Iacdrive|Automation Solution

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.

Variable frequency drive main circuit failure analysis

Variable frequency drive includes main circuit, power circuit, IPM drive and protection circuits, cooling fan and other several parts. The structure is mostly unitized or modular. Incorrect or unreasonable setting will cause the VFD malfunction and failure easily, or can’t meet anticipated operation effect. As a precaution, careful analysis before the failure is particularly important.

Variable frequency drives main circuit mainly consists of three-phase or single-phase bridge rectifier, smoothing capacitor, filter capacitor, IPM inverter bridge, current limitation resistors, contactors and other components. Many common failures are caused by the electrolytic capacitors. The electrolytic capacitor life is determined by the DC voltage and the internal temperature on the capacitor both sides, the capacitor type is confirmed during the circuit design, so, internal temperature inside the electrolytic capacitor is critical important. Electrolytic capacitor will affect the variable frequency drive life directly, generally, temperature increase 10 ℃, VFD life reduce a half. Therefore, on one hand, considering proper ambient temperature in installing, on the other hand, reduce ripple current by taking some measures. Adopt power factor improved AC/DC reactors can reduce ripple current, thereby extend the electrolytic capacitor life.

During variable frequency drive maintenance, usually it’s relative easy to measure the electrostatic capacity of to determine the capacitor deterioration, when the electrostatic capacity is less than rated 80%, insulation impedance is below 5 MΩ, it needs to replace the electrolytic capacitors.

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