Section CSI non-slaient pole synchronous motor drive

Transcription

1 Section CS non-slaient pole synchronous motor drive Perormance with current-source inverter (CS) drive Current-source driven synchronous motor drives generally give higher dynamic response and better reliability because o the higher dynamics o current control possible with current source inverters and the automatic current limiting eature in a CS drive. n variable-speed applications, the synchronous motor is normally driven rom sti current sources rather than voltage sources. Two CS schemes are in general use. n one scheme, sinusoidal currents in the three phases are continuously regulated with SPWM inverters employing MOSFTs. Low power servo drives using permanent magnet synchronous motor all in this category. Three-phase SPWM GBT inverter driven synchronous motor drives in low and medium power synchronous motor drives used in the process and mill drive industries also all in this category. Such SPWM inverter drives require continuous position eed back rom a high resolution (>10 bits) rom optical encoders or magnetic resolvers. The continuous position signal is used to control the amplitude and phase angle o o the three-phase sinusoidal current supply relative to the back em phasors ( ) o each phase. n another scheme, which are ound in large power applications, quasi-square-wave currents o variable amplitude are delivered to the motor using naturally commutated inverters which employ thyrisors. These drives do not require continuous position eedback signals but only use the six commutating signals (rom three shat-mounted Hall position sensors) over one cycle o the supply current. The Hall sensors allow the phase angle o the quasi-square-wave current waveorms to be synchronised with respect to the zero crossings o the line-line voltages o the motor. This control scheme is quite similar to the scheme or the BLDC drive and its position sensor requirement is rather modest and simpler that the sensor or the scheme with continuous control. The scheme o igure is preerred or lower power drives where very high dynamic response is required rom the drive. The three-phase sinusoidal currents o variable amplitude and requency are produced and regulated within the inverter. The inverter typically employs gate turn-o switches, such as the GBT, MOSFTs, and pulse-width modulation techniques within the inverter. Motor phase currents are sensed and used to close independent current controllers or each phase as indicated in the igure Normally, two current controllers suice or a balanced star-connected motor. n this scheme, three-phase sinusoidal AC currents are supplied to the motor, the amplitude and phase angle o which can be independently controlled, as desired. The rotor position sensor is a position encoder which can be o absolute or incremental types. Absolute encoders produce Gray or BCD coded discrete signals o 8 or more bits using as many optical or magnetic transducers as the number o bits. ncremental encoders produce two squarewave signals which are 90 degree displaced rom each other depending on the direction o motion. The requency o these two signals (A and B outputs)are proportional to the speed o the shat. n addition, an index short duration (Z) pulse signal is produced once per revolution. The absolute position o the shat can be determined rom these three signals, to a resolotuion which is given by the number o pulses/revolution o the encoder. The digital position o the rotor rom allows, via the Look-Up-Table, the generation o the three current reerences o the desired amplitude m and angle, The angle reers to the phase 4.4 CS driven synchronous F. Rahman (T, UNSW) motor drive 1 Aug 2013

