Page 202 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

AC machines
Induction, wound rotor, synchronous, permanent magnet synchronous and synchronous reluctance machines.
Special machines
Brush less DC motors, stepper motors, switched reluctance motors are used.
Power Modulators (Controller)
Functions
• It modulates flow or power from the source to the motor is imparted speed - torque characteristics required by
the load.
• During transient operation, such as starting, braking and speed reversal, it reduces the motor current with in
permissible limits.
• It converts electrical energy of the source in the from of suitable to the motor.
• It selects the mode of operation of the motor (i.e) motoring and braking.
Types of power modulators (Controllers)
- In the electric drive system, the power modulators can be any one of the following.
- Controlled rectifiers (AC to DC converter )
- Inverters (DC to AC converters)
- AC voltage controllers (AC to DC converters)
- DC choppers (DC to DC converters )
- Cyclo converters (Frequency conversion)
Electrical sources

Very low power drives are generally fed from single phase sources. Rest of the drives is powered from a 3-phase
source. Low and medium power motors are fed from a 415V supply. For higher ratings, motors may be rated at
3.3KV, 6.6 KV and 11 KV. Some drives are powered from battery.

Sensing unit
- Speed sensing (from motor)
- Torque sensing

- Position sensing
- Current sensing and voltage sensing (from lines or from motor terminals from load)
- Temperature sensing
Control unit : Control unit for a power modulator are provided in the control unit. It matches the motor and power
converter to meet the load requirements.
Block diagram of DC drive : The block diagram of a DC drive system is in Fig 3

Fig 3

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