Cement / Stone

Fans (sometimes also called blowers) usually have a very high moment of inertia; it is often many times higher than the moment of inertia of the driving motor. Therefore most of the fan drives start with a very long start time of up to app. 100 seconds. The torque speed curves (counter torque) of nearly all fans have a quadratic shape. Most fans can be started with reduced load by adjusting the blades. This adjustment provides a lower counter torque which results in a lower start current. Normal start currents for fans (I_a/I_n) are in the range of app. 1,5 - 3  times the motor rated current (I_n). Below you will find a characteristic example for a calculation.  Due to the possibly high moment of inertia of the fan and the resulting very long start time the moment of inertia  J [kgm²] (which can be estimated in the first step) should be present for an offer. 

Torque

The starter starts the motor with reduced voltage, 61% of the rated voltage, in this example (U_s/U_n = 0,61). The torque speed curve of the motor will be reduced by the factor T_s/T_DOL = (U_s/U_n × F)².
T_s/T_DOL = (0,61 × 0,91)² = 0,31 (F is a factor depending on the motor). The torque of the motor must always be higher than the counter torque. The start voltage U_s/U_n has been determined accordingly. The difference between the reduced motor torque T_s and the counter torque is the acceleration torque T_a.

Start time

The start time t_s is calculated from the acceleration torque T_a and the total moment of inertia J = J_M + J_L = 150 + 2.500 kgm² = 2.650 kgm².

Current

The motor current is I_Mot = U_s/U_n × I_DOL × F = 0,61 × 5 × 0,91 = 2,8 × I_n

Mains start current:

A transformer has the same power at primary and secondary, which leads to the fact that the products of current and voltage are the same.

U_n × I_s = U_s/U_n × I_mot => I_s = U_s/U_n × I_Mot / U_n

I_s= 0,61 × 2,8 = 1,7 × I_n

The mains start current is therefore much lower than the motor start current.

For other start methods (electronic soft starter, starting reactor etc.) the following is valid: The mains current equals the motor current:
I_s = I_Mot = 2,8 × I_n

For that reason these starting methods create a higher mains voltage drop.

If a controllable coupling (f.ex. hydraulic) is located between the electrical motor and the driven load, the characteristic of the load does not influence the start process at all. We only need to consider the moment of inertia of the motor and the very small moment of inertia of the coupling half at the motor side for the start calculation. This results in a short start time of app. 5-15 seconds. The start will be carried out almost without load. The coupling generates only a very small counter torque. Therefore the start current can be greatly reduced. Typical start currents (I_a/I_n) for the start of controllable coupled drives are in the range of app. 1-1,5 times of the motor rated current (I_n). Below you will find a characteristic example of a calculation:

Torque

The starter starts the motor with reduced voltage, 47 % of the rated voltage, in this example (U_s/U_n = 0,47). The torque speed curve of the motor will be reduced by the factor T_s/T_DOL = (U_s/U_n × F)².
T_s/T_DOL = (0,47 × 0,9)² = 0,18 (F is a factor depending on the motor). The torque of the motor must always be higher than the counter torque. The start voltage U_s/U_n has been determined accordingly. The difference between the reduced motor torque T_s and the counter torque is the acceleration torque T_a.

Start time

The start time t_s is calculated from the acceleration torque T_a and the total moment of inertia J = J_M + J_L = 80 + 5 kgm² = 85 kgm².

Current

The motor current is I_Mot = U_s/U_n × I_DOL × F = 0,47 × 5 × 0,9 = 2,1 × I_n

Mains start current:

A transformer has the same power at primary and secondary, which leads to the fact that the products of current and voltage are the same.

U_n × I_s = U_s/U_n × I_mot => I_s = U_s/U_n × I_Mot / U_n

I_s= 0,47 × 2,1 = 1 × I_n

The mains start current is therefore much lower than the motor start current.

For other start methods (electronic soft starter, starting reactor etc.) the following is valid: The mains current equals the motor current:
I_s = I_Mot = 2,1 × I_n

For that reason these starting methods create a higher mains voltage drop.

Mills exist in a variety of different designs and types. We have prepared detailled information for the start procedure of a shredder. Needless to say, our starter can also be used for further mill types, if the start torque requirement of the mill is not too high. High moments of inertia and therefore long start times do not pose a problem for our starter. If a controlable (f. ex. hydraulic) coupling is mounted between mill and motor, the mill can be started with a very low start current (I_a/I_n) in the range of app. 1-2 times of the motor start current at direct on line (I_DOL/I_n). We will gladly provide you with an offer and technical support.

• typical start calculation

Machines driven by electric motors exist in a variety of different designs and types. For the most common ones we have prepared detailled information for the start procedure. Needless to say, our starter can also be used for further types of machines. We will gladly provide you with an offer and technical support.

• typical start calculation
• all machines