Closed loop control

Closed loop speed control of VSI drives

Closed loop speed control of CSI drives

• 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

Rotor side speed control

Disadvantages

• 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 lossPag = 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 system2. Scherbius system

• These two system can be further classified into two methods are,

1. Conventional method2. 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,

Pm = (1 – S) Pin

• 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