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AQ: Measuring inductance in per coils of switched reluctance motors

Fundamentally, inductance is the proportionality constant relating flux linkage to current, i.e. lambda = L * i, where lamda is flux linkage, L is inductance, and i is current. Inductance is a property of the geometry. One can write L = N^2 *R, where N = number of turns in the coil and R = the magnetic reluctance “seen” by the coil. So here are a couple of comments. The reluctance varies in an SR motor as the rotor turns, so you will have to take measurements at several positions. If the current is small, L is a constant. If i is large then a small increase in i produces a small increase in lambda, which is smaller than when the current is small. So, you’ll need to decide on the level of current; or, you’ll need to make measurements at several levels of current. For example, at the rated value of current and half the rated value, etc. Now flux linkage is the product of turns and the effective flux through the coil, lamda = N * phi, where phi is the effective flux through the coil. In the case, you have two coils in series where the coils are inside a motor. You only have two leads, one each to the two coils in series. So, you will need to disassemble the motor and tap into a wire that connects the two coils in series. One way to find the flux linkage is the following. Apply a step of voltage to one of your coils. Take traces of the voltage and current. Then apply the formula v = R’ * i + d(lamdda)/dt, where v is the applied voltage to the coil, R’ is the coil resistance (you’ll have to measure this), and i is the current. Then, integrate to find lamda: d(lamda) = int(v – R’*i)dt. You’ll have to do this to both coils.

As you know the inductance of SRM depends on two parameters: 1.coil current 2.rotor position .it means that you have a lot of possible situation that each situation has particular value of inductance .if you want to measure inductance at particular position, I think you should excite one phase with ac supply and use circuit equations (kvl) to find inductance. if you use a dc supply you should measure the flux and it’s hard to do.

AQ: What is a soft starter?

Motor starter (also known as motor soft starter) is a electronic device integrates soft start, soft stop, light-load energy saving and various protection functions for motor controls. Its main components are the three phase reverse parallel thyristors between power supply and being controlled motor and related control circuits. Control the conduction angle of the three phase reverse parallel thyristors by different methods, to achieve different functions by the changeable of the input voltage on the controlled motors.

The difference between soft starter and frequency inverter

Soft starters and AC motor speed control, it can change output voltage and frequency at the same time; actually, soft starter is a regulator for motor starting, only changes output voltage but not the frequency. The frequency inverter has all the features of soft starter, but its price is much more expensive than the soft starter, and the structure is much more complex.

Frequency inverter allows the AC motor smooth start up, control startup current growing from zero to motor rated current, reduce impact to the power grid and avoid the motor being burned out, also provide protection in motor running process. Besides these functions, the main function of frequency inverter is adjusting the motor running speed according to actual operation conditions, to achieve energy saving effect.

AQ: Motor die-cast rotor non-grain-oriented VS grain-oriented

If the material is non-grain-oriented, the path of least resistance for the magnetic flux varies widely from point to point across the sheet: in one place it may go left-to-right across the sheet surface, in another top-to-bottom, and in still another through the sheet. Other points may be anywhere and everywhere in between.

If the material is grain-oriented, the material is aligned such that there is a significant reduction in the energy requirement for passing flux in one direction relative to any other.

Most machines work best with a uniform flux distribution at all points of the airgap surface: this is achieved by stacking both stator and rotor using non-grain-oriented laminations in any arrangement. However, for a grain-oriented material, each lamination has to be rotated by some angle with respect to the one above and below it in the stack (think of it like a spiral staircase).

Regardless of how the winding is made for the rotor (form wound, bar and ring, or die-cast), it is the STACKING process for the core steel that affects grain orientation.

As to skewing BOTH stator and rotor … why? It is a more costly and complex manufacturing process to produce a skewed core vs an unskewed one, regardless whether the skew is in the rotor or stator. Once the skew is begun, there is no real cost difference between a full slot skew and a fractional slot skew.

If you really want to skew both, though – opt for a half-slot skew in one direction in the rotor, and a half-slot skew in the opposite direction for the stator. Note that this means there is only ONE way to assemble rotor and stator together – with the skews opposing. (With the full slot skew on either rotor or stator and an unskewed opposite piece, the rotor can be inserted from either end of the stator with the same effect.)

AQ: Sensorless motor control with TI and Microchip

Question:
I need to learn about the sensorless control of permanent magnet AC (PMAC) motors. Can you recommend a tutorial and/or open source code for the sensorless motor control using the
a) TI TMS320 series processor, or
b) Microchip dsPIC33EP128 series processor?

