module_5.pdf

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Advanced Diploma of Mechanical
Engineering Technology
(DME - 52884WA)
DMEEMD605: AC Electrical Motors and Drives
Webinar 2
Motor Selection, Troubleshooting and Speed Control

Fundamentals of Electric Motors:
• Production of torque in electric motors
• Construction and control of ac motors.
Motor Selection, Troubleshooting and Speed Control:
• The winding connected to the ac supply creates the magnetic field. The
other is simply a shorted winding.
• The current induced in the shorted winding interacts with the magnetic
field and sets up a torque.
Module Structure

Motor Selection, Troubleshooting & Speed Control

Select AC motors for a given application and troubleshoot motor
problems
• Explain motor induction torque/speed curves and the salient points on
the curves
• Explain the important points to be considered for motor selection
• List problem areas in a motor and maintenance needs.
• Identify probable causes for a motor failure.
AC Motor Applications and Problems

SCIM Components

IM Torque Speed Characteristics

• Locked rotor torque of motor: torque developed when the motor is at
standstill and full voltage is applied.
• Pull up torque: Lowest torque during the starting phase.
• Pull-out torque (or breakdown torque): Maximum toque the motor can
generate in operation. For torque demand higher than this value, the
motor will stall.
• Breakaway torque: The value of torque in a mechanical system which is
needed to be overcome to initiate rotation (important for sleeve
bearings)
Important Torque Parameters

Wound Rotor-Speed/Torque Control

Wound Rotor IM

• Motor and load torque characteristics must be matched.
• Highest torque expected in operation must not cause motor to stall.
• Rated values of voltage, current and output power must not be
exceeded on a continuous basis.
• Winding temperature must not exceed the value permitted for the class
of insulation (at the rated ambient temperature).
• Starting duration (for accelerating from zero to final speed) must not
cause overheating of windings .
• Direction of rotation/ single or bi-directional?
• Other – continuous / intermittent operation, environment, mounting
etc.
Motor Selection Considerations

• AC synchronous: High HP, constant power and speed (pumps,
compressors)
• AC cage: Small and medium capacity, medium torque, fixed speed
• AC wound rotor: High torque (hoisting, conveyors, rotary cement kilns)
with speed control.
Motor Applications

• Constant torque, variable power: Hoists, conveyors
• Constant power: traction drives, machine tools
• Constant torque-constant power: Drives in paper industry
• High break-away torque: extruders and screw pumps
• Variable torque, variable power: Fans, pumps
Type of Loads

Equipment Torque/Power vs Speed
Constant power
variable power

• Must have adequate starting torque to overcome break away torque.
• Motor torque > Load torque and the difference must be adequate to
accelerate the load.
– Moment of inertia of motor and load decides starting time.
• The point of intersection of motor and load torque must comply with
rated motor parameters.
• It should also lie on the stable part of the motor curve.
– Torque should have a negative slope for stability.
Motor’s Ability to Drive the Load

During acceleration and deceleration:
• Moment of inertia (J) of the load imposes an additional dynamic
acceleration torque on the motor
• Dynamic acceleration torque is extra torque required to change the
kinetic energy state of the load
During acceleration:
Load Inertia
Nm
t
)
n
-
n
(
60
2
J
=
T
A
1
2
A

Nm
T
+
T
T A
L
M 
J - Moment of inertia kgm2
TA – Acceleration Torque Nm
n2 – final motor speed m/s
n1 – starting motor speed m/s
tA – acceleration time in sec

• Design A: Normal torque, high starting current, low slip
• Design B: Normal torque, low starting current, low slip
• Design C: High torque, low starting current, low slip
• Design D: High torque, low starting current, high slip
NEMA Motor Categories

Torque-Speed: NEMA Characteristics
National Electrical Manufacturers Association (NEMA)

