By its very nature resulting from solid rotor windings and lack of power supply to its windings, an induction motor is most suitable for operation under steady condi- tions and with a small slip. In such a case the angular speed results from the fre- quency of the supply to the stator windings, number of pole pairs and value of the slip. Traditionally, it was applied in drives in which neither frequent changes of speed nor variable control were required (examples of such devices include pumps, blowers, compressors, belt conveyors, cranes, industrial hoists). There was virtually no possibility of controlling induction motors within wide range of speeds while concurrently preserving high energetic efficiency until 1970s. Drives in which the control of speed was necessary most frequently applied slip ring in- duction motors, in which it is possible to control rotational speed as a result of use of external elements. However, such systems are either complex, costly and prob- lematic in control due to the use of cascaded systems. Alternatively, they have lower energetic efficiency due to additional resistance in the rotor’s circuit. In ad- dition, the start-up properties of an induction motor under direct connection to the network are adverse due to the initial period of oscillations of electromagnetic torque with a high amplitude and high value of the start-up current. Despite these drawbacks the induction motor has become the most common machine in electric drive systems due to the fundamental advantages including long service life and
reliability as well as low price and accessible supply source. Following the devel- opment of power electronics and control elements enabling arbitrary shaping of voltages and currents, induction motors became widely applied in complex drives due to a new angular speed control potential and general robustness at heavy duty.
This section will be devoted to the presentation of the methods of forming charac- teristics of induction motors and will cover the devices that make it possible to realize the required characteristics. The possibility of modeling characteristics re- sults directly from the relation defining angular speed
) 1 2 ( ) 1
( s
p
s fs
f
r = − = −
Ω ω π (3.138) where:
p fs
f
ω = 2π - synchronous angular speed of a rotating field. Each of the values in relation (3.138) offers the possibility of modeling mechanical character- istics: number of pole pairs p, slip s as well as the frequency fs of the supply volt- ages. The control of slip s is possible to a large extent as a result of the external in- terference in the rotor circuit and also voltage changes but within a small range of rotational speeds. The presentation of methods used for modeling characteristics associated with rotor slip changes will follow in the subsequent sections. Concur- rently, a separate section will be devoted to an extensive presentation of control as a result of modifying the frequencies of the supply voltages. The application of the various number of pole pairs p for changing motor speed appears to be most straightforward to explain. A series of synchronous speeds ωf for a given supply frequency consists of a discreet values. For the successive number of pole pairs p
= 1,2,3,4,5,6,... and for example for the frequency of the supply fs = 50 [Hz] they are, approximately:
… , 4 . 52 , 8 . 62 , 5 . 78 , 7 . 104 , 1 . 157 , 16 .
=314 ωf
This finds application in multi-pole motors, in which the windings can be switched to two or three synchronous speeds, which leads to a stepwise change of rotor speed. This type of drive is applied in cranes and industrial hoists mainly with two speeds – transit speed with a higher value and a slower approach speed.
3.3.1 Control of Supply Voltage
The control of the supply voltage can offer only limited possibility of adjusting ro- tational speed of an induction motor. This results from the basic mechanical char- acteristic of the motor (Fig. 3.28) which indicates that the slip under a given load can be increased up to the limit of s < sb, which means it has to keep below the break-torque slip beyond which a loss of the stability occurs and the motor stops.
In addition, this type of control is achieved at the expense of efficiency loss since under a constant load the losses in the motor are ΔP > Pf s. This comes as a con- sequence of the increase of the current and losses in the motor windings. At the same time, the control of the supply voltage is currently used in order to reduce the start-up current and perform a soft-start. This is realized with the use of an electronic device called a soft-starter. A diagram of such a device is found in Fig. 3.33.
Fig. 3.33 Basic diagram of a soft-starter for an induction motor
The introduction of semiconductor elements (SCRs, IGBTs, GTOs, MOSFETs etc.) for the two directions of current flow for each line supplying the motor wind- ings makes it possible to employ current flow with a selected delay angle α in rela- tion to the zero crossing of the supply voltage curve. As a result, at some expense of altering the current and voltage from sinusoidal shape, it is possible to control the value of voltage and synchronize the motor with the network at the instant of connecting the particular motor phase windings during start-up. Soft-starters may, accordingly, realize the following functions related to the start-up and stopping of an induction motor:
- synchronization of the connection of particular phase winding to the network and thus enabling the reduction of the variable component of the torque (see 3.2.2.5)
- limitation of the start-up current in a selected range,
- braking with the use of direct current (see section 3.2.2.6) and conduct con- trolled stop of a drive.
Not all of the above functions have to be realized by a single type of soft starter.
In the most economic versions designed for smaller drives, a soft starter some- times contains switches in the two supply lines, which only leads to limitation of the start-up currents and does not provide symmetry of the supply voltages. The following Figs. 3.34-3.38 present the examples of application of a soft-starter for an medium power induction motor with a delay angle α = 40º and the basic value of the moment of inertia J = Js. The figures present a comparison between start- up versions without synchronization during the connection of phases to the net- work and the one with synchronization involving the connection of line L1, L2 for phase angle δ1,2 = 0.48π [rad] and a later connection of the third supply line L3 for angle: δ3 = a + δ1,2 -0.1 [rad]. As a result, we obtain a very soft starting curve dur- ing the initial stage of the start-up of the motor (Fig. 3.34) accompanied by a very favorable torque waveform (Fig. 3.36). The synchronized connection for such a large delay angle α = 40º also results in the reduction of the duration of the
start-up (Fig. 3.35, Fig. 3.36) since the value of the constant component of the motor torque increases during the initial stage of the start-up. The current wave- form in the phase winding of the motor for such a supply is presented in Fig.
3.37. The delay angle in the range of around 40° is virtually the sharpest one for which it is possible to conduct start-up of the motor during idle run within a sen- sible time, due to the considerable reduction of the value of electromagnetic torque of the motor. The approximate illustration of the effect of delay angle α on characteristics of the motor is presented in Fig. 3.38. Soft-starters find application in drives with an easy start-up due to the considerable reduction of the torque fol- lowing the fall of the value of the supply voltage.
a)
b)
Fig. 3.34 Line current of the medium power induction motor during free acceleration with a soft starter (α = 40°): a) without synchronization b) with synchronization: δ=0.48π; δ3 = a + δ1,2 - 0.1
a) b)
Fig. 3.35 Relative velocity curve for the medium power motor during the soft-start free acceleration, under conditions like in Fig. 3.34