Iacdrive|Automation Solution

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.

Soft starter settings

Reference voltage adjustment
Reference voltage is the basic condition of the equipment is able to start or not. Reference voltage adjustment requires the electric motor rotates immediately after voltage applied and the load start up. If the motor does not rotate after voltage applied, we should increase the reference voltage setting value; if the motor start speed is too fast, then reduce the reference voltage setting value. Reference voltage adjustment should be repeated for several times until the load starts immediately after voltage applied. For example, a smoke blower has a 110kW motor in debugging process with soft starter, reference voltage adjusts to 75% rated voltage, the starting current is 500A, motor start up fast; reference voltage adjusts to 40% rated voltage, motor start up in slow speed, starting current rise from 200A to 600A smoothly, and current return back after motor start is completed, therefore, it’s fully meet the soft-start requirements.

Starting time adjustment
Motor acceleration torque and starting time has direct relationship. Electronic soft starter can make the motor with voltage ramp start from initial voltage to full voltage at the set time (0.5 to 2408). Like it can reduce water impact if we extend the time of water pump flow from 0 to 100%, increase the pump speed variation time means increase the starting time which can be achieved by adjusting the starting time of the soft starter. Starting time should be adjusted according to the specific loads and repeated tests, in order to achieve smooth acceleration within starting time.

Soft stop
Soft starter allows the output voltage decreases gradually to achieve soft stop, in order to protect the equipment. Such as the impact of the water pump, when the pump stops suddenly, the water flow inertia in the pipe will raise the pipe and valves pressure suddenly and cause pipeline damaged. Soft stop to extend parking time will solve such the impact.

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.

Induction motor surge testing

I’d be very careful about surge testing motors in industrial environments. There is specific guidance from IEEE, NEMA and EASA that talks about surge testing being potentially destructive when done on motors in the field. More specifically, motors with unknown insulation conditions. Surge and hi pot testing are geared for shop testing on repaired or new motors. I’d recommend monitoring online impedance imbalance and current imbalance. We’ve seen many case studies where these two parameters were early indicators of stator faults. I agree that offline, phase to phase resistance and inductance can be great indicators of stator faults. The downside of offline testing is the fact the motor has to be shutdown.

We also recommend looking for faults conducive to stator failures. For example, if you have a high restive imbalance on the circuit this can increase heat inside the motor. The increased heat further stresses the insulation system and can lead to bigger insulation or stator failures. If we could have found the small problem, ie. resistance imbalance, then we could have prevented the stator fault.

Stator is a tricky fault zone because faults typically develop so quickly. With a good overall motor testing program you can find the faults that lead to stator issues and get them corrected early.
I was trying to point out that impedance imbalance and current imbalance can act as good indicators for stator issues. It seemed to me that most people in the discussion we’re focusing on offline tests and there wasn’t much mention of online stator testing.

I always think that these discussions are best if they focus on the technical aspects and remain fairly vendor neutral. That’s why I didn’t really bring up any vendors in my post. I think these discussions are a great way for people to gather a great deal of knowledge from a large sample of reliability professionals. I hope more threads like this pop up because I’m always interested in new technology and finding ways to better diagnose motor faults.

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.

Voltage transient/inrush current in induction motors

The voltage transient which occurs whenever there is a sudden change in current in an inductive device. Inductors resist a sudden current change.

V=L di/dt

In electric motors this occurs at start up when the contactors close and shut down when the contactors open. Soft starters reduce the start up transient, but not the shutdown transient.

This also occurs with variable frequency drives which switch the current rapidly and repeatedly.

Voltage transients of 2 to 5 times line voltage are common. This is a primary reason for failure of weakened motor insulation systems. Test standards require high voltage Hipot and Impulse testing of insulation systems in order to ensure that a motor can withstand these transients.
Inrush is something we have always had to deal with, especially with motors that are direct on the line start. The inrush can be as high as seven times the nameplate current. The damage created can be minimal if the motor is started up in the morning and them runs all day.

