Iacdrive|Automation Solution

What is the best laptop for field work?

Dell D630 – it is the best laptop for field use I have used. And for some applications standard RS232 port is a must. We have Freja 300 test set which totally refuses to communicate with PC via widely available cheap USB-to-serial adapters. The only usable adapter I have found is semi-industrial type, costing about 50 Euro. Not that a price is so much concern, but it is not very convenient to deal with additional boxes, power supply units for them, etc. when commissioning at field.

But I do not expect you will have problems connecting Omicron via converters. We have been used CPC256 via various USB-RS232 converters without serious problems.
For communication with relay protections from Siemens and AREVA never had problems too. Cannot remember how it was with older ABB relays (last case we used them was 4 years ago), but newer ABB series are all with Ethernet communications.

So my advice will be – by special laptop for field work, not mix it with that for everyday office use. Load it with the minimal necessary software – MS Word, Excel, Adobe Reader, Omicron’s Test Universe and software for relays which will test.
For all these needs most older type laptops (4-5 years older) would be sufficient and you can buy for 200-300 Euro solid business class laptop. And also very important: look for non-glossy displays only!

Power Transformer power losses

Power losses of ferromagnetic core depend from voltage and frequency. In case where is no-load secondary winding, power transformer has a power losses in primary winding (active and reactive power losses) which are very small, due to low current of primary winding (less than 1% of rated current) and power losses of ferromagnetic core (active and reactive power losses) which are the highest in case of rated voltage between ends of primary winding…

Of course, we can give voltage between the ends of primary winding of power transformer (voltage who is higher from rated voltage), but we need include some limits before that:

1. if we increase voltage in the primary winding of power transformer (voltage who is higher from rated voltage), we need to set down frequency, otherwise ferromagnetic core of power transformer will come in area of saturation, where are losses to high, which has a consequence warming of ferromagnetic core of power transformer and finally, has a consequence own damage,

2. if we increase voltage in the primary winding of power transformer (voltage who is higher from rated voltage), also intensity of magnetic field and magnetic induction will rise until “knee point voltage”: after that point, we can’t anymore increase magnetic induction, because ferromagnetic core is in area of saturation…

In that case, current of primary winding of power transformer is just limited by impedance of primary winding… By other side, in aspect of magnetising current, active component of this current is limited by resistance of ferromagnetic core, while is reactive component of this current limited by reactance of ferromagnetic core.

There is a finite amount of energy or power that can be handled by various ferromagnetic materials used for core material. Current increases greatly with relatively small voltage increases when you are over the knee of the magnetization curve characterized by the hysteresis loop. Nickel/steel mix materials saturate at lower flux densities than silicon steel materials. 50ni/50fe materials saturate at about 12kG; 80Ni/20Fe as low as 6kG. Vanadium Permendur material saturates at levels as high as 22kGauss- Nano-crystallines- 12.5kG (type), Ferrites -typically over 4kG at room, decreasing as temperature rises. What causes saturation?: Exceeding material limits.

Overcurrent protection of generators

Overcurrent protection uses as back-up protection for protection generators from faults between two windings of stator (two phases of stator). Setting of overcurrent protection depends from two settings: current setting of relay protection and time setting of relay protection.

Current setting of relay protection represents minimal value of current under which relay protection will send signal to breaker to act and this value is higher from value of rated current in generator (higher from maximum allowed value of current in generator).

Time setting of relay protection represents time after that relay need to send signal to breaker to break fault. Of course, when we talk about time setting of relay protection, we need to have on mind time delay. Time delay represents time during other protections need to act before overcurrent protection acts in case where is overcurrent back-up protection for protection of generator.

Then there is voltage restrained time overcurrent protection (ANSI 51V) which is commonly applied on generators. The pickup setting of these relays reduces (becomes more sensitive) when the applied voltage reduces. It is supposed to aid in sensing faults that are electrically close to the generator terminals as there is insufficient fault impedance to maintain the voltage at the generator. It is especially useful in tripping out faults that have persisted long enough for the generator fault decrement curve to get to the portion where the synchronous reactance is the characteristic impedance. When this happens the fault current will be at the same levels as normal load currents and increased sensitivity is needed.

Transformer Saturation

Bushing insulation testing

In bushing insulation test there are three major current elements which any of those could affect the test result. These current elements are Capacitive current

Motor connection

Many years ago I had an experience of 4nos 37kW fin-fan motors wrongly connected at site to a star. After running for almost 1 year, the operators reported these motors were very warm and felt unusual. We removed one of them to the workshop and opened for inspection. All windings were OK but the rotor lamination surface had turned to light blue colour which showed a sign of abnormal heating.

I asked different experts in the industries for advices. From the advices, we suspected the motor could be designed for a delta connection even though the nameplate indicated a Star connection for 415V. We contacted the motor manufacturer by quoting the motor serial no. The manufacturer confirmed that the motors were designed for delta connection at 415V. The manufacturer apologized for the error in nameplate and gave us a free spare motor.

One clear sign that could lead us to believe that the motor was in a wrong star connection instead of delta was, for a 2 or 4-pole motor the no load running current should be more or less around 30% of FLC. When we tested run the motor in the workshop, the no load current was less than 15%xFLC.

