Miller College
EMA Calculations
The following problems are to be solved with the aid of a calculator. All equations and
working are to be shown.
1. The flux produced by a magnet is 10mWb. Determine the flux density if the area of the
pole is 250 mm2
2. Determine the flux of a magnet if the flux density at the poles is 2T, and the area of the
poles
3. A coil of 120 turns has a current of 250mA flowing through it. Determine the magneto
motive force produced by the coil.
4. How much current must flow in a coil of 1000 turns to produce a magneto motive force
of 125At?
5. A coil of 150 turns has a current of 3.5A flowing through it. Determine the magneto
motive force produced by the coil.
6. Determine the flux produced by a coil of 1000 turns when 1.5 amperes flows through it.
The reluctance of the magnetic circuit is determined to be 45 000At/Wb.
7. Determine the current that must flow through a coil of 1500 turns to produce a flux of
15mWb. The reluctance of the magnetic circuit is determined to be 5 000At/Wb.
8. Determine the flux density at the poles of an electromagnet which produces a flux of
2
15mWb if the area of the poles is 200mm .
2
9. A magnetic circuit has a core area of 250mm and a flux density of 2T. If the reluctance
of the core is 60 000 At/Wb, determine the current flowing through the coil of 600 turns.
10. An electromagnet has a core length of 400mm, is wound with 2000 turns and carries a
coil current of 200mA. Determine the magnetising force of the magnetic circuit.
11. Determine the current flowing in a coil of 600 turns which produces a magnetising force
of 2000 At/m in a core 150mm long.
2
12. A magnetic core is 300mm long with a cross sectional area of 50mm and has a
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permeability of 125.7x10 . Determine the reluctance of the core.
13. A conductor 250mm long moves at right angles with a velocity of 20m/s through a
magnetic field with a flux density of 1.5 Tesla. Determine the emf induced in the
conductor.
14. Determine the velocity of a conductor of 200mm length which is moving at a uniform
speed through a magnetic field of 1.25 Tesla flux density at right angles to produce a
voltage of 10V
15. Determine the flux density of a magnetic field if a conductor 25mm long cuts through the
flux at right angles with a velocity of 15m/s to produce a voltage of 6V.
16. A coil of 150 turns is lined by a flux of 300mWb. If the flux is reduced to 100mWb in
100mS, determine the voltage induced in the coil.
17. A transformer with input (primary) 600 turns and output (secondary)50 turns. The coils
are linked by a common core flux of 25mWb, which is reduced to zero in 5mS.
Determine the voltage induced in both coils.
2
18. Determine the inductance of a 100m length of straight 2.5mm single cored conductor.
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Assume that the number of turns equals 1, and the permeability of air is 12.57x10 H/m
2
19. An aired cored coil of 400 turns has a cross sectional area of 1000mm and a length of
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125.7mm. If the permeability of free air is 12.57x10 H/m, determine the inductance of
the coil.
2
20. A coil of 354 turns and a length of 350mm has a cross sectional area of 50mm .
Determine the inductance of the coil with:
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(a)
an air core (r = 12.57x10 H/m)
(b)
an iron core (r = 180)
(c)
a mu-metal core (r = 100x10 )
3
21. An inductor with 600 turns has a flux linking it of 25mWb. If this flux is reduced to zero
in 5 milliseconds, determine the voltage induced in the coil
22. Determine the flux change required to generate 240V in an inductor of 150 turns if the
flux changes in 10 milliseconds.
23. Determine the inductance of a coil of 120 turns when a reduction of the coil current from
2.0A to 0.4A produces a reduction of the core flux from 25mWb to 9mWb.
24. An inductor of 1.5H has a flux change of 20mWb when the current changes by 5A. For
these conditions, determine the number of coil turns of the inductor.
25. Calculate the value of shunt resistance necessary to extend a 3.5mA / 15 ohms
movement to read 3.5A full scale deflection
26. A galvanometer with a resistance of 17.5 ohms has a full scale deflection current of 2.4
milli amperes. Determine:
(a)
the full scale deflection voltage.
(b)
the resistance of the shunt required to use the meter as a 100mA ammeter.
(c)
the resistance of the multiplier if the meter is used as a 100V voltmeter.
(d)
the sensitivity of the voltmeter.
27. Calculate the value of multiplier resistance necessary to extend a 2.8mA / 25Ω meter
movement to read 12V full scale deflection
28. A galvanometer with a full scale deflection current of 2 milliamperes has a full scale
deflection voltage of 6 millivolts. Determine:
(a)
the resistance of the coil in the galvanometer.