A motor that runs on a There is one situation that creates a huge inductive spike. Take a motor, lets say it is driving a fan, and it is coasting to a stop. The operator decides to push the start button while it is still coasting. It is a misconception that because the motor is already in motion that you will reduce the starting inrush. You will cause more damage to the insulation system by doing this than you could ever imagine.

The inrush current at start-up for a motor is not an inductive spike. In fact, the small inductance in a motor winding is a slight impedance to the inrush (hence the term), though very slight unless it is a high inductance winding.
An inductive spike is the spike that occurs when voltage is quickly switched between windings. The inductance will not allow current to change instantaneously and must go somewhere.
Changing voltages when the motor is moving because the inductance is an energy storage device. If you reverse voltage on a winding in a permanent magnet motor while the motor is active, the voltage on the winding is momentarily doubled, in theory, but the released energy in the winding can cause huge spikes when the back EMF is no longer opposed by the applied voltage, etc.

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.

Soft starter VS variable frequency drive

Soft Starter reduces electric motor starting current to 2-4 times during motor start up, reduces the impact to power grid during motor start up, avoid the motor being burned out, and provide protection in motors running process.

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.

So, from the function side, variable frequency drives are much better than soft starters.

One essential difference between a soft starter and a VFD in this regard is, that the VFD delivers “nearly” sinusoidal voltages (and currents) to the motor, which makes it possible to develop high starting torques during the acceleration, even higher than nominal full load torque, depending on the application, while a soft starter only supplies fractions of the basic waveform, which serves to reduce the current to the motor significantly, but still at the nominal frequency. This will reduce the available starting torque dramatically until the motor is up to around two-thirds of nominal speed, or maybe even higher.

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.

Motor testing and repairing

Are you having noticeable performance problems with these motors? The size and type of motor are critical as mentioned, a cast rotor with the right testing can pick up voids in the bar and resistance rings, not necessarily a problem as most mass produced cast bar rotors will have some sort of voids in the bars, and the motors are fine, the red flag comes up when using these black box tests, which picks up what appears to be a problem but is actually just a normal condition from the manufacturing process.

I have very little faith that any one test on an assembled motor, can tell the user everything about the condition of the internals, or health of the motor.

When you consider all the testing the health field can use, such as a full body scan, many times it leads to false alarms and more expensive testing.

I could ask a few dozen questions on the age, type, past testing, past history of the motors in question, but if you are basing the health or life expectancy of any motor by only the use of testing without a visual of the internals of a motor, those questions need to be addressed to the supplier of the testing equipment.

I believe in predictive maintenance, by vibration charting, insulation value testing, surge testing, all charted and plotted over time.

When you have insulation values at 100 megohms in March, and then 500 in July, it is likely the ambient conditions have effect on the readings. Dependent on the ambient conditions and area the motors are located, humidity in March is gone in July. So plotting the readings over time will give a plot to see if the trend is downward regardless, or it could be the readings in March are fairly constant, the readings in July are constant, but there is no downward plot of the insulation value.

When you get insulation values in March of 100 megohms, and again in July but the megger readings are now 60, then a user would want to decrease the time between testings, starting with say quarterly, once you develop a plot, if that plot changes downward, then it is time to test maybe even weekly as it may show some kind of insulation breakdown, or contaminates that would call for a visual inspection and possible cleaning/repair of the motor.

Same with plotting surge tests.

Same with plotting vibration testing.

But the answers to these questions where the test results are confusing at best, need to be addressed to the testing equipment provider.

I have yet to see any demonstration of a total motor health testing device, that did not have some caveat dependent on the speed or other design factors of particular motors.

Maybe these tests were not confusing prior to now, if so, I doubt two identical motors would fail/start to fail with the same exact type of problem.

Again I could ask a dozen questions such as are the motors new, is this the first time you have results that make no sense, and as much of the total history of the motors and testing programs you have in place.

When it comes to rotors, testing is critical, and often when problems are found with the motor, and all testing points to the rotor, often simply repairing the rotor will not resolve the problem.

In speaking with many engineers over several decades, a large manufacture of large electric motors, have decided once a rotor is identified as the problem, rebarring, or any single repair is usually unsuccessful, and their procedure is to scrap the rotor completely.