Iacdrive|Automation Solution

Induction machines testing

Case: We got by testing 3 different machines under no-load condition.
The 50 HP and 3 HP are the ones which behave abnormally when we apply 10% overvoltage. The third machine (7.5 HP) is a machine that reacts normally under the same condition.
What we mean by abnormal behavior is the input power of the machine that will increase dramatically under only 10% overvoltage which is not the case with most of the induction machines. This can be seen by the numbers given below.

50 HP, 575V
Under 10% overvoltage:
Friction & Windage Losses increase 0.2%
Core loss increases 102%
Stator Copper Loss increases 107%

3 HP, 208V
Under 10% overvoltage:
Friction & Windage Losses increase 8%
Core loss increases 34%
Stator Copper Loss increases 63%

7.5 HP, 460V
Under 10% overvoltage:
Friction & Windage Losses decrease 1%
Core loss increases 22%
Stator Copper Loss increases 31%

Till now, we couldn’t diagnose the exact reason that pushes those two machines to behave in such way.
Answer: A few other things I have not seen (yet) include the following:
1) Are the measurements of voltage and current being made by “true RMS” devices or not?
2) Actual measurements for both current and voltage should be taken simultaneously (with a “true RMS” device) for all phases.
3) Measurements of voltage and current should be taken at the motor terminals, not at the drive output.
4) Measurement of output waveform frequency (for each phase), and actual rotational speed of the motor shaft.

These should all be done at each point on the curve.

The reason for looking at the phase relationships of voltage and current is to ensure the incoming power is balanced. Even a small voltage imbalance (say, 3 percent) may result in a significant current imbalance (often 10 percent or more). This unbalanced supply will lead to increased (or at least unexpected) losses, even at relatively light loads. Also – the unbalance is more obvious at lightly loaded conditions.

As noted above, friction and windage losses are speed dependent: the “approximate” relationship is against square of speed.

Things to note about how the machine should perform under normal circumstances:
1. The flux densities in the magnetic circuit are going to increase proportionally with the voltage. This means +10% volts means +10% flux. However, the magnetizing current requirement varies more like the square of the voltage (+10% volt >> +18-20% mag amps).
2. Stator core loss is proportional to the square of the voltage (+10% V >> +20-25% kW).
3. Stator copper loss is proportional to the square of the current (+10% V >> +40-50% kW).
4. Rotor copper loss is independent of voltage change (+10% V >> +0 kW).
5. Assuming speed remains constant, friction and windage are unaffected (+10% V >> +0 kW). Note that with a change of 10% volts, it is highly likely that the speed WILL actually change!
6. Stator eddy loss is proportional to square of voltage (+10% V >> +20-25% kW). Note that stator eddy loss is often included as part of the “stray” calculation under IEEE 112. The other portions of the “stray” value are relatively independent of voltage.

Looking at your test results it would appear that the 50 HP machine is:
a) very highly saturated
b) has damaged/shorted laminations
c) has a different grade of electrical steel (compared to the other ratings)
d) has damaged stator windings (possibly from operation on the drive, particularly if it has a very high dv/dt and/or high common-mode voltage characteristic)
e) a combination of any/all of the above.

One last question – are all the machines rated for the same operating speed (measured in RPM

Active power losses in electrical motor

Equivalent active power losses during electrical motor’s testing in no-load conditions contain next losses:
1. active power losses in the copper of stator’s winding which are in direct relation with square of no-load current value: Pcus=3*Rs*I0s*I0s,

2. active power losses in ferromagnetic core which are in direct relation with frequency and degree of magnetic induction (which depends of voltage):
a) active power losses caused by eddy currents: Pec=kec*f*(B)x
b) active power losses caused by hysteresis: Ph=(kh*d*d*f*f*B*B)/ρ

3. mechanical power losses which are in direct relation with square of angular speed value: Pmech=Kmech*ωmech*ωmech,

Comment:
First, as you can see, active power losses in ferromagnetic core of electrical motor depend of voltage value and frequency, so by increasing voltage value you will get higher active power losses in ferromagnetic core of electrical motor.

