Torque and speed of a DC motor
Encyclopedia
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.
If applied voltage is kept constant, motor speed has inverse relation with flux.
where:
respond to load changes in different ways, depending on the arrangement of the windings
.
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).
s in rail transport
of every kind, but are being phased out in favour of AC induction motor
s supplied through solid state
inverter
s. 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.
Torque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
and torque density of an electric motor
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
is not necessarily dependent on its speed. It is rather a function of flux
Magnetic flux
Magnetic flux , is a measure of the amount of magnetic B field passing through a given surface . The SI unit of magnetic flux is the weber...
and armature
Armature (electrical engineering)
In electrical engineering, an armature generally refers to one of the two principal electrical components of an electromechanical machine–generally in a motor or generator, but it may also mean the pole piece of a permanent magnet or electromagnet, or the moving iron part of a solenoid or relay....
current.
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:- Back EMFCounter-electromotive forceThe counter-electromotive force also known as back electromotive force is the voltage, or electromotive force, that pushes against the current which induces it. CEMF is caused by a changing electromagnetic field. It is the effect of Lenz's Law of electromagnetism...
drops instantly, the speed remaining constant because of the inertia of heavy armature. - Due to decrease of EMF armature current I is increased because of I = (V − E)/R.
- A small decrease of flux is more than counterbalanced by a large increase of I which means net increase of torque.
- If torque increases the speed also increases.
If applied voltage is kept constant, motor speed has inverse relation with flux.
where:
- N = revolutions per minute (RPM) ,i.e. motor speed
- K = proportional constant
- R = resistance of armature (ohmOhmThe ohm is the SI unit of electrical resistance, named after German physicist Georg Simon Ohm.- Definition :The ohm is defined as a resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere,...
s) - V = electromotive force (voltVoltThe volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...
s) - I = current (ampereAmpereThe ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...
s) - Φ = flux (weberWeber (unit)In physics, the weber is the SI unit of magnetic flux. A flux density of one Wb/m2 is one tesla.The weber is named for the German physicist Wilhelm Eduard Weber .- Definition :...
s)
Characteristics of DC motors
DC motorsBrushed DC electric motor
A brushed DC motor is an internally commutated electric motor designed to be run from a direct current power source.-Simple two-pole DC motor:The following graphics illustrate a simple, two-pole, brushed, DC motor.DC Motor Rotation...
respond to load changes in different ways, depending on the arrangement of the windings
Series and parallel circuits
Components of an electrical circuit or electronic circuit can be connected in many different ways. The two simplest of these are called series and parallel and occur very frequently. Components connected in series are connected along a single path, so the same current flows through all of the...
.
Voltage in steady state condition
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 motorTraction motor
Traction motor refers to an electric motor providing the primary rotational torque of a machine, usually for conversion into linear motion ....
s in rail transport
Rail transport
Rail transport is a means of conveyance of passengers and goods by way of wheeled vehicles running on rail tracks. In contrast to road transport, where vehicles merely run on a prepared surface, rail vehicles are also directionally guided by the tracks they run on...
of every kind, but are being phased out in favour of AC induction motor
Induction motor
An induction or asynchronous motor is a type of AC motor where power is supplied to the rotor by means of electromagnetic induction. These motors are widely used in industrial drives, particularly polyphase induction motors, because they are robust and have no brushes...
s supplied through solid state
Solid state (electronics)
Solid-state electronics are those circuits or devices built entirely from solid materials and in which the electrons, or other charge carriers, are confined entirely within the solid material...
inverter
Inverter (electrical)
An inverter is an electrical device that converts direct current to alternating current ; the converted AC can be at any required voltage and frequency with the use of appropriate transformers, switching, and control circuits....
s. 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).See also
- Alternating currentAlternating currentIn alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
- Brushed DC electric motorBrushed DC electric motorA brushed DC motor is an internally commutated electric motor designed to be run from a direct current power source.-Simple two-pole DC motor:The following graphics illustrate a simple, two-pole, brushed, DC motor.DC Motor Rotation...
- Brushless DC electric motorBrushless DC electric motorBrushless DC motors also known as electronically commutated motors are electric motors powered by direct-current electricity and having electronic commutation systems, rather than mechanical commutators and brushes...
- Equations of a DC Motor under loading