Motor selection characteristics; rheostatic and regenerative braking

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.

   +----[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.