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Motor testing and repairing

Are you having noticeable performance problems with these motors? The size and type of motor are critical as mentioned, a cast rotor with the right testing can pick up voids in the bar and resistance rings, not necessarily a problem as most mass produced cast bar rotors will have some sort of voids in the bars, and the motors are fine, the red flag comes up when using these black box tests, which picks up what appears to be a problem but is actually just a normal condition from the manufacturing process.

I have very little faith that any one test on an assembled motor, can tell the user everything about the condition of the internals, or health of the motor.

When you consider all the testing the health field can use, such as a full body scan, many times it leads to false alarms and more expensive testing.

I could ask a few dozen questions on the age, type, past testing, past history of the motors in question, but if you are basing the health or life expectancy of any motor by only the use of testing without a visual of the internals of a motor, those questions need to be addressed to the supplier of the testing equipment.

I believe in predictive maintenance, by vibration charting, insulation value testing, surge testing, all charted and plotted over time.

When you have insulation values at 100 megohms in March, and then 500 in July, it is likely the ambient conditions have effect on the readings. Dependent on the ambient conditions and area the motors are located, humidity in March is gone in July. So plotting the readings over time will give a plot to see if the trend is downward regardless, or it could be the readings in March are fairly constant, the readings in July are constant, but there is no downward plot of the insulation value.

When you get insulation values in March of 100 megohms, and again in July but the megger readings are now 60, then a user would want to decrease the time between testings, starting with say quarterly, once you develop a plot, if that plot changes downward, then it is time to test maybe even weekly as it may show some kind of insulation breakdown, or contaminates that would call for a visual inspection and possible cleaning/repair of the motor.

Same with plotting surge tests.

Same with plotting vibration testing.

But the answers to these questions where the test results are confusing at best, need to be addressed to the testing equipment provider.

I have yet to see any demonstration of a total motor health testing device, that did not have some caveat dependent on the speed or other design factors of particular motors.

Maybe these tests were not confusing prior to now, if so, I doubt two identical motors would fail/start to fail with the same exact type of problem.

Again I could ask a dozen questions such as are the motors new, is this the first time you have results that make no sense, and as much of the total history of the motors and testing programs you have in place.

When it comes to rotors, testing is critical, and often when problems are found with the motor, and all testing points to the rotor, often simply repairing the rotor will not resolve the problem.

In speaking with many engineers over several decades, a large manufacture of large electric motors, have decided once a rotor is identified as the problem, rebarring, or any single repair is usually unsuccessful, and their procedure is to scrap the rotor completely.

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

VFD external electromagnetic inductive interference

If there are interference sources around the variable frequency drive, they will invade into the filter on variable frequency drive input side to reduce high harmonics, thereby to reduce the noise impact from the power lines to the electronic equipment; and install radio noise filter on VFD output side to reduce its output line noise for the same.

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

Design a PMSM to 10000 rpm high speed

Synchronous speeds are a function of the applied frequency and the number of poles, governed by the equation

120 * (frequency in hertz) / (poles) = (speed in rpm).

Adjust the ratio of frequency to poles to achieve the desired speed.
(example: a 4-pole design would require a line frequency of 333.33 Hz ... which means operating on either an adjustable speed drive or on a dedicated high-frequency power system.)

Once you've got the electro-magnetics sorted out, it's a matter of manufacturing to the mechanical constraints associated with the rotational speed.

Well, depending on the power rating, and on the required reliability, I believe it's very simply. The biggest problem would be to get a variable frequency drive, or other power supply to provide a 3-phase output frequency of about 500Hz.
An automotive alternator should be able to operate relatively reliably at your required speed, and it can probably deliver around 1.5kW at that speed.
In order to make it permanently magnetised, we just have to disassemble the rotor, take the rotor windings out, and replace them with some ring-shaped permanent magnet. We may possibly also use a number of individual smaller permanent magnets embedded in some non-magnetic material such a copper or aluminium between the two half-shelves of the rotor. Depending on the construction of the alternator, we may need some machine shop to pull the rotor halves apart, perhaps machine some material away to make room for the permanent magnet(s), and to press the assembly back together again, and to balance it afterwards.

