Torque and speed of a DC motor

The torque and torque density of an electric motor is not necessarily dependent on its speed. It is rather a function of flux and armature current.

Contents

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.

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

where:

Characteristics of DC motors

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

Voltage in steady state condition

V = E_b %2B I_a R

where,

V-input voltage

Eb-back emf

Ia- armature current

R-total resistance

total resistance R is equal to armature resistance (Ra) & external resistance (Rph).

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 favour of AC induction motors supplied through solid state inverters. The counter-EMF aids the armature resistance to limit the current through the armature. When power is first applied to a motor, the armature does not rotate. At that instant, the counter-EMF is zero and the only factor limiting the armature current is the armature resistance. Usually the armature resistance of a motor is less than 1 Ω; therefore the current through the armature would be very large when the power is applied. Therefore the need arises for an additional resistance in series with the armature to limit the current until the motor rotation can build up the counter-EMF. As the motor rotation builds up, the resistance is gradually cut out.

The output speed torque characteristic is the most notable characteristic of series wound d.c. motors. The speed being almost entirely dependent on the torque required to drive the load. This suits large inertial loads as the speed will drop until the motor slowly starts to rotate & these motors have a very high stalling torque.

Permanent magnet motor

A permanent magnet DC motor is characterized by its locked rotor (stall) torque and its no-load angular velocity (speed).[1]

See also

References

  1. ^ "Understanding D.C. Motor Characteristics". MIT. http://lancet.mit.edu/motors/motors3.html. Retrieved 2008-12-11.