(b)
the full scale deflection current of the galvanometer when connected with a
0.125 shunt.
(c)
the current in the circuit when the meter indicates 1.8 milliamperes.
(d)
the current in the shunt when the meter indicates 1.8 milliamperes.
29. A single conductor of 150mm length is rotated through a field flux of 0.8T at a velocity
of 10m/s. Determine the emf induced in the conductor.
30. Determine the flux density of the magnetic field required to generate 12.6V in a
conductor with an effective length of 2m which moves through the magnetic field at 90
with a uniform velocity of 10.5m/s.
0
31. A generator is wound with 6 series connected coils, each wound with 40 turns. If the
length of the armature is 200mm, the density of the flux is 1.25 Tesla and the armature
rotates with a velocity of 2m/s, determine the generated output voltage of the generator.
32. A separately excited generator has an effective field flux of 0.02T, and is spins at 400
rpm. If the machine constant is 12, determine the generated voltage.
33. A 28V shunt connected motor draws a line current of 50A. Determine the back emf for
these load conditions of the armature resistance is 0.0505and the shunt field resistance
is 56Ω.
34. A 240V series connected motor has a line current of 40A. If the series field resistance is
0.4Ω and the armature resistance is 0.6Ω, determine the back emf generated within the
armature.
35. A 250V compound motor has a shunt field resistance of 100Ω, a series field resistance of
0.5Ω and an armature resistance of 0.8Ω.
(a)
If the back emf is 216V, determine the current drawn from the supply;
(b)
If the load CURRENT increases to 45A, determine the back emf now generated
within the armature
36. A 325V compound motor takes 25A from the supply. If the shunt field resistance is 108
, the armature resistance is 0.4 and the series field resistance 0.8, calculate the:
(a)
shunt field current
(b)
shunt field copper loss
(c)
armature current (IA)
(d)
armature and series field copper losses
(e)
back emf (Eg).
37. A 100kW, 500V DC shunt motor draws a current of 210A from the supply at full-load. If
the full-load speed of the motor is 1000rpm, determine the:
(a)
torque developed by the motor
(b)
power input to the motor
(c)
motor efficiency
38. A 250V DC shunt motor draws a current of 25A from the supply at full-load. If the motor
develops a full-load torque of 35Nm at a speed of 1500rpm, determine the:
(a)
power input to the motor
(b)
power output from the motor
(c)
motor efficiency
39. A 400V DC motor takes 9.3A from the supply and is 80% efficient. Calculate the:
(a)
power input to the motor
(b)
output power from the motor
(c)
motor losses
40. A 400 volt dc motor runs at 1400 rpm at a maximum efficiency of 86% with constant
losses of 350 watts. Hint, maximum efficiency occurs when constant (fixed) losses
equals variable losses. Determine:
(a)
the total losses at this load
(b)
the input power to the motor
(c)
the output power from the motor
(d)
the current taken from the supply
(e)
the torque delivered to the load
41. A 28V shunt connected motor has a full load current of 50A. If the resistance of the shunt
field is 56Ω and the armature circuit resistance is 0.08Ω, determine the
(a)
armature current at start with no starting resistance employed

value of starting resistance required to limit the starting current to 150% of full
load armature current. 
42. A 150V series motor has a full load current of 75A. If the armature circuit resistance is
0.25Ω, determine the:
(a)
Starting current drawn from the supply with no starting resistance employed;

The value of starting resistance required to limit starting current to 175% of full
load current. 
EMA Calculations Answers
1
40T
26
42mV,0.2126,41.65k(416.7/V
2
600 micro webers
27
4260.7
3
300At
28
3,50mA,45mA,43.2mA
4
125mA
29
1.2V
5
525At
30
0.6T
6
33.3mWb
31
240V
7
0.05A
32
96V
8
75T
33
25.5V
9
50mA
34
200V
10
1000At/m
35
21.73A,194.75V
11
500mA
36
3A,975W,22A,193.6W,
387.2W,298.6V
12
47732 At/Wb
37
955Nm,105kW,95.24%
13
7.5V
38
6.25kW,5.495kW,87.9%
14
40m/s
39
3720W,2976W,744W
15
20T
40
700W,5kW,4.3kW,12.5A,29.34Nm
16
300V
41
350A,297m
17
3000V,250V
42
600A,0.893
18
31.4x10 H
19
1.6mH
20
22.5H,4.05mH,2.25H
21
3kV
22
16mWb
44
23
1.2H
45
24
375 turns
49
25
0.01515
50
Miller College
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