What is an Induction Motor Drive?
Induction motor drives are defined as systems used to control the performance of induction motors, including their speed, torque, and position. These drives can adjust the frequency and voltage supplied to the motor, enabling precise control of the motor’s speed and torque, which was not possible in traditional induction motors. This flexibility allows for improved efficiency, reduced energy consumption, and better performance in various applications.
Before further discussing induction motor drives, we need to understand induction motors. In simple terms, an induction motor is a three-phase, self-starting, constant-speed, AC motor. The reason for describing induction motors as constant speed is because normally these motors have a constant speed depending on the frequency of the supply and the number of windings.
In the past, it was not possible to control the speed of the induction motors according to one’s needs. That’s why their use was limited and despite having many other advantages over DC motors they could not be used because of this disadvantage.
However, the field of drivers has improved due to the availability of thyristors or SCRs, power transistors, IGBTs, and GTOs the variable speed induction motor drives have been invented.
Though the cost of these drivers is more than DC drivers, still the use of induction motors is increasing and they are replacing DC motors because of their advantages. While discussing this topic we will look through the starting, braking, and speed control of induction motors.
Starting of Induction Motors
We know that the induction motor is self-starting i.e. when the supply is given to the motor it starts to rotate without any external help. When an induction motor is started as there is no resistance initially (i.e., during starting), there is a tendency for huge current flow through the rotor circuit which may damage the circuit permanently. To overcome this problem various methods have been introduced to limit the starting current. Some of the methods are
- Star delta starter
- Auto-transformers starter
- Reactor starter
- Saturable reactor starter
- Part winding starter
- AC voltage controller starter
- Rotor resistance starter is used for starting of wound rotor motor.
The induction motor drives are normally designed to run on a delta connection, but during starting the supply is given from a star connection because then the starter voltage and current reduce by 1/√3 times than the delta connection. When the motor reaches a steady state speed the connection changes from star to delta connection.
Another type of starting method of induction motors is the auto transformer starting. Since we know that the torque is proportional to the square of the voltage.
In autotransformers, the starting voltage and current are reduced to overcome the problem of overheating due to very high current flow.
During starting the ratio of the transformer is set in a way that the starting current does not exceed the safe limit. Once the induction motor starts running and reaches a steady state value, the autotransformer is disconnected from the supply. The circuit diagram is given here with
Another method of starting is soft-start using saturable reactor drivers. In this method, high reactance is introduced into the circuit so that the starting torque is close to zero.
Now the reactance is reduced smoothly during starting and the starting current increases and the torque also varies steplessly. In this method, the motor starts without any jerk and the acceleration is smooth, that’s why it is called a soft star.
An unbalanced starting scheme for a soft start is another type of starting method where the impedance is introduced only in one of the supply phases. During starting the impedance is kept very high so that the motor operates as a single-phase motor, the speed-torque characteristics at that time are similar to curve A in the graph.
When the speed attains a steady state value the impedance is removed completely, at that time the curve is similar to B, which is the natural characteristic of the machine. This starting method is also without any jerk and the operation is very smooth.
Part winding starting method is special for squirrel-cage induction motors. In this method, two or more starter winding are connected in parallel. When the motor starts, any one of the winding is connected as a result the starter impendence is increased, and the starting current is reduced. When the steady speed is acquired by the motor both the windings are connected.
Specifically for wound rotor motors, a rotor resistance starter is used. In this method, external resistors are used in the rotor circuit to limit the starting current. The maximum value of resistance is chosen to limit the current at zero speed within the safe value.
As the speed increases, the sectional resistance is the temperature rise is lower than other methods of starting high acceleration, frequent starts and stops starting with heavy loads can be done with this type of starting method.
Braking of Induction Motors
When it comes to controlling an electric machine by electric drivers braking is a very important term because it helps to decrease the speed of the motor according to will and necessity. Braking of induction motors can be classified mainly into three types
- Regenerative braking.
- plugging or reverse voltage braking
- Dynamic braking which can be further classified as
- AC dynamic braking
- Self-excited braking using capacitors
- DC dynamic braking
- Zero sequence braking
To explain regeneration braking for induction motors, we can take the help of the equation
Here, θs is the phase angle between the stator voltage and stator current, the simple words whenever this phase angle exceeds 90o (i.e. θs>90o) regenerative braking can take place.
