What is a wound rotor induction motor

Induction machines are part of the category that comprises AC electronic machines which is where the transformation of electrical energy into mechanical energy, or the reverse. The energy conversions occur within induction machines by means of the magnetic field. They are due to the dynamic force that the field exerts the conductors (windings) that are placed inside the machine. In these machines, currents flow which are driven to be pushed by electromotive forces (sem) caused by the magnetic field. An induction machine is comprised of 2 windings (electric circuits) generally located in grooves that are components of the ferromagnetic core that are separated with the air gap. The windings aren’t galvanically connected to one another. One winding are connected with the network for industrial use and the other is directly shorted. The transformation of electrical energy into mechanical energy or vice versa happens due to being able to induce semi’s inside windings that are shortcircuited (unpowered) windings that force the flow of current through the winding.

Apart from that electric motors can also be divided by supply current. There is DC electric motors and AC electric motors that are split into the synchronous and asynchronous. AC motors are also divided according to the principles operating into synchronous induction, and commutator models. Additionally, you can find a distinction between motors based on the shape of the rotor. These are ring-induction motors as well as the squirrel-cage induction motors. Alongside the previously mentioned classifications, there is a second category of electric motors. which are known as universal motors. typically used in appliances for home use. They can be powered by direct current or alternating current.

Induction motor

Induction motors, also known as asynchronous motors, are the most well-known type of electric motors that are used in many industries, not just. In fact, synchronous AC motors are also used in a variety of appliances for the home. They are distinguished by their straightforward and easy-to-maintain designs and power levels that can vary from milliwatts up to megawatts.

Induction (asynchronous) motors are typically employed to drive machines without regulating speed. They’re typically seen in motors equipped with squirrel-cage (short-circuited) rotating rotors with power outputs that range in the range of just a few milliwatts up to several hundred Kilowatts, with nominal voltages ranging of 0.4 and 6 kV.

Motors with wound rotors also known as ring motors, can be made in the range of power between 2 kW and several MW, all at the same voltages for supply.

A ring motor is different from an squirrel cage motor due to the fact that it has three-phase windings within the grooves of the rotor The edges of which have been short-circuited, and the beginnings can be led to and linked to slip rings on the shafts of the rotor. Brushes are positioned on the rings in order to connect the rotor’s rotating winding to the speed or starter controller.

Ring-rotor motors that are specifically designed to operate without speed control, are equipped with built-in circuits for short-circuiting the rings and lifting the brushes.

Ring-rotor motors, in contrast to squirrel-cage motors, a lower current at the start and the ability to control speed. However, they are complicated to operate and also cost more.

Construction and operation of induction motors

Induction motors are constructed out of two main components – an rotor that is moving and a stationary stator. The stator winding is a three-phase alternation current that creates an oscillating magnetic field which in turn is a rotating one. Then, as an effect of induction, the field that is rotating creates an electromotive force within the winding of the rotor. In the presence of this force the current of electricity is pushed through the rotor’s winding. The rotor’s magnetic field as well as that of the stator’s field work together with each other, creating an magnetic torque as well as motion. This is why how an asynchronous motor as its name suggests is based upon induction , which creates an electromagnetic force that is further interacting, leading to motion.

Basic parameters of engines

If your motor connects to a 3-phase network the stator winding creates the field, whose speed of rotation depends on the number pole pairs. When the rotor located within the stator, is able to rotate freely, as well as its windings have been shortcircuited (either directly or via resistance) there is a torque created by the effect of the field’s rotation on the currents generated by these windings, which causes the rotor’s position to change in directions of field.

The speed of the rotor should be at least a percentage lower than the field’s synchronous speed.

The rotor speed should be several percent lower than the synchronous speed of the field. The power factor cosφN and efficiency ηN depend on the degree of its load on the shaft.

Motors’ fundamental parameters comprise:

Rated power:

P_N = √3 U_N I_N η_N cosφ_N ∙ 10^-3

_{Stator phase winding voltage:}

When connected in delta: Us= UN

At the star connection:

Stator phase winding current:

At the triangle connection:

At the star connection: I_st = I_N

Operating states of wound rotor induction motors

Idling of the induction motor

Induction motors are idling. is known as a condition of operation that includes:

