Squirrel Cage Induction Motor

These are essentially similar to squirrel-cage induction motors and are driven by a wind turbine prime mover at a speed just above synchronous speed, normally upto 1% rated slip for today's large wind turbines. Because the speed variation from no load to full load is very small, the term ‘fixed’ speed tends to be widely used. The generator is coupled to the wind turbine rotor via a gearbox as shown in Figure 5.29. The design and construction of the stator and rotor of an induction generator are similar to that of a large induction motor having a squirrel-cage rotor.

The principal advantages of the squirrel-cage induction motor are its simplicity of design and robust construction. Its torque/speed and current/speed characteristics are such that, on starting, a torque of typically twice full-load torque is produced but a large current (typically, six to eight times full-load current) is drawn from the supply. It is this latter aspect of the current drawn upon starting which is important in deciding upon the type of starting equipment needed for a squirrel-cage motor because

1.

The drawing of a large current from the supply will cause a corresponding voltage drop at the point of common coupling with other consumers. This will affect other drives within the installation or consumers fed from the same supply authority.

2.

If the motor drives plant of high inertia (e.g. fans) the time during which the large current is drawn may be extended. Such a current flowing for an extended period may cause the unwanted operation of overload or over-current protection relays within the supply system.

In the case of a squirrel cage induction motor, the geometry is regular and periodic. This was taken advantage of in the mesh generation strategy. The mesh was formed using a mapping function which subdivided each region of the motor. The regions are defined in Figure 4.2.1.

where AB and GH refer to the arc lengths, NB and NA are the specified number of segments along these arcs, and R is as shown in Figure 4.2.2. The total number of layers is then

where NINT returns the nearest integer to the argument. The length, d₁, is found as

The number of segments on this layer is

To find the next layer, we recompute W, replace N by N-1 and R by R −d₁ and so on. The first step in predicting the magnetic vibration characteristics of a particular electric machine is to analyze the dynamic magnetic field acting in the machine. The magnetic field data may then be used to compute forces and force distributions, which are then in turn applied to a mechanical model of the machine and used to predict vibrations.

The rotor tooth and slot section exhibits symmetry over each half slot pitch. The geometry is specified in Figure 4.2.3.

As the rotor turns, the air gap elements must be continualy remeshed. In the analysis which follows, we show that for electromagnetic purposes, the rotor is modelled in its own reference frame. The rotor core and slot-tooth regions are not remeshed, only rotated. The stator core and slot-tooth regions remain stationary. It is only the air gap region (excluding the part of the air gap attached to the rotor mesh and stator mesh as described above) which is continuously changing. The mesh is generated in such a way that the air gap region lies between two circles with uniform node spacing. The remeshing is accomplished (when needed) without adding any new nodes or elements. The algorithm only reconnects existing elements using the new nodal positions. This is illustrated in Figure 4.2.4 for two instants of time.

We consider a three-phase squirrel cage induction motor. The motor is rated at 5 hp. The parameters of the motor are listed in Table 10.1. In this example the entire motor winding is represented. The inputs are the instantaneous voltages at the three terminals. The currents in the windings are unknown. The rotor is free to turn. Each rotor bar is represented as an independent circuit connected to an end ring that has a constant resistance and inductance. The mesh in the air gap may or may not be remeshed at each time step, depending on the distortion of the elements. In any case, the remeshing is done so that the number of nodes and elements remains the same. The sequence of plots in Figures 10.4, 10.5, and 10.6 shows the motor operating at full load at various positions in a cycle.

To maximize the life and performance of AC squirrel-cage induction motors in process industry applications, one must have:

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A motor with the proper design (A, B, C, D, or E) for the application

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An enclosure properly configured to protect the motor from environmental contamination

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An adequately sized motor to handle the load requirements

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A conservatively designed (or applied) motor that will allow it to work well within its designed thermal capacity

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A proper installation that assures good alignment, eliminates vibration, and does not transfer unwanted external loads to the motor

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An adequately designed and maintained (if required) bearing lubrication system

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A motor environment that provides adequate air circulation and maintains the effectiveness of motor heat removal or ventilation systems

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A regulated power supply that supplies the correct voltage, maintains voltage balance between phases and provides protection against over currents

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A load cycle designed to avoid frequent starts and other causes of motor overheating.

The vast majority of the motors used in industry are squirrel-cage induction motors (Figs. 1 and 2) due to their low cost, high reliability, and fairly high efficiency. These motors also use more of the total electrical energy than any other motor type in homes and buildings. The three-phase induction motor, invented in the 1890s, is composed of three sets of stator (the stationary part of the motor) windings arranged around the stator core (a stack of steel laminations with slots for the windings).

Type 1 fixed-speed induction generators are essentially similar to squirrel-cage induction motors. These are driven by a wind turbine prime mover at a speed just above synchronous speed, normally up to 1% rated slip for today’s large wind turbines. Because the speed variation from no load to full load is very small, the term ‘fixed’ speed is widely used. The generator is coupled to the wind turbine rotor via a gearbox as shown in Fig. 6.4. The design and construction of the stator and rotor of an induction generator are similar to those of a large induction motor with a squirrel-cage rotor.

Using Eq. (5.112b), to represent a single-cage fixed-speed induction generator, the instantaneous short-circuit currents delivered by the machine for a three-phase solid fault at its terminals are given by

where

and ${\theta }_o$ is the instant of fault occurrence on machine stator phase r voltage waveform and the machine parameters are as given in Chapter 5, Modelling of rotating ac synchronous and induction machines.