Knowing what a DC motor starter is will give you more insight on the type of starter your motor needs, and the applications as well.A starter is just a device for starting and accelerating a motor. The major job of a starter is to draw away heavy currents that might damage the DC system.
Definition of a DC Motor Starter?
A DC motor starter is a circuit that’s connected in series with a DC motor to lower the starting current and maintain its speed, in order not to send the motor into an overdrive.
The principle of a DC motor starter is that it lowers the voltage and current of the motor as soon as it kickstarts. Any starter used in DC motors is termed a DC motor starter. So, to start and stop your DC motor safely, you need a DC motor starter.
The Working Principle of a DC Motor Starter
A DC motor starter is the resistance that’s connected in series with the armature winding, but just at the time of starting the motor.Just as we stated earlier, the role of a DC starter is to reduce the current of the motor at its starting level.
The starter is only relevant at the time of starting the motor, and once it gets to the appropriate value, it becomes ineffective. So, when in motion already, the starter resistance will go out of the circuit, and the back emf will be formed to control the armature current.
Under normal circumstances, the DC motor starter will be delegated to the “RUN” position, the value of the starter resistance is dropped to zero, and it does not affect the operation.
Why Do DC Motors Need Starters?
A DC motor has no back emf, and at the starting point of the motor, the armature current is driven by the resistance of the circuit.Naturally, an armature’s resistance is low, but when there’s high voltage applied to the motor when at a fixed point, the current of the armature becomes too high, and this might damage the system of the DC motor.
So, as a result of the high current, an extra resistance is put in the armature circuit at the start of the motor.This starting resistance is then cut off when immediately the DC motor gains momentum.
When the speed of the DC motor increases, it automatically increases the back emf, and this pattern will result in a slow but precise decrease of the armature current until the motor gets to a stable speed.
Therefore, it’s safe to say that with the process, the armature current will get to its preferred value. What this means is that the back emf is what helps in limiting the current that passes through the armature to prevent the damaging of brushes, commutators, and windings.
The only time you can start a DC motor without a DC motor starter is when you are dealing with very small DC motors; in this case, an additional resistance is not needed.
Small DC motors gather speed faster because of their small rotor inertia, so connecting them directly with the help of a switch won’t cause any damage.Remember, the essence of a starter is to maintain the safe value of the motor and reduce the starting current until the motor picks up a stable speed.
Click to learn: Small DC Motors: What It Is & Working Principle & Types
The resistance in a motor starter is divided into different parts, and as the speed of the motor picks up, those parts are cut off one after the other. Eventually, the resistance will be cut off totally – but only when the motor builds up to a stable value.
Types of DC Motor Starters
There are three main types of DC motor starters;
A 2-point starter
A 3-point starter
A 4-point starter
A 2-Point Starter for DC Motors
The major function of a 2-point starter is to prevent the DC motor from starting with a high current. To achieve this, the 2-point starter limits the high starting armature current to a stable value by connecting a series resistance at the time of starting.
As the motor begins to accelerate and gain momentum, the added resistance will gradually reduce. The reason why it’s termed a 2-point starter is because there are only two sections for external connections. It features a spring-controlled arm, a resistor, and two coils – trip and hold on coil.
When starting the DC motor, the starter arm is usually in the OFF position while the DC supply is switched on. The armature current then flows through the whole starter resistance to reduce its current to a low point.
Advantages of 2-Point Starter
- It goes off immediately after the power supply is turned off
- It protects the motor system from overload
- The 2-point starter prevents the motor from drawing a high starting current
Applications of 2-Point Starter
- It’s used in railways for starting and stopping the rail
- They are used in DC series motors
A 3-Point Starter for DC Motors
A 3-point starter is a device built to start and maintain the speed of a DC shunt motor. Connecting to the resistance (starter) in series helps reduce the high current and protects the equipment against any damage.
The EMF plays a major role, as it extends when the armature of the motor starts rotating the magnetic field, thereby opposing and countering the voltage supplyIt’s called a 3-point starter because 3 points are brought out for external connections – L, A, and F.
Point L (line terminal) is the terminal that should linked to the positive terminal of the DC power supply. Point A (armature terminal) is the point that’s to be linked to the armature winding, while point F (field terminal) needs to be connected to the field winding.
The 3-point starter also features a resistance “R” for stabilizing the initial current and a handle “H” that’s placed in the OFF condition with a spring. When starting the motor position, the field winding receives the full supply voltage, while the armature current is fixed to an exact secure current by the added resistance R.
You can move the handle of the 3-point starter from one position to another, as this will help increase the speed of the motor until it reaches the RUN position. When in this position; there are things to note.
- The DC motor receives the total speed
- The voltage supply located in the circuit runs through both the motor’s windings
- The added resistance will be fully cut off
The handle in the circuit is fixed in RUN condition with the help of an electromagnet that’s powered by an NVC. The NVC (no volt trip coil) can be connected in series together with the field winding.
If the incident drops below the required value, the NVC will get charged. As a result of the Spring, the handle will be released and pulled into the OFF condition.
Disadvantages of a 3-Point Starter
- The motor speed is directly dependent on the field resistance, so when amplified it reduces the flow of current in the shunt field
- The electromagnet might liberate the arm of the handle through normal motor operation and remove it completely from the power supply
- Introducing high resistance to achieve a high speed will cause the field current to drop drastically
- It’s not an ideal starter for variable-speed motors
Advantages of a 3-Point Starter
- By adding additional equipment, the 3-point starter helps control the speed of the motor
- It protects the motor from short-circuit faults
Applications of 3-Point Starter
- They are used in spinning mills
- 3-point starters are used with DC shunt motors
A 4-Point Starters for DC Motors
4-point starters are almost identical to their 3-point variants; the major difference is that the holding coil of the 4-point starter is detached from the shunt-field circuit.The 4-point starter uses 4 terminals for the running of the DC motor, and unlike the 3-point starter, the connection of a 4-point starter can be done in parallel with the field coil.The three parallel arrangements that can be formed include;
- The shunt field and variable resistance coil
- The holding coil and limiting resistance
- The armature, starting resistance, and shunt field winding
The four terminals of the 4-point starter are; the line terminal, armature terminal, field terminal, and the operational point connected to the NVC.
Disadvantages of a 4-Point Starter
It’s unable to effectively control the speed of the high current in a DC motor
Advantages of a 4-Point Starter
It’s ideal for variable-speed motors
Applications of 4-Point Starter
It’s used in rolling mills
4-point starters are mainly used in DC compound motors
DC motor starters are important if you want to keep your DC motors free of short-circuit faults and overload issues. There are different available DC motor starters on the market, in varying sizes. So, take your time and research which option suits you best.
We have another article here which is: What are DC Motor Drives ?Hope it helps you.