The back emf (E is developed as motor armature starts to rotate in presence of the magnetic field. Initially, the back emf is zero, therefore starting armature current is high.
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.
12. In a manual shunt motor starter relay positions are _______
Overload relay is connected in series and no-volt relay in parallel with the load
Overload relay is connected in parallel and no-volt relay in series with the load
Overload relay and no-volt relay are both connected in series with the load
Overload relay and no-volt relay are both connected in parallel with the load
Answer:1. Overload relay is connected in series and no-volt relay in parallel with the load
Explanation:
In simple manual shunt motor starter like a 3-point starter overload relay coil is kept in series with DC mains while the no-volt coil is kept in parallel with DC mains.
Overload protection is provided by the overload trip coil OLC and the NVC. The overload coil is a small electromagnet. It carries the armature current and for normal values of armature current, the magnetic pull of OLC is insufficient to attract the strip.
When the armature current exceeds the normal rated value (that is when the motor is overloaded, the strip is attracted by the electromagnet of OLC and closes the contacts Thus, NVC is short-circuited. This results in the release of the handle, which returns to the OFF position and the motor supply is cut Off.
13. In which of the following starters is No Volts Coil (NV connected in series with the field coil?
Two-point starter
Five-point starters
Three-point starter
Four-point starters
Answer:3. Three-point starter
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.
14. What will happen if DC motor is used without starter?
Heavy sparking at brushes
It’ll start smoothly
Will not start at all
Depends on load
Answer:1. Heavy sparking at brushes
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.
For several reasons, a large current cannot be allowed to flow in a motor even for a short starting period.
(i) It would cause intolerably heavy sparking at the brushes which may destroy the commutator and brush gear.
(ii) Sudden development of large torque causes mechanical shock to the shaft, reducing its life.
(iii) Such heavy current cannot be generally permitted to be drawn from the source of supply.
15. The coils of DC motor starter are wound with wire of _______
Copper
Kanthal
Manganin
Nichrome
Answer:3. Manganin
Explanation:
The coils of the DC motor starter are wound with wire of Manganin because of the low-temperature coefficient of resistance and a low thermoelectric EMF.
16. In shunt and compound motor starting the shunt field should be made on with full starting resistance in ______
Series with field
Parallel with field
Series with armature
Parallel with armature
Answer:3. Series with armature
Explanation:
In the case of the shunt motor, the top end of the shunt field is connected to the first contact on the starting resistance. This is to ensure that the field winding receives the full supply at the moment of switching on.
If the fields were connected to the last stud of the starting resistance, then on starting, the field would receive only a proportion of the supply voltage, the field current would be correspondingly weak and the torque might be too small to start the motor against the friction of the moving parts.
In shunt and compound motors starting the shunt field should be switched on with full starting resistance in the armature circuit. A short time delay in this position allows the field current to build up to the steady value of the inductive field transients.
17. Which of the following types of starters are used in DC shunt motors?
Two-point starters
Five-point starters
Pointless starters
Four-point starters
Answer: 4. Four-point starters
Explanation:
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.
18. Which of the following types of starters are used in DC series motors?
Two-point starters
Five-point starters
Pointless starters
Four-point starters
Answer: 1. Two-point starters
Explanation:
The two-point starter is used in series motors because, in the case of the series motor, the armature winding and field winding are connected in series. Therefore series motor achieves a dangerously high speed. So series motor should not be started without any load.
19. A starter is required for a 220-V shunt motor. The maximum allowable current is 55 A and the minimum current is about 35 A. The armature resistance of the motor is 0.4 Ω. What will be the number of sections of starter resistance required?
5
4
6
8
Answer: 3. 6
Explanation:
Given
Voltage = 220 V
Armature Resistance Ia = 0.4 Ω
Maximum current I1=55 A
Minimum current I2 =35 A
γ = Upper current Limit/Lower current Limit
= 55/35 = 1.57
R1= 200/55= 4 Ω
Now, γn-1 = 4/0.4 = 10.
By substituting γ value, we get n = 6.
20. The function of a starter in a D.C. motor is to
Control its speed
Increase its starting torque
Limit the starting current to a safer value
Reduce armature reaction effect
Answer:3. Limit the starting current to a safer value
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 Ia= V/Ra. Since armature resistance is very low, the armature current drawn is very high and will damage the machine.
