Author: ABBdriveX

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AQ: 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.

AQ: SCR broken failure in soft starter

SCR’s are limited to a maximum current rating, as well as a maximum voltage rating. In addition, the number of starts per hour is also limited. A combination of voltage spikes, too many starts per hour, or too much current during a start will destroy a soft starter. Phase imbalance for either voltage or current will cause an SCR to fail, as will a single phase condition on a 3-phase motor. What also needs to be considered is the load being started. If it is a high starting torque load it may require a heavy duty version of soft starter to get it going.

SCRs rarely “break” but they do short out, or rather, become full time conductors. The only thing that can cause this is excess tightening torque or clamping pressure. If on the other hand that the soft starter is giving an indication that one SCR is shorted, then that is where the comments from Terence Smith come to play. It will be either a voltage spike, a current spike, or excess heat caused by excessive starting current or starts per hour.

But reactors will not really help and will increase the throughput losses in the soft starter, I would not waste time on that. Starting a spinning motor is not an issue with soft starters either. Both of these are potential issues with VFD, totally different animal.

If the SCR fault covers the unbalanced starting current too, there is another possibility. At the motor connection box, on the side of the motor there are 6 bolts with screws, for connecting cable, star-delta cooper sheets, and motor coils. The lowest places on the bolt are the clamps of the motor coils, which is followed by a bolt. Over this bolt there are the star-delta sheet, bolt, cable connection clamp and upper the 3-rd bolt. In many cases the lowest screw, at the coil clamp is not tight enough. The maintenance electricians never check them, because it doesn’t belong to the cable installation. In many cases they occurred output phase fault in inverters and phase faults in soft starters.

AQ: 129 slot 48 Pole combination in motor design?

Koil can make the synthesis (i.e. design the winding layout from slot-pole combination) only for symmetrical windings. To have a symmetrical 3-phase winding the back EMFs must be equal and out of phase of 120 electrical degrees. Looking at the star of slots, this means that the spokes in the star (or phasors, one for each slot) must be equally spaced and the number of spokes must be multiple of the phase number.

Considering this example, the machine periodicity t is computed as:
t= HCF{Q,p}=HCF{129,24}=3.
Then the number of spokes in the star of slot is Q/t=129/3=43.

In order to have a balanced winding (assuming m=3 as number of phases) Q/t must be divisible by 3. Such condition can be written in general as Q/(m t) integer.

In this case we have Q/(mt)=129/(3 3)= 129/9=14.333 which is not integer, so that the winding is not symmetrical as here described.
Maybe there are some different/non standard arrangement of the winding.

AQ: Compensate electric motors effect of high altitude

Case: Two electrical motors that design for altitude <1000 m but now this two electrical motor have installed on altitude 1880 m and this electrical motors become very hot. The electrical machines power is 15300KW & 9700KW and they cooled by force air and water cooler.

First – machines designed for higher-than-normal altitude (i.e. in excess of 1000 m = 3300 ft above sea level) are designed with lower allowable temperature rises. The rule-of-thumb approximation is 1 degree C for every 100 m above 1000.

This means a typical Class B rise (max 80 C over 40 C ambient) will be designed for a max 71 C rise over ambient at 1880 m altitude.

Since temperature is more-or-less proportional to the square of the current, the design either reduced in output power to limit the current, or is “overdesigned” so that the resultant output power is the effective de-rate condition. In this case, the “sea level” rating of 15300 kW would become 15300 * (71/80)^2 = 15300 * 0.94 = 14382 kW. Likewise, the 9700 kW machine would be rated for 9118 kW.

The ability to cool the machine effectively is based on two things: the amount of coolant in direct contact with the heat source(s), and the pressure of the coolant flow. At altitude, the density of the coolant is reduced significantly, hence the requirement to operate at lower power ratings. The pressure of the airflow over the windings, etc is ALSO reduced at higher altitude, making the cooling more inefficient.

Speeding up the blower (i.e. going from 6 pole speed to 4 pole speed, for example) will overcome some of this by increasing both airflow and pressure. However, the power draw on the blower drive motor may also necessitate an increase in size to accommodate the new loading parameters (including the effects of high altitude on it!). Note that if the air movement within the machine enclosure is dependent solely on the MACHINE rotor speed (i.e. a shaft mounted fan), there will be a need to develop and apply a separately-powered fan to accommodate the required changes.

The probability of voltage breakdown / corona / flashover is increased above 1800 m as well, which means at least taking a cursory look at both creepage and strike distances.