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

What’s PG card in variable frequency drive?

PG is short for Pulse Generator, generally it is used for measuring rotational speed. The most common PG card is optical encoder.
PG card is a part of vector variable frequency drive, to convert the encoder different form signals to suitable for the controller, like: electrical level conversion, analog digital conversion, optical isolation, etc.
Vector control variable frequency drive is a high-performance drive which can be comparable with DC converter.
In the vector control, it requires a motor speed feedback to the variable frequency inverter drive, this speed feedback is achieve by adding a rotary encoder (PG) to the motor, which means PG card feedback vector control VFD. In order to simplify the system, the feedback can be formed by operation of the inverter output signal, this control is called none PG card feedback vector control VFD, the performance has a slight gap than PG card feedback, but configuration is simple.

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

Variable frequency drive cooling fan maintenance

Variable frequency drive cooling system mainly includes heat sinks and cooling fans, wherein the cooling fan service life is short. The fan generates vibration, noise increases and finally stops when approaching end-life, then the VFD drive tipped in IPM overheat. The cooling fan service life is limited by the bearing, which is about 10000 ~ 35000 hours. When the variable frequency drive continuous operation, we need to replace the fan or bearing in two to three years. To extend the cooling fan life, some VFD's fan only operation when the VFD turn on, but not the power on.

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

Electrical machine software

You can categorize the electrical machine software into 2 basic types:

1) FEA packages that may or may not have a front end for analyzing motors. These are available from companies like Vector Field (now Cobham), Infolytica and a few others.
2) Motor design specific software such as the SPEED software, RMxprt and MotorSolve from Infolytica.

In the first category, the FEA packages are expensive because they are general purpose modeling packages. The motor add-on is usually limited mostly to the building the model and perhaps some specialized post-processing for motors. Their main advantages are:

1) 2D and 3D versions.
2) The user is free to define what analysis he wants to perform since they have very advanced general post-processors.

Their main disadvantages are:
1) Cost, they can get very expensive depending on the options you require.In some cases, the motor design module is a cost option.
2) Although they have general post-processors, many users require a lot of training in order to be able to get useful information.
3) Geometry input can be a lot more complicated since the front-ends typically have a limited number of geometries available.

The second category, the motor design software, is specifically designed for motor analysis. It can be magnetic circuit based such as SPEED and RMXprt or full finite element based such as MotorSolve. The magnetic circuit type of software has been available for a long time but it has only been recently that full FEA based motor design packages have become available.

The general advantages of software of this type are:

1) Template based input so the user simply chooses the motor geometry, stator and rotor and sets the parameters for the geometry. The input is therefore very simple but limited to the templates that are implemented in the package.
2) Post-processing is specialized and presented in a form that a motor designer can use it.

The general disadvantages of this type of software is:

1) No specialized post-processing is available directly from these packages unless added by the software provider in a new release.
2) Geometries are limited to the templates and adding templates may be very difficult and has to be done by the software provider.

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

Motor Rotor Bar issue in Current Signature Analysis

The condition of the rotor bars will determine how much torque your motors will deliver. As a person who has been in the electric motor repair business all my life it is something I constantly check. Normally when you talk about rotor bar health it refers to open rotor bars however I have found that in aluminium die-cast rotors there can be voids in the end-rings. Todays vibration equipment and your CSA equipment is so sensitive that it will pick up these voids. In a repair shop environment and with a motor with a good stator winding it is relatively simple to check for open rotor bars. if at all possible we will check for open rotor bars before we take a motor apart by performing a single phase rotor test. You apply approximately 20% of line voltage to two phases of the motor. Rotate the rotor through 360 degrees and monitor the current. If the current is steady the rotor is in good health. If you have one or more open rotor bars the current will drop as the open bars pass the energized part of the stator. A 10% swing would indicate open rotor bars.
Just in case there is a second cage in the rotor you can also put a voltmeter across one of the energized phases and the open phase. Just like the current, the voltage should stay steady.
When a motor is developing open rotor bars it will become noisy on start up. Noisier with each bar that becomes open. It can sound like a cement mixer or as if there is no lubrication in the bearings.