2 angle o the current phasor with respect to the back em phasor o each phase. The actual phase currents are orced to ollow these reerences by the high-gain closed-loop PWMcurrent controllers. + DC-Link T 1 T 3 T 5 AC supply ia re P W M T 1 T1 T 4 T 4 T 6 T 2 DC-Link i c i bre i cre i b i c P W M P W M T 3 T 6 T 5 T 2 iare i bre i cr e Sinusoidal Current Reerences (re, i bre & i cre ) o amplitude = m m LOOK UP TABL Figure Current regulated PWM (CRPWM) synchronous motor drive The scheme o igure is used in high power synchronous motor drives. A variable DC current source is established by closed loop control o a DC power supply. The phase angle controlled AC-DC thyristor bridge converter terminated with a large DC link inductor serves as the regulated but sti current source. The scheme is suitable or large synchronous motors or which thyristor switches are used in the inverter. The current loop is established by sensing the DC link current and by using a closed loop current controller which continuously regulates the iring angle o the ront-end controllable AC-DC rectiier. Switching o the six thyristors are synchronised with the phase back em waveorms, the zero crossings o which are obtained rom three Hall sensors (H) mounted on the shat. t will be shown later that with this type o control, the motor torque is proportional to level o the DC link current. Note that the dotted trace o igure represents the undamental component o the quasisquare current o phase a. Switching o the six thyristors in converter driving the motor takes place autonomously, arranged by position sensor and the converter switching logic. The angular displacement o the phase current waveorm (or its undamental component) with the respect to the back em waveorm o the corresponding phase is indicated in igure Due to the presence o the large DC-link inductor, phase currents may be considered to remain essentially constant between switching intervals. The quasi-square phase current waveorms contain many harmonics, which may be responsible or some torque pulsations or this drive at low speed. 4.4 CS driven synchronous F. Rahman (T, UNSW) motor drive 2 Aug 2013

3 DC Link nductor DCLlink ~ Current controller re + T 1 T 3 T 5 T 1 -T 6 T 4 T 2 T 6 DCLink Converter Switching Circuit H Figure CS synchronous motor drive with DC link current control e a d DCLink = d 1 d Figure The above scheme uses a thyristor converter or driving the motor. The motor is usually operated with over excitation, so that the thyristor switches can commutate with the help o the motor back em or turning the thyristors o at the end o a conduction period. Note that with overexcitation, the motor current leads the back em so that when a phase current passes through zero, the anode-cathode voltage across the conducting thyristor is avourable i.e., the out going thyristor is reverse biased by the the back em o the motor. This type o drive is normally used in very large power applications or which the only suitably rated, natuarally commutated thyristor switches are available.. The motor can be reversed easily by reversing the phase sequence o switching o the inverter. The drive can also be braked regeneratively by increasing the iring angle o the input rectiier above 90 o while maintaining the DC link current at the desired braking torque level until braking is no longer required. The rectiier now absorbs the energy o the overhauling motor, regeneratively. 4.4 CS driven synchronous F. Rahman (T, UNSW) motor drive 3 Aug 2013

4 4.4.2 Brushless DC operation o CS driven synchronous motor n the ollowing analysis, it is assumed that the supply current waveorm or each phase is sinusoidal and o controllable amplitude or RMS value. t is also assumed that the phase angle between current and induced back em in each phase can be arbitrarily chosen. All o these tasks are arranged through continuous rotor position eedback and continuous control o stator currents in closed loops. n other words, phase current reerences, and hence actual rotor currents, are assumed to be synchronised with the rotor position (angle). [n a more general sense, this type o control is equivalent to controlling the motor currents in the rotor reerence rame]. j q X s q-axis e a q R d R i j d X s a t q d-axis (a) Bem and current o phase a Figure d (b) Phasor diagram a Consider the cylindrical rotor synchronous motor the phasor diagram o which is redrawn in igure 4.4.4(b). The angular relationship between and is also indicated in this igure. t should be noted rom the results that ollow that the developed torque at any speed is independent o R since a high gain (sti) current source drive is used. The developed power and torque in terms o the commutation angle are given by P 3 cos (4.4.1) T 3 cos 31cos 3p1cos (4.4.2) / p 2 m 1 1 The ratio that 1 1 at any operating speed is constant and proportional to the rotor lux amplitude, so T Kˆ cos (4.4.3) Note that angle can be arbitrary chosen. 4.4 CS driven synchronous F. Rahman (T, UNSW) motor drive 4 Aug 2013

