Blog – Page 12 – AC DRIVES SOLUTION.

AQ: Designing Gate drivers for IGBT

Q:
When designing gate drivers for IGBT’s, how reliable are the gate driver IC’s ? Now there are a lot of gate driver IC’s available in the market. For example i am using the Hybrid IC M57962L for driving IGBT’s for 3 phase inverter application. The peak output current of this Hybrid IC is 5A and it’s written in data sheet that it can be used for driving IGBT’s up to 200A, 1200V and many features in it.

For an initial design and for lower power rating the configuration is working fine. But, before going for higher power rating, i want to make sure about the reliability of Gate driver IC’s in general.
Is it advisable to design gate drivers using commercially available IC’s or go for a design which includes a gate drive transformer . What are the issues that may arise when using driver IC’s.

A:
I’ve seen and developed designs using these hybrid gate drives quite successfully with long term field reliability in applications requiring from 800 V to 1.25 kV voltage isolation in power conversion products for the semiconductor capital equipment market. Powerex offers various different isolated drivers like the M57962L – my personal favorite is the VLA-502 which also contains the isolated DC/DC converter used to power the isolated gate drive electronics.

There are only two problems that I remember in the last 10 years with these types of commercial drivers – and both problems, if I remember correctly were with the stand alone DC/DC converter intended to be used with the stand alone isolated driver. One problem was a voltage isolation issue from primary to secondary inside the DC/DC switcher. Powerex acknowledged the problem, and upgraded the design. I simply do not recall the part numbers involved. The second problem was with regard to how the isolated VEE rail was established – the monopolar output of the DC/DC converter was offset negative, and ground referenced with a zener diode – and when the IGBT gate would become active at high frequency (25 kHz for that particular application), the gate charge was high enough to sag the negative supply rail against the zener shunt.

Bottom line: Use a good isolated DC/DC converter, with solid VCC and VEE regulated outputs. The isolated drivers themselves are solid in my experience – a nice, simple solution with typically better rise and fall times than gate drive transformers. They also have the added benefit of being capable of holding positive or negative DC bias if the application requires it.

AQ: Overcurrent protection of generators

Overcurrent protection uses as back-up protection for protection generators from faults between two windings of stator (two phases of stator). Setting of overcurrent protection depends from two settings: current setting of relay protection and time setting of relay protection.

Current setting of relay protection represents minimal value of current under which relay protection will send signal to breaker to act and this value is higher from value of rated current in generator (higher from maximum allowed value of current in generator).

Time setting of relay protection represents time after that relay need to send signal to breaker to break fault. Of course, when we talk about time setting of relay protection, we need to have on mind time delay. Time delay represents time during other protections need to act before overcurrent protection acts in case where is overcurrent back-up protection for protection of generator.

Then there is voltage restrained time overcurrent protection (ANSI 51V) which is commonly applied on generators. The pickup setting of these relays reduces (becomes more sensitive) when the applied voltage reduces. It is supposed to aid in sensing faults that are electrically close to the generator terminals as there is insufficient fault impedance to maintain the voltage at the generator. It is especially useful in tripping out faults that have persisted long enough for the generator fault decrement curve to get to the portion where the synchronous reactance is the characteristic impedance. When this happens the fault current will be at the same levels as normal load currents and increased sensitivity is needed.

AQ: Harmonic current

I hate to call them harmonic currents. The do submit to Fourier analysis, but you are probably dealing with AC to DC power supplies. If you look at the current pulses, you will see that each pulse is about 1-2 milliseconds in duration in alternating directions. If you sum these all in the neutral there is the appearance of what looks like 180 Hertz in the neutral. If you use different sized power supplies on each phase, you can see that it is just the addition of the three phases. So the neutral current when you have non power factor corrected power supplies is the sum of the three phases. Unless the current waveforms overlap, there is no cancellation of current in the neutral, hence the neutral current is the sum of the phase currents. The reasoning behind this is the rectifier diodes in the front of the power supply and the DC storage capacitors size relative to the DC load on the capacitor. The general rule of thumb is that the capacitor is about 800 to 1000 microfarads per amp of current in the capacitor.

Realize that the extra heating in the three phase delta-wye transformers is due to the extra circulating current in the primary delta causing excessive heating of the primary conductor. The world calls transformers designed to deal with this “K” factor transformers. Let the world of electrical engineers bury all this simple stuff behind the maze of Fourier analysis. Change the incoming voltage slightly and your Fourier analysis is garbage. The issue here is switches and storage caps— not some magical mathematical garbage.

