Types of Motor Starters

 

              

13 Sept, 2020.

In this post, we will understand various types of starters used for a 3-phase induction motor. (Click on the images for a more zoomed view)

A 3-phase induction motor is one of the most commonly used types of motor in industrial applications. Various means are used to start and stop the motor either automatically or manually. But, at the same time, it is also of utmost importance to operate it safely for use. Traditional means involved only use of relays to start or stop the motor. Starters provide this basic function with an additional feature of overload protection and safe start/stop. A relay will not provide you protection from overload or heavy current; but starter will. So, in this topic, we will look at some of the most common and widely used motor starters.   

Starter is not a device or an individual component. You can refer the above image for study. It is a set of electrical and electronic circuitry; comprising of contactor, overload relay, fuses and control wiring. Nowadays, various advanced electronic devices are also used along with it.  

Single phase Motor Starter:

Let us understand the basic requirement of a starter first; apart from just starting and stopping. In a single phase induction motor, two main factors of the motor work in proportional to each other; rotor induced emf and slip (relative speed between rotor and stator). The emf is the one which drives the current through the rotor. When the motor is in a standstill condition, the speed of the motor is zero. So, the slip is at it’s maximum value, which induces a very high voltage (emf) at the initial condition. Thus, a very high current flows across the rotor at the starting condition. Due to this high current requirement of motor, a very high current is drawn by stator from the supply; which can be 5-8 times higher than the full load current of the motor. Such high current can damage the motor windings, overload it and also reduce the voltage in the supply line. Such irregular current and voltage changes can disrupt other devices in the supply line.  In simpler terms, imagine a scene where you want to push a big idle stone to another place. You will require a great amount of force to push it initially; to move it out from an ideal state. Here, we are relating this with motor torque. Higher the torque required to start the motor, higher will be the current drawn.

So, apart from switching on-off a motor, a starter is required to limit the starting current that is applied to it. It does that by limiting the voltage applied to the motor in initial condition, for a short time. As discussed earlier, due to this, the slip will reduce gradually. Voltage applied will increase gradually with corresponding gradual reduction in slip and after the motor reaches the normal running state; normal voltage will be applied to it with normal current drawn from the mains supply. It also provides protection against overload condition; as, in overload state, very high current flows which can damage the motor windings. 

This principle is same for a three phase induction motor.

Types of Starter in Induction Motor:

Now, let us have a look at some of the most commonly used starter types.

·         DOL (Direct On-Line):

This is the origin of electrical starter and is typically used for motor ratings of less than 5HP. The technique that we discussed earlier of applying full voltage from the start is nothing but DOL starter. It simply consists of a contactor and overload relay. Contactor will provide switching action and overload relay will provide protection against overload spikes. It is not used for higher motor ratings; as it can damage the motor due to starting current condition, which we discussed earlier.  

·         RDOL (Reverse Direct On-Line):

In applications where a small rating motor is required to operate in both the directions (forward and reverse), this starter is used. It is the same as DOL; the only difference being that two contactors are used. The wiring between them is interlocked in such a way that reverse direction can be achieved by just changing the phase sequence of any two wires (R, Y or B).

·         Stator Resistance:

This method achieves the function of reduced starting current by employing variable resistance (rheostat) across stator windings. Due to resistance connected, voltage drops across it which reduces the amount of voltage applied to the windings; thus reducing the current flowing through it. Gradually, the resistance value is decreased; and as the motor reaches it’s normal running speed, the resistance is reduced to zero and full load current will now flow across the motor windings.

Refer the below image for the logic. The resistance is connected in series with each winding. The initial position of this resistance is kept at maximum; which will apply minimum voltage at the start. As the motor picks up speed, the contact of the resistance is moved to minimum position and becomes zero at full load speed.

The only disadvantage of this method is that; due to high power dissipation in the resistors, a large power loss takes place in the motor. 

·         Auto Transformer:

This method is same as the stator resistance; the only difference being that an auto transformer is used in place of resistance. This too reduces the amount of voltage applied initially. Instead of the slider position, a control switch is used which switches the connection to full load supply once the motor reaches 80 percent of it’s rated speed.

·         Star Delta:

This is one of the most famous and commonly used starters for higher motor ratings. In this technique, the motor is run in star connection for a short period of time; and after the time elapses, the motor is run in delta connection. The inrush current is 1/3^rd of the normal current initially due to star connection. Refer the below image. As per electrical formulas, due to star connection, the current per phase is (V / (root 3 * impedance)). But, in delta connection, the current per phase is (V / impedance). So, we conclude that the starting current drawn from the mains supply is one third of the normal current in delta connection. A hardware timer is installed in the electrical circuit to run the motor in star connection initially. This method is one of the most popularly used in industrial automation. One more reason being, that it is very cost effective. But, one thing to remember is that it is suitable mostly for delta connected motors and also the factor by which starting voltage reduced (one third) cannot be changed.

·         Soft Starter:

Coming to advanced methods, a soft starter is another type of reduced voltage starter as we discussed earlier. But, semiconductor power switches (triac) are used here for this purpose. As usual, they are connected in series in back to back mode with each line phase.  The firing angles to these thyristors are controlled so that a reduced voltage will be applied initially. As the motor reaches rated speed, the angles are altered in such a way that full voltage will be applied to it. Due to use of electronic devices, the overall efficiency of the starter increases with reduced jerk or torque pulsations. The overall assembly comes with protection like overload, low voltage and single phasing. All this is possible due to use of electronic devices.

·         VFD Starter:

This is the most advanced ones of starter that we know. Apart from soft starting, a variable frequency drive will also vary the speed of the motor in running condition; by regularly controlling the amount of voltage applied to the motor. A soft starter cannot vary the speed of the motor once it reaches normal run condition. But a VFD can achieve this task. Due to programming parameters, each and every aspect of motor can be controlled here. For your information, the starting time is controlled here by acceleration time setting and the motor can even be stopped smoothly (instead of instant halt) by deceleration time setting in the VFD.

I have covered the general types of starters normally used in practice. I have also not attempted to cover every type of starter deeply; you can learn it easily once you get familiar with them. I have just given you an insight of these types of starters. A great electrical understanding is required by the engineer who designs electrical drawings for starters. Learn the basics and explore a new type of study in this type of automation.

 

Thank you guys; I hope you enjoyed reading the practices normally used for this type of study in industrial automation.