And finally – if, after all this, the machine is still overheating … time to look at the cleanliness of the liquid side of the heat exchanger. This may mean cleaning or replacing the tubing and headers, determining liquid flow rates (and pressures) and ensuring they are within original design criteria (roughly 3.8 litres per minute for each kW of loss in the rotating machine).

AQ: What’s a variable frequency drive (VFD)?

Variable frequency drive is an electric device to change AC power frequency to control AC motor speed, In addition, it also can change the AC power voltage.

In the past, variable frequency drive was included in motor generators, rotating converters and other electrical equipment. With the emergence of semiconductor electronic devices, VFD can be completely manufactured independent.

Variable Frequency Drive allows the electric 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 protect in  motor running process. Besides these functions, the main function of variable frequency drive is adjusting the motor running speed according to actual operation conditions, to achieve energy saving effect.

Generally, variable frequency drive contains two components: rectifier and inverter. The rectifier converts incoming AC power to DC power, then the inverter converts DC power to the desired frequency AC power. In addition to these two parts, variable frequency drive may also contain transformer and battery. Wherein the transformer changes the voltage and isolates input/output circuit, the battery compensates energy loss inside the VFD drive circuit.

The variable frequency drive not only changes the AC power frequency, but also can change electric AC motor rotation speed and torque. In such conditions, the most typical VFD structure is a three-phase two level source variable frequency drive. The VFD controls each phase voltage by the semiconductor switch and pulse width modulation (PWM).

In addition, variable frequency drive also can be used in aerospace industry. For example, the electrical equipment inside aircraft needs 400Hz AC power, but generally the power on ground is 50Hz or 60Hz. Therefore, when the aircraft is parked on ground, the variable frequency drive will convert 50Hz/60Hz to 400Hz AC power to suitable for the aircraft.

AQ: VFD overcurrent trip during acceleration/deceleration

First, we should know it’s caused by loads or itself. If it’s the variable frequency drive problem, we can check the trip current from the VFD operation history, to see if the current exceeds the VFDs rated current or electronic thermal relay settings value. If three-phase voltages and currents are balanced, we should consider overload or sudden change situations, such as motor stall. If the load inertia is big, we should extend the acceleration time appropriately, this is suitable for a good VFD. If the trip current is within the variable frequency drive rated current or electronic thermal relay setting range, then it maybe the IPM module or relevant parts failure. In this case, we can measure the variable frequency drive output terminals (U, V, W), and resistance of the P, N terminals on DC side to determine whether the IPM module damaged or not. If the module is good, then we can know it is the drive circuit trouble. If IPM module overcurrent or ground wire short circuit causes the VFD trip in deceleration, generally it’s the top half-bridge module or drive circuit fault; If IPM module overcurrent during acceleration, then it is the next half-bridge module or drive part fault. For such failures, mostly it’s the external dust entering the variable frequency drives or environment moisture.

AQ: VFD control loop circuit faults analysis

The affection on variable frequency drive life in the control loop circuit is the power part, the buffer capacitor in smoothing capacitor and IPM board. The ripple current pass the capacitor is a fixed value which won’t be affected by the main circuit, so its life is mainly determined by the temperature and power-on time. Since the capacitors are soldered to the circuit board, it difficult to determine the capacitor deterioration by measuring the electrostatic capacity. Generally, we calculate its life base on the ambient temperature and service time.

Power supply circuit provides power to the control circuit board, IPM drive circuit, operation display panel and cooling fan, the power is obtained from the main circuit DC voltage rectified by the switching power supply. Therefore, if one power short circuit, besides itself damaged, also affect other parts power supply, such as misoperation causes power source and the public ground short circuit, result in switching power supply circuit board damaged, the fans power supply short circuit etc. Generally it’s easy to find out by observing the power supply circuit board.

Logic control circuit board is the core of a variable frequency drive, it includes CPU, MPU, RAM, EEPROM etc large scale integrated circuits, the failure rate is very rare due to high reliability. But sometimes all control terminals closed simultaneously during startup which will cause the VFD drive appear EEPROM fault, in such case, just reset the EEPROM.

IPM circuit board contains drivers and buffer circuit, and over-voltage, phase loss protection circuits. Logic control panel PWM signal input to IPM module by voltage drive signal optical coupling, so, we should measure the IPM module optical coupling during module detection.

AQ: 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.

AQ: 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.

AQ: 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.