An essential electrical tool for properly starting DC motors is a DC motor starter, which reduces the hazardously high starting current that happens when a motor starts up. DC motors may encounter currents 10–15 times their typical operating current without adequate beginning control, which could cause equipment damage and safety risks. To start DC motors safely while minimizing the starting current, several starting techniques have been devised. Starting a DC motor is very different from starting other kinds of electrical motors.
What is Starting Current?
The initial high current that flows when a DC motor starts
is known as the starting current, and it must be kept to a minimum to avoid
damage.
Why does a DC motor have such a high starting current?
E = Eb + IaRa
where Ia is the armature current, Ra is the armature
resistance, and E is the supply voltage. Additionally, Eb provides the back
emf.
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| Circuit Diagram to Evaluate Equations of DC Motor Image used courtesy of electrical4u |
Since the back emf of a DC motor is created by the rotation
of the current-carrying armature conductor in the presence of a field, it is
remarkably similar to the generated emf of a DC generator. This DC motor's rear
emf is provided by
At startup, back EMF (Eb) = 0, so: Ia = E/Ra
With a minimum supply voltage of 220 volts, a DC motor's armature resistance is typically kept extremely low, at about 0.5 ohms. With this configuration, the initial current is 440 amps (220V / 0.5Ω). As a result, a significant amount of current flows through the motor.
Why does a DC motor have such a high starting current?
A DC motor's back electromotive force (EMF) is 0 when it is stopped. The motor voltage equation states that only the supply voltage and armature resistance affect the starting current. Drawing more than 400 amps from a 220V supply would result in an exceptionally high beginning current because armature resistance is normally quite low, at about 0.5 ohms. The motor windings, commutator, and brushes may sustain damage as a result of this high beginning current.
The back EMF rises in proportion to the motor's speed, which lowers the supply voltage to the armature coils and limits the current. However, because of the greater rotor inertia in larger motors, starting transients last longer. Starters help increase motor reliability by regulating the starting current.
There is no back EMF to offset the applied voltage when a
DC motor is turned on because the armature starts out at zero speed. A starting
current that is several times the motor's rated current is the outcome of this.
Premature failure may result from this high current's damage to the commutator
and armature winding. As a result, starters are necessary to keep the initial
inrush current within a safe range.
Starting Methods of DC Motor
High beginning current and torque can disrupt the motor system and cause damage. Several DC motor starting techniques have been devised to avoid this. The primary technique is to increase the overall resistance to (Ra + Rext) by introducing external electrical resistance (Rext) to the armature winding. This process limits the armature current to a safe level.
The net operating voltage now drops while the motor keeps running and gaining speed because the back EMF gradually builds up and rises in opposition to the supply voltage. At this point, Rext is gradually reduced until it reaches zero, when the back emf generated is at its highest, in order to keep the armature current at its rated value.
Ia= E – Eb(Ra+ Rext )
The starter facilitates the adjustment of the external
electrical resistance when a DC motor is starting.
DC Motor Starters
- Two-Point Starter
- Three-Point Starter
- Four-Point Starter
Two-point starter
There are several ways to start DC series motors, such as using a two-point starter, which is a basic starter for small series DC motors. The supply occupies one of its two contact points, while the armature and field coils share the other. The motor is directly started by using a manual lever. The start arm is shifted to the right to turn on the motor.
As the start
arm keeps moving to the right, this action first connects the maximum
resistance in series with the armature, which is subsequently progressively
decreased. Another name for this kind of starter is a 2-point starter. To
ensure safe and controlled operation, the no-load release coil holds the start
arm in the run position and releases it when the voltage is removed.
Three-point starter
The lever is slowly turned to the right to turn on the attached DC motor. The armature winding is connected in series with resistances R1 to R5, and the field winding is directly connected across the supply when the lever reaches point 1. The armature winding is connected in series with full resistance at startup.
The resistance is progressively removed from the armature circuit when the lever is pulled farther to the right. All resistance is eliminated from the armature circuit, and the armature is connected directly to the supply when the lever reaches position 6. The lever is held in this position by the electromagnet 'E' (no-voltage coil). In the event that the supply voltage is lost or drastically decreased, this electromagnet releases the lever.
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| Three-Point Starter Image used courtesy of electrical4u |
Four-point starter
The connection of the no-voltage coil (electromagnet E) is the main distinction between a 3-point and a 4-point starter. The no-voltage coil and field coil of a 4-point starter are not connected in series. Rather, when the lever moves and comes into contact with the brass arc (which is situated beneath the resistance studs), the field winding is directly connected to the supply.
A current-limiting resistance Rh is connected to the hold-on coil, also known as the no-voltage coil. This arrangement makes sure that the current flowing through the hold-on coil is unaffected by changes in the shunt field current. Because of this, the hold-on coil's electromagnetic pull is still strong enough to keep the spring from needlessly turning the lever back to the off position.
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| Four-Point Starter Image used courtesy of electrical4u |
Conclusion
Using a starter with variable resistance is the main way to reduce starting current and guarantee safe motor running. Shunt-wound DC motors and compound-wound DC motors are started using three-point and four-point starters, respectively. The starter of a series-wound DC motor does not use a load release coil.
The disadvantages of three-point starters are field current sensitivity and single-speed limitation. Four-point starters are especially helpful in situations when a field rheostat is required to regulate the field current to run the motor faster than its rated speed.
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