Saturday, September 30, 2006
New guidelines for variable speed drives + motors
The rapid rate of development of variable speed drives and motors brings with it many challenges for manufacturers, installers and users. Variable speed AC drives are now being designed in to equipment and plant in ever increasing numbers, fuelled by the two key benefits of costs reduction (through energy saving and lower maintenance demands) or process improvements (better product quality, higher dynamics, more flexible production etc). Many modern automated production systems, batch processes and continuous manufacturing operations, such as web printing, rely on the accurate control of motor/drive systems.
As industry demands ever more precision and power, at reduce cost of purchase and operation, so motor and drive producers are striving to build components and sub systems that deliver the expected performance.
While the connection between motor and inverter supply is usually straightforward, there are still a number of issues which need to be carefully considered.
Whilst the vast majority of VSD driven motors are applied without issue, some special cases require additional guidance.
To date, little independent advice on handling such issues has been available.
Some information on the motor is available from the technical report IEC34 -17 1998 "Guide for the application of cage induction motors fed from converters", but this does not reflect current state of the art and is inadequate for modern inverters.
A little further guidance on VSD issues may be obtained from BS EN 61800-2 " Adjustable speed electrical power drive systems - Part 2: General requirements".
Users may therefore feel that an important area has lacked clear guidance so far.
Considerable work relating to the characterisation and specification of cage induction motors and variable frequency converter supplies is being pioneered here in the UK, by a joint working group comprised of industry experts.
Both GAMBICA, the Association for the Instrumentation, Control and Automation Industry, and REMA, the Rotating Electrical Machines Association, have been working on standards and guidelines relating to motors and variable speed drives (VSD).
However, although both associations were working independently towards the same ultimate objective, it fell to Steve Barker (product marketing manager for variable speed drives) at Siemens Automation and Drives in Manchester, to suggest that they teamed up and formed a joint technical committee.
Mr Barker, who is Chair of the GAMBICA Association's VSD Group, explains: "Although it seemed obvious that GAMBICA and REMA should pool their expertise, the idea did appear to be somewhat revolutionary at the time it was suggested."
The concept was enthusiastically embraced by the manufacturers and a taskforce (also convened by Steve Barker) was commissioned to investigate the issue of motor insulation under inverter operation.
The results of the groups joint efforts can already be seen, with the publication of their first Technical Guide, entitled: "Variable Speed Drives and Motors: Motor Insulation and PWM Inverter Drives".
The guide, which is the forerunner to a much more detailed technical report (also now available) on the subject, is one of a series of planned publications that will provide OEM equipment designers, systems integrators and end users with help on the correct selection and use of motor/drive systems.
The co-operation between GAMBICA and REMA has not gone unnoticed internationally.
Through the various international connections of both REMA and GAMBICA, the IEC (International Electrotechnical Commission), who are responsible for setting so many of the product standards now found in factory automation and process control, have also taken up the challenge.
Previously, one IEC committee concerned itself with motors another group concentrated on drives and power electronics.
Impressed with what they saw happening in the UK, the two IEC committees took the decision to create a joint working group to address the specific issues applicable to motors designed to be operated from inverter supplies.
The working group consists of nearly twenty international experts active in the field of both drives and motors.
Together with end users, most of the major manufacturers are represented with delegates from across Europe, North America and Japan.
Based on his similar experience in the UK, Steve Barker was appointed as the group convenor challenged with steering the team to achieve the set objectives.
"At the inaugural meeting in London, last September, the priorities were set to concentrate on cage induction motors designed for VSD operation operating at less than 1000 Volts."
Mr Barker explains.
" The aim of the team is clarify the key points of consideration and the outcome of this working group's activity, WG26, will hopefully be new guidelines to address our core issues."
The core issues are: Additional motor insulation stress due to inverter operation Increases and effects on motor losses (efficency) Oscillating transient motor torques Increased stray motor Bearing currents Electro-magnetically excited motor audible noise.
Under some unfavourable conditions, the above can cause problems or unnecessary irritation to the user and all these points require further investigation.
