FAQ - Frequently Asked Questions

Q1: How to become an authorized Mecc Alte service center?
A1: First of all address your request with all your data to aftersales@meccalte.it. Then you will be contacted by the Mecc Alte branch responsible of your country.

Q2: How to connect the generator to the load? What about single phase reconnection?
A2: Detailed information about this subject can be found on Mecc Alte instructions manuals (look at the alternator connections tables), that can be downloaded from "Downloads Area" of Mecc Alte web site (www.meccalte.com).
As for single phase reconnection, you can also do reference to video n. 6, available in the "Help Video" section of support.meccalte.com.

Q3: How to connect and set up my new original AVR?
A3: Do always reference to Mecc Alte instructions manuals to perform correctly these operations. These manuals can be downloaded from "Downloads Area" of Mecc Alte web site (www.meccalte.com). Videos n. 1, 2 and 3, available in the "Help Video" section of support.meccalte.com, can also give very useful information about the digital regulators setting.

Q4: How can I adjust the frequency?
A4: Frequency is connected to the speed (RPM) of the prime mover and to the number of pole of alternator.
If the prime mover is an engine, the frequency can be adjusted at the desired value by a proper setting of engine speed.
Typical speed of 4-pole alternator is 1,500 RPM at 50 Hz and 1,800 RPM at 60 Hz; typical speed of 2-pole alternator is 3,000 RPM at 50 Hz and 3,600 RPM at 60 Hz.

Q5: Do I need a computer to set my digital AVR? What potentiometers are for?
A5: In order to have the AVR fully working, it is enough the set properly its 4 potentiometers (VOLT, STAB, HZ and AMP).
Only in case of advanced functions set up it is necessary to be equipped with PC, software and so on.

Q6: When is the three phase sensing necessary?
A6: The use of three phase sensing is advised in presence of unbalance loads. That in order to reduce unbalance voltage.

Q7: How to connect the PD (Parallel Device)?
A7: Do always reference to Mecc Alte ECO-ECP Instructions manual (look at the terminal box with parallel device tables and electrical diagram tables) and Synchronous Generators Parallel Operation manual (look at the parallel devices table) to perform correctly this operation. These manuals can be downloaded from "Downloads Area" of Mecc Alte web site (www.meccalte.com). In case of any doubt please contact the Mecc Alte branch / service center closest to you.

Q8: What do we mean with "Global Warranty"?
A8: Mecc Alte Global Warranty covers Mecc Alte products at the level of standard warranty in every country in which it is used.
Complete list of authorized Mecc Alte service centers around the world is available in the "Service Network" section of support.meccalte.com; in the "Warranty" section it is instead possible to consult Mecc Alte Standard Warranty and Mecc Alte Standard Conditions of Sale.

Q9: What is the impact of leading power factor on my alternator?
A9: A leading power factor load applied to an alternator can create overvoltage conditions. The final result of that can be an alternator failure, but also a damage to the load.
For this reason the capability of alternator to absorb leading power factor load is very limited.
For any additional information, please, contact the Mecc Alte branch / service center closest to you.

Q10: Where do I find the Serial Number of the alternator?
A10: The Serial Number of alternator can be found on its data plate; data plate is usually stuck on the frame of the alternator itself. The Serial Number is available in the field identified by "ID N" or "S/N" (on the left, top side).

Q11: What information do I need to collect before asking for a warranty claim?
A11: Basic information to supply before sending any warranty claim is Serial Number of alternator, alternator type, working hours and general description of failure with pictures, if it is possible.

Q12: What is the Direction of Rotation of the Alternator?
A12: All standard Mecc Alte alternators are fitted with bidirectional fans, and are designed to operate properly with the shaft rotating in either a clockwise or counter clockwise direction.
The only exception is for standard S20W, S20F-FS and T20F-FS alternators, where, for rotor design reason, the only rotating direction allowed is clockwise as viewed from the alternator drive end (viewed from the side of coupling to the engine).
Take also note that for three phase operation all standard three phase Mecc Alte alternators have a 1-5-9 phase sequence when the alternator shaft is rotating clockwise as viewed from the alternator drive end (viewed from the side of coupling to the engine).

