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Problem 1

A PMMC instrument with a 300-turn coil hasa 0.15 T magnetic flux density in its air gaps.The coil dimensions are

D^ = 1.25 cm and

l^ = 2

cm. Calculate the torque when the coil current is500 μA.

Problem 2

A PMMC instrument has a 0.12 T magneticflux density in its air gaps. The coil dimensions are

D^ = 1.5 cm and

l^ =

2.25 cm. Determine the number of coil turns requiredto give a torque of 4.5 μNm when the coilcurrent is 100 μA.

Solution

B = 0.12 T I = 100x

-6^ A l = 0.0225 m D = 0.015 mτ^ = 4.5 x 10

-6^ Nm τ^ = BAIN = BlDIN N =

τ^ /BlDI = 4.5x

-6^ Nm /(0.12T x 0.0225 m x

0.015 m x 100 x 10

-6^ A) = 1111

Solution R= 750m^

Ω^ , Ifsd

= 500

μA

(a) 50 mA ranges,Vm

= IR m^

= I^ m fsd

R= 500 m^

μA x 750

Ω

= 375 x 10

-3^ V

I= I – Ish^

= I – Im

= 50mA - 500fsd

μA

= 49.5 x 10

-3^ A

I^ m

Rm Rsh1 Rsh

I

Ω

× ×

=^

− −

(^576). 7 (^105). 49

10 375

3 3 1 1

V A

V I R

m sh sh

Solution

(b) 30 mA ranges,^ I

= I – Ish

= I – Im^

= 30mA - 500fsd

μA

= 29.5 x 10

-3^ A

Ω

× ×

=^

− −

(^712). 12

(^105). 29

10 375

3 3

2 2

V A

V I R

m sh sh

Solution

Problem 5

A dc ammeter consists of an Ayrton shunt inparallel with a PMMC instrument that has a1.2 k

Ω^ coil resistance and 100 μA FSD. The Ayrton shunt is made up of four 0.

Ω^ series-

connected resistors. Calculate the ammeterrange at each setting of the shunt.

Solution

• Tutorial
• Problem
• A 12 V source supplies 25 A to a load.Calculate the load current that would bemeasured when using an ammeter with aresistance of (a) 0.
• Ω (b) 0.
• Ω (c) 0.

Solution