1. High Voltage Coil with Extra Insulation on initial turns
SC A1 ROTATING ELECTRICAL MACHINES
PS1 Development in Electrical Machines
Design Improvements in IGBT based Variable Frequency Drive Motors (India)
A.K. GUPTA D.K.CHATURVEDI,
NTPC Ltd. INDIA
Background
Variable Frequency Drive (VFD) applications has find place world over for energy saving, smooth start and process requirements.
The VFD application in several areas without analysing motor suitability has led to several failures in past.
The paper discuss the design requirements of motor to be considered prior to finalisation for VFD application.
The paper elaborate the different stress motor winding is subjected to, when it is fed from a VFD.
High Voltage Coil with Extra Insulation on initial turns
415V motor rated for 1600V peak/ rise time of 0.1 micro-sec.
dV/dt for inverter grade motor insulation shall be rated for 12.8kV/micro-sec.
Excessive voltage drops in long length due to high freq. ground leakage current
dV/dt filter reqd. for length >200ft
Reactor at VFD output for length<200ft.
ORIGIN OF SURGES
REQUIREMENTS
SC A SESSION
Sine wave formation
Sine wave at high Switching Freq.
WHEN IGBT SWITCHED FROM ON TO OFF i.e. CONDUCTION TO NON-CONDUCTION MODE, A TRANSIENT RECOVERY VOLTAGE TRV APPEARS.
dQ/dt = C dV/dt
This facilitates dissipation of the charge
Design improvements suggested:
Provide the turn to turn insulation as per IEEE 522/ New IEC Better insulation to Ground is to be ensured.
Semi-conductive coating on slot portion of coil
Higher voltage grade insulated magnet wire confirming to NEMA MG1-part31 for random wound low voltage motors.
Insulated bearing to avoid bearing current flow or fluting
Use filter at VFD output as per manufacturer’s recommendation
Selection of motor size considering harmonics & lower cooling
Extra insulation on few turns to withstand high amplitude train of surges caused by refracted wave phenomena.
Inverter Grade motors are designed to comply following:
Higher size for heating due to harmonics and lower cooling
Turn to Turn insulation to take care the Surge voltages
Critical speed should not fall in desirable operating range
The paper describes the issues to be addressed while introducing VFD in existing installation.
The critical speed of motor & driven equipment together has to be determined for quick ramp up.