AC Drives for HVAC Fan and Pump Applications
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September 18, 2012
Armature-Controlled vs. Field-Controlled DC Motor
Direct current (DC) drives offer the ability to control the speed and torque of heavy-duty DC motors in various industrial applications.
DC drive motor speed control can be achieved by applying voltage to the terminals of the DC motor or by external resistance in the armature. You can also achieve speed control by varying the flux per pole of the motor. While the first two methods involve adjusting the motor’s armature, changing the flux adjusts the motor field. These methods of direct current motor speed control are referred to as “armature control” and “field control.”
Explore about how to control the speed of a DC motor below.
What Is a DC Motor?
A DC motor is a device that takes electrical energy and converts it into mechanical energy. It does this through a conductor material that carries current internally and transmits it 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, reverses current in the armature coils each half-turn to keep the torque unidirectional.
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.
Understanding DC Motor Armature
The DC motor armature carries electric current from the DC supply/drive through conductors embedded in armature slots. This produces torque, making the armature spin.
Parts of the armature of a DC machine include:
- A stack of thin steel laminations forms the core.
- Thin loops of wire through grooves in the core.
- A copper drum/commutator.
- Brushes that supply current to the spinning commutator/armature windings.
- The shaft, a metal stick that holds everything in place.
- A small fan to keep everything cool.
The armature of a DC motor turns electricity into movement by sending electricity from the DC supply/drive through the armature wires. The magnetic field exerts force on the current‑carrying conductors, producing torque that turns the shaft. The copper drum/commutator keeps the motion smooth and going in the same direction, creating a steady spin.
How the Armature-Controlled Method Works
There are several different techniques within armature control. You can vary the resistance or voltage in the armature circuit. The resistance control method is used in applications that require motor speed variation for short periods of time.
Implementing armature resistance 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 reduces the motor speed in proportion to the voltage that’s being applied.
To vary the voltage instead, you change the voltage while keeping the field flux roughly constant. This involves using a variable DC source and creates a high starting, low-speed torque that is easy to regulate. This method is efficient and precise, but it requires power electronics.
Advantages of Armature-Controlled DC Motors
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: With the armature control method, the field current remains 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, they have a few disadvantages when compared to field-controlled motors. Potential cons include:
- Higher initial costs: Armature control 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.
How the Field Control Method Works
In addition to adjusting the armature, you can also control drive speed with field control. To use this method, you can weaken the field to increase the motor's speed. Alternatively, strengthening the field will reduce speed. Additional field control options include varying the reluctance of the magnetic circuit or 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
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. 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.
- 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.
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. However, 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.
Options for DC Series Motor Control
There are various speed controls for motors depending on the motor configuration. Some options for DC series motor controllers include:
- 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 by 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.
Selecting the Right DC Drive for Speed Control
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. They may be regenerative or non-regenerative, with varying horsepower and mounting designs.
Factors to consider when choosing the right DC drive include:
- Defining the job: Understand your load type, speed range and duty cycle.
- Checking the motor: Match the drive and motor for size and electrical capacity.
- Choosing speed regulation: Pick between open-loop and closed-loop systems.
You should also consider the cost and life cycle of the drive. Some drives are more cost-effective than others, but the best option for you depends on your unique equipment and needs. Remember to consider the total cost of ownership, including maintenance expenses.
Contact the Carotron Team for Assistance Today
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