By the way if someone wanted to use the wire sizing guidelines of the National Electrical Code in the US to size wire for 100% power supply load, the neutral wire would be 8 gauge sizes larger than the phase conductors. People need to start demanding PFC power supplies. Fix a switching problem with switches.

AQ: Bushing insulation testing

In bushing insulation test there are three major current elements which any of those could affect the test result. These current elements are Capacitive current

AQ: Neutral current is less than phase current?

In a balanced 3-phase system with pure sine waves, the neutral current is zero, ideally.
If there is phase imbalance, it shows up in the neutral, so check for imbalance.

The other major cause of high neutral currents is full wave rectification, where the current of each phase is flowing only at its peak voltage. In this case, the neutral current can be as high as three times the phase currents, theoretically.

If you can see the frequency of the neutral current, line frequency currents indicate imbalance. Current due to full wave rectification is high in third harmonics, so it may show mostly 3 x line frequency, or be a ratty square wave at 3 x line frequency.

High neutral currents, and some resulting fires, are largely responsible for the adoption of power factor correction requirements. If your loads are balanced and pfc corrected, you should not have neutral currents.

The neutral current (In) is summation of the phase currents. And obviously, the three phases are decoupled now; and not loading Y makes Iy=0.
So In = Ir + Ib (vectorial sum). Now depending on the amount of loading, nature of loads and their respective power factors, a variety of possibilities (for neutral current magnitude and phase) arise; which may include the case of In being higher.
The statement “neutral current is usually less than phase currents” is naive and not universal.

Nonlinear loads (i.e. rectifiers as Ed mentioned above) draw significant harmonic current. In many cases the current Total Harmonic Distortion (THD) is >100%. In a 3-phase, 4-wire system, the triplen harmonic currents (3, 9, 15, 21…) sum in the neutral wire because they are all in-phase. This is why the neutral current can be much higher than the phase currents even on an otherwise balanced load application. If you can put a current probe on the neutral and look at the waveform – you can see how much fundamental vs. harmonic current there is.

AQ: Torque ripple information from low resolution speed signal

Q:
I am trying to develop a controller for switched reluctance motor which minimizes torque ripple. My design is acquiring torque ripple information from speed signal. In simulation a high pass filter for speed gives me good ripple information. But in experiments I am using a 500 PPR optical absolute encoder to get the position and then calculate the speed using microcontroller (dspace) capture module. But the filtered speed signal does not provide much ripple information. Can you suggest any method to extract ripple information from low resolution speed signal.

A:
1. In simulation, do you consider motor inertia? Inertia filters out torque ripple’s impact on speed, resulting in a smooth speed signal. 2. Generally speaking, a low resolution position sensor produces speed signal of more noise, especially at low speed. I would expect more noise out of your high pass filter.

An encoder generally does not specify an accuracy for the A to A! channel or B to B! channel or it is so broad a spec that it is useless. If you have the ability to trigger a clock on A and B to determine the period between A and B channels the difference between successive reads will give you a good indication of your ripple.

In some cases of motor – encoder installations the mechanical alignment of the encoder to the exact center of motor shaft can cause misalignment noise to occur in the resolved speed signal. In theory the ripple signal could provide useful information however in practice there are too many other influences. Even the shaftless encoder mounting has some of these difficulties.

AQ: Snubber circuit for IGBT Inverter in high frequency applications

Q:
First i had carried out experiments with a single IGBT (IRGPS40B120UP) (TO-247 package) 40A rating without snubber and connected a resistive load, load current was 25A , 400V DC and kept it on continuously for 20min . Then i switched the same IGBT with 10KHz without snubber and the IGBT failed within 1min. Then i connected an RC snubber across the IGBT (same model )and switched at 10KHZ. The load current was gradually increased and kept at 10A. This time the IGBT didn’t fail . So snubber circuits are essential when we go for higher switching frequency.

What are the general guide lines for snubber circuit design in You are not looking close enough at the whole system. My first observation is that you are using the slowest speed silicon available from IR. Even though 10KHz is not fast, have you calculated/measured your switching losses. The second and bigger observation I have, is that you think your circuit is resistive. If your circuit was only resistive, any snubber would have no effect. The whole purpose of a snubber is to deal with the energy stored in the parasitic inductive elements of your circuit. Without understanding how much inductcance your circuit has, you can’t begin designing a cost and size effective snubber.