• The stator winding is supplied by voltage from the mains,
• the rotor, which is not loaded with any torque spins at a very high speed , close to the synchronous speed
• there are extremely small amounts of slip (about equal to 0.001) as well as frequency of the fs of the rotor, and losses in the steel rotor,
• the electromotive force having a low magnitude is produced within the rotor. A very tiny amount of I2 current flows through it, so losses in the winding of the rotor are minimal,
• the power generated to the motor’s shaft is negligible (the motor shaft isn’t fully loaded),
• the power generated from the motor is used entirely to cover the losses in the stator winding ∆Pcu0, and in the stator steel, ∆PFe0 as well as mechanical losses ∆Pm,hence:

P_o = ∆P_cu0 + ∆P_Fe0 +∆P_m

Idle losses ∆Po (do not depend on the load) and are:

∆P = P_o – ∆P_cu0

     The idle current in induction machines at rated voltage is:

I_o = (025 – 0,5)I_N

      The power factor at idle is:

cosφ_o = 0,1 – 0,2

     A lower power factor means that the motor running at idle draws almost entirely inductive reactive energy from the grid, which causes power supply losses to increase and reduces power consumption from the mains power source.

In this case it is essential to employ compensating equipment (e.g. capacitor banks and compensaters synchronous) and ensure that loaded induction motors are removed from the grid.

Short circuit condition of induction motor

The condition of a short-circuit in the induction motor can be referred to as an induction motor condition. It is characterized by:

• The stator’s winding gets power from the mains
• the circuit of the rotor is short-circuited and the rotor has not been started, i.e. when n = 0 and si = 1 The current in the rotor is high, and it may be as high as (4 + 10)IN,
• the electrical energy used by the motor in the short-circuited condition is completely converted to energy (mechanical electricity is not used since the rotor doesn’t move),
• due to the relatively small loss in the central core area, it is possible to be roughly believed that the energy consumed by Po is designed to cover loss of load.

∆Pcu1 – in the stator winding;

∆Pcu2 – in the rotor winding,

ie:

P_o = ∆P_cu1 + ∆P_cu2

The short-circuit voltage (uz) of an induction engine (uz) can be described as the current that has to apply to the windings of one side such as the stator. This is so that, when the opposite side is short-circuited, and the rotating rotor is deactivated the current rated flows to the supply side.

The short-circuit voltage of an motor is calculated in relation to the voltage that it is rated at:

The short-circuit voltage for induction motors varies from 10 to 25 percent UN.

Current in the short-circuit of an Induction Motor

An induction motor operating in the short-circuit state, supplied with the rated voltage is drawn a short-circuit current

The equation is: Is = (4 (or 10)IN (and reversed when it is supplied with an in-circuit short-circuit voltage, it draws the current that is rated). The relationship between short-circuit voltage (Iz) (Iz), the rated (IN) (IN) as well as short circuit voltage (uz percent) is as the following:

The short-circuit problem in squirrel-cage motors occurs each when that the circuit is connected even if voltage reduction devices aren’t utilized. The short-circuit current of the ring motor is decreased through increasing resistance in the winding circuit.

Load condition of induction motor. Stable and unstable operation

The load-condition that an induction engine experiences happens in the event that it is connected to the machine being driven and the stator winding is supplied by the line voltage.

Under load conditions these parameters for the motor’s induction are as the following:

• the power conditions for the supply of the motor are generally unchanged,
• the speed at which the equilibrium between the torque produced to the motor by M, and that of the braking force Mh are established.
• at every load change the torque released by the motor is adjusted to the braking torque.
• the slip flow of currents through the windings power ratio that the motor has, the actual energy consumed, as well as the power balance changes,
• the force produced by the motor is changed until the balance of moments is established M = Mh.

The characteristics of an induction motor are determined by its operating characteristics, which is the relation of these parameters to the amount of torque (or power P). The most often used characteristic of motion of induction motors can be seen in the figure M = f(n) and the equation U1N = const. and f1N = const. This is also known as mechanical characteristics.

The characteristics of the motor’s operation that operate in the load state are influenced by their stability and instability, as well as the the motor-driven unit as well as to transient or steady states that include:

In steady state, the speed of rotation (n) that is generated by the machine is constant and the torque produced through the motor identical in magnitude, but in a different direction from the torque for static loads of the machine.

In the state of transient – the speed of rotation changes in the transient state; there is no equilibrium of the torque generated through the motor (M) and the brake the torque (Mh).

There is a difference in these instances:

M ‒ M_h = M_d

     is called the dynamic moment (Md).

If Md > 0 (that is, M > Mh ), the system accelerates;

If Md < 0 (i.e., M < Mh ), the system decelerates.