Second, you can’t compare two electrical motors with different rated voltage and different rated power because active power losses in the ferromagnetic core, as I have already said above, depend of voltage value and frequency while active power losses in the copper of stator’s windings depend of square of no-load current value which is different for electrical motors with different rated power.

Third, when you want to compare active power losses in no-load conditions of two electrical motors with same rated voltage and rated power, you need to check design of both electrical motors because it is possible that one of them has different kind of winding, because, maybe in the past, one of them was damaged, so its windings had to be changed, what could be the reason for different electrical design and that has a consequence different no-load current value.

Friendly system without technicians diagnose

How to make our systems so friendly that they do not need technicians to help diagnose problems? Most of the more obvious answers have been well documented here but to emphasize these points it is normally the case that diagnostics and alarms can dominate the amount of code constructed for an application. That is the amount of code required to fully diagnose an application may be as much if not more than the code required to make the application work in the first place!

I have seen HMIs with diagnostic screens showing an animated version of the Cause & Effects code that allows users to see where the trip condition is. I have also seen screens depicting prestart checks, Operator guides, etc all animated to help the user. Allen-Bradley even have a program viewer that can be embedded on an HMI screen but you would probably need a technician to understand the code.

From a control system problem perspective it is inevitable that you would need to use the vendor based diagnostics to troubleshoot your system. Alarms may indicate that there is a system related problem but it is unlikely that you could build a true diagnostics application in your code to indicate the full spectrum of problems you may encounter. For example if the CPU dies or the memory fails there is nothing left to tell you what the problem is 🙂

From an application/process problem perspective if you have to resort to a technician to go into the code to determine your problem then your alarm and diagnostics code is not comprehensive enough! Remember your alarming guidelines an Operator must be able to deal with an abnormal situation within a given time frame, if the Operator has to rely on a Technician to wade through code then you really do need to perform an alarm review and build a better diagnostics system.

Motor design

When I was doing my PhD in motor design of reluctance machines with flux assistance (switched reluctance machines and flux switching machines with magnets and/or permanently energised coils) my supervisor was doing research on the field of sensorless control (it wasn’t the area of my research but it got me thinking about it all). At the time I had thought (only in my head as a PhD student daydream) that I would have to initially force a phase (or phases) to deliberately set the rotor into a known position due to the phase firing then start a normal phase firing sequences to start and operate the motor for a normal load without the need for any form position detection (all this was assuming I had the motor running from stationary to full speed at normal expected load with use of a position sensor to start with so I could link phase firing, rotor position and timings all together to create a “map” which I could then try to use to re-program a firing sequence with no position detection at all but only if I could force the rotor to “park” itself in the same position every time before starting the machine properly – the “map” having the information to assume that the motor changes speed correctly as it changes the firing sequences as it accelerates to full speed). But any problem such as unusual load condition or fault condition (e.g. short circuit or open circuit in a phase winding) would render useless such an attempt at control with no form of position detection at all. The induction machine being sensorless and on the grid being measured.

How/where do we as engineers need to change?

System Design – A well designed system should provide clear and concise system status indications. Back in the 70’s (yes, I am that old), Alarm and indicator panels provided this information in the control room. Device level indicators further guided the technician to solving the problem. Today, these functions are implemented in a control room and machine HMI interface. Through the use of input sensor and output actuator feedback, correct system operation can be verified on every scan.

Program (software) Design – It has been estimated that a well written program is 40% algorithm and 60% error checking and parameter verification. “Ladder” in not an issue. Process and machine control systems today are programmed in ladder, structured text, function block, etc. The control program is typically considered intellectual property (IP) and in many cases “hidden” from view. This makes digging through the code impractical.

How/where do we as engineers need to change? – The industry as a whole needs to enforce better system design and performance. This initiative will come from the clients, and implemented by the developers. The cost/benefit trade-off will always be present. Developers trying to improve their margins (reduce cost – raise price) and customers raising functionality and willing to pay less. “We as engineers” are caught in the middle, trying to find better ways to achieve the seemingly impossible.

