise at particular speeds, most either accept the sound levels, which range from pitch to volume to whatever, as an irritant.

Often shielding of the motor can contain any noise that might be a factor in other areas around the motor.

I am not a manufacturer of motors, except for the modification of specialty applications. For example we changed out several hundred motors for the National Weather Service contained in a dipole antenna body. Existing motor was a single phase permanent split capacitor synchronous motor, 110 volt, 1800 RPM DESIRED, due to a feedback tachometer mounted on the motor to verify the speed as these receivers accepted upper air feedback of weather conditions, from weather balloons launched two to three times a day. Location and tracking of the balloons were critical, if the tach feedback was off by one rpm [from 1800] the tracking electronics could not deal with the inconsistency.

I attempted to purchase motors for this application, but because the motor was mounted vertically in a solid cone, no ventilation, plus they were single phase, with induction synchronous rotors, voltage was a consideration, and the units were mounted from Hawaii to Guam to Florida, across the US and Territories.

I took the existing single phase PSC SYNCH MOTOR, which few ever had the torque, or would stay at 1800 rpm, or fail do to the heat.

While they only needed around 300 plus active motors, they needed half as many as spares, considering the past history of failures and the lack of ability to deliver accurate timely weather data over an exact path.

It was not a case of excessive NOISE, it was a case of perceived sound, it sounded different, so for those involved with any Governmental Agency knows that form, fit, function is their mantra and excuse to not accept anything.

We had several complaints of noise, turns out the noise was in no way a danger or at levels of any concern, just different.

While testing 4. 6. 8. 2 pole motors for “noise” in a controlled environment, is only data from those conditions, out in the wild west, those conditions are going to change, mounting, structure, all explained above will affect the motor’s “noise” levels, or perceived “noise” levels.

In the fact that no load, [NEMA] testing is not going to be exacting as other possible more exacting, different parameter type testing, if noise is a concern, is under full load, which again is a variable.

How many vanilla NEMA motors ever operate at “full load”?

Many run below the full load capacity, let alone service factor capacity, some operate slightly overloaded, few ever see the exact applied voltages, with changing of applied voltages during seasonal or daily changes in many variables effecting voltage supply.

Motor babbitt bearings

Babbitt Bearings, properly cared for and maintained will last much longer than anti-friction bearings. Properly set up and aligned there is no contact between the babbitt and the journal other that a scuff mark at the bottom that is caused by minimal contact at starting. As long as the motor is properly aligned to the load, the oil is kept clean and continually fed to the bearing the bearing will last a very long time. There are two simple checks that should be done to check if all is well on Babbit Bearings. The first one is oil sampling. Cheap insurance which will tell you what contaminants are in the oil. The second one is an annual check on the air gap at the bottom of the motor. If the air gap is found to be getting smaller your bearings are wearing.

Babbitt bearings are normally found in larger motors and almost always in direct coupled applications.
In high speed motors the replacement of anti-friction bearings is essential every 1-2 years depending on the severity of the application. It is not uncommon to find a 2-pole Babbit Bearing motor, 20 years old or more with the original bearings. These motors can be overhauled and the windings cleaned up on a regular basis but the original bearings are re-installed.
Babbitt bearings are also affected by shaft currents and we often find a NDE babbitt bearing insulated from the housing.
Babbitt Bearings are also much quieter than anti-friction bearings. Another area where sleeve bearings are very common is in the fan motors in home furnaces. If ball bearings were installed in these motor you would get and annoying clicking sound coming through your ductwork. The bearings in these motors are not made from babbitt, there are made form an oil-bronze material.

Babbitt Bearings are much more expensive than anti-friction bearings and you can’t buy them off the shelf like a 6316. If they are worn, (normally because of mis-aligniment or lack of proper lubrication), they need to be re-babbitted. For a high speed motor with a 3″ journal the re-babitting can cost in the region of $1,200.00 to $2,000.00 per bearing.

When a new motor is being purchased it will cost much more with Babbitt Bearings than it will wit anti-friction bearings and users should be aware that, in the event of a bearing failure, it will not be a 2-3 day turn around on the motor.

Babbit bearings have an initial high cost but properly looked after they are more economical over a long period of time.

Aluminum or copper wire in motors?

Some motor manufacturers go from CU to Al because they try to reduce costs.
Then it just works the other way. Al wire needs to have a larger diameter than Cu if you want the same motor performance.
Then you may face problems with the slot opening and the slot fill, there may not even be enough space at all for the Al wire. You may need to change your lamination.

Furthermore, the blade gap of your inserting tools may not be suitable anymore so you will need a new set of toolings.
Also the end turns will have more volume which may cause problems at the end turn forming and even assembling process.
Besides, due to the properties of Al wire, your rejects will increase during the manufacturing process.
These are just a few examples.

