Everything you need to know about DC motors

While all electric utility systems deliver alternating current (AC), direct current (DC) motors are still used in a variety of industrial and marine applications where high torque and variable speed are required. DC motors are ideally suited for these applications because they can be easily controlled to provide a wide range of speeds and torques. DC motors are also very efficient, which can save energy and reduce operating costs.

Here are some of the common applications of DC motors:

• Mine hoists
• Steel rolling mills
• Ship propulsion
• Cranes
• Conveyors
• Elevators

Here is a simplified explanation of why DC motors are ideal for applications where high torque and variable speed are required:

• High torque: DC motors can produce high torque at low speeds, which makes them ideal for applications where a lot of power is needed to start or move a load.
• Variable speed: DC motors can be easily controlled to provide a wide range of speeds, which makes them ideal for applications where the speed of the motor needs to be adjusted to match the load requirements.

DC motors are more complex and expensive to build than AC motors, primarily due to the commutator, brushes, and armature windings. DC motors also require more maintenance than AC motors, particularly the brush/commutator assembly. In contrast, AC induction motors do not have a commutator or brushes, and most use cast squirrel-cage rotor bars instead of wound copper wire windings. DC motors are classified according to their field type, which can be permanent magnet, series, shunt, or compound.

The three most important parameters for predicting DC motor performance are speed, torque, and horsepower.

• Speed is the rotational speed of the motor’s shaft and is measured in revolutions per minute (rpm).
• Torque is the turning force supplied by the motor’s shaft and is measured in pound-inches (lb-in) or pound-feet (lb-ft).
• Horsepower is the rate at which work is done and is measured in watts. One horsepower is equivalent to 746 watts.

DC motors are well-suited for applications where high torque and variable speed are required, such as mine hoists, steel rolling mills, ship propulsion, cranes, conveyors, and elevators.

• More complex and expensive to build than AC motors
• Require more maintenance, particularly the brush/commutator assembly
• Classified according to field type: permanent magnet, series, shunt, or compound
• Well-suited for applications where high torque and variable speed are required

Permanent-Magnet DC Motor

Permanent magnet DC motors use permanent magnets to create the magnetic field in the motor and electromagnets to control the rotation of the armature. The armature is a rotating cylinder that has coils of wire wrapped around it. When current flows through the coils, they create a magnetic field. The magnetic field of the armature interacts with the magnetic field of the permanent magnets to cause the armature to rotate.

The commutator is a device that switches the direction of current flow through the armature coils. This allows the armature to continue rotating in the same direction, even as the magnetic field of the permanent magnets changes.

Here is a simplified explanation of how a permanent magnet DC motor works:

1. Current flows through the armature coil, creating a magnetic field.
2. The magnetic field of the armature interacts with the magnetic field of the permanent magnets, causing the armature to rotate.
3. The commutator switches the direction of current flow through the armature coils, allowing the armature to continue rotating in the same direction.
4. Steps 2 and 3 repeat until the motor is turned off.

The motor is composed of two primary components: a housing that contains the field magnets and an armature that consists of wire coils wound in slots in an iron core and connected to a commutator. Brushes, which are in contact with the commutator, transfer current to the coils. Permanent magnet (PM) motors generate greater torque than wound-field motors. However, PM motors have limited load-handling capacity and are therefore primarily used for low-horsepower applications.

The force that rotates the motor armature is the result of the interaction between two magnetic fields (the stator field and the armature field). To produce a constant torque from the motor, these two fields must remain constant in magnitude and in relative orientation. This is achieved by constructing the armature as a series of small sections connected to the segments of a commutator, as illustrated in Figure 5-15.

• Electrical connection is made to the commutator by means of two brushes.
• It can be seen that if the armature rotates through one-sixth of a revolution clockwise, the current in coils 3 and 6 will have changed direction.
• As successive commutator segments pass the brushes, the current in the coils connected to those segments changes direction.
• The commutator can be regarded as a switch that maintains the proper direction of current in the armature coils to produce constant unidirectional torque.

The direction of rotation of a permanent magnet (PM) DC motor can be controlled by reversing the polarity of the voltage applied to the armature. This is because the armature is an electromagnet, and the direction of its magnetic field depends on the direction of current flow. The speed of a PM DC motor can be controlled by varying the voltage applied to the armature. The higher the voltage, the faster the motor will run. This is because a higher voltage will cause more current to flow through the armature coils, which will create a stronger magnetic field. The stronger magnetic field will interact more strongly with the magnetic field of the permanent magnets, causing the armature to rotate faster

To control the direction of rotation:

1. Apply a voltage to the armature.
2. If the motor is not rotating in the desired direction, reverse the polarity of the voltage.

To control the speed of rotation:

1. Apply a voltage to the armature.
2. Increase the voltage to increase the speed of rotation.
3. Decrease the voltage to decrease the speed of rotation.