Sensorless control

I am curious about the definition of “sensorless control”. When you talk about sensorless control, are you in fact meaning a lack of physical position sensor such as e.g. a magnet plus vane plus hall effect? i.e. not having a unit whose sole objective is position detection.
Is the sensorless control based around alternative methods of measurement or detection to predict position using components that have to exist for the machine to function (such as measuring or detecting voltages or currents in the windings)?

I had long ago wondered about designing a motor, fully measuring its voltage and current profiles and phase firing timings for normal operation (from stationary to full speed full load) using a position sensor for getting the motor to work and to determine the best required phase firing sequences and associated voltage/current profiles then program a microprocessor to replicate the entire required profile such that I would attempt to eliminate the need for any sensing or measurement at all (but I concluded it would come very unstuck for any fault conditions or restarting while it was still turning). So in my mind don’t all such machines require a form of measurement (i.e. some form of “sensing”) to work properly so could never be truly sensorless?

A completely sensor-less control would be completely open-loop, which isn’t reliable with some motors like PMSMs. Even if you knew the switching instants for one ideal case, too many “random” variables could influence the system (just think of the initial position), so that those firing instants could be inappropriate for other situations.

Actually, induction machines, thanks to their inherent stability properties, can be run really sensor-less (i.e. just connected to the grid or in V/f). To be honest, even in the simple grid-connection case there is an overcurrent detection somewhere in the grid, which requires some sensing.

But there can also be said the term sensorless relates to el. motor itself. In another words, it means there are not any sensors “attached” to the el. motor (which does not mean sensors cannot be in the inverter, in such a case). In our company we are using the second meaning, since it indicates no sensor connections are needed between the el. motor and the ECU (inverter).

What is true power and apparent power?

KW is true power and KVA is apparent power. In per unit calculations the more predominantly used base, which I consider standard is the KVA, the apparent power because the magnitude of the real power (KW) is variable / dependent on a changing parameter of the cos of the angle of displacement (power factor) between the voltage and current. Also significant consideration is that the rating of transformers are based in KVA, the short circuit magnitudes are expressed in KVA or MVA, and the short circuit duty of equipment are also expressed in MVA (and thousands of amperes, KA ).

In per unit analysis, the base values are always base voltage in kV and base power in kVA or
MVA. Base impedance is derived by the formula (base kV)^2/(base MVA).

The base values for the per unit system are inter-related. The major objective of the per unit system is to try to create a one-line diagram of the system that has no transformers (transformer ratios) or, at least, minimize their number. To achieve that objective, the base values are selected in a very specific way:
a) we pick a common base for power (I’ll come back to this point, if it should be MVA or MW);
b) then we pick base values for the voltages following the transformer ratios. Say you have a generator with nominal voltage 13.8 kV and a step-up transformer rated 13.8/138 kV. The “easiest” choice is to pick 13.8 kV as the base voltage for the LV side of the transformer and 138 kV as the base voltage for the HV side of the transformer.
c) once you have selected a base value for power and a base value for voltage, the base values for current and impedance are defined (calculated). You do not have a degree of freedom in picking base values for current and impedance.

Typically, we calculate the base value for current as Sbase / ( sqrt(3) Vbase ), right? If you are using that expression for the base value for currents, you are implicitly saying that Sbase is a three-phase apparent power (MVA) and Vbase is a line-to-line voltage. Same thing for the expression for base impedance given above. So, perhaps you could choose a kW or MW base value. But then you have a problem: how to calculate base currents and base impedances? If you use the expressions above for base current and base impedance, you are implicitly saying that the number you picked for base power (even if you picked a number you think is a MW) is actually the base value for apparent power, it is kVA or MVA. If you insist on being different and really using kW or MW as the base for power, you have to come up with new (adjusted) expressions for calculating base current and base impedance.

