Page 206 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Moreover, the AC motor torque is the product of stator flux and rotor current. For maintaining the rated torque at
all speeds the constant flux must be maintained at its rated value, which is basically done by keeping the voltage
- to - frequency (V/f) ratio constant.
Block diagram of AC drive

The Insulated - Gate - Bipolar- Transistor (IGBT) is in the past two decades come to dominate VFD as an inverter
switching device.
IGBTs (insulated gate bipolar transistor) provide a high switching speed necessary for PWM (Pulse width
Modulation) inverter operation. IGBTs are capable of switching on and off several thousand times a second. An
IGBT can turn on in less than 400 nanoseconds and off in approximately 500 nanoseconds. An IGBT consists of
a gate, collector and an emitter. When a positive voltage (typically +15 VDC) is applied to the gate the IGBT will
turn on. This is similar to closing a switch. Current will flow between the collector and emitter.

An IGBT is turned off by removing the positive voltage from the gate. During the off state the IGBT gate voltage is
normally held at a small negative voltage (-15 VDC) to prevent the device from turning on. So the gate can control
the switching on/off operation of an IGBT.

Fig 1 shows the block diagram of AC drive and Fig 2 shows the internal connection diagram. There are three basic
sections of the AC drive; the rectifier, DC bus, and inverter.

Fig 1 Fig 2

The rectifier in an AC drive is used to convert incoming AC power into direct current (DC) power. Rectifiers may
utilize diodes, silicon controlled rectifiers (SCR), or transistors to rectify power . An AC drive using transistors in
the rectifier section is said to have an “active front end.
After the power flows through the rectifiers it is stored on a DC bus. The DC bus contains capacitors to accept
power from the rectifier, store it, and later deliver that power through the inverter section. The DC bus may also
contain inductors, DC links, chokes, or similar items that add inductance, thereby smoothing the incoming power
supply to the DC bus.
Inverter : An inverter is a device which converts DC into AC. The inverter contains transistors that deliver power
to the motor. The “Insulated Gate Bipolar Transistor” (IGBT) is a common selection in modern AC drives. The
IGBT can switch on and off several thousand times per second and precisely control the power delivered to the
motor. The IGBT uses a method named “Pulse Width Modulation” (PWM) to simulate a current sine wave at the
desired frequency to the motor.
Advantages and disadvantages of AC drive
Advantages
• They use conventional low cost 3 phase AC induction motors for most applications

• AC motors require virtually no maintenance and are preferred for application where the motor is mounted in
an area not easily reached for servicing or replacement.
• AC motors are smaller, lighter, more commonly available and less expensive than DC motors.

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CITS : Power - Electrician & Wireman - Lesson 93-100 CITS : Power - Electrician & Wireman - Lesson 93-100