I have no idea what a rotor bar health index is. I would assume that it is a severity level that has been developed by the people who manufacture your test equipment.

Neither am i familiar with the Motor Current Signature Analysis. We use a surge tester which has an attachment for checking rotors but I don't put much faith in it.

Open rotors can be a nightmare for electric motor repair facilities. Open rotor bars are not always visible and can be very difficult to detect. Our core tester has clamps that allow us to induce a low voltage and high current into the rotor cage but it is not conclusive. We could use a growler to energize the rotor and throw iron filings over the core. On a big rotor it takes a bit of time and customers don't like paying for it, especially when you don't find any problems.

If your motors are die-cast aluminium and they are starting up every day without struggling to get up to speed and they are not noisy during start up, your equipment might be picking up voids in the aluminium.
If you have copper or copper aloy rotors with brazed end-rings and I might suggest that you be concerned. Once you get one open rotor bar it only gets worse as time goes by.

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

Improve induction motor efficiency

The efficiency of an induction motor is determined by intrinsic losses that can be reduced only by changes in motor design. Intrinsic losses are of two types: fixed losses - independent of motor load, and variable losses - dependent on load. Fixed losses consist of magnetic core losses and friction and windage losses. Variable losses consist of resistance losses in the stator and in the rotor and miscellaneous stray losses. So by reducing these losses we can improve efficiency of induction motor.

Changing the rotation direction will not improve efficiency.
Core loss and copper, those are the dominant losses. Improve them and you will get better efficiency. Changing the slot shape etc will help considerably, as will using copper in the rotor. BUT, you can't do either one without affecting the performance of the motor, specifically the starting torque and current as well as the maximum torque and current. In addition, if the motor is designed to have aluminum cage, then changing the cage material to copper won't help the efficiency much since the rotor slot and end rings are not optimally designed.

Improving slot fill will help your copper loss, by putting bigger wires in the stator slot, the wire resistance will reduce and the copper loss will go down. Reducing the end turn height of the windings will also help reduce copper losses.
Stray losses are the only one which can improve efficiency without affecting size of the induction motor. This can be reduced by reducing harmonies in the machine, which can be controlled by selecting slot combination, winding layout, size of air gap, saturation, concentricity of air gap etc.

If an induction motor has to run in both direction and uses a bi directional fan it is inefficient. uni directional fans are used in higher ratings to improve efficiency. further direction of rotation is determined by the driven equipment and cannot be changed at will. Minimising losses both core and copper and stray losses, better cooling ,improvement in cooling fan design a combination of all this suitably balanced will improve efficiency but there is always a limitation on max value imposed by certain conditions of application, materials, willingness of customers to pay.

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

What factors cause Current unbalance

1. Voltage unbalance in supply side (1% volts could easily be 10% current).
2. Physical differences between individual stator coil shapes and connections causing small (but noticeable) resistance changes.
3. Unsymmetrical magnetic circuit - not as big a deal in the smaller "ring" lamination designs, unless highly saturated.
4. Lightly loaded machines will exhibit far higher unbalance than those loaded closer to the full nameplate rating (mostly due to the magnetizing current requirements and associated core/stray loss).

For quick solution measure the current in the three phases, then change the three supply terminals by shift the three terminal to rotate the motor in the same direction, and measure again the current, if the high current move with a certain phase (example: phase L1 of supply read high current in the two case above) the problem is from supply, you can then measure the voltage at motor terminal to be sure that the control circuit and cable are good.

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