The remit of IEC's WG26 is to characterise each of these issues in detail, identify possible solutions and finally to suggest design changes where appropriate.
It is hoped that new international guidelines or standards will be in place by within two years, although Mr Barker admits that this is a tough task.
His objective is to provide authoritative information which will assist users to apply equipment correctly and will persuade manufacturers to improve designs where needed.
In the UK study, GAMBICA and REMA evaluated the situation where the quality motors offered by REMA members were controlled by PWM inverters from the GAMBICA suppliers.
Although motor insulation stress is increased with VSD operation (dependent on cable length), the key findings of the team were:- Motors running on a supply voltage of 500V or less are not affected by these stresses Enhanced winding insulation systems are required for supply voltages over 500V A key challenge for the team was to ensure that the guidance was clear and straightforward.
As presented in their technical guide, they point to the need for users of motors running above 500V to pay special attention to the way the motor winding insulation has been constructed.
It is recommended that for supply voltages between 500V and 690V, motors are chosen, from the range offered by REMA members, with an enhanced insulation system.
These readily available motors may cost between 10 and 20 per cent more than a standard design, but they offer the required assurance against costly failures.
If a motor is being considered from alternative suppliers, it is important, say the working group, to establish the pulse withstand capability by discussing the application with the supplier.
These can then be compared to the benchmark pulse withstand characteristics which are included as part of the technical guide.
However, in some cases, for example where a drive is being retro-fitted to an existing motor, or the characteristics of the motor cannot be identified, these recommendations cannot be followed.
In these cases, alternative solutions such as output filters are available and it is usually advisable to consult with the drive and/or motor manufacturer.
Beware, some special cases merit further attention.
For example, drives that use alternative non PWM control strategies (such as direct torque injection schemes), could actually further double the motor's peak voltage stress and therefore special mitigation measures may be needed.
The guide also offers advice on other applications such as active front end drives where further limitations apply.
Finally, the working group points out that it is especially important to note that the use of inverter drives with "Ex" rated motors used in hazardous environments may invalidate the hazardous area certification and the application must be referred to the equipment supplier Mr Barker feels that it is more important than ever to consider the motor and drive as an integral package.
"The persuasive benefits offered by frequency converters will ensure that the expansion of inverter control continues as VSD technology develops.
As industry demands ever more precision and power, at reduce cost of purchase and operation, so motor and drive producers are striving to build components and sub systems that deliver the expected performance.
While the connection between motor and inverter supply is usually straightforward, there are still a number of issues which need to be carefully considered.
Whilst the vast majority of VSD driven motors are applied without issue, some special cases require additional guidance.
To date, little independent advice on handling such issues has been available.
Some information on the motor is available from the technical report IEC34 -17 1998 "Guide for the application of cage induction motors fed from converters", but this does not reflect current state of the art and is inadequate for modern inverters.
A little further guidance on VSD issues may be obtained from BS EN 61800-2 " Adjustable speed electrical power drive systems - Part 2: General requirements".
Users may therefore feel that an important area has lacked clear guidance so far.
Considerable work relating to the characterisation and specification of cage induction motors and variable frequency converter supplies is being pioneered here in the UK, by a joint working group comprised of industry experts.
Both GAMBICA, the Association for the Instrumentation, Control and Automation Industry, and REMA, the Rotating Electrical Machines Association, have been working on standards and guidelines relating to motors and variable speed drives (VSD).
However, although both associations were working independently towards the same ultimate objective, it fell to Steve Barker (product marketing manager for variable speed drives) at Siemens Automation and Drives in Manchester, to suggest that they teamed up and formed a joint technical committee.
Mr Barker, who is Chair of the GAMBICA Association's VSD Group, explains: "Although it seemed obvious that GAMBICA and REMA should pool their expertise, the idea did appear to be somewhat revolutionary at the time it was suggested."
The concept was enthusiastically embraced by the manufacturers and a taskforce (also convened by Steve Barker) was commissioned to investigate the issue of motor insulation under inverter operation.