Transients in the operation of rotating machinery most often occur during:

• start-up,
• speed control,
• load changes,
• changes in power supply conditions,
• stopping of the system.

Depending on the characteristics of the induction motor – driven machine system, there can be two ranges of operation:

• stable operation, if the system returns to equilibrium after a brief disturbance,
• unstable operation, if after a short disturbance the system stops or tends to coast.

_{Stable operation range of induction motor at 0 < s < sk}

If the induction motor is filled with a braking force Mh = const lower than the initial torque Mr (at the point of beginning n = 0 and the s value is 1) It will be supplied by line voltage, and the motor starts to spin in the direction of dynamic torque Md. The moment Md gives acceleration to the rotor, and consequently the speed of the rotor rises and slip s decreases.

The length of an electric motor in the form M = f(s) can be described in the following manner:

• The motor’s operating point is moved in the direction of that M = f(s) beginning at to the point of s=1 towards the smaller slips, and then across the line (Mk, sk);
• The steady-state at A. The lines (b) the steady state at point A, where M = f(s) and intersects the Mh line. Then, the torque generated by the motor M = Mh while the torque dynamic Md is M + Mh is 0, rotor’s speed is fixed at the point A, at a speed of n1;
• The steady state is at B. the torque Mh grows until it reaches the value M’h. Then M’h > M, i.e., it will be higher than the torque generated by the motor. The motor’s speed decreases, and slip grows. The operating point is shifted towards more slip, and the torque produced by the motor will be equal to the torque of braking in the braking system. A new steady state will be created at point B. It will be characterized by the sII as well as nII.

Range of motor unstable operation at sk < s < 1

In the event that an instantaneous equilibrium state M = M’k is observed at the point C (the braking torque increases, Mh) The motor’s speed is reduced and the motor operating point shifts towards the slip s=1.

The torque produced by the motor as well as the rotor’s speed will decrease until, when 1 s, the motor is stopped.

In a different scenario, if the braking torque is reduced when it is at equilibrium at the point C, the force generated by the motor will be higher than the braking torque M exceeds Mh which causes the rotor’s speed to increase.

The motor’s operating point will move through it’s point (Mk, the sk ) towards smaller slips. This will result in the simultaneous increasing the torque and speed. The steady state change will take place at the point B.

Overload capacity of an induction motor

The torque of motors is proportional magnitude of the voltage Any decrease in the line voltage will drastically reduce the amount of torque generated by the motor. This is why the motor’s operating point must be in the steady operating range that is far away from its critical torque in order that the decreased voltage doesn’t make the engine stop.

The majority of the time, the nominal torque (MN) that an induction engine produces is two times smaller than the crucial torque (Mk). The motor’s overload power is calculated using the formula:

     The overload capacity in a motor is assumed to be about 2.5. In motors performing responsible tasks, such as in crane motors, it can even exceed 4.5.

Protection of induction motors

Electric motors can have a substantial effect on electricity grids, for example, current overcurrents when motors start as well as voltage dips and fluctuations and substantial reactive power consumption from motors that are idle.

Motor protection basics include:

• short-circuit protection,
• overprotection,
• (c) protection from the negative effects of the effects of voltage reduction.

To protect motors from short-circuits that have voltages less than 1 kV, three-phase fuse or three-phase solenoid devices are utilized.

For motors that have an output of more than 1 kV The function of short-circuit protection is fulfilled with fuses in 3 phases, paired with a disconnector, or time delay relays. For motors that have an output of greater than 2 MW , and when six terminals are exposed with longitudinal differential protection, two phases is utilized. Motors that exceed 1MW have ground fault protection. It is that is supplied by an electric Ferranti transformer.

The setting current of the motor short circuit protection is to remain as minimal as it is but in a way that it doesn’t operate at high levels of currents during normal operation , and also when the motor is starting up.

The current that is rated for fuse-links to protect AC motors is determined by the connection:

I_bn > I_ns

in which:

Irs – motor inrush current, in A;

Ins – rated motor current, in A;

kr – inrush current multiplication factor;

α – starting coefficient (under average conditions equal to 2 for fast-acting inserts

and 2.5 for inserts with delayed action).

     Triggers or electromagnetic relays that are short-circuit protection for the motor are set to current:

I_we ≤ 1,2 ·I_rsmax

Where: Irsmax – the highest inrush current of the motor.

Triggers, thermal relays and temperature sensors are employed to provide overload protection for motors with a rating of up at 1kV. If motors are rated above one kV, delayed protection is utilized.

