---
title: "Motor selection characteristics; rheostatic and regenerative braking"
subject: "Utilization of Electrical Energy"
chapter: 2
key: "semester_7/utilization_of_electrical_energy:motor-selection-characteristics-braking"
source: https://www.bctengineer.com/q/semester_7/utilization_of_electrical_energy/motor-selection-characteristics-braking
site: BCT Engineering
---

# Motor selection characteristics; rheostatic and regenerative braking

Utilization of Electrical Energy · Chapter 2: Electric Drive System

- Asked in: 2072 Kartik
- Video: [Types of electric braking- Regenerative braking,dynamic braking,plugging](https://www.youtube.com/watch?v=pKlc4uxJkNs) — Read Electric Vehicle
- 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/motor-selection-characteristics-braking
- Whole chapter: https://www.bctengineer.com/md/semester_7/utilization_of_electrical_energy/chapter-2

## Question

Write the electrical and mechanical characteristics to be considered for selection of motor. Explain Rheostatic braking and Regenerative braking.

## Short answer

Selection considers starting torque and current, speed-torque characteristics, speed control, braking, efficiency, supply, overload, enclosure, bearings, transmission, mounting, and cooling. Rheostatic braking disconnects the motor and dissipates energy in a resistor; regenerative braking returns energy to the supply when speed exceeds no-load speed.

## Answer

#### Characteristics considered in motor selection

**Electrical characteristics**

1. **Starting characteristics**: starting torque (high for cranes, traction, crushers; low for fans) and starting current (limited by supply capacity; starter type).
2. **Running (speed-torque) characteristics**: constant speed (shunt, synchronous, induction), variable speed (series), match with load characteristic for stable operation.
3. **Speed control**: range, smoothness, method (armature voltage, field, V/f, slip control) and efficiency over the range.
4. **Braking characteristics**: availability of plugging, dynamic and regenerative braking.
5. **Efficiency and power factor** at the expected load.
6. **Supply**: AC/DC, voltage, phases, frequency.
7. **Overload capacity** and duty cycle (S1 to S8).

**Mechanical characteristics**

1. **Type of enclosure**: open, drip-proof, TEFC, flameproof, according to environment.
2. **Bearings**: ball/roller/sleeve; axial and radial load.
3. **Transmission of drive**: direct coupling, belt, gear, chain.
4. **Mounting**: foot, flange, vertical/horizontal.
5. **Noise and vibration** levels.
6. **Cooling** method; **size and weight**; **cost** and maintenance.

#### Rheostatic (dynamic) braking

The motor is disconnected from the supply and connected across a **braking resistor**. Driven by the stored kinetic energy, it acts as a generator; the generated energy is dissipated as heat in the resistor, producing a braking torque.

- **DC shunt/separately excited motor**: armature disconnected and connected to resistor $R_B$; field kept excited.

$$
I_a = -\frac{E_b}{R_a + R_B}, \qquad T = K\phi I_a \;(\text{negative})
$$

- **DC series motor**: the field connections are reversed relative to the armature, so the generated current flows through the field in the same direction as before and builds up the flux (self-excited generator).
- **Induction motor**: stator disconnected from AC and fed with DC (**DC dynamic braking**); the rotor cuts the stationary field and the energy is dissipated in rotor resistance.

```text
   +----[Ra]---(Eb)----+
   |                   |
   +-------[ RB ]------+   (field excited separately)
```

Braking torque falls as speed falls (since $E_b \propto N$), so it cannot hold a load at standstill; mechanical brakes are used to stop finally. Simple and cheap, but energy is wasted.

#### Regenerative braking

The motor runs **faster than its no-load speed** (driven by the load, e.g. a lowering hoist or a train going downhill), so the back emf exceeds the supply voltage ($E_b > V$ for DC, or rotor speed above synchronous speed for an induction motor, slip negative). Current reverses, the machine acts as a generator and **returns energy to the supply**.

$$
I_a = \frac{V - E_b}{R_a} < 0 \quad \text{when } E_b > V
$$

- Used in electric traction, hoists, lifts, cranes, EVs.
- Advantages: energy saving, less brake wear.
- Requirements: supply must be able to accept energy (receptive line, or inverter/converter capable of reverse power flow); cannot bring the motor to rest by itself.

| Point | Rheostatic braking | Regenerative braking |
|---|---|---|
| Energy | Dissipated in resistor | Returned to supply |
| Supply | Disconnected | Connected |
| Speed | Any speed down to near zero | Only above no-load (synchronous) speed |
| Efficiency | Wasteful | Energy saving |

## Check yourself

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

1. Which load needs a motor with high starting torque?
   A. Centrifugal fan
   B. Crane or traction
   C. Light blower
   D. Idle pulley
   Answer: B. Cranes, traction and crushers need high starting torque; fans need low. (Key point: Starting torque)

2. In rheostatic braking, what happens to the motor?
   A. It stays on supply and returns energy to it
   B. It is disconnected from supply and connected across a braking resistor, acting as a generator
   C. It is fed reversed supply voltage
   D. It is stopped by a mechanical shoe
   Answer: B. The stored kinetic energy drives the machine as a generator and the energy is dissipated as heat in the resistor. (Key point: Rheostatic braking principle)

3. When does regenerative braking occur in a DC motor?
   A. Eb < V
   B. Eb = 0
   C. Eb > V, so armature current reverses
   D. Supply is disconnected
   Answer: C. The load drives the motor above no-load speed so Eb exceeds V, current reverses and energy returns to the supply. (Key point: Regenerative braking condition)

4. Which statement correctly compares the two braking methods?
   A. Rheostatic returns energy; regenerative wastes it
   B. Both work down to zero speed
   C. Rheostatic works down to near zero speed; regenerative only above no-load (synchronous) speed
   D. Regenerative keeps supply disconnected
   Answer: C. Rheostatic works at any speed down to near zero but wastes energy; regenerative needs speed above no-load speed and saves energy. (Key point: Rheostatic vs regenerative)

## Related in Utilization of Electrical Energy

- [DC shunt motor hoist: raising and rheostatic lowering](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Adc-shunt-hoist-rheostatic-braking-400kg) — Utilization of Electrical Energy, chapter 3
- [Induction motor plugging: braking current and torque](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Ainduction-motor-plugging-braking-current) — Utilization of Electrical Energy, chapter 3
- [Reverse current (plugging) braking of 20 kW DC shunt motor](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Adc-shunt-reverse-current-braking-20kw) — Utilization of Electrical Energy, chapter 3
- [Durations of acceleration, coasting and braking (2 km run)](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Aquadrilateral-curve-2km-40kmph-periods) — Utilization of Electrical Energy, chapter 4, asked 2×
- [Electric braking in AC and DC traction drives](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Aelectric-braking-traction-drives) — Utilization of Electrical Energy, chapter 4
- [Maximum speed and distance before braking (2.5 km)](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Amax-speed-50kmph-2-5km-trapezoidal) — Utilization of Electrical Energy, chapter 4

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