---
title: "Block diagram of electric drive; series vs shunt starting"
subject: "Utilization of Electrical Energy"
chapter: 2
key: "semester_7/utilization_of_electrical_energy:drive-block-diagram-motor-starting"
source: https://www.bctengineer.com/q/semester_7/utilization_of_electrical_energy/drive-block-diagram-motor-starting
site: BCT Engineering
---

# Block diagram of electric drive; series vs shunt starting

Utilization of Electrical Energy · Chapter 2: Electric Drive System

- Asked in: 2079 Bhadra, 8 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/utilization_of_electrical_energy/drive-block-diagram-motor-starting
- Whole chapter: https://www.bctengineer.com/md/semester_7/utilization_of_electrical_energy/chapter-2

## Question

Explain the block diagram of an electric drive. Why series motors and shunt motors are started on load and no-load respectively.

## Short answer

An electric drive consists of source, power modulator, motor, load, sensing unit and control unit that compares command with feedback and adjusts the modulator. A series motor is started on load because at no load low armature current gives very low flux and dangerous overspeed; a shunt motor is started at no load because its flux is constant and light load keeps starting current and heating low.

## Answer

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

```text
          +-----------+     +-------+     +------+
 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.

```text
 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.)

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What does the power modulator do in an electric drive?
   A. Controls power flow from source to motor, limits starting current and selects motoring/braking
   B. Stores load
   C. Measures speed only
   D. Drives the load directly
   Answer: A. Modulators like rectifiers, choppers and inverters give the motor the required voltage, current and frequency. (Key point: Power modulator)

2. Why is a series motor never started at no load?
   A. Flux is too large
   B. Armature current and flux are very small, so speed rises dangerously high
   C. Torque is zero
   D. Starting current is zero
   Answer: B. Flux is proportional to Ia, so at no load N ~ Eb/phi becomes dangerously high. (Key point: Series motor runaway)

3. Why can a series motor start heavy loads?
   A. T is proportional to Ia^2 so starting torque is high
   B. T is independent of current
   C. Its flux is constant
   D. It has no armature
   Answer: A. Since flux is proportional to Ia, T is proportional to phi Ia, i.e. Ia^2. (Key point: Series motor torque)

4. Why is a shunt motor started at no or light load?
   A. Its speed rises dangerously at no load
   B. Starting torque is proportional to Ia only, so heavy-load starting needs large current for long, overheating starter and armature
   C. It has no back emf
   D. It has series field
   Answer: B. Torque per ampere is lower, so starting on load draws large current for longer; speed is steady, so no-load is safe. (Key point: Shunt motor starting)

## Related in Utilization of Electrical Energy

- [DC shunt motor speed at reduced voltage and starting resistance](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Adc-shunt-reduced-voltage-starting-resistance) — Utilization of Electrical Energy, chapter 3
- [Advantages of electric drive](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Aadvantages-of-electric-drive) — Utilization of Electrical Energy, chapter 2
- [Electric drive: parts, advantages, disadvantages](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Aelectric-drive-parts-advantages) — Utilization of Electrical Energy, chapter 2
- [Justify: series motor power; shunt motor for traction](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Ajustify-series-motor-power-shunt-traction) — Utilization of Electrical Energy, chapter 4
- [3-phase half controlled bridge: no-load speed, firing angle](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Ahalf-bridge-motor-no-load-speed) — Utilization of Electrical Energy, chapter 3, asked 3×
- [3-phase thyristor bridge: no-load speed and firing angle](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Afull-bridge-motor-no-load-speed) — Utilization of Electrical Energy, chapter 3, asked 3×

## Related in other subjects

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- [DC series motor started at no load: true/false](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adc-series-no-load-true-false) — Electrical Machines, chapter 4
- [Starting shunt generator on no load and voltage build-up](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ashunt-generator-no-load-buildup) — Electrical Machines I, chapter 3
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