Iacdrive|Automation Solution

Ratio of stator coils and rotor poles in three phase axial flux PM motor design

Question:
I am currently investigating the design of a three phase axial flux PM motor, but replacing conventional materials with high temperature superconductors. I’m interested to know the thoughts of group members regarding design rules/rules of thumb relating to the number of stator coils and rotor poles. Many in the amateur wind turbine community seem to use a 4:3 ratio (magnets:coils), but I can’t seem to find anything ‘official’ on the topic.

An equal number of magnets: coils would cause problems with starting the motor and with cogging/torque pulsations.
The only textbook I’ve found dedicated to the design of axial flux PM motors is Jacek Gieras’s book on ‘Axial Flux Permanent Magnet Brushless Machines’, but this seems only to mention examples of coils: poles ratios (e.g., 12 stator coils and 8 rotor poles, 9/8, etc.).

Answer:
“Design of Brushless Permanent-Magnet Motors” by J.R. Hendershot Jr. and TJE Miller is an excellent design book and pages 3-50 thru 3-55 illustrate the 3 phase winding patterns you describe (8/6, 8/9, and 4/6). Whether axial air gap or radial air gap the principles are the same. I assume with an axial air gap machine you do not want phases overlapping each other, that is the common factor in the three patterns above. This keeps winding simple and compact and is commonly used on smaller 3 phase brushless motors.

These windings do not automatically guarantee a true BEMF sine wave form. If you want a sinusoidal waveform you will have to do some work on tailoring the magnetic design (gap between magnets, skewing, air gap profiling, etc.). Some servo motor manufacturers do just this to get a true BEMF sine wave to match their sine wave controllers for ripple free torque operation.

Another decision is does the coil center have a laminated steel pole or only and air center. Air gap windings should be axially thin and have no hysteresis component which is good for high speed operation. A slotted pole winding can handle more wire bulk but a laminated construction may be difficult to implement, you might look at an AC Powdered Metal for the Armature and teeth.

If you allow phase coils to overlap there are a great many other winding patterns possible (listed in the reference book), some are better for Trapezoid controller drive and some are better for sine wave controller drive (BEMF should match controller drive type). Just depends on you end goals.

AC motors Variable torque and Constant torque

AC Motors – Variable torque: AC motors have a speed torque characteristic that varies as the square of the speed. For example, an 1,800/900-rpm electrical motor that develops 10 hp at 1,800 rpm produces 2.5 hp at 900 rpm. Since ac motors face loads, such as centrifugal pumps, fans, and blowers, have a torque requirement that varies as the square or cube of the speed, this ac motor characteristic is usually adequate.

AC Motors – Constant torque: These ac motors can develop the same torque at each speed, thus power output varies directly with speed. For example, an ac motor rated at 10 hp at 1,800 rpm produces 5 hp at 900 rpm. These ac motors are used in applications with constant torque requirements such as mixers, conveyors, and compressors.

Add a separate AC line reactor/DC choke to VFD

How to add a separate AC line reactor / DC choke in case the variable frequency drive doesn’t have it? Can we use a separate line reactor if it’s not built in with the VFD drive? What all parameters I would have to look into, if I want to add the line reactor? Is there any sizing criteria? How would I have to install it?

It depends on how much THD you want to have and how much money you want to spend. If this is for electric motor protection there are additional methods of spike suppression and better reactors/filters.

Size for amps and voltage.

THD will vary will design and specifications. You want the reactor to filter or tune out the unwanted frequencies, mainly the AC drive carrier frequency. One often overlooked parameter is what rejection frequency the reactor is wound for. You want a reactor wound for the rejection frequency you have your VFD drive set at.

This will make you want to raise the carrier frequency to make the reactor smaller, less turns, and less expensive. Before you do this look at the de-rating tables and other factors involved with a high carrier frequency.

It’s always best to first check with your VFD installation and operation documentation. It is likely that the motor drives manufacturer makes recommendations for reactor ratings. That said 3 to 5% reactance at the VFD drive’s rated input current is always a good solution. If there is no internal bus choke or reactor in the VFD then use 5%. Don’t sweat the voltage drop. The drop is in quadrature to the source voltage and so mostly subtracts at a 90 degree angle. Thus, the drop will be less than half the %reactance.

