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Synchronous motors VS induction motors

1- Synchronous motors generally offer more efficiency than induction ones, and hence in higher ratings (about 5000 hp and higher) they may be more cost effective considering Life Cycle Costs. The exact size of preference to switch to Synchronous shall be determined based on LCC analysis of specific application.
2- A Large reciprocating compressor is a highly variable load and a synchronous motor will keep its speed in this situation while the induction motor would respond with fluctuating speed.
3- Based on API 618 (with reference to IEC and NEMA), a synchronous motor used for reciprocating compressor may tolerate 66% variation in current, while an induction motor is allowed to have only 40% variation in current which in larger compressors may be exceeded (because of variable load).Also Higher efficiency induction motors with less slip, cause more current variations and are prohibited.

Synchronous motors are characterized by limited starting torque, the ability to actively control power factor and less current in-rush than the induction motor. The synchronous motor also requires active matching of torque demand with motor output. Synchronous motors started “across-the–line” also produce oscillatory torques at the twice slip frequency during acceleration (i.e., starting at 120 Hz and decreasing to 0 Hz at full speed). These torques generally require additional transient torsional analysis because of the potential for damage.
Synchronous motors are usually advantageous on slow speed applications (e.g., low speed reciprocating compressors operating from 200-400 RPM) and also on machines larger than about 10,000 to 15,000 HP.  With both motor types, it is important to match the compressor torque versus speed requirements with motor torque versus speed capabilities as discussed in Sections 6.0 and 7.0. Both induction and synchronous motor types can be coupled with a VFD for variable speed operation.

If the motor is being driven by a variable frequency drive with sophisticated drive algorithms, i.e. controllers that can track the load torque variations, then both the efficiency and transient stability problems can be solved together.

The other significant thing is the starting problem. The transient load torque is also present at starting so the motor has to be able to accelerate through the load transients and be capable of starting when the compressor is sitting at the highest load.

By |May 26th, 2016|Iacdrive_blog|0 Comments

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.

By |May 26th, 2016|Iacdrive_blog|0 Comments

Variable frequency drive Vector control VS V/F control

As far as I know all variable frequency drives with vector control can also be run with just V/F control.

A drive in vector control mode has several tuning parameters to increase or decrease motor performance. With factory default parameters a VFD in vector mode will have higher performance than a drive in V/F mode. Sort of like a "sport or racing" computer option in a modern automobile.

Depending on the application using vector control can use a lot more power. If you have a rapidly surging load the vector may be really struggling to keep the speed constant while a variable frequency drive in V/F mode never notices the speed change. If the application has a steady mid-range speed and load or has a slow rate of change a vector and V/F may be very close in amp draw.

If you have an application where you need the vector for starting or stopping quickly but you are using a lot of current at speed you can change vector parameters to reduce the current. In some applications it is cheaper to oversize a V/F drive to get starting or stopping torque if you don't need precise speed control.
I accept the fact that, in the practice, V/f is considered by many the better choice for fan loads, but I see few reasons why V/f approach could result in better efficiency.

One reason could be that, since it doesn't try to regulate anything, practically it can't oscillate due to weak stability, although oscillations may still occur (I've seen a heavily vibrating torque measurement on a fan driven by a V/f variable frequency drive).
Another could be that, while non-linear V/f curves (suitable to non-linear loads as fans) are quite common, the same is not done for the flux reference (magnitude) in vector control.
And, of course, the few parameters of a V/f control are far easier to tune than a vector scheme (which companies don't really share).

However, one interesting thing that can be done with vector control is, for slow dynamics applications, to automatically tune the flux reference to achieve a minimum loss control during the control operation. I don't think this would be possible with V/f.

By |May 26th, 2016|Iacdrive_blog|0 Comments

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.

By |May 26th, 2016|Iacdrive_blog|0 Comments

Cable length between VFD and Motor | Iacdrive

The dU/dt at the output of the variable frequency drive combined with the motor cable length will result in very high voltage peaks at the motor terminals. This is a concern for the isolation in motors not designed to be driven by VFDs.
On the other hand the maximum motor cable length depends also on the switching frequency used due to the charging effect of the motor cable capacitance (this is a limitation on the variable frequency drive side, not on the motor isolation).
The dU/dt at motor terminals normally is very different from the dU/dt that you can calculate from IGBT and its driving characteristics (turn on time, gate resistor, etc) at variable frequency drive terminals. As the cable acts like a distributed LC impedance, the dU/dt calculation on VFD terminals will give you very high values that can be apparently dangerous, but in practice, will not happen at motor terminals.

For long cables, the combination of cable impedance, high frequency input impedance of motor and VFD switching frequency can lead to reflection of voltage pulses that gives origin to large voltage overshoots on motor terminals. The problem increases as increasing switching frequency because the time between voltage pulses will be smaller, so, a voltage pulse reaching the motor will add to the pulse being reflected. This “double pulsing” can results in extreme voltage overshoot and dU/dt that will result in motor insulation failures. For the variable frequency drives side the increasing switching frequency will be a problem (besides power losses) if you have a big capacitor filter at converter output, that can lead to high current pulses at inverter side.