Motor short-circuit protection must be employed in three phases of three-phase systems. However, in DC systems, it should be applied to two poles or in one when the other pole is grounded. Short-circuit protection shouldn’t be utilized in circuits that excite.

Each motor must have its own short-circuit protection. Short-circuit protection that is common to groups of motors is permissible, which means that should there be the short circuit occurring within one of the motors the group protection will be activated. Overload protection must be present for each motor , with the exception of:

• motors that have ratings of lower than A,
• motors that do not exceed 10 kW in continuous operation and are unlikely to overload (pumps or fans),
• motors that form one unit, each with its own safety,
• motors that operate intermittently which are protected with temperature sensors isn’t economically viable.

In three-phase systems, with the neutral point being grounded, overload protection is employed in 3 phases, but not with a natural point that is grounded in two phases. Overload protections are configured to have a limit of not more than 1.1 Ins.

Protection against overvoltage is provided as over-voltage delay single- or two-phase. This protection should be utilized in the following situations:

• The reduction in voltage prevents the motor from working,
• self-starting of a short circuit motor is not recommended,
• self-starting of an ring motor is unacceptable,
• it is recommended to detach a variety of less critical motors to ensure the self-starting of the other motors.

The arrangement of two relays undervoltage, which are which are switched by the line-to-line voltages, is utilized to guard a group of motors or a single motor important from the standpoint of the technology. A system with only one relay undervoltage should be utilized to safeguard specific motors that are less important.

Wound rotor induction motor characteristics

The red color indicates features of the motor at the positive spin rate. If the slip is that is greater then zero (motor speed lower than synchronous speed: oos) the machine acts as a driver, whereas for slips that are negative (motor speed higher than the synchronous speed: o>os) the machine goes from brake operation. The blue color indicates characteristics of the slower rate of field rotation that can be achieved by switching between 2 phases in the voltage of supply to the motor.

Mk – critical torque,

Mr – starting torque,

MN – rated torque,

ωN – rated speed,

ωs – synchronous speed.

Advantages and disadvantages of wound rotor induction motor

Advantages of the induction motor

1. One of the main advantages of an induction motor is the fact that its construction is very simple. Stator designs are identical to that of the synchronous and induction motors. However the synchronous generator needs the use of a slip ring to supply DC energy to be delivered to the motor’s rotor. Slip rings are not necessary in a squirrel-cage induction motor as the windings are always shorted. As compared to the DC motor Induction motors have no brushes, and maintenance requirements are minimal. This results in a simpler design.
2. The motor’s operation is completely independent of the conditions in the environment. This is due to the fact that the motor is robust and durable mechanically.
3. Squirrel cage induction motors do not have slip rings, Brushes and commutators. Because of this, the price for the motor can be affordable. But slip rings are utilized to create Wound Induction Motors to provide an external source of resistance for the windings of the rotor.
4. Due to the absence brush, there aren’t sparks that could ignite the motor. It is also able to operate in hazardous environments.
5. Unlike motors that are synchronous 3 phase induction motors has a large starting torque, great control of speed and decent overloading capabilities.
6. Induction motors are highly efficient with an efficiency of 85-97 percent.

Disadvantages of the induction motor

1. A single-phase induction motor unlike a three-phase motor, is not equipped with a start-up torque. It requires auxiliary equipment to begin a single-phase motor.
2. Under low load conditions the power efficiency of motor decreases to a low level. This is due to the fact that during the initial phase the motor, it draws a significant magnetic current to overcome the reluctance created due to the air space that is created between the rotor and stator. Additionally that an induction motor is able to draw smaller current from power source. The combination of the magnetizing and load present is deferred by approximately 75-80 degrees. This means it’s power efficiency is very low. Due to the large magnetizing current, the losses of copper in the motor will increase. This results to a reduction in motor efficiency.
3. Controlling the speed for an induction motor can be very difficult to achieve. This is due to the fact that the three-phase motor is one that is constant speed that is, for the entire range of load, the rate of change in motor speed is minimal.
4. Induction motors possess large surge currents at the input and are also referred to by the name magnetization surge currents. This leads to a decrease in voltage once the motor is first started.
5. Due to the lower beginning torque of the motor, it is unable to be utilized for tasks that require large beginning torque.

References:

https://riverglennapts.com/pl/basic-induction-motor/53-advantages-and-disadvantages-of-induction-motor.html

http://old.zseii.edu.pl/archive/dydaktyka/maszyny/budowa_sil_induk.htm

https://bezel.com.pl/2018/08/01/silniki-indukcyjne/