slip speed control 10.2

Constant slip speed control

Let,V = rated voltage of the motorf = rated frequency of the motor

• When the motor is operated in constant v/f control with the base speed.– For a fz kf – The motor terminal voltage will be kv Where k is a factor such that 0<k<1 , fz is changes from 0 to rated fz , voltage changes from o to rated voltage VK changes from 0 to 1

• The rotor current equation is,

• The torque equation is,

• In equation 1 & 2 , ks maintained constant then Ir’ & T will be constant.

• Since the slip is small value, rotor current Ir’ will be inphase with the voltage– Since flux is constant value, Im will also constant.

• Therefore stator current is,

• For constant value of ks and f is varied then the motor will operate at constant torque and current.

• At frequency kf,

• Synchronous speed = kωS

• slip s is

• Where ωSl – slip speed

it is difference b/w rotating field speed kωS

and rotor speed ωm.

• It is defined as the drop in motor speed from its no – load speed when the machine is loaded.

• From the above discussion, – For any value of torque T, slip speed is the same

for all frequencies.– Hence, the motor speed torque char., for 0<s<sm

approximately parallel curves– The operation of the motor at a constant slip

speed also implies the operation at a constant rotor frequency as given below.

ks = (kf)s / fwhere, fr – rotor fz in Hz

ωr – rotor fz in rad/ sec

• For s< sm,– (Rr’/ks) >> (Xs + Xr’) here from above equation,

– Above equation suggests that for s<sm,– The speed – torque curves are nearly becomes

straight lines.From the above discussion, for a given slip speed1.Motor current and torque are constant values at all

frequency2.Motor current and torque can be controlled by

controlling the speed.

Variable frequency AC motor drives

• The IM speed can be controlled by varying the supply fz.– This method is mainly applied to the squirrel cage

IM• The variable fz control allows,– Good running and– Transient performance

• The variable frequency IM drives very popular because of,– Application involving explosive and dirty

environments such as mines and the chemical industry

– Special applications requiring maintenance free operation, such as under ground & under water installations.

• The variable fz ac drives applications are,– Pumps– Fans– Mills runner– Blower– Spindle drives– Conveyors– m/c tools and so on..

• Due to availability of power semiconductor devices such as power transistor, power MOSFETs, IGBT & GTO etc.,– Improve the ratings and characteristics

• The cost of the equipment is less compared to the dc drives.

• The variable fz conversion can be made by using.1.VSI2.CSI3.cycloconverter

VSI fed AC drives• An inverter is defined as converter that

converts DC into AC.– An inverter called VSI– If viewed from load side, the AC terminals of the

inverter function as a voltage source.i.e., the i/p voltage should be constant

• The VSI has low internal impedance.– Because of this the terminal voltage of a VSI

remains constant with variations in load

• VSI allows a variable fz supply to be obtained from a DC supply.

• MOSFETs is used in low voltage & power inverters– Power transistor & IGBT – medium power level– GTOs, IGCTs – high power level

• Fig shows a VSI employing IGBTs

• Voltage source inverter can be operated as a– stepped wave inverter or– PWM inverter

• Inverter operated as a stepped wave inverter,– IGBTs are switched in the sequence of T/6 – Each IGBT is kept an duration T/2

• Fig shows stepped wave inverter line voltage waveform

• In the stepped wave inverter the o/p fz can be varied by varying T.– o/p voltage can be varied by varying i/p DC

voltage.• When the i/p voltage is DC– Variable DC i/p voltage is obtained by connecting

a chopper b/w DC supply and inverter.– Shown fig

• Here dc supply is given to the DC chopper– DC chopper – fixed DC to variable DC voltage– This voltage is fed to the filter – Filter – filter out the harmonics in DC link voltage– The DC voltage is fed to the six step inverter.

• The Inverter o/p voltage is variable fz variable voltage– It is fed to the 3Φ IM

• When the i/p voltage is AC– Variable DC i/p voltage is obtained by connecting

a controlled rectifier b/w AC supply & inverter– It is shown in fig

• Here 3Φ AC supply is fed to the controlled rectifier – It converts fixed AC in to variable DC voltage– This voltage is fed to the filter– Filter reduces the harmonics.

• The filtered o/p is fed to the inverter,– The inverter o/p voltage can be varied by varying

DC voltage.• The o/p fz can be varied by time period of the

inverter.• Inverter o/p line and phase voltages are given

by following Fourier series,

Drawbacks of stepped wave (VSI) inverter:• Large harmonics of low fz in the o/p voltage.• Because of low fz harmonics, the motor losses

are increased.• Motor develops pulsating torque due to 5th,

7th, 13th harmonics.• Harmonic content in motor current increases

at low speed and the m/c saturated at light loads at low speeds due to high (V/f) ratio.– These two effects overheat the m/c at low speeds.

• The above drawbacks are eliminated by using PWM inverter.

• The advantages of PWM inverters are,– Harmonics are reduced – Losses are reduced– Smooth motion is obtained at low speeds.

• Fig shows o/p voltage waveform for sinusoidal PWM

• By using this method, the inverter o/p voltage and fz can be controlled.– There is no need of external control– v/f can be controlled inverter itself.

• When the i/p voltage is DC , it is directly connected to the PWM inverter.– It is shown in fig.

• When the i/p voltage is AC, DC supply is get from a diode bridge rectifier.– It is shown below

• Here 3Φ supply is fed to the diode bridge rectifier – It converts fixed AC to fixed DC voltage– This voltage is fed to the filter and then PWM

inverter– PWM inverter gives variable an V & f – By changing the v & f the motor speed can be

controlled.

CSI fed AC drives