INTRODUCTION
The use of frequency inverter enables a continuous control of the motor speed and provides constant torque in the whole speed range.
The rating for variable speed induction motors need to be selected based on the following aspects:
- Heating due to harmonic content of inverter waveform.
- Reduced ventilation due to low speed operation.
- Constant torque/Constant Power/ Variable torque requirement of load.
- Other limiting factors e.g. maximum permissible motor speed, ambient temperature, altitude etc.
While variable speed drives provide highest energy saving for variable torque (Centrifugal pump, fan) applications, for constant torque and constant power application, Variable speed drives offer significant process control improvements. Basic inverters permit operation over a typical 20:1 speed range. With closed loop vector control the effective range may be increased to 1000:1.
The main International Standards that deal with this subject are:
- IEC: 60034-17 - Rotating Electrical Machines - Part 17: Cage induction motors when fed from converters.
- IEC 60034-25 - Rotating Electrical Machines - Part 25: Guide for the design and performance of cage induction motors specifically designed for converter supply
MAIN FEATURES OF INVERTER DUTY MOTOR
- Inverter Friendly Insulation (Dual-coated class-H wires, special impregnation process & high quality of flat insulations. Normally class F & if require class H)
- Insulated Bearing at NDE (Generally For Higher Frame Size)
- Thermal overload protection
- Precision dynamic balanced rotor assembly
- Provisions for encoder mounting
- Forced Cooling if required
- Customization for Special Performance Requirements
CONSTRUCTION
Supply Voltage and Frequency
Standard motors are suitable for operation for a main supply of 3 Phase, 415V+/-10%, 50Hz.+/-5% . However, motors can be offered for any base voltage from 200Volt to 690Volts.
Protection
Motors are provided with IP55 degree of protection as standard. IP56 degree of protection can be provided on request.
Duty
Standard motors are designed for S1 duty. Crane duty motors suitable for S4/S5 duty ratings are available as a separate range. Motor to suit other duty conditions may be offered on request.
Insulation and Temperature Rise
The motors are designed suitable for 45°C ambient temperature and provided with class F insulation with temperature rise limiting to class B limit. Research has shown that thermal margin enhances dielectric performance of inverter fed motors significantly. Class H insulation may be offered on request. Motor re-rating factor due to ambient temperature and altitude is indicated in the motor selection chart. Apart from many other specific design considerations, inverter duty motors are provided with special insulation system to withstand voltage stresses imposed by IGBT based drives comprising of the following.
- Special grade of wire insulation depending on motor rating
- Vacuum impregnation /Multi dip treatment
- Tropicalisation treatment
- Phase separator/Overhang tape
Cooling
Standard motors are suitable for operation with shaft mounted fan with standard IC411 cooling configuration.
Various Forms of Cooling is given below.
- IC411 Totally Enclosed Fan Cooled. Motor cooled by its own shaft driven externally mounted fan. Cooling effect reduces with reduction of speed. Over speeding increases Windage loss and noise level
- IC410 Totally Enclosed Non ventilated (TENV). Free convection (self cooling), no external mounted fan
- IC418 Totally Enclosed Air Over Motor cooled by airstream. Cooling effect reduces with reduced airstream velocity.
- IC 416 Totally enclosed Force cooled. Motor cooled by independent fan. Constant cooling throughout speed range.
Consideration for Variable Speed Operation
Variable speed drives are used for operation both at low and high speed. Cooling for variable speed
motors gets affected while operating at lower speed whereas during high speed operation windage losses, bearing lubrication, increase in noise level, increased temperature due to shaft seals if any require special attention. At low speed (Below 10Hz.) loss distribution changes substantially which may lead to higher surface temperature.
Temperature rise near bearing location should be periodically checked to ascertain that they are within limit. For sustained operation at low speed separate Force ventilation (FV) unit is recommended. FV unit is also used to restrict windage loss and motor noise when the motor runs frequently near to highest permissible operating speed of the bearing system.
Force Ventilation Unit
Force ventilation unit replaces integral shaft mounted fan with an independently mounted separately excited motor driven fan. The unit provides constant airflow to the drive motor throughout speed range. Totally Enclosed Air over motor /TEFC motor, depending on the design of FV unit, is provided to drive the independent fan.
Fan motor rating depends on the rating and size of drive motor. Specially designed axial flow fan with optimized air delivery, low noise level and minimum power input is provided.
Bearing
Metric size Ball and Roller bearings are used. Double sealed bearings are provided with high temperature grease suitable for operation upto 110°C. Grease relief arrangement is provided for motors having re-greasing facility.
Bearing Current
Depending on motor size and supply voltage rating, bearing current may be generated owing to shaft voltage caused by high dV/dT common mode voltage (CMV) and motor parasitic capacitance.
Normally bearing fluting caused by such bearing currents are noted from motor rating 110KW upwards. For frame sizes 280 and above insulated bearing housing is provided as standards. Good installation practice can reduce the risk of bearing current. Some recommendations are as follows.
- Effective grounding of drive system components (Drive, Motor, Driven equipment) Ensure good HF bonding for potential equalization between drive components.
- Use of symmetrical motor cables with armour shield
- Proper installation of motor cables and connectors.
In Built Motor Protection
S1 duty motors in frame sizes 200 and above are recommended with thermistor protection as standard. 3 Nos. PTC140 (Class B Rise) provided in winding overhang are connected in series and thermistor terminals are brought out to terminal box. For operation with Class F rise PTC160 thermistor may be provided.
Accessories
Encoders / tachogenerators are required to provide close loop control between inverter and motor for accurate position control and monitoring. These units are fitted at NDE side on request.
Incremental encoders provide feedback on rotor movement by converting rotational movement into electrical signals used for numerical processing for precise speed control. Incremental encoders operating on photo electric measuring principle provides very high degree of measurement accuracy.
AC Tachogenerators using permanent magnet rotor are used for simple speed monitoring and control. They convert the input speed to a three phase voltage whose magnitude and frequency is proportional to speed.
APPLICATION
Generally belts, sheaves, or gearboxes are used to reduce the speed of the motor, in this case the motor still runs at full speed. With the introduction of VFD the speed can be controlled by varying frequency & voltage fed to the motor the advantage being reduction in energy cost & mechanical issues associated with starting motors across the line.
