|Electrical Machines - Basic vocational knowledge (Institut für Berufliche Entwicklung, 144 p.)|
|5. Asynchronous motors|
|5.3. Operational behaviour|
Inrush current origin
As soon as the current is switched on the rotating field rotates at full speed along the rotor bars of the squirrel cage rotor.
(Cp. Figure 52) indicates that as s = 1 (U = U), a greater inrush current is attained in the rotor which is transmitted transformerwise to the stator side.
Every asynchronous motor accepts a higher current when starting from no-load position.
This inrush current, when utilising the full mains voltage, can be four to eight times as great as the rated current.
This excessive current load can lead to a disruptive voltage drop in the network. Consequently, for example, only motors with a rated performance of up to 2.2 kW may be connected directly to the 380 V network where the making current exceeds the rated current by more than seven times over. Higher powered motors require special measures for cutting back the considerable starting current.
Incorporating the equation
we derive for the rotor current I2
The rotor note only features the small ohmic resistor R2 but also the inductive resistor
XL2 = s · w1 ·L2 = s · 2 · p · f1 · L2
During the switching torque the resistance attains its maximum value as s = 1 and is therefore greater than the ohmic resistance.
Figure 54 - Indicator diagrams of the rotor circuit resistors
Legend as for Figure 52
The power factor cos j2 therefore attains a minimal value and there is similarly only a low starting torque
Despite the considerable inrush current the asynchronous motor only evidences a minimal torque when starting from no-load position.
Measures to restrict the starting current
All drive operations presuppose a sound starting up, that is to say, a sufficiently high motor torque. Consequently measures must be undertaken to boost the starting torque. However, the network load which arises during start operations which may be evidenced in a voltage decline or through the inrush current, shall not exceed the prescribed values. It is therefore essential effectively to limit starting current. A simultaneous increase in starting torque is also often requested.
Starting current restriction becomes possible by
- decreasing U2.0: a lesser stator voltage is fed to the motor (U1 ~ U2.0) during starting operations. This leads to a starting procedure for which additional devices are required to connect the short circuit motor.
- increasing R2: increasing the rotor resistance R whilst starting requires a differently constructed rotor. The short-circuit rotor must be replaced by a differently arranged rotor featuring changeable ohmic resistance facilities.
In the equation M = C2 · F · I2 · cos j2 all physical values have been incorporated which might influence the torque. Such an optimal solution denotes that such values are changed which permit the starting torque to increase without increasing the starting current. This demand is only met if cos j2 is increased. The power factor is boosted by means of an ohmic resistor at the rotor circuit resistance. This in turn makes necessary a different rotor construction from the short-circuit rotor.
Additional facilities make it possible to decrease the high starting current of the squirrel cage rotor motor (Cp. 5.4.1). A reduction of the starting current whilst simultaneously increasing the starting torque is only possible where differently constructed rotors are used which evidence a greater ohmic resistance during starting operations.
Speed behaviour depending on the torque
Operating an asynchronous motor presupposes a certain speed for a given torque. This ratio is given for any one motor.
Figure 55 - Rotational behaviour of an asynchronous motor in dependence on the torque
1 Speed, 2 Torque, 3 Rated speed nn, 4 Starting torque Ma, 5 Rated torque Mn, 6 Breakdown torque Mk
The initial torque Ma is the torque yielded by the motor in no-load operation. The breakdown torque Mk, is the greatest possible motor torque. If this torque is exceeded, the motor comes to a standstill.
The torque at rated load Mn is the motor torque yielded during rated load and rated speed.
Mn = torque at rated load
Pn = rated power in kW
n = rated speed in rpm.
Every asynchronous motor must be able to accept at least a 160 per cent rated torque for short load spells without motor breakdowns, that is to say the rotor stops. The speed does not vary greatly given considerable load variations.
Operating characteristics of an asynchronous motor
The various motor data about the yielded torque are cited in order to provide an overview of the behaviour of the asynchronous motor between no-load and rated load operation.
This figure indicates the operational curves of an asynchronous motor by means of the following data:
Pn = 4 kW, U = 380 V, In = 8.8A, n = 1.430 rpm.
cos jn = 0.83
Figure 56 - Operating curves of a 4 kW squirrel cage motor (380 V; 8.8 A; cos j = 0.83)
1 Current in amperes
2 Power in kilowatts
3 Power curve
4 Current curve
5 Nominal power
Where asynchronous motors are driven at subload, efficiency and the power factor assume lesser values.
Therefore, asynchronous motors should not be overdimensioned or run in no-load operation.
Many drives, for example in the textile or wood-working industries, require speeds considerably greater than 3000 rpm.
Conversely, sometimes low speeds are similarly necessitated. Thus, speed control of the asynchronous motor becomes essential. Speed control possibilities for asynchronous motors can be derived from the following equation:
n = nD (l - s)
The speed established given a certain torque can therefore be influenced by:
- frequency change in the supplied three-phase alternating current through the employment of rotating frequency converters of through alternating or converter circuits with adjustable frequencies.
- altering the number of pole pairs through pole changing. This can ensue in two ways. Either by means of two or several separate stator windings which can be switched on as desired or by switching over parts of a single stator winding (Dahlander pole-changing circuit).
- altering the slip by changing the voltage application UI by means of series resistors or adjustable transformers, resp. by changing the ohmic resistance R2 in the rotor circuit (slip ring rotor).
The most commonly employed adjustment procedures are those of circuit engineering as cited in section 5.4.2.
Any change in the rotational sense in the case of an asynchronous motor can be attained by changing the rotational direction of the rotating field by exchanging any two external conductors in accordance with Figure 57.
Figure 57 - Rotational sense change
L... conductor U; V; W winding connections (terminal board)