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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 • • • • • 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