Iacdrive|Automation Solution

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.

Frequency inverter maintenance

1) In inverter regular inspection, we must cut off power before operation. Wait 4minutes (the bigger the longer, the maximum waiting time is 15 minutes) till the frequency inverter display panel LED indicator lights turn off, to make the main circuit DC filter capacitor fully discharged, and measure with a multimeter to confirm before proceeding.

2) Detach control board and main circuit from the frequency inverter, clean the inverter circuit board and internal IGBT modules, input and output chokes and other parts with brush and dust cleaner. Use cotton swab with alcohol or neutral chemical to clean PCB dirty place.

3) Check the inverter inner wire insulation has overheating traces, corrosion and discoloration or not, if found out, we should handle or replace it in time.

4) As the frequency inverter has vibration, temperature changes and other effects, screws maybe loose, we should tighten all screws.

5) Check input and output chokes, transformers, etc. is overheating, discoloration or smelly.

6) Check the intermediate circuit filter electrolytic capacitor safe valve is bulging out or not, and the outer surface has cracks, leakage, swelling and so on. Generally, the inverter filter capacitor life cycle of about five years, the inspection intervals is one year. The capacity of the capacitor can be measured by digital capacitance measurement, when the capacity drops to 80% rated capacity or less, it should be replaced.

7) Check the cooling fan operation is in good condition or not. The cooling fan lifetime is limited by bearings, we should replace the cooling fan or bearings in 2-3 years. If there are abnormal sounds and vibration, we need to replace in time.

8) Check the frequency inverter insulation resistance is in the normal range or not (all terminals with ground terminals). Note, do not use the megger to measure the circuit board, otherwise it will damage the circuit board electronic components.

9) Disconnect the inverter R, S, T terminals with power supply, and U, V, W terminals with motor cable, measure the insulation resistance between each phase conductor and each phase conductor with the protective ground terminals with the megger, to see if it’s in normal value or not, generally its higher than 1MΩ.

10) After inspection, we should use frequency inverter drive the motor with no load for a few minutes, and check the motor rotation direction.

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.)

Variable frequency drive applications

Due to variable frequency drive maintenance and repair experiences.

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.

Frequency inverter failure analysis

Transistor frequency inverter has the following disadvantages: easy trip, difficult re-start, poor overload capacity. As the rapid development of IGBT and CPU, the inverter drive integrates perfect self-diagnosis and fault prevention features, improve the reliability greatly.

Vector control frequency inverter has “automatic torque compensation function” to overcome “starting torque inadequate” etc. This function is the inverter uses a high-speed microcomputer to calculate the torque required at current time, to modify and compensate the output voltage quickly to offset the frequency inverter output torque changed by external conditions.

In addition, because as the inverter software development more and more perfect, we can pre-set various failures parameters in the frequency inverter, to ensure continuous running after failure resolved. For example, re-start motor in free parking process; automatic reset internal failures and maintain continuous operation; adjust running curve if load torque is too high to detect the mechanical system abnormal.

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.

Avoid variable frequency drive damaged in lightning

Sometimes

Can a VFD reduces motor starting kick?

At zero speed the motor requires torque which is flux (voltage) and current (mostly reactive). Only a little bit of active current to compensate for the motor power losses.
Only the power losses need to be drawn from the grid at that time, which means a very small amount of current. It may produce 200% current on the motor and pull only 10% current from the grid.

Of course, as the motor is accelerating, the motor will require kW and the current pulled from the grid will increase accordingly, as the active power consumed by the motor is increasing.

Regarding the kick of torque on the motor, it is controlled by the maximum current ramp limit or through the speed reference as the ramp rate defines the current and the derivative of that rate is the current rate. For this reason, many large machines will be started using an S-Curve speed reference where the S part will adjust the torque (current) rate to avoid stressing the mechanical components, especially if there is mechanical backlash in the gears.

Actually the starting method depends on the type of motor itself SR or SQ type the voltage supply, the motor capacity and motor function, for the MV Motor a liquid or oil starter was the best solution used before.

In case the operation process required a change in the equipment speed the variable frequency drive (Air or water-cooling) based on the drive capacity is the optimum and reliable solution.

Definitely it reduces starting kick of the motor. Actually, the degree of starting kick of a motor is depending upon the starting speed of the motor. If you start your motor at low speed you will have a low starting kick but if started at high speed, you will have high starting kick. This is generally the condition for low and high kw motors. One factors of varying the speed of motors is by varying the frequency of the motors (from the formula N=120f/p) and VFD drive is use to vary the frequency, thus varying the speed of the motors. But if used for starting only, this is expensive as there is more cheaper way like using the Soft Starter or use a WRIM/Slip-ring motors with LRH/Resistor Starters or other. Normally, variable frequency drive is use on operation with speed reduction/varying requirements at required number of time or continuously.

Motor power cable – bigger or smaller?

When a choosing a power cable for a motor, we prefer using one larger diameter cable than two smaller diameter cables in parallel, although it would cost less to do so. Why?

1. Conductors/Cables/Feeders in parallel connection generally are not recommended unless there is no option, therefore it can be adopted under the following conditions:
i. Cables are of the same material and cross section area.
ii. Are of the same route and length.
iii. The sum of the current carring capacity of the parallel circuits after applying all necessary applicable correction factors should be greater than the nominal regulated current of the protective device.
iv. The current carrying capacity (before derating) shall be not less than 300A (according to the local authority/Service provider requirement/regulation).
v. Capability of addressing the Thermal & electrodynamics constraints in proper way.

2. Some designs call for parallel connection so as to:
i. Overcome the voltage drop.
ii. Avoid the difficulties of installing big size cables (bending, pulling) due corridor limitation,etc.
iii. Meet the Power demand.
iv. Mitigate the cost (Costwise).

3. For electrical Motors, two connections are normally required. One from MDB to Motor CP and other from CP to the Motor.
By virtue of the requirement of Delta/star starter, two cables are required (Mandatory) between CP & motor (one will be dead just after changing to delta connection).
While the connection from the MDB to CP will be one, sized according to the Motor rating.

However, Parallel connection of Feeders need an expert engineer(s) to meet the requirement since Short Circuit fault protection for parallel circuits require further evaluation from the Engineer that the impact of the short circuit current within the parallel section will have severe fault due to fault current path that can occur in addition subtransient contribution of the downstream system.