Q13: Calculation of the mechanical power requested on the engine shaft, knowing alternator apparent power (kVA) and efficiency.
A13: Convert the alternator apparent power (kVA) in active power (kW).
As for 3-phase machines at cosφ = 0.8, the active power is calculated by multiplying the apparent power by the coefficient 0.8. As for 1-phase machines at cosφ = 1, the active power is same as the apparent power.
Then the mechanical power requested on the engine shaft is calculated by dividing the alternator active power by its efficiency. The value achieved, expressed in kW, can be converted in CV dividing it by a coefficient equal to 0.735 or in HP dividing it by a coefficient equal to 0.746.
It is always advisable to select an alternator a little bit oversized against the engine used.

Example: 3-phase alternator rated 100 kVA with 92% efficiency.
The alternator active power is then equal to 80 kW (= 100kVA x 0.8), instead the mechanical power requested on the engine shaft is equal to 87 kW (= 80kW / 0.92). This mechanical power, expressed in metric horsepower, is equal to 118.3 CV (= 87kW / 0.735), expressed in British horsepower, is equal to 116.6 HP (= 87kW / 0.746).

Q14: Calculation of the apparent power (kVA) supplied by an alternator, knowing the mechanical power available on the engine shaft and the alternator efficiency.
A14: Convert the mechanical power available on the engine shaft in kW.
This means that, if the mechanical power is expressed in CV, this value has to be multiplied by a coefficient equal to 0.735, instead, if the mechanical power is expressed in HP, this value has to be multiplied by a coefficient equal to 0.746.
At this point multiply the mechanical power available on the engine shaft, expressed in kW, by the alternator efficiency. It is then achieved the active power supplied by the alternator, expressed in kW.
As for the 3-phase machines at cosφ = 0.8, the apparent power is achieved by dividing the active power calculated by the coefficient 0.8. As for the 1-phase machines at cosφ = 1, the apparent power is same as the active power calculated.
Of course the apparent power calculated has to be lower or equal to the alternator nominal apparent power.
It is always advisable to select an alternator a little bit oversized against the engine used.

Example: engine with mechanical power available on the shaft of 140 HP and 3-phase alternator rated 130 kVA with 93% efficiency.

The mechanical power available on the engine shaft, expressed in kW, is equal to 104 kW (= 140HP x 0.746). The active power supplied by the alternator is then equal to 97 kW (= 104kW x 0.93). The apparent power supplied by the 3-phase alternator at cosφ = 0.8 is consequently equal to 121 kVA (= 97kW / 0.8).

Q15: Why PMG system is no longer necessary?
A15: Supply to electronic regulator: From pmg or auxiliary winding?

The pmg system provides energy supply to the electronic regulator by means of a small alternator with permanent magnets. This unit is usually located towards the rear of the alternator. Mecc Alte ensures the power supplying to the electronic regulator by means of an auxiliary winding buried in the main winding. It follows a brief description of the differences and the common features of those two systems.

Weight and size.

For the same power, a machine with an auxiliary winding is lighter and smaller than a machine with PMG. In fact the auxiliary winding has a negligible weight and its inclusion in the main stator does not have any effect on the size of the machine which therefore remains unchanged. The PMG (consisting of a permanent magnet rotor and a stator) has however a certain weight and its inclusion has a significant effect on both the weight and the length of the machine.

Voltage supply of the system.

The power output from the pmg is not affected by the power generated by the alternator or by distorted loads. The output from an auxiliary winding slightly is. This voltage variation must be considered when designing the electronic regulator (AVR). If the AVR is properly designed, these effects can be completely compensated. Therefore, from the user point of view, there is not any difference between the two systems. Due to a careful design of the electronics, MeccAlte generators with auxiliary winding system can meet exactly the same performances of a pmg-fitted generator during power transients or with distorted load.

Transient power of the system

The pmg system can only provide a limited transient power compared with the auxiliary winding system. This means that the pmg has got a limited starting capability on electrical motors, that require a higher than nominal current to start up. The auxiliary winding system has got a much higher motor starting capability. In a pmg the excitation power is limited to the size of the pmg, while in the Mecc Alte system the excitation power is theoretically equal to the machine power, which is considerably higher than the power needed for excitation.

Short circuit capacity.

Both systems are able to maintain a correct supply to the electronic regulator even in the event of a short circuit within the main stator. In this situation the sensing on the voltage of the electronic regulator is zeroed and the guaranteed short circuit current is greater in both cases than 3 times the nominal current. The Mecc Alte system could also provide higher short circuit current values without compromising the weight, the size, or the cost of the alternator; however this is not possible with the pmg system.