Before changing to Al wire, manufacturers should consider the pros and cons carefully.
Some may invest more than they will safe with the cheaper Al wire.

Copper – at least at the purities and alloys used for electrical conductors – is fairly scarce, which tends to make the price pretty volatile. Aluminum, on the other hand, is fairly abundant in the alloys used for conductors … and hence pretty stable in price (not to mention cheaper than copper).

Neither raw material (copper or aluminum) is used in its pure form for electrical conductors. Both have some other materials added, primarily for mechanical strength. The key factor in determining how much of each to use is the conductivity: 98 percent for the typical copper alloy (ref UNC C11000), 61 percent for the 1970s aluminum alloy (ref EC 1350), or 56 percent for the modern aluminum alloys used in busbar material (ref alloy 6101).

Material properties:
Tensile strength (same cross section) lb/in2: Cu = 50000 Al = 32000
Tensile strength (same conductivity) lb/in2: Cu = 50000 Al = 50000
Weight (same conductivity) lb : Cu = 100 Al = 54
Cross section (same conductivity) % : Cu = 100 Al = 156
Coefficient expansion per deg C x 10^-6 : Cu = 16.6 Al = 23.0

The choice between Al and Cu usually boils down to either cost or weight.

Care must be taken because Al is not as strong (more problems with the forces generated by fault conditions) AND because it has a higher susceptibility to dimensional change under high temperature conditions (such as those occurring during electrical faults).

Another consideration for an aluminum-winding machine are the connection points for real-world transmission: care in terminations is a must. Galvanic action between dissimiar materials is a known difficulty that can be further aggravated by airborne (chemical) contaminants.

DC Motor Armature Testing

For a DC Motor Armature, There is a simple method of determining the condition of the Armature.
Drop Test Method: Give a DC Voltage across the commutator Segments for one pole pitch area from a Power supply or Battery. Connect Positive end of the DC power supply at one end and the Negative end at the opposite end.
For example if the total number of commutator segments are say, 40 in the armature to be tested and the total number of poles is 4, then one pole pitch area will be 10 segments.

Now measure with a Milli volt meter say 0 to 10 millivolts range, the Voltage Drop at the center point, that is between 5th and 6th segment. again rotate the Armature Clockwise or Anti clock wise and measure the next set of segments.
Like this complete measurements for all the 40 segments pairs. simultaneously recording the readings.

If there is any defect in the winding, that is shorted or open, it will show in the readings.
If the reading of Milli volt Meter is uniform for the all the 40 segments pairs, than the armature is good. If there is short between winding or the winding coil between one particular pair of segments, the reading will be less drop in millivolts. If there is any loose or open, the reading will be more than normal readings. Thus one can determine the condition of a DC armature for short or lose or open winding.

When testing a DC armature there is a series of tet should that should be done. The first is. Ground insulation test or more commonly known as a mugger test, usually done at 500VDC. If the ground reading is above 1 meg ohm the armature is good to go to the next test which is a bar to bar test. There are 2 pieces of equipment to conduct this test the best. One of these combined with the mugger test will tell you if the armature is satisfactory return to service. The first bar to bar test is conducted with a “DLRO” digital low resistance ohm meter. The meter will circulate about 8-10 amps thru adjacent successive bars and measure the milli ohm resistance of the circuit. If there is more than a 5% variation then the armature is shorted turn to turn. The next tester which is called a high frequency bar to bar tester. The tester has 4 tet points and as you move it around the armature a high frequency voltage is introduced across the pairs of successive windings and the meter will show a variation if there is a shorted turn. If it passes either of these 2 bar to bar test and the ground insulation test then it can be returned to service.

Ratio of stator coils and rotor poles in three phase axial flux PM motor design

Question:
I am currently investigating the design of a three phase axial flux PM motor, but replacing conventional materials with high temperature superconductors. I’m interested to know the thoughts of group members regarding design rules/rules of thumb relating to the number of stator coils and rotor poles. Many in the amateur wind turbine community seem to use a 4:3 ratio (magnets:coils), but I can’t seem to find anything ‘official’ on the topic.

An equal number of magnets: coils would cause problems with starting the motor and with cogging/torque pulsations.
The only textbook I’ve found dedicated to the design of axial flux PM motors is Jacek Gieras’s book on ‘Axial Flux Permanent Magnet Brushless Machines’, but this seems only to mention examples of coils: poles ratios (e.g., 12 stator coils and 8 rotor poles, 9/8, etc.).

Answer:
“Design of Brushless Permanent-Magnet Motors” by J.R. Hendershot Jr. and TJE Miller is an excellent design book and pages 3-50 thru 3-55 illustrate the 3 phase winding patterns you describe (8/6, 8/9, and 4/6). Whether axial air gap or radial air gap the principles are the same. I assume with an axial air gap machine you do not want phases overlapping each other, that is the common factor in the three patterns above. This keeps winding simple and compact and is commonly used on smaller 3 phase brushless motors.