DC Chokes on variable frequency drives

From a manufacturing economics standpoint, there is often a trade off in the decision to add a DC bus choke or not based on its ability to reduce the DC bus ripple. This is because it can reduce the DC bus capacitance necessary to present a clean DC source to the transistors. For some AC drive manufacturers who have the internal capability to wind their own component chokes, this often represents a component cost benefit compared to buying capacitors from outside vendors and being more subject to market volatility. On the other hand if the AC drive manufacturer IS also a manufacturer of capacitors, it works exactly the other way around.

I believe this is why we often see small component class drives being made without DC chokes primarily by companies, mostly in Asia, for whom capacitors are a very low cost commodity. When EU and US manufacturer make larger variable frequency drives, it’s usually less expensive for them to wind chokes, but that option is often perceived to be too physically large for component class drives so they farm out their designs and production to Asian manufacturers. Ironically then, users will add an external AC reactor anyway, but fail to observe that the overall footprint is now larger than it would have been with a DC choke.

I attribute this to the same false market perception that society uses in buying airline tickets. We now shop on the internet based on one criteria, price of the ticket. The airlines have finally figured that out, so they now appear to have lower ticket prices, but charge us extra for bags, snacks, leg room etc. and we actually are paying MORE than we used to. So to relate that back to the AC drives, the market demanded smaller and smaller packaging of VFD drives, which became a primary selection criteria, leading to the smallest physical package, the ones without DC chokes, being dominant in that low kW realm to the point where virtually everyone else gave up and joined the party.

That said, there is still validity to the added protection for the front end of the AC drive provided by the reactor compared to a DC choke. If there are multiple AC drives in an enclosure however, that benefit can still be realized with one larger reactor ahead of the entire inverter drive input power circuit.

Choose PLC base on top PLC Brand?

Always the top brands will be the most popular PLC and over many years it is my opinion that this is because of their marketing strategy, history, reputation and worldwide acceptance more than any other reasons. This does not mean they are better or worse in any way, just means they are more accepted world wide and more people are experienced with their software. Thus there is some security for the owner in respect to programmer support or future resources etc (people come, people go) and a basis on which management may dictate what hardware is used. There is also the consideration on the capital outlay for programming software which can be very expensive.

Choice most often depends on your application and infrastructure. Example: if an entire factory or whatever was “x-brand” and communicating with each other through “y-protocol”, it may be wise to keep to the same-same. Other brands PLC may talk same protocol but then you need to think about software and the experience of your programmer resources, spares etc.

The alternative may be a more task or machine specific PLC that can communicate the same protocol but at the cost of the programmer not knowing the device or software, or the costs of additional software and also there may be less skilled programmers in this hardware choice constricting the owners future options in using this alternative.

Experienced programmers fall into two basic categories. Just like Joe-Builder who has had 25yrs experience – now Joe, was that 25years experience doing different things or was that 1years experience 25 times? I have encountered this so often, fantastic CV but doesn’t know anything because has been in same job, day in day out, year after year. Very good at THAT job mind you but no real (other) world experience. PLC programmers are often the same, know x-plc (or software language) inside out but nothing else.

Just my opinion but a good programmer is someone skilled in ladder logic, functions / function blocks, structured text, CRC etc and knows when to use it. Someone also familiar with the hardware and its associated costs. Someone who knows how the hardware device scans and can makes efficient use of its resources through the above mentioned skills. Someone also who is mind-full of who will maintain / modify and what can be modified and what should not… etc. Bit of a mouth full I know, but such a person can then make choices of hardware based on the end result required and not be constrained in his/her thinking based on what already exists or what they themselves know or what they or their management consider to be the current reality.

So, a long story to ask another question. Are you really asking which is the most popular brand PLC because a quick google search using the a brand name would tell you that in seconds based on the number of millions of pages available for THAT brand or are you asking which PLC should you choose?