Series DC Motor

Wound-field DC motors are usually classified as series-wound, shunt-wound, or compound-wound. The connection for a series-type DC motor is illustrated in Figure 5-17. A series-wound DC motor consists of a series field winding (identified by the symbols Sl and S2) connected in series with the armature (identified by the symbols A1 and A2). Since the series field winding is connected in series with the armature, it will carry the same amount of current that passes through the armature. For this reason the windings of the series field are made from heavy-gauge wire that is large enough to carry the full motor load current. Because of the large diameter of the series winding, the winding will have only a few turns of wire and a very low resistance value.

A series-wound DC motor has a low resistance field and low resistance armature circuit. Because of this, when voltage is first applied to it, the current is high (I = E/R). The advantage of high current is that the magnetic fields inside the motor are strong, producing high torque (turning force), so it is ideal for starting very heavy mechanical loads.

Figure 5-18 shows the speed–torque characteristic curves for a series DC motor. Note that the speed varies widely between no load and rated load. Therefore, these motors cannot be used where a constant speed is required with variable loads. Also the motor runs fast with a light load (low current) and runs substantially slower as the motor load increases. Because of their ability to start very heavy loads, series motors are often used in cranes, hoists, and elevators, which can draw thousands of amperes on starting. Caution: The no-load speed of a series motor can increase to the point of damaging the motor. For this reason, it should never be operated without a load of some type coupled to it.

Shunt DC Motor

A shunt-wound DC motor consists of a shunt field (identified by the symbols F1 and F2) connected in parallel with the armature. This motor is called a shunt motor because the field is in parallel to, or “shunts,” the armature. The shunt field winding is made up of many turns of small-gauge wire and has a much higher resistance and lower current flow compared to a series field winding.

Since the field winding is connected directly across the power supply, the current through the field is constant. The field current does not vary with motor speed, as in the series motor and, therefore, the torque of the shunt motor will vary only with the current through the armature.

• When the motor is starting and speed is very low, the motor has very little torque.
• After the motor reaches full rpm, its torque is at its fullest potential.
• One of the main advantages of a shunt motor is its constant speed. It runs almost as fast fully loaded as it does with no load.
• Unlike series motor, the shunt motor will not accelerate to a high speed when no load is coupled to it.
• Shunt motors are particularly suitable for applications such as conveyors, where constant speed is desired and high starting torque is not needed.

A separately excited DC motor has the armature and field coils fed from separate supply sources. This type of motor has a field coil similar to that of a self-excited shunt motor. In the most common configuration, armature voltage control is used in conjunction with a constant or variable voltage field excitation. An advantage to separately exciting the field is that the variable-speed DC drive can be used to provide independent control of the field and armature.

Compound DC Motor

A compound-wound DC motor is a combination of the shunt-wound and series-wound types. This type of DC motor has two field windings, as shown in Figure 5-22. One is a shunt field connected in parallel with the arma-ture; the other is a series field that is connected in series with the armature. The shunt field gives this type of motor the constant-speed advantage of a regular shunt motor. The series field gives it the advantage of being able to develop a large torque when the motor is started under a heavy load. This motor is normally connected cumulative-compound so that under load the series field flux and shunt field act in the same direction to strengthen the total field flux.

Figure below shows a comparison of speed–torque characteristic curves for a cumulative-compound DC motor versus series and shunt types.

• The speed of the compound motor varies a little more than that of shunt motors, but not as much as that of series motors.
• Compound-type DC motors have a fairly large starting torque much more than shunt motors, but less than series motors.
• The shunt winding can be wired as a cumulative long-shunt or as a short-shunt compound motor.
• For short-shunt, the shunt field is connected in parallel with only the armature, whereas with long-shunt, the shunt field is connected in parallel with both the series field and the armature.
• There is very little difference in the operating characteristics of long-shunt and short-shunt compound motors.
• These motors are generally used where severe starting conditions are met and constant speed is required at the same time.

Direction of Rotation

The direction of rotation of a wound DC motor depends on the direction of the field and the direction of the current flow through the armature. If either the direction of the field current or the direction of the current flow through the armature of a wound DC motor is reversed, the rotation of the motor will reverse. If both of these two factors are reversed at the same time, however, the motor will continue rotating in the same direction.

For a series-wound DC motor, changing the polarity of either the armature or series field winding changes the direction of rotation. If you simply changed the polarity of the applied voltage, you would be changing the polarity of both series field and armature windings and the motor’s rotation would remain the same.

As in a DC series motor, the direction of rotation of a DC shunt and compound motor can be reversed by changing the polarity of either the armature winding or the field winding. The industry standard is to reverse the current through the armature while maintaining the current through the shunt and series field in the same direction. For the compound-wound motor, this ensures a cumulative connection (both fields aiding) for either direction of rotation.

Motor Counter Electromotive Force (CEMF)

As the armature rotates in a DC motor, the armature coils cut the magnetic field of the stator and induce a voltage, or electromotive force (EMF), in these coils. This occurs in a motor as a by-product of motor rotation and is sometimes referred to as the generator action of a motor. Because this induced voltage opposes the applied terminal voltage, it is called counter electromotive force, or CEMF.

