Iacdrive|Automation Solution

What’s PG card in variable frequency drive?

PG is short for Pulse Generator, generally it is used for measuring rotational speed. The most common PG card is optical encoder.
PG card is a part of vector variable frequency drive, to convert the encoder different form signals to suitable for the controller, like: electrical level conversion, analog digital conversion, optical isolation, etc.
Vector control variable frequency drive is a high-performance drive which can be comparable with DC converter.
In the vector control, it requires a motor speed feedback to the variable frequency inverter drive, this speed feedback is achieve by adding a rotary encoder (PG) to the motor, which means PG card feedback vector control VFD. In order to simplify the system, the feedback can be formed by operation of the inverter output signal, this control is called none PG card feedback vector control VFD, the performance has a slight gap than PG card feedback, but configuration is simple.

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.

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.

Design a PMSM to 10000 rpm high speed

Synchronous speeds are a function of the applied frequency and the number of poles, governed by the equation

120 * (frequency in hertz) / (poles) = (speed in rpm).

Adjust the ratio of frequency to poles to achieve the desired speed.
(example: a 4-pole design would require a line frequency of 333.33 Hz … which means operating on either an adjustable speed drive or on a dedicated high-frequency power system.)

Once you’ve got the electro-magnetics sorted out, it’s a matter of manufacturing to the mechanical constraints associated with the rotational speed.

Well, depending on the power rating, and on the required reliability, I believe it’s very simply. The biggest problem would be to get a variable frequency drive, or other power supply to provide a 3-phase output frequency of about 500Hz.
An automotive alternator should be able to operate relatively reliably at your required speed, and it can probably deliver around 1.5kW at that speed.
In order to make it permanently magnetised, we just have to disassemble the rotor, take the rotor windings out, and replace them with some ring-shaped permanent magnet. We may possibly also use a number of individual smaller permanent magnets embedded in some non-magnetic material such a copper or aluminium between the two half-shelves of the rotor. Depending on the construction of the alternator, we may need some machine shop to pull the rotor halves apart, perhaps machine some material away to make room for the permanent magnet(s), and to press the assembly back together again, and to balance it afterwards.

Electrical machine software

You can categorize the electrical machine software into 2 basic types:

1) FEA packages that may or may not have a front end for analyzing motors. These are available from companies like Vector Field (now Cobham), Infolytica and a few others.
2) Motor design specific software such as the SPEED software, RMxprt and MotorSolve from Infolytica.

In the first category, the FEA packages are expensive because they are general purpose modeling packages. The motor add-on is usually limited mostly to the building the model and perhaps some specialized post-processing for motors. Their main advantages are:

1) 2D and 3D versions.
2) The user is free to define what analysis he wants to perform since they have very advanced general post-processors.

Their main disadvantages are:
1) Cost, they can get very expensive depending on the options you require.In some cases, the motor design module is a cost option.
2) Although they have general post-processors, many users require a lot of training in order to be able to get useful information.
3) Geometry input can be a lot more complicated since the front-ends typically have a limited number of geometries available.

The second category, the motor design software, is specifically designed for motor analysis. It can be magnetic circuit based such as SPEED and RMXprt or full finite element based such as MotorSolve. The magnetic circuit type of software has been available for a long time but it has only been recently that full FEA based motor design packages have become available.

The general advantages of software of this type are:

1) Template based input so the user simply chooses the motor geometry, stator and rotor and sets the parameters for the geometry. The input is therefore very simple but limited to the templates that are implemented in the package.
2) Post-processing is specialized and presented in a form that a motor designer can use it.

The general disadvantages of this type of software is:

1) No specialized post-processing is available directly from these packages unless added by the software provider in a new release.
2) Geometries are limited to the templates and adding templates may be very difficult and has to be done by the software provider.

What factors cause Current unbalance

1. Voltage unbalance in supply side (1% volts could easily be 10% current).
2. Physical differences between individual stator coil shapes and connections causing small (but noticeable) resistance changes.
3. Unsymmetrical magnetic circuit – not as big a deal in the smaller “ring” lamination designs, unless highly saturated.
4. Lightly loaded machines will exhibit far higher unbalance than those loaded closer to the full nameplate rating (mostly due to the magnetizing current requirements and associated core/stray loss).

For quick solution measure the current in the three phases, then change the three supply terminals by shift the three terminal to rotate the motor in the same direction, and measure again the current, if the high current move with a certain phase (example: phase L1 of supply read high current in the two case above) the problem is from supply, you can then measure the voltage at motor terminal to be sure that the control circuit and cable are good.