Induction motors stator current

Since the kW developed by a 3-0 winding is 50% more than by a 2-0 winding for the same value of stator current /, the economics of this principle is employed in an induction motor for general and industrial use. As standard practice, therefore, in a multi-phase system, only 3-0 induction motors are manufactured and employed, except for household appliances and applications, where mostly single-phase motors are ttsed. 

The declared efficiency and power factor of a motor are affected by its loading. Irrespective of the load, no-load losses as well as the reactive component of the motor remain constant. The useful stator current, i.e. the phase current minus the no-load current of a normal induction motor, has a power factor as high as 0.9-0.95. But because of the magnetizing current, the p.f. of the motor does not generally exceed 0.8-0.85 at full load. Thus, at loads lower than rated, the magnetizing current remaining the same, the power factor of the motor decreases sharply. The efficiency, however, remains practically constant for up to nearly 70% of load in view of the fact that maximum efficiency occurs at a load when copper losses (f R) are equal to the no-load losses. Table 1.9 shows an approximate variation in the power factor and efficiency with the load. From the various tests conducted on different types and sizes of motors, it has been established that the... 

This is a very useful nomogram to determine the performance of a motor with the help of only no-load and short-circuit test results. In slip-ring motors, it also helps to determine the external resistance required in the rotor circuit to control the speed of the motor and achieve the desired operating performance. Slip-ring motors are discussed in Chapter 5. The concept behind this nomogram is that the locus of the rotor and the stator currents is a circle. Consider the equivalent circuit of an induction motor as shown in Figure 1.15, where... 

If a large induction motor is switched on such a system it is possible that its rotor may lock up at the sub-synchronous speed and keep running at higher slips. This situation is also undesirable, as it would cause higher slip losses in addition to higher stator current and overvoltage across the series capacitors. 

Induction motors usually entail insulated wiring windings for both the rotor and the stator, with the stator connected to an external electric power source. Between the narrow gap of the stator and the rotor, a revolving magnetic field is established. A current can be established only when the waves of the rotor and stator windings are not in phase—not at a maximum simultaneously. 

The synchronous motor is a constant-speed machine. Unlike the induction motor which inherendy has slip from losses, the synchronous motor uses an excitation system to continually keep the rotor in synchronous speed with current flowing through the stator. Within its designed torque characteristics, it will operate at synchronous speed regardless of load variations. 

In general, the stator of a synchronous motor is quite similar to the stator of an induction motor. The polyphase current flowing in the stator winding sets up a rotating magnetic field in the same way as the induction motor. 

When the motor first starts there is a sudden surge of current into the stator winding (inrush current). This is usually 6 to 6.5 times the amps the motor sees when it is running. This causes similar surge in the squirrel cage rotor hars, so great care must he taken to size all the hars, WMC uses copper hars, correctly. This magnetic circuit in the rotor induces a current thus, the name induction motors. 

Induction Motors. An induction motor is an alternating-current motor in which a primary winding on one member (usually the stator) is connected to the power source and a polyphase secondary winding or a squirrel-cage secondary winding on the other member (usually the rotor) carries induced current. There are two types ... 

Figure 6.93 Longitudinal section drawing of a flameproof cage induction motor with integrated eddy current clutch, stator housing water cooled.

AC methods include standard squirrel-cage induction motors, wound rotor induction motors, synchronous motors and commutator motors. Speed variation is obtained by the control of applied voltage to the stator or the control of current and voltage in the rotor by external circuit connections. 

If an induction motor is running in a stable steady state with a low shp, then the various fundamental currents and voltages within the motor can be calculated from the conventional equivalent circuit. When the motor is supplied from a source of harmonic voltages the impedance elements in the circuit need to be modified to account for the frequency of each harmonic that is present. The various reactances are directly proportional to the harmonic frequency. The stator and rotor resistances may be assumed constant, although in practice they will increase with the frequency, the rotor more than the stator, see Reference 9, Figure 1.26 therein. 

When a running induction motor has a short-circuit applied to its terminals the air-gap flux creates an emf that drives a current into the fault. The motor is then driven by the inertia of its load. The speed may be assumed to be unchanged for the duration of the fault current, which in practice for small motors is only a few cycles at the supply frequency i.e. less than 60 milliseconds. For large motors the duration may as long as 250 milliseconds, see Reference 23. This is due to the higher X-to-R ratio in the short circuit than is the case with small motors. The impedance to the fault current consists of the transient reactance (equal to the sub-transient reactance) and the stator resistance. This will be shown below. 

In the case of constantly fluxed synchronous motors, the stator cmrent will follow the motor torque more closely. After adding excitation losses, the synchronous motor efficiency is still shghtly better than the induction motor. The drive inverter for a synchronous motor supplies the armature or torque producing current, compared to the induction motor apph-cation where the inverter must supply torque producing current and magnetising current In the synchronous motor application, the motor operates at unity power factor, which reduces current demand in the inverter section. As a result, there are fewer losses in the inverter and motor, and to a lesser extent fewer losses in the converter. 

The magnetic flux generated in the stator of an induction motor, such as that shown in Fig. 3.22 earlier in this chapter, rotates immediately the supply is switched on, and therefore the machine is self-starting. The purpose of the motor starter is not to start the machine as the name implies, but to reduce heavy starting currents and to provide overload and no-volt protection as required by lET Regulations 552. 

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