To echo one of the thoughts of Felipe, you need to know the exact purpose of the snubber. Is it to slow down the dV/dt on turn off or is it to limit the peak voltage? Depending on which you are trying to minimize and your final switching frequency, will dictate which snubber topology will work best for you. The reason that so many snubber configurations exist, is that different applications will require different solutions. I have used snubbers in various configurations up to 100KHz.

AQ: Why BLDC motors are noiseless compare to Induction motor?

If referring to the acoustic noise generated at or around the PWM frequency of the PWM frequency. There are more laminations in an induction machine. This may account for some of the difference.

I also don’t know what the relative power difference is between the BLDC and the induction machine. If it’s about a 5Hp BLDC and a 100Hp induction machine, then you can bet that the PWM frequency of the BLDC is likely above the audible range and the PWM frequency of the induction inverter is well within the audible range.

These are just a few reasons that you may find subtle differences between the two. There are many factors and more information is needed to really help understand your specific situation.
I also believe there are simply sophomoric and unprofessional answers. My statement is based on the general rule that there is greater surface area between laminations of squirrel cage induction machine then there are in BLDC machines. Of course, if you want to state that you have a thin lamination on a long stack length design for a BLDC then there may be an argument that such a motor design when compared to a typical induction machine of the same power has a similar surface-to-surface lamination area. It is these laminations moving due to eddy currents at the PWM frequency that causes the audible noise.

BLDC can come with very small inductance which requires a higher PWM frequency, if you compare both them with controller that may cause different.
If you build 2 motor using exact mechanical shapes and electrical parameter they should be very close. You can build 2 induction machines from 2 different vendors to same electrical spec and they will not sound the same.

AQ: System configuration of grounding

There are different types of system configuration for grounding like TT,IT,TN-C etc. How do we decide which configuration is suitable for the particular inverter (string or central). What are the factors that help us to decide the configurations?

One of the main concerns in a system is to avoid large low impedance ground loops.
These are created by the return signal path connected to the chassis (metal work) at multiple points. The large current loop allows noise currents to radiate H fields and hence couple into other electronics. The antenna effect will be proportional to loop area.

Single point grounding of the return path to chassis prevents this. However single point grounding conflicts with good RF practice where you want to ground to chassis at the sending and receiving ends of a signal path. There is therefore no universal best practice.

In my field, spacecraft, the standard practice has all primary power electronics galvanically isolated from the spacecraft chassis. Individual modules must maintain the isolation with transformer coupled DC/DC converters. The centre tap of each PSU secondary output is then single point grounded to the module metalwork. We talk about primary side and secondary side electronics where only secondary side is grounded to the metalwork.

Anything powered directly from the primary bus must be isolated with a maximum capacitance to chassis of 50nF to avoid excess HF loop currents forming.

In general it depends on country specific law and standards required by Power Supply Operators. From design point of view it all depends on which point of grid you are going to connect and what type of inverter is used.

AQ: Lighting control panel to distribution board

There are a couple of construction differences which may be present, depending on the style of “lighting control panel”.

First, a distribution board typically has poly-phase branch breakers with the intention of feeding either other sub-panels or large loads — such as a motor with a motor controller.

A lighting control panel will have mostly single-pole breakers with phase-to-neutral branch circuits feeding lighting circuitry. There is the added possibility of having either ‘smart’ breakers or integral contactors included on the branch circuits to allow for a control means for area lighting beyond local control of an individual fixture/small group of fixtures, such as an office or conference room.

In general
1. The final branch circuits to be identified and rating load to be estimated.
2. Adequate utilization/diversity Factor to be applied if applicable (depends on the application).
3. To ensure the load balance over the 3 Phase as possible.
4. For Fluorescent light fixtures arrangement of the said fixtures with respect (RYB phases) is necessary to mitigate rendering/glaring and frequency affect.
5. Then size of cable from DB to LCP can be determined/sized, rating of the protective devices can be selected and type of CB(s) subject to type of lighting fixtures.
6. Verification of Voltage drop within the prescribed limit, otherwise select the next standard cable size.

A distribution board typically has poly-phase branch breakers with the intention of feeding either other sub-panels or large loads and lighting control panel (is also one type of distribution panel) will have mostly single-pole breakers with phase-to-neutral branch circuits feeding lighting circuitry.