# DC Motor Operation

By on March 28th, 2012

Coils of wire on the rotor carry a d.c. current which generates a magnetic field. A stator magnetic field is created using either permanent magnets or electrical field windings.

Any current-carrying conductor placed within an external magnetic field experiences a force. Torque is produced in the motor by this mechanism.

As a d.c current provides a constant magnetic field, this needs to change each half revolution and is called commutation. Commutation is typically accomplished by metal brushes against the contacts. Where a field winding is used to generate the stator magnetic field, different motor characteristics can be obtained depending on how this winding is connected.

## Operation Principals

For the equivalent circuit the parameters are given by:

Where:

- lap winding

-       wave winding

given that

• n = number of conductor
• p = number of pole pairs
• ω = motor speed, rad s-1
• φ = flux per pole, Wb
• T = torque Nm
and

The power input to armature is given by:

The armature power loss is given by:

The output power is given by:

The torque is given by:

Typically, IaRa is ±3% of Va and the power dissipated in field winding ±3.5% of the motor rating.

## Field Windings

Permanent magnets or field windings of a dc motor provide the necessary magnetic fields for the motor to operate(often call excitation):

• permanent magnets - provides a continuous and constant magnetic field by the use of permanently magnetized materials (typically on only the smallest motors)
• separately excited - separate windings and power supply provide the magnetic field for the motor
• shunt winding - this is connected in parallel with the motor armature to provide the excitation
• series winding - this is connected in series with the motor armature to provide the excitation
• compound winding - is a hybrid between shunt and series where to field windings are connected both in parallel and in series with the armature

## Speed Control of DC Motors

Separately excited, shunt

• Constant power drive - Ia,Va is held constant and If varied
• Constant torque If is held constant and Ia, Va varied

## DC Converter output Voltages

Semi converter,                [DC converter equations do not look correct.  Please can someone check.]

Full converter, - with infinite inductive filtering

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Steven has over twenty five years experience working on some of the largest construction projects. He has a deep technical understanding of electrical engineering and is keen to share this knowledge. About the author

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