Malcolm Barnes. Practical Variable Speed Drives and Power Electronics
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3-Phase AC induction motors 59
− The inertia factor F
I
, which is the ratio of the total moment of inertia
to the moment of inertia of the motor rotor.
− The moment of inertia of the motor rotor (J
M
)
− The permissible average moment of resistance T
V
, during the change
of speed given with rated load torque.
• Designation example: S7 – 500 c/h – (F
I
= 2) – (J
M
= 0.08 kgm
2
) – (T
V
=
0.3T
N
)
S8: Uninterrupted periodic duty with recurring speed and load changes
• A sequence of identical duty cycles, each comprising a period of operation at
constant load corresponding to a predetermined speed of rotation, followed by
one or more periods of operation at other constant loads corresponding to
different speeds of rotation.
• The period of the duty cycle is too short for thermal equilibrium to be
obtained.
• This type of duty cycle is used for pole changing motors.
• The following items should also be specified for this duty cycle
− The number of load cycles per hour (c/h).
− The inertia factor F
I
, which is the ratio of the total moment of inertia
to the moment of inertia of the motor rotor.
− The permissible average moment of resistance T
V
, during the change
of speed given with rated load torque.
− The cyclic duration factor for each speed of rotation.
− The moment of inertia of the motor rotor (J
M
).
− The combinations of the load and the speed of rotation are listed in the
order in which they occur in use.
60 Practical Variable Speed Drives and Power Electronics
• Designation examples:
− S8 – 30 c/h – (F
I
= 30) – T
V
= 0.5T
N
– 24 kW – 740 rev/m – 30%
− S8 – 30 c/h – (F
I
= 30) – T
V
= 0.5T
N
– 60 kW – 1460 rev/m – 30%
− S8 – 30 c/h – (F
I
= 30) – T
V
= 0.5T
N
– 45 kW – 980 rev/m – 40% – (J
M
= 2.2 kgm
2
)
2.11 Cooling and ventilation of electric motors (IC)
All rotating electrical machines generate heat as a result of the electrical and mechanical
losses inside the machine. Losses are high during starting or dynamic braking. Also,
losses usually increase with increased loading. Cooling is necessary to continuously
transfer the heat to a cooling medium, such as the air. The different methods of cooling
rotating machines are classified in the standards IEC 34.6 and AS 1359.21.
For AC induction motors, cooling air is usually circulated internally and externally by
one or more fans mounted on the rotor shaft. To allow for operation of the machine in
either direction of rotation, fans are usually of the bi-directional type and made of a
strong plastic material, aluminum, or steel. In addition, the external frames of the motor
are usually provided with cooling ribs to increase the surface area for heat radiation.
The most common type of AC motor is the totally enclosed fan cooled (TEFC) motor,
which is provided with an external forced cooling fan mounted on the non-drive end
(NDE) of the shaft, with cooling ribs running axially along the outer surface of the motor
frame. These are designed to keep the air flow close to the surface of the motor along its
entire length, thus improving the cooling and self-cleaning of the ribs. An air-gap is
usually left between the ribs and the fan cover for this purpose.
Internally, on smaller TEFC motors, the rotor end-rings are usually constructed with
ribs to provide additional agitation of the internal air for even distribution of temperature
and to allow the radiation of heat from the end shields and frame.
Special precautions need to be taken when standard TEFC induction motors are used
with AC variable speed drives, powered by VVVF converters. For operation at speeds
below the rated frequency of 50 Hz, the shaft mounted fan cooling efficiency is lost. For
constant torque loads, it is sometimes necessary to install a separately powered forced
cooling fan (IC 43) to maintain adequate cooling at low speeds. On the other hand, for
prolonged operation at high speeds above 50 Hz, the shaft mounted fan works well but
may make excessive noise. Again, it may be advisable to fit a separately powered cooling
fan.
Larger rotating machines can have more elaborate cooling systems with heat
exchangers.