Answer:
I have used Microchip and TMS320 to develop VFD. They provide you with a demo kit, PCB and a motor. It take me half a day to get the demo PCB running with my PMSM. Then I copy their design to my own.

The Microchip solution provides you with demo code. I used that before, but it require quite a bit of C programming, and motor tuning take even longer. The demo code and application note are no where near the performance of the Ti solution (I do not work for Ti -so I am not advertising). I take me a week to get my motor spinning with the demo kit from Microchip.

Then there are the International Rectifier solution that is available from many years. The IR sensorless motion control solution have implemented a FOC motor control in FPGA. So you don’t need to write code for motor control. In the chip, it also has a 8051 cpu. You write the program in C; 1 page of code will get a washing machine working. It takes me 1 day to get a PMSM motor running with this solution.

I will use the TI solution for high end motor control – such as a US$40,000 dollar, 100HP direct drive PCP used in the oil field.
I will use the IR solution for a water pump, washing machine – things that is a few kw.
I will use the microchip for solution for toys, because Microchip is so much fun to play with.

AQ: Die-cast rotor design

The method of creating a die-cast rotor is as follows:

1. An assembly of steel laminations (which may or may not be grain-oriented) containing the openings for both rotor bars and ventilation (as required) is made and clamped together to form a cylindrical iron core.
2. The assembly is inserted into a mold, which has space both above and below the core for the end (shorting) ring assembly.
3. The molten conductor material (aluminum or copper, usually) is injected into the mold and allowed to flow through the bar openings. It also fills the end ring spaces.
4. The entire assembly is allowed to cool so that the conductor solidifies.
5. The “cast” core is then shrunk onto a steel shaft.

Now we have a “cast” rotor assembly, ready for bearings and mounting into machine.

AQ: ACS800-104-0105-3 (ABB VFD Drives)

Question:
I have a problem with ABB ACS800-104-0105-3 drive model, the output current reading on the VFD is always double the reading of the clamp ampere(i.e. drive reading= 40 A, clamp ampere reading=20 A), what is the procedure that i can follow to detect the cause of this error?

Answer:
I don’t know about ABB drives, but hope this thing will help you.
1. The variable frequency drive may have problem with current sensor, just replace with another drive for comparison.
2. Make sure you use, true RMS type clamp meter.
3. If there is leakage current (through cable insulation and air) between each phase. This normally because of the cable insulation already degraded. Add output reactor and replace the cable with suitable insulation can fix this kind of problem.
4. If there is leakage current between this VFD drive and the other drives, that both motor cable is quiet long and run in parallel together.

To Collect more data and get more idea, you can do this:
1. Clamp all the 3 phase motor cable together using clamp. The reading will show you the leakage current. Normally about 10% of motor rated current at full load.
2. Check the current on each phase, and see if the current is balance for each phase.
3. Run the variable frequency drive without the motor cable, check the current reading and clamp meter.
4. Run the AC drive with the motor cable but without the motor, check again the reading and clamp meter.
5. Run the drive with motor, check if any oscillation in motor current.
6. Check current input to the AC drive inverter.
7. Turn of the other drive (if the motor cable run parallel together with other VFDs), and see if any change in current.

AQ: Electric motor rotor and stator

When building a traditional electric machine (motor or generator), the idea is to distribute the flux very evenly over both the rotor and stator surfaces where they contact the air gap. This means using either grain-oriented steels and rotating each lamination slightly from the previous one to provide a relatively even flux path, or using a non-grain-oriented steel and having the flux distribute on its own.

Grain-oriented steels are good for lowering magnetizing flux – provided the grain in each lamination is aligned in the same direction. This can also help with reducing stray loss and eddy loss (flux that travels parallel to the shaft and does no useful “work”).

Most electrical steels used in stator and rotor construction also have an insulating coating applied; some of these are organic materials and some are inorganic (solvent-based) materials. The choice is typically made based on a combination of temperature gradient and local environmental laws. The inorganic (solvent) materials can generally withstand higher temperatures but are far less eco-friendly in the manufacture of the coating material or in the curing of the coating after it is applied.

Since most coatings are applied after the rolling-to-thickness process, these are usually cold-rolled steels. The use of cold- vs hot-rolled material can also be based on tooth / slot geometry: for very narrow teeth that require “post processing” for a coating, hot rolled is often used because the material will retain its geometry better through the temperatures used to cure the coating.