5 Case A: Maximum Torque per Ampere (MTPA) operation ( = 0) = 0, the motor developed torque is maximum per ampere o stator current (MTPA operation). T Kˆ Nm (4.4.4) which is very similar to the torque expression o a separately excited brushed DC motor. n other words, the developed torque o a cylindrical-pole synchronous motor can be controlled directly by controlling the amplitude o the stator phase current. The maximum torque per ampere (MTPA) characteristic is achieved when = 0. Note that operation with maintained at zero angle (see Figure 4.4.5(b)) at all times is key to this brushless DC motor like operation. v a e a q X s q R = 0 = q = 0 a Figure 4.4.5(a) Figure 4.4.5(b) However, the motor input current phasor now invariably lags the voltage phasor at the motor terminals. [see the phasor diagram o igure 4.4.5(b)]. Note that, which is determined by the level o excitation. t also determines the angle to some extent. Clearly, when maximum torque per ampere characteristic is required, a power actor less than unity has to be accepted. Case B: Operation with ield weakening Operation above base speed is normally obtained with ield weakening. n this speed range, because o the limited DC link voltage available, the rotor ield must be weakened otherwise the amplitude o the phase induced em will exceed the DC link voltage and current control will not be eective. Field weakening is a means o keeping o at the rated level or speeds higher than the base speed. Note that Kˆ implies that speed can be increased by decreasing. For synchronous motors with rotor ield winding, weakening o the rotor ield is easily arranged by reducing the ield current which is supplied via slip rings rom a separate but controllable converter. The ield current is normally regulated with a ast current control loop, so that it can eectively and quickly change the ield current when the motor is accelerated to higher than base speed or decelerated rom this speed ast. Otherwise the motor voltage could exceed the rated value during the accleleration to, and deceleration rom, above the base speed. 4.4 CS driven synchronous F. Rahman (T, UNSW) motor drive 5 Aug 2013

6 jx s jx s jx s Under excited SM < Figure 4.4.5(c) Over excited SM > For a permanent magnet motor, rotor lux can be reduced by armature reaction. For example, i is made to lead, the d-axis component o, i.e., d, will lead by 90. The mm due to d then opposes the rotor d-axis mm, as indicated in igure Note that Nssd Ld d Ns. the airgap is small, the negative d-axis component o the armature current may reduce the rotor lux as required. The air gap is large in a surace magnet motor, and ield weakening is consequently not very eective or such motors. Permanent magnet motors with magnets buried into the rotor have smaller airgap and allow operation with ield weakening (i.e., operation above the base speed). q-axis q d a d-axis Case C: Unity power actor operation Figure The power actor with which a motor operates is an important issue, especially or a large drive. The power actor is given by the cosine o the angle between the input voltage and current to the motor. A large results in a poor power actor which means that in order to deliver a load, the motor must draw a larger current than what would be drawn rom the inverter i the power actor were high. For a large drive, a poor power actor may not be acceptable rom the consideration o the cost associated with supply o high input current rom the supply. 4.4 CS driven synchronous F. Rahman (T, UNSW) motor drive 6 Aug 2013

7 Operation o the synchronous motor with a current source inverter allows power actor compensation directly, using the commutation angle () adjustment. This is more direct than via excitation control or voltage source inverter drives. Consider the ollowing two cases. lags q X s e a v a R d q R q j d X s d a Figure 4.4.7(a) Figure 4.4.7(b) n Figure 4.4.7, the overall power actor is lagging, since is a lagging angle. The power actor angle is larger than. Note that since the motor is under-excited and lags, and d magnetizes the rotor. leads the phasor is chosen to lead, the overall power actor can be much better, as indicated in Figure 4.4.8, including unity. Note that the motor now operates with less than maximum torque per Ampere (MTPA) characteristic. v a e a d R j d X s j q X s q R d q leading a Figure 4.4.8(a) Figure 4.4.8(b) Note also that the d-axis componernt o now tends to demagnetise the rotor (leid weakening). t should be noted that angle adjustment as a means or power actor correction is only applicable to machines with suitable (i.e., level o rotor excitation) and synchronous reactance parameters. 4.4 CS driven synchronous F. Rahman (T, UNSW) motor drive 7 Aug 2013