Definitions

# Torque and speed of a DC motor

Torque of an electric motor is independent of speed. It is rather a function of flux and armature current.

$N = frac\left\{K\left(V-IR\right)\right\}\left\{varphi\right\}$

where:

• N = number of turns
• K = proportional constant
• R = resistance of armature (ohms)
• V = electromotive force (volts)
• I = current (amperes)
• Φ = flux (webers)

## Effects

Increase in flux decreases the speed but increases the torque. If torque is decreased by decreasing the field current the following sequences are found:

1. Back EMF drops instantly, the speed remaining constant because of the inertia of heavy armature.
2. Due to decrease of EMF armature current I is increased because of I = (V − E)/R''.
3. A small decrease of flux is more than counterbalanced by a large increase of I which means net increase of torque.
4. If torque increases the speed also increases.

If applied voltage is kept constant, motor speed has inverse relation with flux.

## Characteristics of DC motors

DC motors respond to load changes in different ways, depending on the arrangement of the windings.

### Shunt wound motor

A shunt wound motor has a high-resistance field winding connected in parallel with the armature. It responds to increased load by trying to maintain its speed and this leads to an increase in armature current. This makes it unsuitable for widely-varying loads, which may lead to overheating.

### Series wound motor

A series wound motor has a low-resistance field winding connected in series with the armature. It responds to increased load by slowing down and this reduces the armature current and minimises the risk of overheating. Series wound motors were widely used as traction motors in rail transport of every kind, but are being phased out in favor of AC induction motors supplied through solid state inverters.

### Permanent Magnet motor

A permanent magnet DC motor is characterized by its locked rotor (stall) torque and its no-load angular velocity (speed), as described in the link: http://lancet.mit.edu/motors/motors3.html

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