The system used to describe the method of cooling is currently being changed by IEC,
but the designation system currently in use is as follows:
• A prefix comprising the letters IC (index of cooling)
• A letter designating the cooling medium, this is omitted if only air is used
• Two numerals which represent
1. The cooling circuit layout
2. The way in which the power is supplied to the circulation of the cooling
fluid, fan, no fan, separate forced ventilation, etc
3-Phase AC induction motors 61
Code Description Drawing
IC 01
- Open machine
- Fan mounted on shaft
- Often called ‘drip-proof’ motor
IC 40
(New : IC 410)
- Enclosed machine
- Surface cooled by natural
convection and radiation
- No external fan
IC 41
(New : IC 411)
- Enclosed machine
- Smooth or finned casing
- External shaft-mounted fan
- Often called TEFC motor
IC 43 A
(New : IC 416A)
- Enclosed machine
- Smooth or finned casing
- External motorized Axial fan
supplied with machine
IC 43 R
(New : IC 416R)
- Enclosed machine
- Smooth or finned casing
- External motorized Radial fan
supplied with machine
IC 61
(New : IC 610)
- Enclosed machine
- Heat Exchanger fitted
- Two separate air circuits
- Shaft-mounted Fans
- Often called CacA motor
Figure 2.17:
Designation of the most common methods of cooling
62 Practical Variable Speed Drives and Power Electronics
2.12 Degree of protection of motor enclosures (IP)
The degree of protection (also called index of protection – IP) which is provided by the
enclosure of the motor, is classified in the standards IEC 34.5 and AS 1359.20.
The system used to describe the Index of Protection is as follows:
• A prefix comprising the letters IP (index of protection)
• Three numerals which represent
1. The protection against contact and ingress of solid objects, such as dust.
2. The protection against ingress of liquids, such as water.
3. The mechanical protection and its resistance to impact.
This third numeral is often not used in practice.
First number :
protection against solid objects
Second number :
protection against liquids
Third number :
mechanical protection
Definition
Definition
DefinitionIP
IP
IPTest s Test s
Test s
0
1 11
2 2
2
3 33
44
55 5
00
No protection
No protection
No protection
Protected against
solid objects of
over 50 mm
(eg : accidental
hand contact)
Protected against
vertically dripping
water
(condensation)
15
0
60
0
Ø 50 mm
Ø 12 mm
Ø 2.5 mm
Ø 1 mm
Protected against
solid objects of
over 12 mm
(eg : finger)
Protected against
solid objects of
over 2.5 mm
(eg : tools, wire)
Protected against
solid objects of
over 1 mm
(eg : small tools,
thin wire)
Protected against
dust (no deposits
of harmful
material)
Protected against
water dripping
up to 15° from
the vertical
Protected against
rain falling
up to 60° from
the vertical
Protected against
water splashes
from all directions
Protected against
jets of water from
all directions
15cm
15cm
20cm
40cm
150g
250g
250g
500g
Impact energy :
0.225 J
Impact energy :
0.375 J
Impact energy :
0.500 J
Impact energy :
2 J
1m
0.15m
..m
6
77
8
9
6
Totally protected
against dust.
Does not involve
rotating machines
Protected against
jets of water
comparable to
heavy seas
Protected against
the effects of
immersion to
depths of between
0.15 and 1 m
Protected against
the effects of
prolonged
immersion at depth
..m
40cm
40cm
5000g
1500g
Impact energy :
6 J
Impact energy :
20 J
Figure 2.18:
Summary of the index of protection
3-Phase AC induction motors 63
This system of degrees of protection does not relate to protection against corrosion.
For example, a machine with an index of protection of IP557, is protected as follows:
5: Machine is protected against accidental personal contact of moving parts, such as the
fan, and against the ingress of dust
Test Result: No risk of direct contact with rotating parts (test finger)
No risk that dust could enter the machine in harmful quantities
5: Machine is protected against jets of water from all directions from hoses 3 m away
and with a flow rate less than 12.5 liters/sec at 0.3 bar
Test Result: No damage from water projected onto the machine during operation
7: Machine is resistant to impacts of up to 6 joules
Test Result: Damage caused by impacts does not affect running of the machine
2.13 Construction and mounting of AC induction motors
Modern squirrel cage AC induction motors are available in several standard types of
construction and mounting arrangements. These are classified in accordance with the
standards IEC 34.7 and AS 1359.22.
Mounting position needs to be specified to ensure that drain plugs, bearings and other
mechanical details are correctly located and dimensioned during assembly.