• Motor losses result in heating and the heat has to be removed to keep
temperature within permissible limits to avoid winding failures.
Cooling method depends on rating:
• Totally enclosed with shaft mounted fan
• Open cycle air cooled (small motors typically dc)
• External cooling with shaft mounted fan (LV and Lower range MV
motors)
• Closed cooling air-air heat exchanger (MV medium capacity)
• Closed cooling air-water heat exchanger (Medium voltage high capacity)
Cooling

• Essential for insulating live windings from active magnetic parts (slots).
• Insulation is selected to withstand highest possible winding
temperature (hot spot).
Classes B and F are most commonly used in motors:
• Class A Insulation: 105 Deg C.
• Class E Insulation: 120 Deg C.
• Class B Insulation: 130 Deg C.
• Class F Insulation: 155 Deg C.
• Class H Insulation: 180 Deg C.
Insulation Types

Electrical:
• Rated voltage, frequency, current, insulation class .
• Note: Output torque varies in proportion to voltage in the case of
synchronous motors and Voltage2 for induction motors.
Output:
• Power in kW, RPM, shaft torque, efficiency.
Environment:
• Ambient temperature, RH, altitude, special conditions.
• Derating of output may be required based on the environment
conditions .
Motor Parameters for Selection

• Usually Direct-on-line (full voltage)
• High starting current
• Current falls after motor reaches close to rated speed
Considerations:
• Mechanical shock on the driven equipment
• Electrical stress on the system
• Thermal effects
Motor Starting Methods

Motor Current During Start

• Reduced voltage starting possible:
– Star delta
– Auto transformer
– Electronic soft start.
• Starting torque varies as V2
• Not suitable for high torque requirements.
• May cause stalling.
Other Starting Methods

Winding temperature during starting is critical:
• Frequent starting may cause excessive winding temperature and
winding failure.
Torque adequate to meet acceleration requirements:
• Insufficient torque may cause stalling
• It may also cause prolonged starting time and result in excessive
heating.
Starting Considerations

• Cleaning dust deposit on frame.
• Seal the motor and terminals against water ingress.
• Correct lubrication.
• Alignment check , belt condition/tension.
• Vibration recording and analysis (large motors).
• Monitor insulation resistance.
Motor Maintenance-Main points

• Protection from pollution - Keep motor clean free from dirt and oil.
• Protection from moisture - Check for dampness around the motor or
inside motor.
• This can reduce insulation strength of motor winding.
• As far as possible, keep motor dry internally as well as externally.
• Also, run motor for few hours periodically if not in use for long time so
that the moisture goes off.
• Beware of excessive ambient temperature
Motor Winding Upkeep

• Inspection of bearings - Check bearing condition on a regular basis.
• Bearings should be lubricated only with prescribed lubricant.
• At the same time lubrication should be always done in proper quantity.
• Excessive as well as less than recommended quantity can harm the
parts lubricated.
Motor Lubrication

Motor Failures

Motor Condition Monitoring Methodologies

Explain the principle and selection of Variable Speed Drives for AC motors
• Examine the need for speed control and the control approach used in
AC machines.
• Explain the principle of a Variable Speed Drive for AC motors.
• Examine the torque/speed/power curves of a typical VSD.
• Explain the criteria for selecting a VSD for a given application.
AC motor Variable Speed Drives

• Demands of the driven equipment.
– Example: Electric cranes, traction systems.
• Provide soft starting by gradual speed variation.
• Saving of energy.
– Many pump applications use throttle valves for flow control
– Results in energy losses
– Speed variation can cause change of flow without loss of energy (next
slide illustrates).
Why do we need Speed Control?

