Motor chokes

Thyristor converters are the most commonly used systems for supply and control of electric motors rotational speed. Motor chokes (ED1S or ED3S) are often installed between the motor and the converter system in order to improve the mechanical characteristics and dynamic properties of the thyristor power transmission system.
The motor chokes are applied commonly in the converting power transmission systems, both DC and AC. They are installed in the circuits connecting the converter with motor. Depending on the power transmission system type, with which they cooperate, they perform a lot of tasks, including ensuring the motor current continuity and smoothing its pulsation, minimization of the short-circuit current in converter load system as well as the limitation of commutation overvoltages and compensation of the supply line capacitance.

Tasks of motor chokes in controlled rectifying systems.
The ripples of rectified current in the circuit of motor supplied by a controlled rectifier causes the sparking under the brushes and it makes the commutation process difficult. Appropriately selected ED1S motor choke installed in the rectifier load circuit allows to reduce the effective value of the first current harmonic to the allowed level of (2–15) % of the rated current, depending on the motor power and motor angular velocity adjustment range. Knowing the amplitude of a variable component of rectified supply voltage (Udz), the circuit inductance necessary to maintain the allowed value of k current harmonic ΔIk (%) in the circuit is determined from the following equation (1).
 

(1)
where: ω – , m – number of phases, k – harmonic multiply, Idn – rated value of converter current,  ΔIk(%) – allowed value of appropriate current harmonic.

Knowing the necessary circuit inductance (Lob) and inductance of machine armature (Lt) the inductance of motor choke (ED1S) limiting the current pulsation in the converter load circuit (2) (rys1) can be determined.

(2)

It is important to remember that magnetic material of the core and the motor choke construction should allow to maintain a constant inductance at the armature current equal to twice value of the rated current. This condition results from converter overcurrent capacity.


Fig. 1 Simpified diagram of symmetrical 3-phase bridge.

Role of motor chokes in AC power transmission systems.
Output voltage of converters is a series if rectangular pulses of adjustable width and frequency. The rate of voltage run pulses accretion is very high and endangers to the insulation of supplied machines. Limitation of voltage accretion rate and thus the risk of damage to the motor insulation is achieved by installing the ED3S motor choke between the motor and inverter (Fig .2).


Fig. 2 Simplified diagram of converter supplying the squirrel-cage motor.

ED3S motor chokes are applied also in order to limit the shot-circuit currents until the protections are triggered and and the current is switched off in the circuit. In most cases the selection of appropriate inductance of motor choke is the only possibility to protect the thyristors (power transistors) of converting systems (Fig. 2). The selection of ED3S motor choke inductance depends on maximal value of short-circuit current in the system. This current cannot exceed the unique peak current value of thyristor (ITSM).

In practice it is often necessary to supply the voltage to drives which are located far away from the supply source. Long supply lines are characterized by great capacitancies, which contributes to a significant increase of power losses in the system. In addition to machine insulation protection the ED3S motor choke compensates the capacitance of the supply line and limits the harmonics and commutation overvoltages in the motor circuit. A smoothing choke (ED1W) is installed in the intermediate circuit of the converter in order to smooth the pulsation and ensure the continuity of rectified current. Optimal selection of its inductance has a significant impact on the operation of the entire power transmission system.
 

ELHAND motor choke, type ED3SM-0,29mH / 250A

ELHAND motor choke, type ED3S-0,15mH / 400A

 

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