AC Drives for HVAC Fan and Pump ApplicationsAugust 22, 2012
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Armature Controlled vs. Field Controlled DC Motor
DC drives offer the ability to control the speed and torque of heavy-duty DC motors in various industrial and other similar applications. Speed control can be achieved using DC drives in a number of ways. Voltage can be applied to the terminals of the DC motor or external resistance can be applied in the armature.
Another method is to vary the flux per pole of the motor. The first two methods involve adjusting the motor’s armature while the latter method involves adjusting the motor field. These methods are referred to as “armature control” and “field control.”
What Is a DC Motor?
A DC — direct current — motor is a device that takes electrical energy and converts it into mechanical energy. They do this through a conductor material that carries current internally and transmits to coiled wires called windings. The surging current creates magnetic fields that interact with the magnets on the rotor.
If the magnets and the field attract, the motor rotates one way. Conversely, two fields that repel force the motor to spin in the opposite direction. The commutator — an internal component — supplies consistent current to the windings to continue generating the magnetic fields and rotate the motor.
DC motors are standard in industrial equipment, thanks to two unique characteristics. These motors can start, reverse or stop on demand, which is essential to production. They also support speed control, another necessity for precision machine performance.
What Is DC Motor Speed Control?
Many applications require a DC motor's speed to adjust, maximizing machine function and performance. Doing so intentionally and as necessary requires speed control. Operators can do this manually or rely on automated technology devices. Speed control of a DC motor differs from speed regulation, which is keeping a continuous speed despite load variances.
Speed controllers come in two primary forms — armature controls and field controls. Changes in the terminal voltage or external resistance impact function as armature controls. Conversely, changing the magnetic flux is a method of field control.
DC Motor Working Principle
DC motors work on the principles of several laws of electricity. Faraday's law on electromagnetism states a conductor carrying current undergoes mechanical force when meeting a magnetic field. Fleming's "left-hand rule" says the conductor's motion is always perpendicular to the magnetic field and the current.
Under Lenz's law, the resulting electromagnetic field (EMF) resists voltage, creating a phenomenon called back EMF. This back EMF lends DC motors a unique ability to balance torque across different loads.
Armature Control for DC Motors
With armature control the voltage is varied using several methods. One way is by implementing armature resistance, which involves connecting a variable resistance in series to the circuit of the armature. Once resistance has been increased, the current flow through the circuit is reduced and the armature voltage drop is less than the line voltage. This in turn reduces the motor speed in proportion to the voltage that’s being applied. The armature resistance control method is used in applications that require motor speed variation for shorter periods of time, not continuously. Other methods of armature control are armature voltage control and shunt resistance control.
How Is DC Motor Speed Calculated?
To determine the speed of a DC motor, you need the net voltage — the supply voltage plus the back EMF. From that number, subtract the armature current multiplied by the armature resistance. Divide that result by the magnetic flux per pole to find the speed of a DC motor.
Advantages of Armature Controlled DC Motors
Armature control is a closed loop system, while field control is an open loop system. Closed loop systems are often the preferred choice for operators and business leaders who are looking for stability and the convenience of an automated process. Armature controlled motors offer nearly unmatched accuracy and control, along with a wide speed variation range.
Additional advantages of armature controlled DC motors include:
- Constant field current and torque: With the armature control method, the field current and torque levels remain constant throughout the application. Regardless of the speed of the motor, you can rely on these factors.
- Fast and simple speed variation: Armature controlled DC motors are known for their exceptional speed control, which allows operators to vary the speed as necessary in both directions.
Disadvantages of Armature Controlled DC Motors
While armature controlled motors are fast and ideal for short, fixed processes, they have a few disadvantages to consider when comparing armature control vs. field control:
- Higher initial costs: The armature control method is often more expensive than the field control method.
- Low energy efficiency: One reason why armature control is most commonly used for shorter lengths of time is that speed variation tends to waste large amounts of power. This power loss makes the process less energy efficient and more costly overall.
Field Control Method
When using the field control method for DC motors, the field is weakened to increase the speed or it can be strengthened to reduce the motor’s speed. Attaining speeds that are above the rated speed can be achieved by providing variable resistance in series to the field circuit, varying the reluctance of the magnetic circuit, or by varying the applied voltage of the motor to the field circuit (with constant voltage being supplied to the armature circuit).
Advantages of Field Controlled DC Motors
As an open loop system, the field control method is ideal for operators who need cost-effectiveness, smooth performance and steady output. Field controlled DC motors are more common for longer processes due to their reliability and convenience. Unlike armature controlled motors, field control provides speeds that are above the normal range.
The main advantages of this method include:
- Lower costs: The field control method is a highly economical form of motor control. It’s easy to use and manage, and the lower operational costs make it cost-effective in the long-term. For manufacturers or engineers on a tight budget, this is an ideal solution.
- Minimal power loss: The speed of a field controlled DC motor is varied through the magnetic field rather than the armature. As a result, this method typically wastes a smaller amount of power. Extra energy efficiency can save both time and money while helping the environment.
Disadvantages of Field Controlled DC Motors
Field controlled DC motors are easy and hassle-free, which makes them a popular choice for motor operators and manufacturers. On the other hand, there are certain cases when a different motor control method may be more effective. Disadvantages of field control include:
- Limits on speed: If your application requires you to adjust the motor below the normal speed, you may be better off choosing an armature controlled method. Field controlled DC motors can only operate above the normal speed. Higher speeds can also result in less torque.
- Reduced stability: The field control method allows operators to obtain higher speeds than the norm. Yet its overall range can be lowered due to a lack of stability. With a weaker field, you may only be able to safely exceed certain speeds.
Compare several DC drives to find one that will provide efficient, cost-effective control for your DC motor application. DC drives are built with specific functions and features to meet a variety of needs. DC drives may be regenerative or non-regenerative, with varying horsepower, mounting designs, and more.
Control Systems Application Guide
Other Types of Speed Control
There are various speed controls for motors depending on the motor configuration. Some typical ones for series motors include:
- Armature resistance control method: This approach requires controlling the resistance in connection to the supply. It's most common to lower speed under light loads.
- Shunted armature control: You combine a rheostat, which varies the voltage supply, under this technique.
- Armature terminal voltage control: This method relies on a separate voltage supply with variable input.
- Field diverter method: With this approach, you reduce the field flux through shunting around the series to lower resistance and increase speed. As a result, you get a higher than normal speed that increases as the load lessens.
- Tapped field control: You raise speed by reducing the number of field winding turns with outside tapping.
Methods for controlling speed in shunted motors are similar. To control via the armature, you can use either resistance or voltage controls. Under a field control approach, most industries rely on rheostats to reduce resistance outside of load.
Contact the Carotron Team for Assistance Today
Have a question or not sure what you need? We can help! Contact a customer representative or engineer at Carotron, Inc. at 1-888-286-8614, and let us review your application and offer the right components to do the job. You can also contact us online using our form.