The determination of the resulting dU/dt at motor terminals from the dU/dt at VFD drive terminals is very difficult if you try to use simulations. For this task you’ll need the high frequency parameters of cables (that also depends on installation details) and motor, that will not be available from standard datasheets and are very difficult to obtain from measurements. In practice almost all VFD manufacturers make extensive measurements and establish some criteria in order to orient applications. The approach is to determine if it is necessary or not to have an output filter for a known application (cable length).

For instance, a common specification is:

For cable lengths up to 100 meters (and motor suitable for variable frequency drive applications) it is not necessary a filter; for lengths from 100 to 200 meters, a series reactance can be used; for greater lengths it is necessary an LC filter at VFD terminals. The limit lengths can be different from different manufacturers and voltage levels (LV/MV). Iacdrive, for instance, can give complete orientation for application of its drives considering the needed cable length for the application.

By |May 26th, 2016|Iacdrive_blog|0 Comments

Compensate electric motors effect of high altitude

Case: Two electrical motors that design for altitude <1000 m but now this two electrical motor have installed on altitude 1880 m and this electrical motors become very hot. The electrical machines power is 15300KW & 9700KW and they cooled by force air and water cooler.

First - machines designed for higher-than-normal altitude (i.e. in excess of 1000 m = 3300 ft above sea level) are designed with lower allowable temperature rises. The rule-of-thumb approximation is 1 degree C for every 100 m above 1000.

This means a typical Class B rise (max 80 C over 40 C ambient) will be designed for a max 71 C rise over ambient at 1880 m altitude.

Since temperature is more-or-less proportional to the square of the current, the design either reduced in output power to limit the current, or is "overdesigned" so that the resultant output power is the effective de-rate condition. In this case, the "sea level" rating of 15300 kW would become 15300 * (71/80)^2 = 15300 * 0.94 = 14382 kW. Likewise, the 9700 kW machine would be rated for 9118 kW.

The ability to cool the machine effectively is based on two things: the amount of coolant in direct contact with the heat source(s), and the pressure of the coolant flow. At altitude, the density of the coolant is reduced significantly, hence the requirement to operate at lower power ratings. The pressure of the airflow over the windings, etc is ALSO reduced at higher altitude, making the cooling more inefficient.

Speeding up the blower (i.e. going from 6 pole speed to 4 pole speed, for example) will overcome some of this by increasing both airflow and pressure. However, the power draw on the blower drive motor may also necessitate an increase in size to accommodate the new loading parameters (including the effects of high altitude on it!). Note that if the air movement within the machine enclosure is dependent solely on the MACHINE rotor speed (i.e. a shaft mounted fan), there will be a need to develop and apply a separately-powered fan to accommodate the required changes.

The probability of voltage breakdown / corona / flashover is increased above 1800 m as well, which means at least taking a cursory look at both creepage and strike distances.

And finally - if, after all this, the machine is still overheating ... time to look at the cleanliness of the liquid side of the heat exchanger. This may mean cleaning or replacing the tubing and headers, determining liquid flow rates (and pressures) and ensuring they are within original design criteria (roughly 3.8 litres per minute for each kW of loss in the rotating machine).

By |May 26th, 2016|Iacdrive_blog|0 Comments

Soft starter settings

Reference voltage adjustment
Reference voltage is the basic condition of the equipment is able to start or not. Reference voltage adjustment requires the electric motor rotates immediately after voltage applied and the load start up. If the motor does not rotate after voltage applied, we should increase the reference voltage setting value; if the motor start speed is too fast, then reduce the reference voltage setting value. Reference voltage adjustment should be repeated for several times until the load starts immediately after voltage applied. For example, a smoke blower has a 110kW motor in debugging process with soft starter, reference voltage adjusts to 75% rated voltage, the starting current is 500A, motor start up fast; reference voltage adjusts to 40% rated voltage, motor start up in slow speed, starting current rise from 200A to 600A smoothly, and current return back after motor start is completed, therefore, it's fully meet the soft-start requirements.

Starting time adjustment
Motor acceleration torque and starting time has direct relationship. Electronic soft starter can make the motor with voltage ramp start from initial voltage to full voltage at the set time (0.5 to 2408). Like it can reduce water impact if we extend the time of water pump flow from 0 to 100%, increase the pump speed variation time means increase the starting time which can be achieved by adjusting the starting time of the soft starter. Starting time should be adjusted according to the specific loads and repeated tests, in order to achieve smooth acceleration within starting time.

Soft stop
Soft starter allows the output voltage decreases gradually to achieve soft stop, in order to protect the equipment. Such as the impact of the water pump, when the pump stops suddenly, the water flow inertia in the pipe will raise the pipe and valves pressure suddenly and cause pipeline damaged. Soft stop to extend parking time will solve such the impact.

By |May 26th, 2016|Iacdrive_blog|0 Comments

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.

By |May 26th, 2016|Iacdrive_blog|0 Comments

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.

By |May 26th, 2016|Iacdrive_blog|0 Comments

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.

By |May 26th, 2016|Iacdrive_blog|0 Comments