DC Series Motors: High Starting Torque but No Load Operation Ill-Advised

June 20, 2011  |  Articles

Direct current (D.C.) series motors get their name from the way their armature and field windings are connected together: in a series circuit. 1 This type of connection gives a D.C. series motor the following characteristics:

  • High Starting Torque
  • No Load Operation
  • Poor Speed Regulation


They are widely used for starting heavy, industrial, high torque loads such as cranes, hoists, elevators, trolleys and conveyors, but are also used for automobile starters. The primary disadvantage of D.C. series motors is they cannot operate safely in an unloaded condition.

Construction

The basic components of a D.C. series motor are the armature, field windings, brush assembly, frame, endbells and bearings. 2 The armature is the rotating component of the motor and is made of a steel shaft with notched laminations that the armature windings are wound on. On one end of the shaft is the commutator, which consists of copper segments insulated from each other. A brush assembly holds the electrically conductive, carbon graphite brushes, which slide on the commutator segments and provide a means to connect the D.C. power supply to the rotating commutator. The commutator and brushes function as an electro-mechanical switch that changes the diretion of current flow in the armature as it rotates. The field windings or magnet is a coil and laminated pole assembly powered by the same D.C power supply as the armature. To correct for armature reaction, interpoles are used to shift the neutral magnetic plane to eliminate brush arcing. 3 The motor frame is a circular, steel structure that mechanically supports the field poles. The endbells enclose all the components of the motor and are bolted onto the frame. Bearings are pressed into the endbells to provide free movement of the armature.

Operation

Motor action 4 governs the operation of a D.C. series motor and states that a current-carrying coil will generate a magnetic field and if this coil is placed in another magnetic field, a force or torque will be exerted on the coil. This torque will be proportional to both the current in the coil and the strength of the magnetic field it is placed within. The D.C. series motor’s armature (rotating component) is the previously mentioned current-carrying coil and the field winding (stationary component) of the motor is the other magnetic field. So, when the armature and field windings are energized by a D.C. power supply, current will flow through these windings and generate their respective magnetic fields and will be magnetically positioned in such a manner to cause torque. But this torque will only be sustained if the magnetic relationship of the armature and field are maintained. This is accomplished by the commutator, which switches the current flow and reverses the armature’s magnetic polarity every time the commutator segment passes through a brush, causing the armature to be attracted to the stationary field magnet, thus sustaining unidirectional torque or rotation. 5

Characteristics

To understand the proper application of D.C. series motors, one must understand the characteristics of torque, speed and armature current relative to load changes. In D.C. series motors, the entire armature current (Iarmature) passes through the series field windings so the magnetic flux (Φ) produced is proportional to armature current:

Flux Φ α Iarmature
The torque produced will be proportional to the product of flux and armature current:

Torque α (Flux Φ) Iarmature
Since Flux Φ α Iarmature and Torque α (Flux Φ) Iarmature, it follows that the Torque will be proportional the square of Iarmature. In other words, in formula form:

Torque α Iarmature2 6
This means that when a D.C. series motor is first started, very high torque will be produced because armature resistance is low, CEMF is zero and the total D.C. supply voltage will drive current through the armature. This armature current would be unimpeded, causing a very high torque. This characteristic makes D.C. series motors ideal for applications requiring high starting torque.

While torque is proportional to the square of the armature current, motor speed is inversely proportional to armature current. 7 Thus, the torque-speed characteristic of a D.C. series motor is:

Speed α 1/√T 8
This characteristic means that as the load on the motor increases, the armature current will increase and the torque will increase causing the motor speed to decrease. Hence, D.C. series motors have poor speed regulation because they are load dependent. It also means that as the load decreases, torque decreases and speed increases. At no load, the motor speed would ramp to an extremely high level that could ultimately destroy the motor. This runaway condition would prove to be a personal safety hazard as well.

Applications

D.C. series motors are ideal for large loads and industrial applications that require high starting torque. In addition, they have poor speed regulation that’s load dependent and exhibit an unstable runaway condition when unloaded. Hence, D.C. series motors should never be used where the loads are intermittent, change frequently, or frequently cycle on/off. For example, a water pump drive that runs constantly and requires only small adjustments to maintain the flow rate would be a good application for a series motor. Conversely, a pump that cycles frequently to maintain a tank water level wouldn’t.9


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