DC Motor Starter MCQ

Answer: 2. DC shunt motor with field weakening control

Explanation: 

A 4-point starter is used to start and control the speed of a Dc shunt motor with field weakening control.

High-speed protection of DC shunt motor is not provided by 4-point starter.

Four-point starter:

The four-point starter works as a current controlling device in the deficiency of back EMF while it starts running off the DC motor. A four-point starter also works as a protecting device. The main difference between a 4-point starter compared to a 3-point starter is, the holding coil is detached from the shunt-field circuit.

The 4-point starter uses four terminals for speeding up the motor. These four terminals namely, the armature terminal (, field terminal (F), and the line terminal (L).

• NVC (No Volt Coil): The connection of a four-point starter can be done in parallel with the field coil
• The line terminal (L) is connected to a positive supply
• The armature terminal ( is connected to the winding of an armature
• The field terminal (F) is connected to the field winding
• It is provided as not to affect the current flowing through the ‘Hold on’ coil even when the field current changes.

Answer:4. To restrict armature current as there is no back emf at starting

Explanation: 

The dc motor draws a very high starting current if started directly online without the use of a starter. Such a high current may damage the insulation provided in the armature winding. Hence a starter is essential to start the dc motor in order to limit the starting current to a safe value.

At the time of starting (n=0), the induced emf of a motor is zero such that the current drawn by the armature, from the rated voltage supply would be I_a= V/R_a. Since armature resistance is very low, the armature current drawn is very high and will damage the machine.

Answer:3. Reduce the starting current

Explanation: 

A starter is necessary to start a DC motor because it restricts the initial high armature current that exists, as the value of starting back-EMF is zero.

A resistor is used in the starter for a DC shunt motor

• When the connected dc motor is to be started, the lever is turned gradually to the right
• When the lever touches point 1, the field winding gets directly connected across the supply, and the armature winding gets connected with resistances R1 to R5 in series
• During starting, full resistance is added in series with the armature winding
• Then, as the lever is moved further, the resistance is gradually is cut out from the armature circuit
• Now, as the lever reaches position 6, all the resistance is cut out from the armature circuit and the armature gets directly connected across the supply

 So, the purpose of including an external resistance at the time of starting a DC motor is to reduce the starting current.

Answer: 1. Up to 5 H.P

Explanation: 

Direct online (DOL) motor starting is when a motor is started at full load, with full line voltage applied to motor terminals. As this causes the motor to draw a large amount of current

DOL Starter causes a significant dip in voltage hence DOL starters are limited to the small rating motors where the distribution system (mains supply) can withstand high starting currents without excessive voltage dips.

For a large rating motor, ranging from 5 HP to 25 HP, oil-immersed DOL starters are used which provides insulation against sparking on contact points, increases the life of the starter. it is only suitable for motor powers up to 4Kw, with a maximum motor size of 10Kw connected this way.

Answer:2. 3

Explanation: 

• There are mainly 3 types of DC motor starters, such as 2 point starter, 3 point starter and 4 point starter.
• The basic concept behind every DC motor starter is adding external resistance to the armature winding during starting.
• 3-point starters and 4-point starters are used for starting dc shunt-wound motors and dc compound wound motors.
• 2 point starter is used for starting dc series motors.
• However, very small dc motors may be started directly by connecting them to the supply with the help of a contractor or a switch. It does not result in any harm because they gather speed quickly due to small rotor inertia.
• In this case, the large starting current will die down quickly because of the fast rise in the back emf.

Answer: 2. Shunt as well as compound motors

Explanation: 

To limit or minimize starting armature current, we use 3-point starter. A three-point starter helps in starting and running the shunt-wound motor or compound wound DC motor. A three-point starter is used to reduce the starting current and it has three terminals.

If we connect the DC series motor terminal to the line(L) and Armature ( terminal of the starter then the Field terminal of the starter will be left open..and therefore no current will flow through NVR (No volt relay) as NVR is connected in series with field control circuit of starter.

So the motor will start and run as long as the operator holds the starting handle and it will stop again after releasing it because NVR is not energized.

Answer: 2. 3 point

Explanation: 

The back emf (E is developed as the motor armature starts to rotate in presence of the magnetic field. Initially, the back emf is zero, starting current is high. To limit this starting current, we use a starter.

A three-point starter helps in starting and running the shunt-wound motor or compound wound DC motor.

Answer:4. No Voltage Coil

Explanation:

NVC in the three-point starter is called as No Voltage Coil

The no-volt trip coil (NV is connected in series with the field winding and armature of the motor.

In the event of switching off, or when the supply voltage falls below a predetermined value, or the complete failure of supply while the motor is running NVC is de-energized. This results in the release of the handle, which is then pulled back to the OFF position by the action of the spring. The current to the motor is cut off, and the motor is not restarted without resistance R in the armature circuit.

The NVC also provides protection against an open circuit in the field winding. The NVC is called no-volt or Undervoltage protection of the motor. Without this protection, the supply voltage might be restored with the handle in the RUN position. Consequently, full line voltage may be applied directly to the armature resulting in a very large current.

Answer:1. Series

Explanation:

The no-volt trip coil (NV is connected in series with the field winding and armature of the motor.

In the event of switching off, or when the supply voltage falls below a predetermined value, or the complete failure of supply while the motor is running NVC is de-energized. This results in the release of the handle, which is then pulled back to the OFF position by the action of the spring. The current to the motor is cut off, and the motor is not restarted without resistance R in the armature circuit.

The NVC also provides protection against an open circuit in the field winding. The NVC is called no-volt or Undervoltage protection of the motor. Without this protection, the supply voltage might be restored with the handle in the RUN position. Consequently, full line voltage may be applied directly to the armature resulting in a very large current.

Answer:1. Low

Explanation:

Starting resistance of a DC shunt motor and DC compound motor is low.

A motor at rest has no back or counter e.m.f. At staring, therefore, the armature current is limited only by the resistance of the armature circuit. The armature resistance is very low, however, and if the full voltage were impressed upon the motor terminals at a standstill, the resulting armature current would be many times the full-load value usually sufficient to damage the machine.

For this reason, additional resistance is introduced into the armature circuit at starting. As the motor gains speed, it’s back e.m.f. builds up and the starting resistance is cut out.