The system used to describe the mounting arrangements is as follows:
• A prefix comprising the letters IM (index of mounting)
• Four numerals which represent,
1. Type of construction
2. Type of construction
3. Mounting Position
4. Mounting Position
A summary of the mounting designations is shown in Figure 2.19.
A previous system of designation used letters B (horizontal mounting) and V (vertical
mounting). This system has been superseded in both IEC 34.7 and AS 1359.22. The old
designations are shown in the table in brackets.
64 Practical Variable Speed Drives and Power Electronics
Foot Mounted Motors
IM 1001 (IM B3)
- Horizontal shaft
- Feet on floor
IM 1071 (IM B8)
- Horizontal shaft
- Feet on ceiling
IM 1051 (IM B6)
- Horizontal shaft
- Feet wall mounted
with feet on LHS when
viewed from drive end
IM 1011 (IM V5)
- Vertical shaft
- Shaft facing down
- Feet on wall
IM 1061 (IM B7)
- Horizontal shaft
- Feet wall mounted
with feet on RHS when
viewed from drive end
IM 1031 (IM V6)
- Vertical shaft
- Shaft facing up
- Feet on wall
Figure 2.19:
Mounting designations for foot mounted motors
Flange Mounted Motors
IM 3001 (IM B5)
- Horizontal shaft
IM 2001 (IM B35)
- Horizontal shaft
- Feet on floor
IM 3011 (IM V1)
- Vertical shaft
- Shaft facing down
IM 2011 (IM V15)
- Vertical shaft
- Shaft facing down
- Feet on wall
IM 3031 (IM V3)
- Vertical shaft
- Shaft facing up
IM 2031 (IM V36)
- Vertical shaft
- Shaft facing up
- Feet on wall
Figure 2.20:
Mounting designations for flange mounted motors
3-Phase AC induction motors 65
Face Mounted Motors
IM 3601 (IM B14)
- Horizontal shaft
IM 2101 (IM B34)
- Horizontal shaft
- Feet on floor
IM 3611 (IM V18)
- Vertical shaft
- Shaft facing down
IM 2111 (IM V58)
- Vertical shaft
- Shaft facing down
- Feet on wall
IM 3631 (IM V19)
- Vertical shaft
- Shaft facing up
IM 2131 (IM V69)
- Vertical shaft
- Shaft facing up
- Feet on wall
Figure 2.21:
Mounting designations for face mounted motors
2.14 Anti-condensation heaters
When rotating electrical machines need to stand idle for long periods of time in severe
climatic conditions, such as a high humidity environment, moisture can be drawn into the
machine and absorbed into and onto the insulation of the stator and rotor windings.
When a machine is de-energized after it has been running for a period of time, the internal
temperature is high. As the machine cools, the low pressure inside the machine draws
external moist air into the machine via the seals around the shaft. The moisture degrades
the performance of the insulation materials by providing a partially conductive path
between the windings and the frame of the machine. When the machine is energized,
electrical breakdown of the insulation can occur. Standby motors or generators, which
have not been used for some time, can fail to operate when they are needed.
Under these conditions, where a motor is expected to stand idle for long periods in an
environment of high humidity, it may be necessary to specify additional winding
impregnation treatment and consideration should also be given to anti-condensation
heaters. These are fitted inside the motor and their connections brought out to terminals.
The heaters are energized from a 240 V supply when the motor is not in use to prevent
condensation forming inside the windings.
Anti-condensation heaters are normally in the form of a tape, which comprises a flat
glass-fiber tape with a heating element woven into it. This tape is then inserted inside a
glass fiber sleeve and wrapped around the stator winding overhang, braced and
impregnated with the stator winding. One heater element is normally fitted to each end of
the stator winding. A typical rating of a heater varies from 25 watts, for small motors, to
200 watts for large motors.
66 Practical Variable Speed Drives and Power Electronics
2.15 Methods of starting AC induction motors
Direct-on-line (DOL) starting is the simplest and most economical method of starting an
AC squirrel cage induction motor. A suitably rated contactor is used to connect the stator
windings of the motor directly to the 3-phase power supply. While this method is simple
and produces a reasonable level of starting torque, there are a number of disadvantages:
• The starting current is very high, between 3 to 8 times the full load current.