And, surprise!, you will find out that you need to define a “base power factor” to do so. In other words, you will be forced back into defining a base apparent power. So, no, you cannot (easily) use a kW/MW base. For example, a 100 MVA generator, rated 0.80 power factor (80 MW). You could pick 80 as the base power (instead of 100). But if you are using the expressions above for base current and base impedance, you are actually saying that the base apparent power is 80 MVA (not a base active power of 80 MW).

PMBLDC motor in MagNet

You can build it all in MagNet using the circuit position controlled switch. You will have to use motion analysis in order to use the position controlled switches. You can also use the back EMF information to find what the optimal position for the rotor should be with respect to the stator field. The nice thing about motion is that even if you do not have the rotor in the proper position you can set the reference at start up.

Another way of determining that position is to find the maximum torque with constant current (with the right phase relationship between phases of course) and plot torque as a function of rotor position. The peak will correspond to the back EMF waveform information.

If you want to examine the behavior of the motor with an inverter then another approach works very well. There are 2 approaches you can use with MagNet: 1) co-simulation, and, 2) reduced order models. The former can be used with matlab with Simulink or Simpower Systems and runs both Matlab and MagNet simultaneously. The module linking the two systems allows 2 way communication between the modules hence sharing information. The latter requires that you get the System Model Generator (SMG) from Infolytica. The SMG will create a reduced order model of you motor which can then be used in Matlab/Simulink or any VHDL-AMS capable system simulator. A block to interpret the data file is required and is available when you get the SMG. Reduced order models are very interesting since they can very accurately simulate the motor and hook up to complex control circuits.

Transformer uprating

I once uprated a set of 3x 500KVA 11/.433kv ONAN transformers to 800KVA simply by fitting bigger radiators. This was with the manufacturers blessing. (not hermetically sealed – there were significant logistical difficulties in changing the transformers, so this was an easy option). Limiting factor was not the cooling but the magnetic saturation of the core at the higher rating. All the comments about uprating the associated equipment are relevant, particularly on the LV side. Increase in HV amps is minimal. Pragmatically, if you can keep the top oil temperature down you will survive for at least a few years. Best practice of course is to change the transformer!

It is true that you can overload your transformer say 125 %, 150 % or even greater on a certain length of time but every instance of that overloading condition reflects a degradation on the life of your transformer winding insulation. Overload your transformer and you also shorten the life of your winding insulation. The oil temperature indicated on the temperature gauge of the transformer is much lower than the hotspot temperature of the transformer winding which is a critical issue when considering the life of the winding insulation. Transformers having rating of 300 KVA most probably do not even have temperature indicating gauge. The main concern is how effectively can you lower the hotspot temperature in order that it does not significantly take away some of the useful life of your transformer winding insulation.

AM & FM radio

For AM & FM radio & some data communications adding the QP filter make sense.
Now that broadband, wifi, data communications of all sizes & flavours exist – any peak noise is very likely to cause interuptions & loss of integrity of data – all systems are being ‘cost reduced’ ensuring that they will be more susceptible to noise.
I can understand the reasons for the tightening of the regulations.
BUT, it links in to the other big topic of the moment – the non-linearity of managers.
William is obviously his own manager – I bet if his customer was to ask him to spend an indefinite amount of time fixing all the root causes to meet the spec perfectly without any additional cost it would be a different matter.

Unfortunately for most of us the realities of supervisors wanting projects closed & engineering costs minimized we have to be careful in the choice of phrasing.
Any suggestion that one prototype is ‘passing’ suddenly can be translated to job finished, & even in our case where the lab manager mostly understands, his boss rarely does & the accountant above him – not at all.

It gets worse than that – at the beginning of a project (RFQ) – the question is “how long will EMC take to fix?” with the expectation if a deterministic answer; the usual response of a snort of derision & how long is a piece of string generally translates to 2 weeks & once set in stone becomes a millstone (sorry mile-stone).

We already have a number of designs that while not intentionally using dithering, do use boundary mode PFC circuits which automatically force the switch frequency to vary over the mains cycle. These may become problematic at some future variation of the wording of the EMC specs.

While I have a great deal of sympathy for the design it right first time approach, the bottom line for any company is – it meets the requirement (today) – sell it!!