To explain this more clearly and easily we can say that whenever the speed of the rotor exceeds synchronous speed, regeneration braking occurs.
That is because whenever the rotor rotates at a speed more than synchronous speed there is a reverse field occurs which opposes the normal rotation of the motor and therefore braking takes place.
The main disadvantage of this type of braking is that the speed of the motor has to exceed synchronous speed which may not be possible every time. To acquire regenerative braking at a lower speed than synchronous speed, a variable frequency source can be used.
Plugging of induction motors is done by interchanging any two of the supply terminals. When the terminals are reversed the operation of the machine changes from motoring to plugging.
From a technical point of view and for better understanding it can be said that the slip changes from ‘s’ to (2-s), which indicates that due to the reversal of the terminals, the torque also changes its direction and braking occurs.
The first classification of dynamic braking of induction motors is AC dynamic braking any one of the supply phases is disconnected from the supply and then it is either kept open or connected with the other phase.
The first type is known as two lead connection and the second one is known as three lead connection. To understand this braking method clearly, we can assume the system to be a single-phase system.
Now the motor can be considered to be fed by positive and negative sequence voltages. That’s why when the rotor resistance is high the net torque is negative and braking can be acquired.
Sometimes capacitors are kept permanent by being connected across the supply terminals of the motor. This is called self-excited braking using capacitors of induction motors. This type of braking works mainly by the property of the capacitors to store energy.
Whenever the motor is disconnected from the supply the motor starts to work as a self-excited induction generator, the power comes from the capacitors connected across the terminals.
The values of the capacitor are so chosen that they are sufficient to make the motor work as an induction generator after being disconnected from the supply. When the motor works as an induction generator the produced torque opposes the normal rotation of the motor and hence braking takes place.
Another type of dynamic braking is DC dynamic braking. In this method, the stator of running induction motor drives is connected to the DC supply.
The consequences of connecting a DC supply to the stator are as follows: the DC current produces a stationary magnetic field, the rotor keeps rotating, and as a result, there is an induced voltage in the rotor winding, therefore the machine works as a generator which opposes the motion of the motor and braking is acquired
Speed Control of Induction Motors
We have discussed about the starting and braking of induction motors but what about controlling the speed during the running time?
Speed control of induction motors can be done in six methods which are:
- Pole changing
- Stator voltage control
- Supply frequency control
- Eddy current coupling
- Rotor resistance control
- Slip power recovery
We know that the speed of the induction motor is inversely proportional to the number of poles. So it is possible to increase or decrease the speed of the induction motor if the number of the poles are decreased or increased respectively. The motor in which the provision of changing the number of poles is present, they are called a ‘pole changing motor’ or ‘multi-speed motor’.
Another method of controlling the speed of induction motor drives is the stator voltage control. Stator voltage is directly responsible for the rotating speed of the rotor.
Torque is proportional to voltage squared and the current is proportional to the voltage. So, if the stator voltage is reduced the speed reduces and similarly if the stator voltage is increased the speed also increases.
The speed of an induction motor is proportional to the product of the supply frequency and air gap flux. But as there is a chance of magnetic saturation while decreasing the supply frequency, that’s why not only the frequency but the v/f (i.e. the ratio of supply voltage and frequency) is controlled and this ratio is tried to be kept constant. And if the speed is needed to be changed the ratio of v/f is changed accordingly.
The eddy current speed control method is done by placing an eddy current clutch between an induction motor running at a fixed speed and the variable speed load. Now what is this eddy current clutch?
It is nothing but an induction motor drive in which both the stator and the rotor are allowed to rotate. The rotor is coupled with the main induction motor.
When eddy currents are produced in the rotor drum, they interact with the stator field, and a torque is produced which rotates the main motor. By controlling the DC current through the stator winding the speed of the motor can be controlled.
Depending on the rotor resistance, the speed of the rotor falls or increases. The variation of speed torque characteristics with respect to changes in rotor resistance is shown in the figures below. This speed-controlling method is better than many other methods because of its low cost.