Depending on the size of the motor, this can result in voltage sags in the
power system.
• The full torque is applied instantly at starting and the mechanical shock can
eventually damage the drive system, particularly with materials handling
equipment, such as conveyors.
• In spite of the high starting current, for some applications the starting torque
may be relatively low, only 1.0 to 2.5 times full load torque.
To overcome these problems, other methods of starting are often used. Some common
examples are as follows:
• Star-delta starting
• Series inductance starting (e.g. series chokes)
• Auto-transformer starting
• Series resistance starting (e.g. liquid resistance starter)
• Solid state soft-starting (e.g. smart motor controller)
• Rotor resistance starting, requires a slipring motor
Most of the above motor starting techniques reduce the voltage at the motor stator
terminals, which effectively reduces the starting current as well as the starting torque.
From the equivalent circuits and formulae for AC induction motors, covered earlier in
this chapter, the following conclusions can be drawn about reduced voltage starting:
• Both the stator current and output torque during starting are proportional to
the square of the voltage. During star-delta starting, the voltage is reduced to
0.58 of its rated value. The current and torque are reduced to 0.33 of
prospective value.
)
(
2
Start
Voltage
I
∝
)
(
2
Start
Voltage
T
∝
2.16 Motor selection
The correct selection of an AC induction motor is based on a thorough understanding of
the application for which the motor is to be used. This requires knowledge about the type
and size of the mechanical load, its starting and acceleration requirements, running speed
requirements, duty cycle, stopping requirements, and the environmental conditions. The
following checklist and reference to the preceding sections provides a guide to the
selection procedure.
When selecting an electric motor, the following factors should be considered:
• Type and torque requirements of the mechanical load
• Method of starting
3-Phase AC induction motors 67
• Acceleration time
• Type of construction of AC induction motor
− squirrel cage rotor
− wound rotor with sliprings
− foot mounted
− flange mounted
• Environmental conditions
− ambient temperature
− altitude
− dust conditions
− water
• Required degree of protection of the enclosure
• Insulation class
• Motor protection
• Method of cooling
• Mounting arrangement
− horizontal
− vertical
• Cable connections
• Direction of rotation
• Duty cycle
• Speed control (if required)
In general, the selection of the motor is dictated by the type of load and the environment
in which it will operate. The selection of a cage motor or slipring motor is closely related
to the size of the machine, the acceleration time required (determined by load) and the
method of starting (determined by the electrical supply limitations).
From the point of view of price, reliability, and maintenance, the cage motor is usually
the first choice. In general, slipring motors are required when:
• The load has a high starting torque requirement, but the supply dictates a low
starting current
• The acceleration time is long due to high load inertia, such as a fan
• Where duty dictates frequent starting, inching or plugging
These are general comments because cage motors can be successfully used in all the
above situations.
Slipring motors are sometimes used for limited speed control. The slip can be
controlled by controlling the external rotor resistance. As demonstrated earlier, the overall
efficiency of this method is poor, so this method can only be used if the speed does not
deviate too far from the rated speed. The slip power is dissipated as heat in the external
rotor resistors.
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3.1 Introduction
This chapter deals with the active components (e.g. diodes, thyristors, transistors, etc) and
passive components (e.g. resistors, chokes, capacitors, etc) used in power electronic
circuits and converters. Power electronics is that field of electronics which covers the
conversion of electrical energy from one form to another for high power applications. It
applies to circuits in the following power ranges:
• Power ratings up to the MVA range
• Frequency ratings up to about 100 kHz
Power electronics is a rapidly expanding field in electrical engineering and the scope of
the technology covers a wide spectrum. Therefore, the emphasis will be on the
components used in converters used for the speed control of electric motors. Components
used for other applications such as power supplies, high frequency generators, etc will not
be covered in great detail.
3.2 Definitions
The following are the common terms used in the field of power electronics.
• Power electronic components, are those semiconductor devices, such as
diodes, thyristors, transistors, etc that are used in the power circuit of a
converter. In power electronics, they are used in the non-linear switching
mode (on/off mode) and not as linear amplifiers.
• Power electronic converter or ‘converter’ for short, is an assembly of power
electronic components that converts one or more of the characteristics of an
electric power system. For example, a converter can be used to change
− AC to DC