Traction Motors

This article is a discussion paper on the various types of electric motors, and their characteristics, with a focus on the types suitable for traction motor use in models. Only DC motors are considered in this discussion paper. AC motors, while very common in industry and home appliances, are not really suited to traction use in models, are not easily controlled, and are not considered.

There are 2 main types of DC motors:- Wound field, and Permanent Magnet field. There are also other types of DC motors such as stepper motors, brushless motors, etc which usually require complex electronic controls, and are used for specialised purposes, and are beyond the scope of most model engineers and this article.

Every motor has a magnetic field, generated by electric wire coils or by permanent magnets, and an armature (or rotor) which rotates within that field. Just as 2 like poles of a magnet repel each other, and opposite poles attract, in a motor exactly the same thing happens but it is arranged around a circle, and the armature field is rapidly reversed as required by the commutator as the armature rotates.

The field in DC motors is produced 2 ways:

Wound Field: The magnetic field is produced by current flowing through a coil of wire, and
Permanent Magnet Field: The magnetic field is produced by permanent magnets.

 

Wound Field Motors

In wound field motors, the field coil can be connected 2 ways, in SERIES with the armature (Series wound) or in PARALLEL with the armature (Shunt wound). Each type of connection produces very different torque/speed curves. A third type has 2 field windings, one in series and one in parallel (Compound wound), and has some of the characteristics of series and shunt motors. Often, there are physically 2 field coils, one on either side of the motor to ensure manufacturability and an even field.

Series Motors

In a series wound motor, the current flows through the field and the armature. The low-resistance field winding is relatively few turns of heavy wire, as the higher armature current must flow though it. The main characteristics of series winding are:

  • Speed varies automatically with the load, increasing as the load decreases.
  • VERY high starting torque, making it ideal for traction use.
  • Best used where a heavy power demand is necessary to bring the machine up to speed.
  • There is no inherent speed limiting.

Series motors offer very high starting torque and good torque output per ampere input, and respond to increased load by slowing down thus reducing the armature current and minimising the risk of overheating. Series wound motors are widely used as traction motors in rail transport of every kind (and have been for over a century). The high starting torque combined with theoretically unlimited top speed make them ideal for model traction use. If series wound motors can be obtained, they are the best choice.

As there is no inherent speed limiting, the speed can theoretically increase, and continue to increase without bound. Series-wound motors should never be run without a load due to the risk of over-speed, and may ultimately cause self-destruction of the motor. Just be aware of this in no load bench testing, or do rolling testing on rails.

If the power source is reversed, the current is reversed in both the field AND the armature, and the direction of rotation is unchanged. To reverse a series motor, power to either the field OR the armature is reversed but not both!

Shunt Motors

In a shunt wound motor, the current flow is split and some flows through the field winding to create the magnetic field, while the majority flows through the armature. The field winding is many turns of finer wire. The main characteristics of shunt winding are:

  • Runs practically constant speed for any setting of the controller, regardless of the load
  • Reasonably constant torque over whole speed range
  • Not suited for widely varying loads

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 not suited for widely varying loads, which may lead to overheating.

This type of motor runs practically constant speed for any setting of the controller, regardless of the load. It is the type generally used in commercial practice for fans, pumps etc, and is usually recommended where starting conditions are not severe. Speed of the shunt-wound motors may be regulated by inserting resistance in series with the armature, thus decreasing speed. Shunt motors are not really suited, and not recommended, for traction use as they are effectively constant speed.

If the power source is reversed, the current is reversed in both the field AND the armature, and the direction of rotation is unchanged. To reverse a shunt wound motor, either the field OR the armature is reversed, but commonly only the field is reversed, as the currents involved are much lower.

Another option is a separately excited field winding of a shunt motor, where the field coil only is supplied from the full supply voltage all the time, and the armature ONLY is supplied from the variable voltage from the controller. The characteristics are now very similar to a Permanent Magnet Motor (see below), and much more useful for traction use. The main advantages of a separately excited shunt winding are better speed control, and more usable speed/torque characteristics. A shunt motor can be converted to a separately excited field by providing separate field connections.

 

Compound Motors

In a compound wound motor, both shunt and series fields are used. The main characteristics of compound winding are:

  • Characteristics are a combination of both series and shunt wound
  • Higher starting torque (than shunt)
  • Poorer speed regulation (than shunt)
  • Speed limiting is inherent in design

When comparing the advantages of the series and shunt motors, the series motor has greater torque capabilities, while the shunt motor has more constant speed over various loads. These two characteristics can be found in the same motor by having both a series field and a shunt field winding in the same motor. Thus, we have the compound motor, where a combination of the shunt wound and series wound types combines the characteristics of both. Characteristics may be varied by varying the field produced by each of the two windings. These motors are generally used where there are heavier starting conditions and relatively constant speed is required at the same time. This type offers a combination of good starting torque and speed stability. Standard compounding is about 12% (i.e. the series field provides 12% of the total field). Heavier compounding of up to 40 to 50% can be used for special high starting torque applications at the expense of poorer speed regulation, but often cannot be achieved with an existing motor.

This type of motor runs practically constant speed, regardless of the load. It is the type generally used in commercial practice and is usually recommended where starting conditions are not usually severe. Speed of the compound-wound motors may be regulated by inserting resistance in series with the armature, thus decreasing speed. Again, as this is still predominately a shunt-wound motor, it is not well suited to traction use.

If the power source is reversed, the current is reversed in both the fields AND the armature, and the direction of rotation is unchanged. To reverse a compound wound motor, either both fields OR the armature is reversed, but usually only the armature is reversed due to the lower switching complexity.

Permanent Magnet Motors

In a permanent magnet motor, there are no wound field coils, the field is produced by permanent magnets. They do have a conventional wound armature with commutator and brushes, and current flows the armature. The main characteristics of a permanent magnet motor are:

  • Excellent starting torque
  • Motor losses are less with better operating efficiencies (No field current)
  • Can be dynamically braked, or even reversed while going forward at low armature voltage (10%) to provide braking (called "plugging")
  • Good relationship between supply voltage and speed
  • Speed regulation not as good as compound motors (not a problem for traction use)
  • Maximum speed is inherently limited

If the power source is reversed, the current is reversed in the armature only (as the field is fixed), and the direction of rotation is reversed.

The issue of speed regulation is not an issue for model traction use. The speed is varying all the time anyway in response to changing loads, track gradients and commands of the driver. Methods to control the speed and direction of motors can be found on the Control of Model Electric Locomotives page.

The type of motor of choice for model traction use is the series wound motor, just like full size practice. The high starting torque and non-limited top speed make them the best choice. The main problem here is that series motors of a suitable size and voltage are not readily available. Obtaining a shunt or compound wound DC motor and rewinding the field coils for series use is a practical option, and has been used with success. Car generators (NOT alternators) have wound fields and can be obtained from wreckers, the fields rewound to series configuration with heavier wire and used as traction motors.

The next best choice is permanent magnet motors, and is a common choice. Separately excited shunt motors work equally as well. Permanent magnet motors of suitable size and voltage rating are more readily obtained, and give satisfactory service.

The use of shunt wound or compound motors is not the best choice for model traction use, and are not recommended.

Modern full size practice in recent times has been to use AC variable-frequency, variable-phase motors, with sophisticated computer-controlled speed controllers. This has only been possible due to advances in power semiconductors and the use of microcomputers. The use of series motors in full size is declining, but their desirable characteristics remain. AC traction is not practical at this stage for models due to availability and the requirement of very sophisticated controllers required.