an analysis of magnetic saturation in induction motors during dc signal injection
TRANSCRIPT
An analysis of MAGNETIC SATURATION in
Induction Motors during DC signal
injection
Introduction An overview of active thermal protection
techniques An analysis of magnetic saturation during dc
signal injection Effects of magnetic saturation on stator
resistance and temperature
What is magnetic saturation??? saturation is the state reached when an
increase in applied external magnetizing field (H) cannot increase the magnetization of the material further, so the total magnetic field (B) levels off.
What happens to the projectile when the coil is boosted to an extremely strong magnetic field?
It is a characteristic property of ferromagnetic materials
limits the maximum magnetic fields achievable in ferromagnetic-core electromagnets and transformers
saturation is exploited in some electronic devices
• magnetic recording media, such as hard disks and magnetic recording tape
• Saturation is employed to limit current in saturable-core transformers,
• saturable core inductors, fluorescent light ballasts, and power control systems.
Thermal protection techniques are crucial in Induction Motors
Conventional protection techniques causes tripping & under protection because
• thermal behaviors of IM cannot be accurately modified
• Cooling capabilities are neglected
A new method of thermal protection - By injecting dc bias voltage into the IM The DC model can be used to estimate
the stator winding resistance & stator winding temperature
This approach is highly accurate and robust
Due to dc signal injection magnetic saturation is present in the IM
Conventional method: connect a power diode between one of
the motor leads to create a dc bias in the input voltage of IM
Limitations :1. dc signals have to be given continuously
– heat dissipation in both power diode and IM
2. continuous and uncontrollable pulsation is induced
To overcome the drawbacks it is proposed to intermittently inject
dc signals Torque pulsation can be controlled by
adjusting the magnitude of dc signal Thermal protection techniques can be
applied for mains fed, soft-starter-connected, inverter fed IM
Dc signal is injected intermittently
The magnitude of injected dc signal is dependent on resistance R which can be adjusted depending on stator resistance and stator current
Under normal operation, MOSFET is kept on to minimize the heat dissipation in the circuit
The dc signals are injected periodically Typical waveforms of input current and voltage
Current consists of an ac current and dc bias current
Contains anti-parallel solid state power switches During dc injection period, only one contactor
(corresponding to one phase) is kept open, the other two works normally
The torque pulsation can be controlled by adjusting the delay angle
Open loop motor drives are widely used for controlling the rotor speed of IM
To inject dc signals, a dc voltage command can be simply added to the original input voltage
Magnetic saturation during normal operation:
• Total flux is induced by 3 phase currents which rotates at synchronous speed
• IM is designed to have slight magnetic saturation – for reducing cost, to limit losses, improve the performance
• Since magnetic flux is constant, level of magnetic saturation is also constant
solid line- with saturation dashed line- without
saturation
• With consideration of saturation , magnitude of magnetic flux is reduced but it is constant. Hence the effect of saturation is consistent
• Therefore even with saturation, stator, rotor & mutual inductances are constant.
Magnetic saturation during DC signal injection:
• Injected dc signals induce a bias in both stator current and flux
• Ψ= actual magnetic flux Ψdc= stationary flux Ψac= rotating flux• The magnitude of the flux is varying due to
injected dc voltage. Hence different saturation levels are induced in each cycle
• Lower level of saturation- magnitude of total flux is small
• Higher level of saturation- magnitude of total flux is larger
• Inductance varies periodically in each cycle
When dc voltage is injected , higher level of saturation is induced around positive peak & lower level of saturation is induced around negative peak of phase current
Absolute value of current increases around positive peak & decreases around negative peak
Multiple harmonics at even orders are induced in both stator current and air-gap flux
The interaction between these harmonics and fundamental frequency induces torque pulsation
From fig in normal operation, the rotating flux at different locations of the core are same
During dc signal injection magnitudes vary at different positions of the core
Hence, the heat dissipation in the core is not uniform
Magnetic saturation also increases core losses
The extra heat dissipation is due to injected dc signal and core losses
This can be tackled by injecting dc signals intermittently for a small amount of time, which is sufficient to obtain an accurate estimation of stator temperature
Stator resistance:The dc component in the d axis stator voltage is
given by
In steady state, as the saturation period varies periodically, the stator flux is still periodic at the fundamental frequency. Therefore vds
dc
So even considering the magnetic saturation, the stator winding resistance can be estimated as
• From fig during dc signal injection the positive peak value is increased due to higher level saturation of saturation compared to normal operating condition; the negative peak value is reduced due to lower level of saturation
Stator temperature: the stator temperature can be estimated as
Where Ts0 & Rs0 represents Ts & Rs at room temperature
α= Temperature coefficient of resistivity
The stator resistance &temperature can be monitored using dc signal injection
Although magnetic saturation affects stator currents it does not have any impact on their dc components
Accuracy of stator resistance & temperature estimation can be guaranteed even in the presence of magnetic saturation
1. Ieee 2009 paper on “ an analysis of magnetic saturation in induction motors during dc signal injection”
2. www.ieee.org3. www.google.com4. www.wikipedia/magnetic saturation in
induction motors. in