Three-phase AC machines

1. Three-phase AC machines

Level 3 Diploma in Electrical Installations (Buildings and Structures)
PowerPoint
presentation
Unit 302: Principles of electrical science
Three-phase AC machines
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2.

Level 3 Diploma in Electrical Installations (Buildings and Structures)
Motor construction
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3.

Level 3 Diploma in Electrical Installations (Buildings and Structures)
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4.

Level 3 Diploma in Electrical Installations (Buildings and Structures)
Synchronous speed
F
N(rps) =
P
Where:
F ´ 60
N(rpm) =
P
N = Revolutions
rps is revolutions per second
rpm is revolutions per minute
F = Frequency in Hertz (Hz)
P = The number of pairs of poles
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5.

Level 3 Diploma in Electrical Installations (Buildings and Structures)
EXAMPLE 1 – A four-pole AC three-phase machine is fed with a
supply at a frequency of 50Hz. Calculate synchronous speed in:
a) rps
b) rpm
NB: the machine has four poles, so that is two pairs of poles.
a)
F
P
50
=
2
= 25 rps
rps =
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6.

Level 3 Diploma in Electrical Installations (Buildings and Structures)
EXAMPLE 1 – A four-pole AC three-phase machine is fed with a
supply at a frequency of 50Hz. Calculate synchronous speed in:
a) rps
b) rpm
b)
F ´ 60
P
50 ´ 60
=
2
= 1500 revs per minute
rpm =
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7.

Level 3 Diploma in Electrical Installations (Buildings and Structures)
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8.

Level 3 Diploma in Electrical Installations (Buildings and Structures)
Slip
Slip is defined in two ways:
per unit slip
% slip
and can be calculated as follows:
(ns - nr )
Per unit slip =
ns
Where:
nS
nr
=
=
(ns - nr )
% slip =
´ 100
ns
Synchronous speed
Rotor speed
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9.

Level 3 Diploma in Electrical Installations (Buildings and Structures)
EXAMPLE 2 – A two-pole induction motor runs at 2880 rpm when connected to
the 50 Hz mains supply. Calculate the:
a) per unit slip
b) percentage slip.
F ´ 60
P
50 ´ 60
=
1
= 3000 rpm
Sync speed, ns =
a)
(ns - nr )
Per unit slip, S =
ns
(3000 - 2880)
3000
= 0.04
=
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10.

Level 3 Diploma in Electrical Installations (Buildings and Structures)
EXAMPLE 2 – A two-pole induction motor runs at 2880 rpm when connected to
the 50 Hz mains supply. Calculate the:
a) per unit slip
b) percentage slip.
Sync speed, ns = 3000 rpm
(ns - nr )
% slip =
´ 100
ns
b)
(3000 - 2880)
´ 100
3000
= 4%
=
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11.

Level 3 Diploma in Electrical Installations (Buildings and Structures)
EXAMPLE 3 – A four-pole 50Hz induction motor has a per-unit slip of 0.03 on
full load. Calculate the full load speed.
F ´ 60
Sync speed, ns =
P
50 ´ 60
=
2
= 1500 rpm
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12.

Level 3 Diploma in Electrical Installations (Buildings and Structures)
EXAMPLE 3 – A four-pole 50Hz induction motor has a per-unit slip of 0.03 on
full load. Calculate the full load speed.
(ns - nr )
Per unit slip, S =
ns
S ´ ns = ns - nr
S ´ ns + nr = ns
nr = ns - S ´ ns
= 1500 - (0.03 ´ 1500)
= 1500 - 45
= 1455 rpm
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13.

Level 3 Diploma in Electrical Installations (Buildings and Structures)
Wound rotor motor
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14.

Level 3 Diploma in Electrical Installations (Buildings and Structures)
AC three phase generator
DC
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15.

Level 3 Diploma in Electrical Installations (Buildings and Structures)
Any questions?
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