Flow Control Methods: Centrifugal Pumps
A – throttling,
B – bypassing,
C – on-off control and
D – VSD control

• Many drives require speed control to meet process needs.
• DC machines with variable voltage input served this need for a long
time. But DC machines are prone to problems.
• AC wound rotor motors with rotor resistance control was another
choice → energy wastage in rotor resistors.
• Speed control of the rugged squirrel cage ac motor can be done by
varying the input supply frequency.
• Control of AC motor speed by voltage variation is not practical. Awaited
the development of solid state power devices to become a reality
Motor Speed Control Principles

To vary the speed there are two main methods:
1. Wound rotor motor with resistance control
– Waste of energy in resistance
– Higher maintenance efforts required.
2. Frequency variation
– Speed is proportional to frequency
• The latter is commonly used.
• Made possible by solid state devices for rectification/inverting.
IM Speed Control

• The supply from AC mains is rectified using semiconductor power
devices.
• The DC output is reconverted to AC using an inverter (semiconductors
used as switches) .
• The output frequency can be controlled in the inversion process.
• A standard AC motor fed from the inverter output will run at a speed
determined by the frequency of the inverted supply.
Variable Speed Drive-Basic principle

• Employ various techniques to maintain V/F ratio constant (Constant
magnetic flux).
• Commonly known as VVVF (Variable voltage, variable frequency)
drives.
• General name is Variable speed drives (VSD).
Components:
• Power rectifier
• DC bus
• Inverter
• Controls and protection
Variable Speed Drives

AC Variable Speed Drive
• Main Components of an AC Variable Speed Drive

• While using variable frequency for speed control, voltage needs to be
varied too.
• The magnetic field strength depends on V/F:
– The ratio of V and F needs to be maintained constant corresponding to
motor rated values
– A lower V/F causes the torque to be reduced
– A higher V/F will produce higher torque.
• VSD can drive induction as well as synchronous motors but the former
is preferred except for very large ratings.
Voltage and Frequency

• VSD creates a rotating magnetic field of the same strength but at a
lower frequency.
• The induced rotor current is about the same as at normal frequency for
a given torque .
• The motor will thus produce the rated torque even at zero speed
without overheating the motor.
• By having a higher V/F ratio the torque can be increased to a value
necessary for breakaway (limited by VSD capability and magnetic
saturation).
VSD Torque

• VSD operation can go very much beyond the rated motor frequency.
• It can also permit operations at very low speeds.
• There are limitations arising from the motor design in both cases.
• VSD can be applied both to induction and synchronous motors.
– Field can be applied right from starting unlike conventional motors.
Speed Range

• Why is the limitation on high speed?
• And at low speeds?
Limitations on Speed Range

High speed:
• Constraint is mechanical design (higher centrifugal force, bearing
damage).
Low speed:
• Constraint is reduced cooling
• To address this, shaft mounted fans are used or forced cooling.
High and Low Speed Operation

Selection is based on the Continuous Torque Loadability:
• Load Torque must be within Torque Capacity of VSD
• This should be true at all Speeds
Continuous Torque Loadability of standard AC motor used with VVVF
Converters is always lower than its rated torque:
• Speed range 0 - 50Hz reduced by decreased cooling
– If necessary auxiliary fan provides cooling air volume.
– Higher maintenance efforts required.
• Speeds > 50Hz Torque reduced by lower air-gap flux
• At all speeds loadability reduced by harmonic heating.
– Good engineering practice to allow small de-rating of 5%.
Torque Loadability

Torque Loadability Curve
The region between zero speed and the rated speed is called the constant
torque mode and above the rated speed the motor is on a constant power
mode.

Region between zero and the base speed of 50Hz:
• Constant Torque operation
• Power increases in proportion to speed.
Inverter output frequency can increase above base speed:
• In fact to frequencies as high as 400Hz
• Above the base speed, the output voltage remains fixed
• V/f ratio will fall in direct proportion to the speed
• Reduce the output torque in proportion to speed
• Constant power region or the field weakening region
• Similar terminology to DC drive terminology.
Constant Torque Region

Constant Power Region

• No electric drive, fixed or variable speed, can be correctly specified
without knowing something about machine load.
Output Torque of an AC VSD is considered to be adequate when it:
• Exceeds the breakaway torque of the machine load
• Can accelerate the load from standstill to its preset speed within the
required acceleration time
• Exceeds the load torque by an adequate margin during continuous
operation at any speed in the speed range
• Does not exceed the thermal ratings of all electrical components.
Nature of the Machine Load

