Lesson 2.1

What is the primary purpose of a galvanometer?

• To measure heavy currents

• To detect electric currents

• To generate electric currents

• To control electric circuits

What happens when the switch is closed?

• The current stops flowing to the moving coil

• The coil experiences an attractive force

• The coil moves to the position

• The permanent magnet poles react with the coil

What determines the amount of force experienced by the coil?

• The position of the coil

• The permanent magnet poles

• The current passing through the coil

• The switch being closed

What happens when the switch is closed?

• The coil moves to the position shown in Figure 2

• The coil experiences a force proportional to the current passing through it

• Current flows to the moving coil of PMMC

• Opposite poles attract

A PMMC instrument with a 100-turn coil has a magnetic flux density in its air gaps of B = 0.2T. The coil dimensions are D= 1cm and L = 1.5cm. Calculate the torque on the coil for a current of 1mA.

A PMMC instrument has a coil of area 10mmx8mm. The flux density in the air gap is 0.15 Wb/m². If the coil is wound for 100 turns, carrying a current of 5mA. Calculate the deflecting torque.

A PMMC instrument with a 150 turns and internal resistance,Rm = 20Ω deflects at 120° with 100mV supply. If the coil surface area, A = 5.4 x 10-4 m & spring constant k = 0.375 x 10-6 Nm/rad, calculate the magnetic field density, B.

A PMMC instrument has a flux density of 0.12T in its air gaps. The coil has D = 1.5cm and L = 2.25cm. Determine the number of coil turns needed if the torque is to be 4.5 x 10-6 Nm when the coil current is 100μA.

A PMMC instrument with a 300-turn coil has a magnetic field density of B = 0.15T in its air gaps. The coil dimensions are D = 1.25cm and L = 2 cm. Calculate the torque when the coil current is 500μA.

Calculate the torque which is developed by the moving coil in a PMMC meter movement when: (a) N = 5000 turns, B = 4000 Wb/m2, A = 2cm2,and I = 50A (b) N = 1000 turns, B = 5000 Wb/m2, A = 2.5cm2, and I = 1mA.