Conclusion

The two systems are more or less equivalent from the performance point of view. The pmg system has only a theoretical advantage in applications with distorted loads; actually the Mecc Alte auxiliary winding system is able to provide the same performances with distorted loads and transient power as the pmg system by means of a specifically designed electronic regulator. The Mecc Alte system certainly has advantages in terms of the motor starting capability and the excitation power available, as well as in size and costs.

Q16: Which is the method for sizing a generator working with distortion loads?
A16: Simplified method for the sizing of electrical machines working with distortion loads.

In order to avoid problems with the overheating of alternators with distortion loads, precise calculations should be made with regard to the sizing. By distortion load, we mean any type of load producing current harmonics. Generally, such loads are: diode rectifiers, thyristor converters, inverters, uninterruptible power systems (UPS), etc.
If there are not accurate specifications about the distortion load that the alternator has to feed, Mecc Alte bases the sizing of the electrical machine purely from a thermal point of view that leads to a machine over-sizing. Considering the different types of distortion, it is generally assumed that 20% over-rating should be applied to the machine.
Over-sizing is necessary to obviate the temperature increase of the windings, due to the current harmonics introduced by the distortion load.
If it is necessary to meet a specification on the overall voltage distortion, it is necessary to know the individual value of the current harmonics amplitude introduced by the load.

The total harmonic distortion (THD) is defined as follows:

(1) THD% = 100 • SQRT(Σ Un²/U²)

Where Un is the RMS amplitude of the n-th voltage harmonics and U is the RMS amplitude of the first voltage harmonic. In a three phase generator, if the neutral is not distributed, multiples of 3rd harmonic must not be considered. Thus, the previous equation becomes:

(2) THD% = 100 • SQRT ((U5²+U7²+U11²+ … Un²)/U²)

In a synchronous machine the voltage droop produced by the current harmonics is determined by the direct - axis sub-transient reactance (X"d):

(3) Un = X”d • n • In

Where In is the component of the n-th current harmonic. Unsaturated sub-transient reactances should be taken into account.

If all the quantities are expressed in percentage of the rated values, the equation 2 becomes:

(4) THD% = SQRT ((U5%)²+(U7%)²+(U11%)²+ … +(Un%)²)

and the equation 3:

(5) Un% = (X”d%/100) • n • In%

If it is known the relative amplitudes of the current harmonics, the components of the voltage harmonics can be easily calculated. These amplitudes in a converter are a function of the commuting configuration. If the THD is specified, it is possible to calculate the required X"d from a given content of current harmonics.

Combining together equation 4 and from the equation 5 it comes:

(6)THD%=SQRT (((X”d%/100) • 5 • I5%)²+(((X”d%/100) • 7 • I7%)+ … +(((X”d%/100) • n • In%)²)

(7) THD% = (X”d/100) • SQRT ((5 • I5%)²+(7 • I7%)²+ … +(n • In%)²)

and finally:

(8) X”d% = (100 • THD%)/ SQRT ((5 • I5%)²+(7 • I7%)²+……………+(n • In%)²)

By means of the formula n. 8, it is possible to calculate the correct value of X"d (direct - axis sub-transient reactance) that is needed to suit the THD voltage distortion specification. As it has been proved, it is necessary to know current harmonic content of the load.

As an example, let's consider a distorted load of 70 kVA at 400 V, 50 Hz.

If there is not any other specification given, it is advisable to select an alternator of the size ≥84 kVA (+ 20%) at 400 V, 50 Hz (=> ECP34-1S/4). This is necessary to obviate the temperature increase of the windings, due to the current harmonics introduced by the distortion load.

Let's suppose that the same load has to meet a maximum voltage distortion specification. The specs for such a load are:
- Maximum voltage THD of 10%.
- 6 pulse distortion load characterized by the following individual current harmonics:

Applying the equation n. 8, it is possible to get:

(9) X”d%=(100 • 10)/ SQRT ((5 • 20)²+(7 • 14)²+(11 • 9)²+(13 • 8)²+ … +(23 • 4)²+
+(25 • 4)²)

X”d% = 3,58%

Then it is calculated an X"d equal to 3.58%, that results to be met by the model ECP34-2L/4. The X"d of this alternator is equal to 6.8% at 150 kVA, 400 V, 50 Hz and, then, it will be equal to 3.2% (< 3.58%) at 70 kVA, 400 V, 50 Hz. The reactance expressed in percentage (%) changes proportionally in respect to the power.

The use of a damped caged rotor is strongly suggested when power supplying a distorted load. The damping cage on the rotor is a standard supply for all the machines of series 28, 32, 34, 38, 40, 43 and 46.