These windings do not automatically guarantee a true BEMF sine wave form. If you want a sinusoidal waveform you will have to do some work on tailoring the magnetic design (gap between magnets, skewing, air gap profiling, etc.). Some servo motor manufacturers do just this to get a true BEMF sine wave to match their sine wave controllers for ripple free torque operation.

Another decision is does the coil center have a laminated steel pole or only and air center. Air gap windings should be axially thin and have no hysteresis component which is good for high speed operation. A slotted pole winding can handle more wire bulk but a laminated construction may be difficult to implement, you might look at an AC Powdered Metal for the Armature and teeth.

If you allow phase coils to overlap there are a great many other winding patterns possible (listed in the reference book), some are better for Trapezoid controller drive and some are better for sine wave controller drive (BEMF should match controller drive type). Just depends on you end goals.

AC motors Variable torque and Constant torque

AC Motors – Variable torque: AC motors have a speed torque characteristic that varies as the square of the speed. For example, an 1,800/900-rpm electrical motor that develops 10 hp at 1,800 rpm produces 2.5 hp at 900 rpm. Since ac motors face loads, such as centrifugal pumps, fans, and blowers, have a torque requirement that varies as the square or cube of the speed, this ac motor characteristic is usually adequate.

AC Motors – Constant torque: These ac motors can develop the same torque at each speed, thus power output varies directly with speed. For example, an ac motor rated at 10 hp at 1,800 rpm produces 5 hp at 900 rpm. These ac motors are used in applications with constant torque requirements such as mixers, conveyors, and compressors.

Add a separate AC line reactor/DC choke to VFD

How to add a separate AC line reactor / DC choke in case the variable frequency drive doesn’t have it? Can we use a separate line reactor if it’s not built in with the VFD drive? What all parameters I would have to look into, if I want to add the line reactor? Is there any sizing criteria? How would I have to install it?

It depends on how much THD you want to have and how much money you want to spend. If this is for electric motor protection there are additional methods of spike suppression and better reactors/filters.

Size for amps and voltage.

THD will vary will design and specifications. You want the reactor to filter or tune out the unwanted frequencies, mainly the AC drive carrier frequency. One often overlooked parameter is what rejection frequency the reactor is wound for. You want a reactor wound for the rejection frequency you have your VFD drive set at.

This will make you want to raise the carrier frequency to make the reactor smaller, less turns, and less expensive. Before you do this look at the de-rating tables and other factors involved with a high carrier frequency.

It’s always best to first check with your VFD installation and operation documentation. It is likely that the motor drives manufacturer makes recommendations for reactor ratings. That said 3 to 5% reactance at the VFD drive’s rated input current is always a good solution. If there is no internal bus choke or reactor in the VFD then use 5%. Don’t sweat the voltage drop. The drop is in quadrature to the source voltage and so mostly subtracts at a 90 degree angle. Thus, the drop will be less than half the %reactance.

DC Chokes on variable frequency drives

From a manufacturing economics standpoint, there is often a trade off in the decision to add a DC bus choke or not based on its ability to reduce the DC bus ripple. This is because it can reduce the DC bus capacitance necessary to present a clean DC source to the transistors. For some AC drive manufacturers who have the internal capability to wind their own component chokes, this often represents a component cost benefit compared to buying capacitors from outside vendors and being more subject to market volatility. On the other hand if the AC drive manufacturer IS also a manufacturer of capacitors, it works exactly the other way around.

I believe this is why we often see small component class drives being made without DC chokes primarily by companies, mostly in Asia, for whom capacitors are a very low cost commodity. When EU and US manufacturer make larger variable frequency drives, it’s usually less expensive for them to wind chokes, but that option is often perceived to be too physically large for component class drives so they farm out their designs and production to Asian manufacturers. Ironically then, users will add an external AC reactor anyway, but fail to observe that the overall footprint is now larger than it would have been with a DC choke.

I attribute this to the same false market perception that society uses in buying airline tickets. We now shop on the internet based on one criteria, price of the ticket. The airlines have finally figured that out, so they now appear to have lower ticket prices, but charge us extra for bags, snacks, leg room etc. and we actually are paying MORE than we used to. So to relate that back to the AC drives, the market demanded smaller and smaller packaging of VFD drives, which became a primary selection criteria, leading to the smallest physical package, the ones without DC chokes, being dominant in that low kW realm to the point where virtually everyone else gave up and joined the party.

That said, there is still validity to the added protection for the front end of the AC drive provided by the reactor compared to a DC choke. If there are multiple AC drives in an enclosure however, that benefit can still be realized with one larger reactor ahead of the entire inverter drive input power circuit.

Choose PLC base on top PLC Brand?

