Page 211 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

The AC input supply is provided should match the voltage and frequency given on the controller’s name plate.
Improper voltage may damage the equipment and insufficient current will cause erratic operation of the drive.
The shielded cable is recommended for the tachometer and all low level signal circuit to eliminate the possibility
of electrical interference.

In some DC drives a speed adjusting potentiometer is provided to vary motor speed by controlling armature input
voltage after the controller has been started. Some time a torque adjusting potential meter is used in place of
speed adjusting potentiometer. It controls motor torque by controlling the DC current in the motor armature.
Starting and controlling the speed of DC motor
When the ‘ON’ button in BOP is pressed, the motor will start running. The desired speed can be attained by using
‘P’ button and D & Ñ buttons.

When the “OFF” button is pressed the motor will stop but AC line voltage remains connected to the controller and
full field voltage is present. Armature voltage is reduced to zero. When pressing the “ON” button again the motor
will accelerate to the preset speed.
Inching operation
For inching operation the ‘JOG’ position should be selected. Then the controller will operate only as long as the
“ON” button is held pressed.
Changing the direction of rotation
In some model a ‘reversing switch’ is provided to change the direction of rotation of the motor. This switch is
responsible for changing the polarity at the motor armature connection. First start the motor by pressing ‘ON’
button. The motor will run in forward direction. To change the direction of rotation, press “OFF” button and ensure
that the motor is completely stopped. Now press the reversing button and then press the “ON” button. The motor
will now run in the reverse direction. The reversing key has a provision which prevents direct transfer from one
direction to the other.
Precautions during installation, connection and operation of DC drive
• Ensure all screws are tightened to the proper torque rating.
• During installation, follow all local electrical and safety codes.
• Ensure that appropriate protective devices (circuit breaker MCB or fuses) are connected between the
power supply and DC drive.
• Make sure that the drive is properly earthed.
• Do not attach or remove wiring when power is applied to the DC drive.
Parameter setting of AC drive

As explained earlier the speed (N) of AC induction motor is directly proportional to the voltage (V) and frequency
(f) of the applied power supply. Within the base speed limit, the torque (T) can be kept constant by maintaining
a constant voltage / frequency (V/F) ratio. By increasing of speed to above base speed limit is also possible
but at the cost of the torque.

(VFD /VVVFD (Variable Voltage Variable Frequency Drive) drives are used for efficient speed control of AC
motors. The advantages of using drives to control the speed is already explained.
The AC drive has a front panel which includes two parts. Display panel and keypad. The display panel is
provided with the parameter display and shows operation status of the AC drive. Keypad provides programming
interface between users and AC drives. The Fig 8. shows the location of buttons and display unit on the front
panel of AC drive.
Mode /Reset button
By pressing this button repeatedly the display will show status at the AC drive such as the reference frequency
and output current. If the drive stops due to a fault, correct the fault first, then press this button to reset the
drive.

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