The results of the groups joint efforts can already be seen, with the publication of their first Technical Guide, entitled: "Variable Speed Drives and Motors: Motor Insulation and PWM Inverter Drives".
The guide, which is the forerunner to a much more detailed technical report (also now available) on the subject, is one of a series of planned publications that will provide OEM equipment designers, systems integrators and end users with help on the correct selection and use of motor/drive systems.
The co-operation between GAMBICA and REMA has not gone unnoticed internationally.
Through the various international connections of both REMA and GAMBICA, the IEC (International Electrotechnical Commission), who are responsible for setting so many of the product standards now found in factory automation and process control, have also taken up the challenge.
Previously, one IEC committee concerned itself with motors another group concentrated on drives and power electronics.
Impressed with what they saw happening in the UK, the two IEC committees took the decision to create a joint working group to address the specific issues applicable to motors designed to be operated from inverter supplies.
The working group consists of nearly twenty international experts active in the field of both drives and motors.
Together with end users, most of the major manufacturers are represented with delegates from across Europe, North America and Japan.
Based on his similar experience in the UK, Steve Barker was appointed as the group convenor challenged with steering the team to achieve the set objectives.
"At the inaugural meeting in London, last September, the priorities were set to concentrate on cage induction motors designed for VSD operation operating at less than 1000 Volts."
Mr Barker explains.
" The aim of the team is clarify the key points of consideration and the outcome of this working group's activity, WG26, will hopefully be new guidelines to address our core issues."
The core issues are: Additional motor insulation stress due to inverter operation Increases and effects on motor losses (efficency) Oscillating transient motor torques Increased stray motor Bearing currents Electro-magnetically excited motor audible noise.
Under some unfavourable conditions, the above can cause problems or unnecessary irritation to the user and all these points require further investigation.
The remit of IEC's WG26 is to characterise each of these issues in detail, identify possible solutions and finally to suggest design changes where appropriate.
It is hoped that new international guidelines or standards will be in place by within two years, although Mr Barker admits that this is a tough task.
His objective is to provide authoritative information which will assist users to apply equipment correctly and will persuade manufacturers to improve designs where needed.
In the UK study, GAMBICA and REMA evaluated the situation where the quality motors offered by REMA members were controlled by PWM inverters from the GAMBICA suppliers.
Although motor insulation stress is increased with VSD operation (dependent on cable length), the key findings of the team were:- Motors running on a supply voltage of 500V or less are not affected by these stresses Enhanced winding insulation systems are required for supply voltages over 500V A key challenge for the team was to ensure that the guidance was clear and straightforward.
As presented in their technical guide, they point to the need for users of motors running above 500V to pay special attention to the way the motor winding insulation has been constructed.
It is recommended that for supply voltages between 500V and 690V, motors are chosen, from the range offered by REMA members, with an enhanced insulation system.
These readily available motors may cost between 10 and 20 per cent more than a standard design, but they offer the required assurance against costly failures.
If a motor is being considered from alternative suppliers, it is important, say the working group, to establish the pulse withstand capability by discussing the application with the supplier.
These can then be compared to the benchmark pulse withstand characteristics which are included as part of the technical guide.
However, in some cases, for example where a drive is being retro-fitted to an existing motor, or the characteristics of the motor cannot be identified, these recommendations cannot be followed.
In these cases, alternative solutions such as output filters are available and it is usually advisable to consult with the drive and/or motor manufacturer.
Beware, some special cases merit further attention.
For example, drives that use alternative non PWM control strategies (such as direct torque injection schemes), could actually further double the motor's peak voltage stress and therefore special mitigation measures may be needed.
The guide also offers advice on other applications such as active front end drives where further limitations apply.
Finally, the working group points out that it is especially important to note that the use of inverter drives with "Ex" rated motors used in hazardous environments may invalidate the hazardous area certification and the application must be referred to the equipment supplier Mr Barker feels that it is more important than ever to consider the motor and drive as an integral package.
"The persuasive benefits offered by frequency converters will ensure that the expansion of inverter control continues as VSD technology develops.
# posted by Medical Information : 4:19 AM
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