Blog – Page 3 – AC DRIVES SOLUTION.

AQ: 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.

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: 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.

AQ: Variable frequency drive power anomalies

Variable frequency drive power anomalies can be divided into following three types: phase loss, low voltage and power off, sometimes they maybe appear mixed. The main reasons for these anomalies are transmission line impact by wind, snow and lightning, sometimes it’s the power supply system appear ground wire and phase short circuit. The lightning is very different due to geographical and seasonal factors. In addition to voltage fluctuations, some power grid or self-generation units will have frequency fluctuations, and these phenomena maybe appear repeated in short times, in order to ensure normal operation, the variable frequency drive power supply also need to make corresponding requirements.

If there is a direct-start motor or cooker or other equipment near the variable frequency drive, to avoid voltage decrease when these devices power on, those devices power supply should be separated with the VFD power supply to reduce influence each other.

For the applications require continues operation in instantaneous power off, in addition to select appropriate VFD drives, we also need to consider the motor load deceleration ratio. When the variable frequency drive and external control loop are adopted instantaneous power off compensation, we need to prevent over current during acceleration by detect motor speed when power on.

For the application requires continuous operation, it’s better to install additional automatic switching uninterrupted power supply devices. Like adopt diode input and single-phase control power variable frequency drives, it can continue work even if in phase loss status, but individual rectifier device current is too high, and the capacitor pulse current also high, it’s not good for the variable frequency drives reliability and service life in long time running, so we should handle it the early the better.

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: 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.

AQ: Variable frequency drive installation requirements

Variable frequency drives are electronic devices, they have stringent requirements in installation environment which is specified in its user manual normally. In exceptional circumstances, if it does not meet these requirements, we must adopt appropriate suppression measures: vibration is the main reason to cause electronic devices mechanical damaged, for big shock and vibration occasions, we should use rubber anti-vibration measures; moisture, corrosion gas and dust will cause electronic devices such as corrosion, poor connection, insulation reduced and then cause short circuit, as a precautionary measure, we should do dust treatment and corrosion control for the control panel, and adopt closed structure; temperature is the key factor to affect electronic devices life and reliability, especially semiconductor devices, we should install the variable frequency drive according to its required installation environment or install additional air conditioning and avoid direct sunlight.

In addition to the above points, inspect the variable frequency drives air filter and cooling fan periodic is also very necessary. For special alpine occasions, to avoid the microprocessor can’t work properly due to temperature too low, we should take necessary measures such as setting the air heater.

AQ: Reduce cost of single and three induction motor

First you must optimize the design for the application. This is true for the electromagnetic and mechanical design. If you are making a general purpose motor then this will be more challenging because you will have to compromise to meet a variety of requirements. But the process is the same. You can design by hand using knowledge and experience or, better you can use the numerous design tools, many of which have perimetric design, variable ranges or optimization methods.
To evaluate your designs you need a cost equation. You simply multiply the weight of material and the cost or relative cost of the materials. Can you reduce the amount of the most expensive materials by making better use of the less expensive ones. Often you can.

With a similar approach you can review the mechanical design and you must be aware the these two activities can become intertwined. It is understanding this tight interrelationship that makes a good machine designer. So you must ask are you using the materials effectively? If for example you have poor cooling, optimization of the electromagnetic design will not get you to the lowest cost machine. Don’t forget about fan design, air flow, thermal transfer and similar items. Mechanical also involves the amount of material in the parts. Can the amount of material be reduced and still maintain strength? And so on…

Finally you look at presses. First are your processes themselves reducing the effectiveness of the materials. Poor processes show up in high stray losses, high iron and copper losses. Do you have a good die casting process? What is you vendor doing? Do the know and how can they help you. And sometimes you should ask them how you can help them. If you design is hard to make well, who’s fault is that. Look at winding, excessive material? Insulation, to thick or thin? Do you have good contact between stator and housing?

That is no one thing that gets to a low cost design. It is like playing sports, you have to learn the fundamentals and execute them well. Once you do that, then you can look at automation, more exotic processes and materials. It is a great team project. Pull together someone with sales, electromagnetic, mechanical, and manufacturing process experience and have a go at it. It is great fun and exciting. You will be surprised at what you will find.

AQ: 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.

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.