Technical Note – adapelektrik

START DELTA STARTER

This kind of starter is used for motors that have low resistant torque or that they start without load. The motor must be disposed to the kind of starter and it must have the terminals adapted for star / delta cabling.

At the start (star mode) the current of start is 1/3 of the current of start of a direct starter (I start = 1,5 ÷ 2,6 I nominal); it is smaller with electro-mechanical starters.

ELEVATOR AUTOTRASFORMER START WITH INTERMEDIATE VOLTAGE

This kind of starter is usually indicated for motors of medium / high power situated at high distance from the control panel.

At the start, the motor is fed with the intermediate voltage (V start > V line) by means of an elevator autotransformer with two level of voltage on the secondary circuit. At the end of the start, the motor exchange at the higher voltage on the secondary circuit of the autotransformer (V nominal > V start > V line); therefore, the autotransformer is always in service.

The current of start is variable with the square of the ratio between the voltage (V nominal / V start)², it is much reduced, by function of voltage ratio chosen. Also, after start phase, the nominal current remains very low in comparison with a nominal current of a same motor fed at more low voltage, with all the advantages regarding the fall of voltage, therefore the dimensions of cable, in the connection cabling between control panel and motor.

The advantages of this kind of starter are:

current of start much low
nominal current very low in comparison with a nominal current of a same motor fed at more low voltage
use of only three cables of reduced section in comparison with the section of cable necessary for a same motor fed at voltage of line
no interrupt of feeding

The disadvantages are:

considerable encumbrance and weight of autotransformer
complexity and cost of realization of the autotransformer at continuous service with intermediate voltage

STATOR IMPEDANCE STARTER

This kind of starter is used for any type of motor and it is largely used for action of submersible pumps, because is favorable the use of only three cables.

At the start, the motor is fed by means of stator impedances in series with the motor stator. The stator impedances cause a fall of voltage that proportionally reduce the current of start (I start = 2,5 ÷ 3,5 I nominal – against I start = 5 ÷ 8 I nominal in the case of direct start).

How much the fall of voltage is greater, so much the current of start is low, but the torque of start is reduced of the squared of the fall of voltage.

After a programmable time, usually around 2 sec. approximately, at the end of the transitory start phase of the motor, the impedances are excluded without any interrupt and the motor are fed at the voltage of line.

Therefore, the advantages of this kind of starter are:

low current of start,
adapted for every type of motor,
use of only three cables,
no interruption of feeding.
The disadvantages are:
low torque of start,
considerable encumbrance of stator impedances.

SOFT STARTER

This kind of starter is used for any type of motor, particularly for electric motors with high current of start.

The Soft – Starter is a voltage controller, it makes the start of motor by means of the progressive increase of the voltage of feeding. This allows the sweet starter of the motor, with the reducing of the peak of current. Also, the arrest of the motor is sweet, by means of the progressive reducing of the voltage of feeding.

At the start and at the arrest of motor, the Soft Starter controls the characteristics of operating of the motor, also the thermal protection. During the run phase, at the natural characteristics of the motor, instead, the Soft Starter may be by-passed by means of by-pass contactor, for preserving the device.

The advantages of this kind of starter are:

low current of start
adaptability to every kind of motor
extreme simplicity of realization
minimum stress for the motor and for the connected mechanical devices, thanks to the sweet starter

The disadvantages are:

low torque of start
cost of the device

DIRECT STARTER

This kind of starter is simplest between those used. The motor is connected to the net and it start with its natural characteristics.

The advantages are obvious:

simplicity of the device
high torque of start (C start = 0,5 ÷ 1,5 C nominal)
fast start
low cost

But it show a series of denied factors, as high current of start (I start = 5 ÷ 8 I nominal) with all consequences of the case.

AUTOTRASFORMER STARTER

This kind of starter is used for motors of medium / high power.

At the start, the motor is fed with reduced voltage by means of an autotransformer that, at the end of the start, is excluded from the feeding circuit.

Usually, V start = 70% V line but, on request, it is possible to make control panel with different ratio between voltages, in accordance with the minimal necessary torque to start the motor.

The current of start and the torque of start are proportional with the square of the ratio between the voltages (V line / V start)², then they are:

I start = 1,7 ÷ 4 I nominal;
C start = 0,5 ÷ 0,85 C nominal.

The advantages of this kind of starter are:

current of start much low
adaptability to every kind of motor
use of only three cables
no interrupt of feeding

The disadvantages are:

considerable encumbrance of autotransformer
transitory phase at the passage from reduced voltage of start to the line voltage
complexity and cost of realization of the autotransformer

INVERTER STARTER

The Inverter, or frequency converter, is a device that can control the start of the motor and, above all, the managing of the motor.

This type of starter is the best for to have energetic savings, for to optimize the energy employment of the motors and to limit the current of start.

With the conversion of the line frequency, the Inverter changes the speed and the torque of the motor, consequently the electrical power absorbed from the motor, not only in the start phase and in the arrest phase.

The Inverter may be driven in voltage or in current by means of analogical signals from analogical transducers of pressure, from transducers of hydraulic capacity, from transducers of temperature. In case of electric pumps, the inverter guarantees constant hydraulic capacities or constant pressure of working, or variable pressure of working in function of the customer requirements. In case of air fan or in case of air cooler, the Inverter guarantees constant temperatures, constant air flows, variable temperatures or variable air flows in function of atmospheric parameters, and unlimited other applications, optimizing the energy employment.

Always the inverter may to maintain the motor in the optimal conditions of working, moreover, thanks to the inverter, the electrical line sees the motor as a resistive load, obviating at the problem of power factor correction in case of great motors.

At last, for short periods of working, the inverter may to run the electric motor to advanced speed in comparison to the nominal speed.

Therefore the range of operating of the motor is more extended, that may to obviate unexpected over loads.

The advantages of this kind of starter are:

great energetic savings
control of all parameters of the motor, in any phase of working
constant or variable torque, in any phase of working, in function of the parameters of setting
adaptability to every kind of motor
no stress for the motor and for the connected mechanical devices
managing of unlimited applications, above all if coupled with a programmable logical controller (PLC)

The disadvantages are:

extreme sensibility to perturbations or radiofrequency that would to arrive to the inverter from the cables of connection with the motor that would to need, in case of great distance (over 70 ÷ 100 m), use of shielded cables or filters near the inverter and near the motor (this is impossible, for example, in case of submersible pumps).