After the rectification of all the 4 motors to delta connection, we had no complaint anymore. It was a good lesson out of this solved problem.

Torque ripple information from low resolution speed signal

Q:
I am trying to develop a controller for switched reluctance motor which minimizes torque ripple. My design is acquiring torque ripple information from speed signal. In simulation a high pass filter for speed gives me good ripple information. But in experiments I am using a 500 PPR optical absolute encoder to get the position and then calculate the speed using microcontroller (dspace) capture module. But the filtered speed signal does not provide much ripple information. Can you suggest any method to extract ripple information from low resolution speed signal.

A:
1. In simulation, do you consider motor inertia? Inertia filters out torque ripple’s impact on speed, resulting in a smooth speed signal. 2. Generally speaking, a low resolution position sensor produces speed signal of more noise, especially at low speed. I would expect more noise out of your high pass filter.

An encoder generally does not specify an accuracy for the A to A! channel or B to B! channel or it is so broad a spec that it is useless. If you have the ability to trigger a clock on A and B to determine the period between A and B channels the difference between successive reads will give you a good indication of your ripple.

In some cases of motor – encoder installations the mechanical alignment of the encoder to the exact center of motor shaft can cause misalignment noise to occur in the resolved speed signal. In theory the ripple signal could provide useful information however in practice there are too many other influences. Even the shaftless encoder mounting has some of these difficulties.

Add filters to frequency inverter to eliminate harmful

The high frequency edges of switched waveforms can cause capacitively coupled currents to flow from windings to frame, returning through the bearings, and these can accelerate corrosion in the bearings, causing early failure. Small filters on the motor leads allowing these currents to return locally to ground will avoid this.

The best way, though, is to use filters which can eliminate sharp transitions and leave only (like +/-10% ripple) fundamental frequency (motor’s RPM at given point) of the motor drives. However if somebody can handle 40 – 50kHz of the switching frequency the filter’s size shrinks dramatically and it is not too expensive anymore. Again, the problem is in ability to handle 100 (or so) kWs and 50kHz together.

Why BLDC motors are noiseless compare to Induction motor?

If referring to the acoustic noise generated at or around the PWM frequency of the PWM frequency. There are more laminations in an induction machine. This may account for some of the difference.

I also don’t know what the relative power difference is between the BLDC and the induction machine. If it’s about a 5Hp BLDC and a 100Hp induction machine, then you can bet that the PWM frequency of the BLDC is likely above the audible range and the PWM frequency of the induction inverter is well within the audible range.

These are just a few reasons that you may find subtle differences between the two. There are many factors and more information is needed to really help understand your specific situation.
I also believe there are simply sophomoric and unprofessional answers. My statement is based on the general rule that there is greater surface area between laminations of squirrel cage induction machine then there are in BLDC machines. Of course, if you want to state that you have a thin lamination on a long stack length design for a BLDC then there may be an argument that such a motor design when compared to a typical induction machine of the same power has a similar surface-to-surface lamination area. It is these laminations moving due to eddy currents at the PWM frequency that causes the audible noise.

BLDC can come with very small inductance which requires a higher PWM frequency, if you compare both them with controller that may cause different.
If you build 2 motor using exact mechanical shapes and electrical parameter they should be very close. You can build 2 induction machines from 2 different vendors to same electrical spec and they will not sound the same.

Are variable speed drives harmful to motor?

Variable speed drive switches very fast which brings high dv/dt on motor. How often do we face with problems coming with VSD? How harmful is the common mode currents in windings and other parts of motor due to high dv/dt. Do we see winding isolation failure? How much does the life of motor reduce? Also, is the filtering of voltage at the output of inverter common or applicable practice in the field?

The waveforms for the INVERTER are not good to the motor…. Makes the motor run hot and less efficient….. and all the above….
In-line filters to reduce harmonics is a must in many cases…
Depending on power levels you can have in line reactors for CM and DM or balanced bridge methods for CM… There is methods of harmonic canceling with reactors called harmonic blockers, where you arrange the 3 phase windings in such a manner to cancel certain harmonics….not all harmonics will be blocked, usually in grouping intervals…you need to be aware of what harmonics are your worst offenders…

Mostly in medium and high voltage motor drives the very fast change of the voltage can induce high capacitive currents inside the motor with harmful results.
A way to reduce this negative effect is to increase the number of voltage steps (levels) such that the dV/dT will decrease proportionally (dT=turn on switching time, dV=one voltage step). The most popular method used is SVPWM (space vector sine PWM) NPC (neutral point clamped) multilevel frequency converter. Line L-C filters are also used for EMC.

The first step in any filter analysis is knowing what harmonic vectors your dealing with.
Mathcad is a great tool for modeling the PWM modulation with the sub carrier and generating the harmonic matrices..vectors…I usually go above the 100th harmonic in some analysis, then doing this over the operating ranges of the motor….you then pick your Worse Case operating point and now you have a matrices to work with…. Summing the harmonic magnitudes will give you an idea of how much garbage your feeding your motor windings.

They could be harmful for high frequency current and voltages which are not economical to be eliminated.
But this weakness is so neglect able to the benefits providing. These benefits are very comprehensive. The harmful harmonics are controlled by the standards, so in order to improve harmonic characteristics, we need an improved standard.