Variable Torque Machine Loads:
• Pumps and Fans are simplest applications.
Following potential problems:
• Breakaway and Starting Torque is usually very low but Slurry pumps can
sometimes be a problem
• Running at Low Speeds is seldom a problem
• Running Above Base Speed can be a problem Power increases as Cube
of the Speed.
Variable Torque Loads: Pumps & Fans

Variable Torque Loads: Pumps & Fans

Constant Torque Machine Loads:
• Constant torque loads need special attention.
The following are some potential problems:
• Starting torque may be higher than expected
– Breakaway Torque (Torque required to initiate rotation)
– Acceleration Torque (dynamic torque)
• Low Speed Running may be a problem if the Load Torque is above
Motor Loadability Curve forced cooling may be required.
Winches & Conveyors

Constant Torque Loads: Conveyors

Variable Torque loads:
• Have low starting torque requirement and easily pulled away and
accelerated to the set speed by any VSD
• Area for concern is the high breakaway torque on some slurry pumps
• Other limiting factor is the total absorbed power at full rated speed must
be within the capacity of the drive.
Constant Torque loads:
• Require full torque at starting but this does not usually present a problem
• Can use torque boost on the parameter settings or select Sensorless
Vector Control.
Requirements For Starting

Starting Torque of the variable speed drive system:
• Mainly limited by the current limit setting on the VSD.
With an over-sized converter:
• There is a limit to torque the motor will produce above its rated torque.
• The motor will usually stall at 2.5 to 3 times its rated torque, depending
on the design.
• For very high starting torques ... a larger motor and converter should be
considered.
Requirements For Starting
Nm
T
I
I
x
1.5
=
T N
Motor
Convr
S

• What is the main reason for suggesting a VSD in a pump application
where part load operation is possible for long periods?
• Energy saving by speed reduction is possible under part load conditions.
• Why is the same reason not valid for a pump which runs at full load
constantly?
• A pump which runs constantly at rated load has no scope for energy
reduction as the speed will have to remain constant.
Reasons for VSD

• Pump application with part load operation for long periods – VSD is
suitable due to energy savings by speed reduction.
• Pump application running at full load constantly - VSD is not required
as there is no scope for energy reduction with the speed remaining
constant.
• It is not possible to have a VSD running at higher than its base speed
to drive a pump motor whose rated speed is equal to the base speed of
the drive.
• The reason is because the pump requires higher torque and higher
power beyond its rated speed.
• The torque output of VSD reduces beyond the rated speed (with power
remaining constant).
VSD Load Scenarios

• AC Variable Speed Drive System correctly selected when:
– Motor Specification is correct
▪ Correct type and size of electric motor, whose output torque, speed and
accuracy are adequate for all load and environmental conditions
– AC Converter Specification is Correct
▪ Correct type and size of AC Converter, whose output voltage, current,
frequency meets the motor requirements for all load and environmental
conditions.
• The first step is ALWAYS to select the correct electric motor.
• The next step is to choose a suitable AC converter to match the
requirements of the motor.
Basic Selection Procedure

Motor selection follows traditional procedure:
• Mounting and Position
• Select correct IP, IC and IM ratings
Output Ratings:
• Motor Torque rating (Nm)
• Motor power rating (kW)
• Number of poles (speed).
So that Load torque on motor shaft remains within continuous torque
capability of motor at all speeds in speed range and high torques of short
duration, such as starting torque, must also be accommodated
Motor Selection Procedure

Selection of the correct motor is affected by the speed range within which
it is expected to run continuously: Can motor drive the load continuously in
the speed range without stalling or overheating the motor?
• Running at sub-synchronous speeds (f < 50Hz):
– Reduces the motor cooling because the cooling fan.
• Running at above-synchronous speeds (f > 50Hz):
– Reduces air gap flux because V/f ratio is reduced, which in turn reduces
the torque output capability of the motor
– Torque is reduced in proportion to the frequency
– Maximum torque allowed at above-synchronous speeds.
Speed Range
Nm
f
50
T
0.6
T P