Always the top brands will be the most popular PLC and over many years it is my opinion that this is because of their marketing strategy, history, reputation and worldwide acceptance more than any other reasons. This does not mean they are better or worse in any way, just means they are more accepted world wide and more people are experienced with their software. Thus there is some security for the owner in respect to programmer support or future resources etc (people come, people go) and a basis on which management may dictate what hardware is used. There is also the consideration on the capital outlay for programming software which can be very expensive.

Choice most often depends on your application and infrastructure. Example: if an entire factory or whatever was “x-brand” and communicating with each other through “y-protocol”, it may be wise to keep to the same-same. Other brands PLC may talk same protocol but then you need to think about software and the experience of your programmer resources,  spares etc.

The alternative may be a more task or machine specific PLC that can communicate the same protocol but at the cost of the programmer not knowing the device or software, or the costs of additional software and also there may be less skilled programmers in this hardware choice constricting the owners future options in using this alternative.

Experienced programmers fall into two basic categories. Just like Joe-Builder who has had 25yrs experience – now Joe, was that 25years experience doing different things or was that 1years experience 25 times? I have encountered this so often, fantastic CV but doesn’t know anything because has been in same job, day in day out, year after year. Very good at THAT job mind you but no real (other) world experience. PLC programmers are often the same, know x-plc (or software language) inside out but nothing else.

Just my opinion but a good programmer is someone skilled in ladder logic, functions / function blocks, structured text, CRC etc and knows when to use it. Someone also familiar with the hardware and its associated costs. Someone who knows how the hardware device scans and can makes efficient use of its resources through the above mentioned skills. Someone also who is mind-full of who will maintain / modify and what can be modified and what should not… etc. Bit of a mouth full I know, but such a person can then make choices of hardware based on the end result required and not be constrained in his/her thinking based on what already exists or what they themselves know or what they or their management consider to be the current reality.

So, a long story to ask another question. Are you really asking which is the most popular brand PLC because a quick google search using the a brand name would tell you that in seconds based on the number of millions of pages available for THAT brand or are you asking which PLC should you choose?

As further comment…
Today I would go task specific by choice. If you want ultra speed, complex math or fast analogue and. or heavy processing etc… then you are looking at a soft logic PLC that will talk the same protocol as the other PLC’s in the factory. If the task is simple logic and minimal analogue and does not require ultra fast scan times (i.e. 10ms+ is acceptable) then many top brands offer a range that will do this.

There are many things you can do in ladder logic that will satisfy a situation admirably. There are lots of things you can do in structured text that is impossible / impractical to do in ladder logic. All soft-logic PLC’s I have experienced are totally useless at complex ladder logic. This is WHY I choose by what the task requires as opposed to choosing because of what constrains my current reality thinking or comfort zone.

The end result is a functional task, machine or project that is maintainable – not what a particular

Frequency inverter with AC line reactor or DC choke?

Is it AC line reactor more important than DC choke in a frequency inverter? If AC line reactor is missing in the inverter, what are possible impacts to the inverter? And how about DC choke?
Quality frequency inverters incorporate either an AC Reactor or DC Reactor (choke). Their inclusion in the basic design of the frequency inverter allows the design engineer to maximize the advantages of the choke. Their function is to reduce the current distortion caused by the input stage rectifiers by slowing the rate of change of current, and thus charging the internal capacitor at a slower rate over a longer time.

The Harmonic Distortion caused by a frequency inverter is related to its size & load, choke size, and the supply network parameters.
With no AC Reactor or DC Choke, the harmonic distortion will be greater.

Another consideration should be a properly sized source transformer that provides enough impedance. The sized source transformer used as an isolation transformer (although a bit more of an investment) should provide 3 to 5% impedance yet also provides Voltage Transient mitigation with ten to one reduction in impulse peaks, as well as noise reduction through the use of a Delta primary to Wye secondary with center tap ground. It provides additional protection for the frequency inverter front (Converter) end while proper ground of the Source to inverter, frequency inverter to Motor and Motor to Voltage Source assists in mitigating high frequency noise, especially when flat braid is used as the grounding straps. This protects your investment and assists in keeping the variable frequency inverter from generating noise into the supply that can compromise your nearby instrumentation, and PLC power supplies, etc. As well you can tap up the transformer giving you a higher input voltage mitigating the voltage drop issues resulting from the higher impedance.

The DC link assists in mitigating DC Bus Ripple and increasing the input impedance enabling a slower inrush for power on and sudden demand current requirements furthering the life of your capacitors, while a sized supply transformer protects the front end of the frequency inverter drive by providing voltage noise protection and adding input impedance for smoother current and adding a capability to change taps to prevent a voltage drop, while input reactors slow inrush current furthering the life of your input components and capacitors but add no protection from Voltage impulses or noise to the drive converter components, and add voltage drop increasing stress on those components. The important thing to remember is that “Proper” systemic design protects your frequency inverters and system components investment.