As further comment…
Today I would go task specific by choice. If you want ultra speed, complex math or fast analogue and. or heavy processing etc… then you are looking at a soft logic PLC that will talk the same protocol as the other PLC’s in the factory. If the task is simple logic and minimal analogue and does not require ultra fast scan times (i.e. 10ms+ is acceptable) then many top brands offer a range that will do this.

There are many things you can do in ladder logic that will satisfy a situation admirably. There are lots of things you can do in structured text that is impossible / impractical to do in ladder logic. All soft-logic PLC’s I have experienced are totally useless at complex ladder logic. This is WHY I choose by what the task requires as opposed to choosing because of what constrains my current reality thinking or comfort zone.

The end result is a functional task, machine or project that is maintainable – not what a particular

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.

Motor Rotor Bar issue in Current Signature Analysis

The condition of the rotor bars will determine how much torque your motors will deliver. As a person who has been in the electric motor repair business all my life it is something I constantly check. Normally when you talk about rotor bar health it refers to open rotor bars however I have found that in aluminium die-cast rotors there can be voids in the end-rings. Todays vibration equipment and your CSA equipment is so sensitive that it will pick up these voids. In a repair shop environment and with a motor with a good stator winding it is relatively simple to check for open rotor bars. if at all possible we will check for open rotor bars before we take a motor apart by performing a single phase rotor test. You apply approximately 20% of line voltage to two phases of the motor. Rotate the rotor through 360 degrees and monitor the current. If the current is steady the rotor is in good health. If you have one or more open rotor bars the current will drop as the open bars pass the energized part of the stator. A 10% swing would indicate open rotor bars.
Just in case there is a second cage in the rotor you can also put a voltmeter across one of the energized phases and the open phase. Just like the current, the voltage should stay steady.
When a motor is developing open rotor bars it will become noisy on start up. Noisier with each bar that becomes open. It can sound like a cement mixer or as if there is no lubrication in the bearings.

I have no idea what a rotor bar health index is. I would assume that it is a severity level that has been developed by the people who manufacture your test equipment.

Neither am i familiar with the Motor Current Signature Analysis. We use a surge tester which has an attachment for checking rotors but I don’t put much faith in it.

Open rotors can be a nightmare for electric motor repair facilities. Open rotor bars are not always visible and can be very difficult to detect. Our core tester has clamps that allow us to induce a low voltage and high current into the rotor cage but it is not conclusive. We could use a growler to energize the rotor and throw iron filings over the core. On a big rotor it takes a bit of time and customers don’t like paying for it, especially when you don’t find any problems.

If your motors are die-cast aluminium and they are starting up every day without struggling to get up to speed and they are not noisy during start up, your equipment might be picking up voids in the aluminium.
If you have copper or copper aloy rotors with brazed end-rings and I might suggest that you be concerned. Once you get one open rotor bar it only gets worse as time goes by.

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.

What’s the difference of variable frequency drive and soft starter

variable frequency drives are two different purpose products. VFD is for AC motor speed control, it’s not only change the output voltage but also change the frequency; Soft starter is a regulator actually for motor starting, just changing the output voltage. Variable frequency drive has all the features of soft starters, but the price is much more expensive than the soft starter and the structure is much more complex.

Variable frequency drive is converting power supply (single phase VFD and three-phase variable frequency drive.

Soft starter is a set of motor soft start/stop, light-load energy saving and various protection functions devices to control motors.

Soft starter uses three opposite parallel thyristors as regulator, plug it into the power source and motor stator. When using soft starter to start the motor, the thyristor output voltage increases gradually, and the motor accelerates gradually until the thyristor is turned on completely. The motor operates at rated voltage to achieve a smooth start, reduce starting current and avoid start overcurrent trip. When the motor reaches rated RPM, the startup process is completed, the soft starter uses bypass contactor to replace thyristor to provide rated voltage to the motor, in order to reduce the thyristor heat loss, extend the soft starter service life and improve efficiency, also avoid harmonic pollution to the power grid.

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.