Max

• In some applications, speed needs to be accurately controlled.
The following questions need to be considered:
• Is normal motor slip acceptable?
• May be advisable to select a larger motor and converter in order to
reduce the slip and improve speed accuracy
• May also be necessary to use speed feed-back from the motor, eg.
tachometer or digital speed encoder
Speed Control Accuracy

Manufacturers have evolved various methods for quickly selecting the
size of motors and frequency converters for a particular machine load:
• All use the same basic procedure
• First select the motor of the correct kW and speed (pole) rating.
Important to consider 2 or more options:
• Larger or smaller kW sizes
• Motors with more or less poles (base speeds)
Select the Correct Motor

Example: Selecting the Correct Motor Base Speed
• Compare the torque-speed (loadability) curves for two squirrel-cage
induction motors (using a VSD):
– 90kW, 4-pole motor (1,475 rev/min)
– 90kW, 6-pole motor ( 985 rev/min).
Example

The following final checks should be made:
• Is the continuous torque rating of the motor greater than the
continuous torque requirements of the load … at all speeds ?
• Is the starting torque capability of the VSD high enough to exceed the
breakaway torque of the load?
• If the VSD is operating in the over-synchronous speed Is the motor
torque capability at maximum speed adequate for the load torque?
• Is the speed accuracy adequate for the application?
Final Checks

VSD will be set to suit the application and motor rating:
• Voltage, frequency, current limit
• Maximum and minimum speed limits
• Motor rated current, current limit
• Acceleration and deceleration time
• Starting torque boost (for high breakaway torque applications)
VSD Essential Settings

VSD Essential Settings

• A compressor motor being switched on for the first time rotates in the
reverse direction.
Solution:
• Since the motor is being switched on for the first time, it indicates that
the phase sequence is incorrect.
• Reversal of the phase sequence either at the panel end or the motor
end can solve this problem.
Examples of some motor failures

• In a process factory, the motor driving a rotary kiln fails and the
insulation is found to have burnt out during summer.
This has happened twice in two years:
• Likely cause is the ambient temperature not considered correctly in
design.
• May have to choose a motor with a higher capacity which when de-
rated for the maximum ambient temperature will be able to meet the
load requirements.

• A conveyor motor has failed twice within a year with insulation burn
out.
• It is a totally enclosed fan-cooled motor.
On inspection, it is observed that the outer fins have a thick layer of metal
dust from the process:
• Likely cause is reduced cooling due to the metal dust.
• More frequent cleaning of the motor and better ventilation and dust
extraction systems can be installed to minimize the dust deposit.

• A 3-phase 400V 50Hz 4-pole cage induction motor of rating 30 kW, with
slip at 5% of the rated load, has a starting torque equal to 110% of the
rated torque.
• The motor is coupled to a pump through a reduction gearbox of
efficiency 80%.
• The breakaway torque is 20N.m at the motor shaft.
• The motor accelerates to the rated speed in 5 s.
Calculate:
1. Synchronous and rated speed
2. Rated Power and Available Torque
3. Moment of inertia seen at the motor shaft
Worked Example

Solution:
Synchronous speed =120 f/P = 1500rpm
Rated speed is Ns x (1-s) = 1500 x (1-0.05) = 1425 RPM
Rated Power kW = (Rated Torque x speed RPM/9550)
So Rated torque = 30 x 9550/1425 = 201 N.m
Starting torque = 1.1 x 201 = 221.1 N.m
Breakaway torque is 20 N.m
Torque available for acceleration = Tstarting - Tbreakaway =221.1-20 =201.1N.m
Efficiency of 80% in transmission, the torque available is 201.1 x 0.80 =
160.88 N.m.
Worked Example

Solution:
• Moment of inertia seen at the motor shaft is:
𝑇𝐴 = J (2π/60) (n2-n1)/ 𝑡𝑎 in N.m
160.88 = J × 2 π ×(1425-0)/(60×5)
J = 160.88 × 300/(2 π × 1425) = 5.4 kg.m2
Worked example

End of Webinar
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