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