Block diagram of electric drive; series vs shunt starting

An electric drive is a system that controls the motion of a machine using an electric motor. Its main parts are the power source, power modulator, motor, load, and control unit with sensors.

Block diagram

          +-----------+     +-------+     +------+
 Source ->|   Power   |---->| Motor |====>| Load |
 (AC/DC)  | modulator |     +-------+     +------+
          +-----------+        |  ^ speed/current/
                ^              |  | position sensing
                |   +----------+--+
                +---| Control unit|<--- Input command
                    +-------------+     (reference)

1. Source: single- or three-phase AC (50 Hz), or DC (battery, DC mains).
2. Power modulator: controls the flow of power from source to motor so the motor gets the voltage, current and frequency needed. Examples: controlled rectifiers, choppers, inverters, cycloconverters, AC voltage controllers; also resistors and contactors in older drives. Functions: limit starting current, convert source energy to suit the motor, select motoring or braking mode.
3. Electric motor: DC (series, shunt, separately excited), induction, synchronous, BLDC, stepper; converts electrical energy to mechanical.
4. Load: the machine (pump, fan, crane, conveyor, machine tool) with its own torque-speed requirement.
5. Control unit: compares the command with feedback from sensors (speed, current, position) and generates control signals for the modulator. Built with microcontrollers, DSPs, PLCs, op-amps.
6. Sensing unit: tachometer/encoder (speed, position), CTs/Hall sensors (current), voltage sensors; used for closed-loop control and protection.

Why a series motor is started on load

• Series motor speed is roughly $N \propto \dfrac{E_b}{\phi}$ and $\phi \propto I_a$.
• At no load, armature current is very small, so the flux is very small and the speed rises to a dangerously high value ("runaway"). Centrifugal forces can damage the armature winding and commutator.
• Also, $T \propto \phi I_a \propto I_a^2$, so it develops high starting torque and is able to start heavy loads.
• Hence a series motor is always started on load and connected directly (gear, not belt) to the load, so the load can never come off.

 N
 |\              series motor
 | \             (N very high
 |  \            at light load)
 |   '--.__
 |         ''---___
 +-------------------> T or Ia

Why a shunt motor is started at no load

• The field is across the supply, so the flux is nearly constant and the speed is nearly constant from no load to full load; there is no danger of overspeed at no load.
• Starting current is limited only by a starter resistance; at starting $E_b = 0$, so $I_a = V/(R_a+R_{st})$. Starting at no (light) load keeps the starting current and the accelerating time small, so the starter and armature are not overheated.
• Torque $T \propto \phi I_a$ (only proportional to $I_a$), so its starting torque per ampere is lower than a series motor's; starting under heavy load would need a large current for a long time.
• Hence shunt motors are started at no load or light load, and the load is applied after the motor reaches speed.

(Important: the field of a shunt motor must never open while running, otherwise the speed rises dangerously.)