---
title: "Closed-loop control of multi-motor drive"
subject: "Utilization of Electrical Energy"
chapter: 2
key: "semester_7/utilization_of_electrical_energy:closed-loop-multi-motor-drive"
source: https://www.bctengineer.com/q/semester_7/utilization_of_electrical_energy/closed-loop-multi-motor-drive
site: BCT Engineering
---

# Closed-loop control of multi-motor drive

Utilization of Electrical Energy · Chapter 2: Electric Drive System

- Asked in: 2081 Bhadra, 5 marks; 2073 Chaitra, 6 marks (asked 2 times)
- Video: [control of electric drive | drive control | closed loop torque control | close loop drive | L-21](https://www.youtube.com/watch?v=TuZg1VebPCg) — Mind of Engineer 
- 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/closed-loop-multi-motor-drive
- Whole chapter: https://www.bctengineer.com/md/semester_7/utilization_of_electrical_energy/chapter-2

## Question

Draw and explain the closed-loop feedback control system for a multi-motor drive.

## Short answer

A master speed reference is multiplied by ratio settings to give each section's speed reference. Each section has an outer speed loop feeding a speed controller whose output is the current reference, an inner current loop controlling the converter, and tacho/encoder feedback; tension sensors trim references between sections.

## Answer

In a **multi-motor drive**, one machine has several motors, each driving a separate part (section) of it, e.g. paper machines, textile machines, rolling mills, cranes (hoist, trolley, travel motors). The sections must run at **precise speed ratios** (or tension), so closed-loop feedback control is used.

#### Block diagram

```text
                 Master speed reference w*
                         |
         +---------------+----------------+
         |               |                |
      [ratio k1]      [ratio k2]       [ratio kn]
         |               |                |
  w1* -->(+)-->[SC]-->(+)-->[CC]-->[Conv]-->M1--> section 1
          ^-  speed      ^-  current            |
          |   ctrl       |   ctrl               |
          |              +-- current sensor <---+
          +------------- tacho/encoder <--------+
         ...
  (same loop for sections 2 ... n)
         |
   [Tension/load sensor between sections]
         +---> trims speed refs of next section
```

#### Working

1. **Master reference**: an operator sets the line speed $\omega^*$. Each section's speed reference is obtained by multiplying by a ratio $k_i$ (draw ratio), so all sections change speed together.
2. **Outer speed loop**: the speed sensor (tachogenerator or encoder) measures the actual speed $\omega_i$. The error $\omega_i^* - \omega_i$ goes to the **speed controller** (PI), whose output is the current (torque) reference.
3. **Inner current loop**: the current controller compares the reference with the measured armature (or stator) current and sets the converter firing angle/duty cycle (for DC drives) or the inverter PWM (for AC drives). It limits current during starting and overload and gives fast torque response.
4. **Power converter** (controlled rectifier, chopper or inverter) supplies the motor.
5. **Tension/position (dancer) feedback** between sections adds a trim signal so the material is neither slack nor over-stretched.
6. **Load sharing**: when several motors drive the same shaft, their current loops are coordinated so each carries its share.

#### Advantages

- Accurate speed ratios and tension; good product quality.
- Fast response to disturbances (current loop corrects supply and load changes).
- Protection against overcurrent; smooth starting of the whole line.
- Easy changing of line speed with one master control; suitable for PLC/computer control.

#### Applications

Paper and textile mills, steel rolling mills, printing presses, cranes, conveyor lines.

## Check yourself

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

1. Why is closed-loop control used in a multi-motor drive?
   A. Sections must run at precise speed ratios or tension
   B. To remove the need for motors
   C. To reduce supply frequency
   D. To avoid using converters
   Answer: A. Different sections of the machine must keep precise speed ratios or tension, which feedback maintains. (Key point: Purpose of multi-motor drive)

2. How does each section get its speed reference?
   A. Each is set separately by hand
   B. The master reference is multiplied by a ratio k_i for that section
   C. It equals the measured current
   D. It comes from the supply frequency
   Answer: B. The master line speed is scaled by a draw ratio for each section, so all sections change speed together. (Key point: Master reference and ratio)

3. What is the role of the inner current loop?
   A. It measures tension
   B. It limits current during starting/overload and gives fast torque response
   C. It sets the master speed
   D. It replaces the speed sensor
   Answer: B. The current controller sets converter firing/PWM, limits current, and responds quickly to disturbances. (Key point: Inner current loop)

4. What does the tension or dancer feedback between sections do?
   A. It trims the speed reference so material is neither slack nor over-stretched
   B. It measures motor temperature
   C. It starts the motor
   D. It changes supply voltage
   Answer: A. Tension/position feedback adds a trim signal to the next section's speed reference. (Key point: Tension feedback)

## Related in Utilization of Electrical Energy

- [Two quadrant armature controlled DC drive; closed loop](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Atwo-quadrant-armature-control-loop) — Utilization of Electrical Energy, chapter 3
- [Open loop and closed loop V/f inverter-fed drive](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Aclosed-loop-vf-inverter-drive) — Utilization of Electrical Energy, chapter 3
- [Individual, group and multi-motor drives](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Aindividual-group-multimotor-drive) — Utilization of Electrical Energy, chapter 2
- [Constant V/f control of induction motor](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Aconstant-v-f-control-induction-motor) — Utilization of Electrical Energy, chapter 3, asked 2×
- [Four quadrant operation in DC motor control](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Afour-quadrant-dc-motor-control) — Utilization of Electrical Energy, chapter 3
- [Ward-Leonard system of speed control](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Award-leonard-speed-control) — Utilization of Electrical Energy, chapter 3

## Related in other subjects

- [Define control system and closed loop block roles](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Adefine-cs-bd-role) — Control Systems, chapter 1, asked 5×
- [Closed loop control system block diagram explained](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Awhat-is-cs-bd-closed-loop) — Control Systems, chapter 1, asked 4×
- [Steps to design closed loop control system](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Aclosed-loop-design-steps) — Embedded System, chapter 7
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