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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.

By |May 26th, 2016|Iacdrive_blog|0 Comments

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.

By |May 26th, 2016|Iacdrive_blog|0 Comments

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.

By |May 26th, 2016|Iacdrive_blog|0 Comments

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.

By |May 26th, 2016|Iacdrive_blog|0 Comments

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.

By |May 26th, 2016|Iacdrive_blog|0 Comments

What’s the difference of variable frequency drive and soft starter

variable frequency drives are two different purpose products. VFD is for AC motor speed control, it's not only change the output voltage but also change the frequency; Soft starter is a regulator actually for motor starting, just changing the output voltage. Variable frequency drive has all the features of soft starters, but the price is much more expensive than the soft starter and the structure is much more complex.

Variable frequency drive is converting power supply (single phase VFD and three-phase variable frequency drive.

Soft starter is a set of motor soft start/stop, light-load energy saving and various protection functions devices to control motors.

Soft starter uses three opposite parallel thyristors as regulator, plug it into the power source and motor stator.  When using soft starter to start the motor, the thyristor output voltage increases gradually, and the motor accelerates gradually until the thyristor is turned on completely. The motor operates at rated voltage to achieve a smooth start, reduce starting current and avoid start overcurrent trip. When the motor reaches rated RPM, the startup process is completed, the soft starter uses bypass contactor to replace thyristor to provide rated voltage to the motor, in order to reduce the thyristor heat loss, extend the soft starter service life and improve efficiency, also avoid harmonic pollution to the power grid.

By |May 26th, 2016|Iacdrive_blog|0 Comments

VFD control loop circuit faults analysis

The affection on variable frequency drive life in the control loop circuit is the power part, the buffer capacitor in smoothing capacitor and IPM board. The ripple current pass the capacitor is a fixed value which won't be affected by the main circuit, so its life is mainly determined by the temperature and power-on time. Since the capacitors are soldered to the circuit board, it difficult to determine the capacitor deterioration by measuring the electrostatic capacity. Generally, we calculate its life base on the ambient temperature and service time.

Power supply circuit provides power to the control circuit board, IPM drive circuit, operation display panel and cooling fan, the power is obtained from the main circuit DC voltage rectified by the switching power supply. Therefore, if one power short circuit, besides itself damaged, also affect other parts power supply, such as misoperation causes power source and the public ground short circuit, result in switching power supply circuit board damaged, the fans power supply short circuit etc. Generally it's easy to find out by observing the power supply circuit board.

Logic control circuit board is the core of a variable frequency drive, it includes CPU, MPU, RAM, EEPROM etc large scale integrated circuits, the failure rate is very rare due to high reliability. But sometimes all control terminals closed simultaneously during startup which will cause the VFD drive appear EEPROM fault, in such case, just reset the EEPROM.

IPM circuit board contains drivers and buffer circuit, and over-voltage, phase loss protection circuits. Logic control panel PWM signal input to IPM module by voltage drive signal optical coupling, so, we should measure the IPM module optical coupling during module detection.

By |May 26th, 2016|Iacdrive_blog|0 Comments

3 phase induction motor designs

For 3 phase motor designs, there is hardly any slot combination that will yield a perfectly smooth torque-speed curve. Keeping the following rules in mind will (mostly) avoid the combinations that tend to amplify magnetic noise, harmonics, and parasitic torques.

Let the number of stator slots be S, and the number of rotor slots be R, and the number of poles be P. Undesirable combinations occur when any of the following are true:

1. S - R = 0
2. S - R = +1 OR -1
3. S - R = +2 OR -2
4. S - R = +P or -P
5. S - R = +(P + 1) or -(P +1)
6. S - R = +(P + 2) or -(P + 2)
7. S - R = -(P * 2)
8. S - R = -(P * 5)
9. S - R = +(P * 3) or -(P * 3) .. or multiples of +/-(3 * P).

We know the stator should have an even number of slots to make winding easier - although for certain pole counts, it too can be an odd integer value. And except for a few cases, the number of rotor slots can be either even OR odd.

Then it comes down to the accuracy of the compound die or indexing die for the slot stamping.

By |May 26th, 2016|Iacdrive_blog|0 Comments

VFD overcurrent trip during acceleration/deceleration

First, we should know it's caused by loads or itself. If it's the variable frequency drive problem, we can check the trip current from the VFD operation history, to see if the current exceeds the VFDs rated current or electronic thermal relay settings value. If three-phase voltages and currents are balanced, we should consider overload or sudden change situations, such as motor stall. If the load inertia is big, we should extend the acceleration time appropriately, this is suitable for a good VFD. If the trip current is within the variable frequency drive rated current or electronic thermal relay setting range, then it maybe the IPM module or relevant parts failure. In this case, we can measure the variable frequency drive output terminals (U, V, W), and resistance of the P, N terminals on DC side to determine whether the IPM module damaged or not. If the module is good, then we can know it is the drive circuit trouble. If IPM module overcurrent or ground wire short circuit causes the VFD trip in deceleration, generally it's the top half-bridge module or drive circuit fault; If IPM module overcurrent during acceleration, then it is the next half-bridge module or drive part fault. For such failures, mostly it's the external dust entering the variable frequency drives or environment moisture.

By |May 26th, 2016|Iacdrive_blog|0 Comments

Choose motors for electric vehicles

My experience with the types of motors in electric vehicle is the following. There are three choices for motors in EVs, permanent magnet PM, integral permanent magnet IPM, and induction motor IM. They each have their pros and cons. A PM has the highest power density; it was used on a military HEV on which I worked. A con for the PM is the back emf during a vehicle run-away. If the vehicle were to go down hill at a high rate of speed a large bemf would be generated that would damage the IGBTs due to excessive DC bus voltage. The integral permanent magnet motor has smaller power density because the magnets are smaller and interior to the rotor, but is a compromise on the excessive bemf during a run away. The IPM has "half" permanent magnet torque and "half" reluctance torque. The IM has the smallest power density, and thus the physically largest for the same power and torque. On the other hand, it does not have an excessive bemf condition during run-away. The IM is also less expensive, but this was not the main consideration on the HEV on which I worked.

The major reason for using PM or IPM motors is power density and efficiency. That results in better mileage, lower weight and additionally less cooling required.
The cost for PM is significantly higher and availability is lower. Especially in Hybrids PM seems to be standard (e.g. Prius) but they have their own motor design.
For run-away the solution Chip suggested is an option. The short circuit currents are not necessary to high for the inverter if the inductance is high enough. That obviously needs a special design for the motor and possibly a short circuit device between motor and drive. Additionally the transients for the short circuit currents can be twice as high as the steady state short circuit currents. Another option would be to disconnect the driveline from the motor mechanically.
Another motor type that has not been discussed here is the high speed switched reluctance motor. Inexpensive to build and high efficiency (although lower power density).

By |May 26th, 2016|Iacdrive_blog|0 Comments