Skewing is the relationship between a rotor “turn” and a stator “turn”. Each manufacturer is different; and different machines (synchronous, induction, Permanent magnet, direct current) approach it differently. For example – it is usually easier to skew the stator laminations of an AC machine, because the insertion of the coils is easier. For a DC machine, skewing of the rotor is preferred for the same reason. The amount of skew is typically one slot pitch … which means that one end of the machine has the slot centerline aligned with the opposite end’s tooth centerline.

Grain orientation only applies to the lamination steels … not the conductor materials.

Energy efficient bearing is really a misnomer. However, they can be thought of as those that are sized to have relatively low friction coefficients and therefore low thermal losses (so that you don’t have to use extra energy to cool the lubricant). In the bigger picture, they would also use a lubricant that is less energy-intensive to produce and / or require less replacement.

AQ: Figure out variable speed drives failures

If there is frequent current-limitation or overcurrent alarm during the variable speed drive running, we should check the loads and inverter IGBT module is normal or not, if its good, then the failure is the Hall magnetic compensation current sensor damaged on the control circuit of the variable speed drive. Hall magnetic compensation current sensor is a device to measure the current value of sinusoidal and non-sinusoidal periodic, which can truly reflect the real current waveform, to provide a control and protection signal to the variable speed drive. Generally, this device in variable frequency drive mostly is Swiss company LEM LA series components, LA Series Hall current sensor magnetic compensation can be divided into three and five terminals, for different variable speed drives capacity, the Hall current sensor magnetic compensation also is difference.

Electronic components are very sensitive to static electricity, it will cause electronic components soft breakdown and then cause the circuit board cannot work. So we should be careful when we replace the circuit board, and ensure wearing grounding wrist strap before working, make sure the strap ground directly and human body is at zero potential, in order to prevent body’s electrostatic damage to the circuit board. If there is no grounding wrist strap, we should touch the variable speed drive metal cabinet before replacing the circuit board, to ease static electricity through the variable frequency drive enclosure.

AQ: Energy Efficient Motor VS Standard motor

This is a very simplified comparison for a very complex issue. Every motor manufacturer is somewhat different in their approach, and there are literally thousands of design details in each machine that can be accommodated as the designer balances efficiency VS performance VS cost VS reliability VS safety VS manufacturability.

To generalize a bit, take a look at the following list. Not everything is there (not by a long shot!) but there should be enough to give you a reasonable overview. Note that some items are “design” related, while others are “operation” related.

1. Use a lower loss material for both stator and rotor laminations.
2. Use a larger copper cross-section for the same power rating.
3. Skew rotor winding with respect to stator winding.
4. Use more magnetic material (diameter, length, or both) to reduce flux densities.
5. Effectively size the machine for a somewhat higher rating than nameplate (because the typical peak of the efficiency curve occurs somewhere between 70 and 85 percent “rated” load).
6. Operate the machine at reduced temperatures and/or increase coolant flow.
7. Limit input frequency and/or voltage variation to tighter tolerance (note that this is a specification approach, not a manufacturing approach).
8. Better bearings / lubrication to reduce friction loss.
9. More care taken with internal geometry – i.e. closed slots, large air gaps, generous tooth dimensions, smooth surfaces, etc – to reduce windage.

AQ: AC drive faults analysis

It will cause a series problems during AC drive operation in various environmental conditions, take an example as: when failure occurs, AC drives protective function is activated, and the AC drive tripped immediately, the electric motor stop slowly, the red LED alarm indication turns on, the display panel shows alarm message code or fault content. Then we can analyze the variable frequency AC drive fault reasons base on the display information, if it is soft failures, we can cut of the AC drive and reset it. If the drive still not works, we need to check it manually or automatic initialization, and input the parameter values after the initialization finished. In this way, the AC drive can work if the failure is not critical. If the AC drive still can’t work after above detection, then we need to check the variable frequency drive damaged parts according to the fault phenomena, to replace components or circuit boards. Troubleshooting should follow the drives failure sequence. Like:

(1) Fault code 36, its main power failure, then the three-phase rectifier bridge modules may be breakdown shorted or opened.

(2) Fault code 14, its ground failure, check the motor windings and insulation with megger to see if it’s damaged or not.

(3) Fault code 37, its the inverter failure, the IGBT module may short-circuit breakdown. If the IGBT module short circuit, the main circuit fuse will burnout too. When a phase gate damaged, the variable frequency AC drive will appear overcurrent phenomenon, then it’s time to check the IGBT modules.