Transcript Closed loop control

Closed loop control
Closed loop speed control of
VSI drives
Closed loop speed control of
CSI drives
Rotor side speed control
• Stator side control is applicable to both
squirrel cage & SRIM
– Because of more advantages
– Squirrel cage motor is always preformed
• rotor side control – the speed control of slip
ring IM
Disadvantages
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Wound rotor machines is heavier
High cost
High rotor inertia
High speed limitation
Maintenance & reliability problems due to
brushes and slip rings.
• SRIM speed control method is very simplest
and oldest method.
– The speed can be controlled by mechanically
varying rotor circuit rheostat.
• The main feature of this m/c is that slip power
becomes easily available from the slip rings.
– Which can be electronically controlled to control
speed of the motor.
Applications of slip power recovery drives are,
• Variable speed wind energy system
• Large capacity wind energy system
• Variable speed hydro pumps / generators
Rotor resistance control
• 3Φ AC supply is fed to the stator and a
variable resistance R2 is connected in the rotor
side.
• By varying the rotor circuit resistance R2
– The starting torque & starting current can be
controlled.
N – T char., of rotor resistance
control
Speed – Stator current char.,
• In this curve,
– By increasing rotor circuit resistance , Tm remains
constant but Tst increases N decreases.
• N – Is curve
– By increase rotor circuit resistance, Is decreases, N
decreases
Drawbacks
• Reduced efficiency because the slip energy is
wasted in the rotor circuit resistance.
• Unbalance in voltage & current.
Advantages
• High line power factor
• Absence of line current harmonics
• Smooth & wide range of speed control
SRIM speed control with rotor circuit
chopper (or) static rotor resistance
control
• The speed can be varied by varying the rotor
circuit resistance
– The rotor resistance can be varied sleeplessly by
using a diode bridge rectifier & chopper as shown
below.
• This method of speed control is very
inefficient because slip energy is wasted in
rotor circuit resistance
• Advantages: high starting torque is available
at low starting current & improved power
factor
• Line power supply is given to the stator of m/c
and
• Rotor circuit, slip voltage is available across
the slip rings.
– This slip voltage is rectified by the 3Φ diode bridge
rectifier.
• The dc voltage is converted to current source
Id by connecting a large series inductor Ld
• It is then fed to shunt chopper with resistance
R,
– The chopper circuit may use IGBT , GTO, thyristor
(or) any other power semiconductor devices.
– Here the dc chopper circuit consists of an IGBT.
• The chopper periodically connects and
disconnects the resistance R.
When the IGBT chopper is on
Resistance is short circuited & the current Id is
passed through it
i.e, Vdc = Vd = 0 and R = 0
When the I G B T chopper is off
• The resistance is connected in the circuit & the
dc link current Id flow through it.
i.e., Vdc = Vd and resistance in the rotor circuit is R.
• Re varied by varying duty cycle of the chopper
• Therefore the developed torque and speed of
the m/c can be controlled by the variation of
the duty cycle of the chopper.
Slip power recovery system
• This system is mainly used for speed control of
SRIM
– The speed of SRIM can be controlled by varying
the stator voltage (or) by controlling the power
flow in the rotor circuit.
• Power delivered to the rotor across the airgap
is equal to the mechanical power delivered to
the load and the rotor cu loss.
• Rotor power = mech., power + rotor cu loss
• Rotor power = mech. Power + rotor cu loss
Pag = Pm + Pcu ----------------(1)
Pag = ωsT
and Pm = ωT
ω = ωs(1-S)
Pcu = S. ωs T
----------------(2)
SPag = slip power
Pm = (1-s) Pag ----------------(3)
Where, T & ωs – electromagnetic torque
developed by the motor & synchronous
angular velocity
• The air gap flux of the m/c is established by
the stator supply and it remains practically
constant.
– If stator impedance drop and supply voltage
fluctuation are neglected.
– Rotor cu loss is proposition to slip
• The speed control of SRIM by connecting the
external resistance in the rotor side.
• Main drawback
– Large slip power is dissipated in the resistance and
– This reduces the efficiency of the motor at low
speeds.
• The slip power can be recovered to the supply
source can be used to supply
– An additional motor which is mechanically
coupled to the main motor.
– This type of drive is known as a slip power
recovery system and
– It improves the overall efficiency.
• The speed of the SRIM can be controlled both
in the
– Sub synchronous region
– Supper synchronous region
• The slip power is taken from the rotor and
feedback to the supply.
– at this condition the motor operates in the sub
synchronous region.
• If electrical power is pumped to the rotor,
– the motor operates in the super synchronous
region
• The torque equation, T = K2ΦI2 COSΦ2
T = K2 I2
• When the motor operates at a constant i/p
voltage
– The flux remain constant & p.f is unity
• The motor operates under constant T, rotor
current(I2) is also constant
I2 = E2/Z2
where Z2 - rotor impedance
E2 - rotor emf
• In the cascade connection,
E2 = KΦ(ωs – ω) + Eext
E2 – Eext = KΦ(ωs – ω)
where, ω – rotor speed
Eext – external emf fed to motor
• The motor speed is dependent on the external
emf Eext
• If E2 – Eext is positive
– Power flows from the rotor to source of Eext and
– The motor operates in sub-synchronous region
• If E2 – Eext is negative
– The power flows from source of Eext to the rotor
and
– The motor operates in the super synchronous
region.
Types of slip power recovery
system
• It is classified into two types are,
1. Kramer system
2. Scherbius system
• These two system can be further classified
into two methods are,
1. Conventional method
2. Static method
Kramer system
• It is only applicable for sub – synchronous
speed operation.
• The classification of Kramer system is,
– Conventional Kramer system
– Static Kramer system.
Conventional Kramer system
• The slip power is converted - dc power by a
rotary converter and fed to the armature of a
dc motor.
• The speed of SRIM is adjusted by adjusting
the speed of dc motor with the help of field
regulator.
• This system also called the electromecanacial
cascade
– Because the slip ring power is returned as mech.,
power to SRIM shaft by the dc motor.
• If the mech., losses in cascade system are
neglected the shaft power o/p of SRIM is,
• The slip power (Ps = Pin S) is added to Pm by
converting it to mech., power by the dc motor.
– This mech., power is fed to the SRIM shaft
– The SRIM the power o/p remains constant
• Hence, it is also called the constant power
cascade.
– This method is only used for large motor of
400KW or above.
• Adv.,
– Any speed, within the working range can be
obtained.
– Improves the power factor.
Modified Kramer system
Static Kramer system
• In rotor resistance control method,
– The slip power is wasted
• Instant of wasted the slip power is can be
converted to 50 Hz AC and pumped back to
the line
– Here , the slip power can flow only in one
direction.
– This method of drive is called static Kramer drive.
• The static Kramer drive offers speed control
only for sub synchronous speed.
– i.e., speed can control only less than the Ns speed.
Static Kramer system