Lecture 4 SC Series motor and power El

Electrical Drives and motor

Lecture 4 Speed Control of DC Series Motors Solid-State Control

Electrical Drives and motor

Speed Control of DC Series Motors

To control the speed of DC Series Motors: 1) Adding a Resistance in the armature Circuit 2) Adjusting the armature voltage 3) Adjusting the field current2

Electrical Drives and motor

Speed-Torque Characteristics of DC Series Motors

Speed=f(Torque)1800 1600 1400 1200 1000 800 600 400 200 0 0 0.2 0.4 Torque, N.m

Speed, rpm

n

Speed-Torque Characteristics of Dc series motors0.6 0.8

Electrical Drives and motor

Equivalent Circuit of a DC series motor

Ea =Vs- Ia (Ra+Rs+RaX)=Vs-Ia R4

Electrical Drives and motor

Controlling Speed by adding Resistance to the Armature Circuit

ω=

Ra + R f Vt = ω0 ω KCI a KC

If a resistance is inserted in series with the armature circuitRa + R f + Radd Vt ω= = ω0 ω KCI a KC5

Electrical Drives and motor

Controlling Speed by Adjusting Armature Voltage

For a constant torque, the current is constant, and the first term of the equation decreases with the decrease of the voltageRa + R f + Radd Vt ω= = ω0 ω KCI a KCThe voltage control can be done by any technique described for shunt motor drives The voltage should be kept at or below the rated voltage: speed reduction below the rated 6 speed

Electrical Drives and motor

Speed control by adding a resistance in the field circuit: This is the same for the armature circuit.

Controlling Speed by Adjusting Field CurrentThe flux control method makes a motor operate at a speed higher than its rated speed. By current divider rule R fadd If = Ia R fadd + R f If and Ia are different now

Electrical Drives and motor

Controlling Speed by Adjusting Field CurrentA reduction of Radd results in current reduction of the field and an increase in the motor speed. This method is suitable for speed increase. This type of speed control must be performed with care so the current of the motor is not excessive

Electrical Drives and motor

Lab3: Speed Control of DC series motorspeed=f(torque) at different resistances1800 1600 1400speed, rpm

1200 1000 800 600 400 200 0 0 0.1 0.2 0.3 0.4

n Rs+2 Rs+4

0.5

0.6

0.7

0.8

Torque, N.m

Electrical Drives and motor

Solid-State ControlSolid-State control or used of the converter To control the voltage in the armature circuit In some cases to control the field voltage High EfficiencyThe power source can be DC or AC 1) DC separately excited or shunt motor 2) DC series Motor10

Basic configuration of DC motor with a converter

The armature circuit is connected to the to the converter, which is fed from an AC source. The field circuit is excited from the AC source through a full wave rectifier circuit .

Electrical Drives and motor

The load is a DC motor

The converter could be a simple SCR trigged by a control circuit (not shown).

Vt = VS (uα u β ) + Ea [1 (uα u β )] Vt = (Vmax sin ωt )(uα u β ) + Ea [1 (uα u β )]11

Electrical Drives and motor

When the cathode is negatively charged relative to the anode, no current flows until a pulse is applied to the gate. Then the SCR begins to conduct, and continues to conduct until the voltage between the cathode and anode is reversed or reduced below a certain threshold value. Using this type of thyristor, large amo

unts of power can be switched or controlled using a small triggering current or voltage.12

Electrical Drives and motor

Thyristor- A thyristor is a four-layer semiconductor device, consisting of alternating P type and N type materials (PNPN). A thyristor usually has three electrodes: an anode, a cathode, and a gate (control electrode). The most common type of thyristor is the silicon-controlled rectifier (SCR). When the cathode is negatively charged relative to the anode, no current flows until a pulse is applied to the gate. Then the SCR begins to conduct, and continues to conduct until the voltage between the cathode and anode is reversed or reduced below a certain threshold value. Using this type of thyristor, large amounts of power can be switched or controlled using a small triggering current or voltage. Thyristors are used in motor speed controls13

Electrical Drives and motor

Thyristor driveA thyristor drive is a motor and controller combination including the drive shaft, where Ac supply current is regulated by a thyristor phase control to provide variable voltage to a DC motor. Thyristor drives are very simple and were first introduced in the 1960s. Now they are replaced by chopper drives for high performance systems and inverters for high reliability with AC motors. They are still employed in very high power applications, such as locomotives, where the high power capability of the thyristor and the simplicity of the design can make them a more attractive proposition than transistor based controllers.

Electrical Drives and motor

Electrical Drives and motor

When the SCR is open, the current is 0 and the load voltage Vt i= 0, when the SCR is closed the Vt =Vs

Vs is a sine wave Vs=Vmaxsin ωt At an angle α, the SCR is triggered (closed). No current existed in the circuit before the SCR was triggered

uα = u (ωt α ) u β = u (ωt π )

uα and uβ are step functionsu β = 0 forωt < 180o uα = 0 forωt < α16

Electrical Drives and motor

Average voltage across the load

Electrical Drives and motor

Full-wave bridge

Electrical Drives and motor

Waveforms of full-wave SCR circuit

Electrical Drives and motor

Electrical Drives and motor

Single-Phase, Half-Wave, ac/dc conversion for inductive loads without freewheeling diode

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