The overall effect of the CEMF is that this voltage will be subtracted from the terminal voltage of the motor so that the armature motor winding will see a smaller voltage potential. Counter EMF is equal to the applied voltage minus the armature circuit IARA drop. The armature current, according to Ohm’s law, is equal to:

IA = (VMTR − CEMF) / RA

where:

• IA = armature current
• VMTR = motor terminal voltage
• CEMF = counter electromotive force
• RA = armature-circuit resistance

Counter EMF is directly proportional to the speed of the armature and the field strength. That is, the counter EMF increases or decreases if the speed is increased or decreased, respectively. The same is true if the field strength is increased or decreased. At the moment a motor starts, the armature is not rotating, so there is no CEMF generated in the armature. Full line voltage is applied across the armature, and it draws a relatively large amount of current. At this point, the only factor limiting current through the armature is the relatively low resistance of the windings. As the motor picks up speed, a counter electromotive force is generated in the armature, which opposes the applied terminal voltage and quickly reduces the amount of armature current.

Armature Reaction

The magnetic field produced by current flow through the armature conductors distorts and weakens the flux coming from the main field poles. This distortion and field weakening of the stator field of the motor are known as armature reaction. As segment after segment of the rotating commutator pass under a brush, the brush short-circuits coil after coil in the armature. Note that armature coils A and B are positioned relative to the brushes so that at the instant each is short-circuited, it is moving parallel to the main field so that there is no voltage induced in them at this point. When operating under loaded conditions, due to armature reaction, the neutral plane is shifted backward, opposing the direction of rotation. As a result armature reaction affects the motor operation by:

• Shifting the neutral plane in a direction opposite to the direction of rotation of the armature.
• Reducing motor torque as a result of the weakening of the magnetic field.
• Arcing at the brushes due to short-circuiting of the voltage being induced in the coils undergoing commutation

When the load on the motor fluctuates, the neutral plane shifts back and forth between no-load and full-load positions. For small DC motors, the brushes are set in an intermediate position to produce acceptable commutation at all loads. In larger DC motors, interpoles (also called commutating poles) are placed between the main field poles, to minimize the effects of armature reaction. These narrow poles have a few turns of larger-gauge wire connected in series with the armature. The strength of the interpole field varies with the armature current. The magnetic field generated by the interpoles is designed to be equal to and opposite that produced by the armature reaction for all values of load current and improves commutation.

Speed Regulation

Motor speed regulation is a measure of a motor’s ability to maintain its speed from no load to full load with-out a change in the applied voltage to the armature or fields. A motor has good speed regulation if the change between the no-load speed and full-load speed is small, with other conditions being constant. As an example, if the speed regulation is 3 percent for a motor rated 1500 rpm with no load applied, then this means that the speed will drop by as much as 45 rpm (1500 × 3%) with the motor fully loaded. The speed regulation of a direct current motor is proportional to the armature resistance and is generally expressed as a percentage of the motor base speed. DC motors that have a very low armature resistance will have a better speed regulation. Speed regulation is the ratio of the loss in speed, between no load and full load, to the full-load speed and is calculated as follows (the lower the percentage, the better the speed regulation):

Percent speed regulation = (No-load speed – Full-load speed) x 100 / Full-load speed

Varying DC Motor Speed

The base speed listed on a DC motor’s nameplate is an indication of how fast the motor will run with rated armature voltage and rated load amperes at rated field current. DC motors can be operated below base speed by reducing the amount of voltage applied to the armature and above base speed by reducing the field current. In addition, the maximum motor speed may also be listed on the nameplate.

Perhaps the greatest advantage of DC motors is speed control. In armature-controlled adjustable-speed applications, the field is connected across a constant-voltage supply and the armature is connected across an independent adjustable-voltage source. By raising or lowering the armature voltage, the motor speed will rise or fall proportionally. For example, an unloaded motor might run at 1200 rpm with 250 V applied to the armature and 600 rpm with 125 V applied. Armature-controlled DC motors are capable of providing rated torque at any speed between zero and the base (rated) speed of the motor. Horsepower varies in direct proportion to speed, and 100 percent rated horsepower is developed only at 100 percent rated motor speed with rated torque.

Shunt motors can be made to operate above base speed by field weakening.

• The motor is normally started with maximum field current to provide maximum flux for maximum starting torque.
• Decreasing the field current weakens the flux and causes the speed to rise.
• The reduction in field current results in less generated counter EMF and a greater armature current flow for a given motor load.
• One method for controlling field current is to insert a resistor in series with the field voltage source. This may be useful for trimming to an ideal motor speed for the application.
• Others methods use a variable-voltage field source.

First the motor is armature voltage–controlled for constant-torque, variable-horsepower operation up to base speed. Once base speed is reached, field-weakening control is applied for constant-horsepower, variable-torque operation to the motor’s maximum rated speed.

DC Motor Drives

In general, DC magnetic motor starters are intended to start and accelerate motors to normal speed and to provide protection against overloads. Unlike motor starters, motor drives are designed to provide, in addition to protection, precise control of the speed, torque, acceleration, deceleration, and direction of rotation of motors. In addition, many motor drive units are capable of high-speed communication with programmable logic controllers (PLCs) and other industrial controllers.

A motor drive is essentially an electronic device that uses different types of solid-state control techniques. This drive is made up of two basic sections: the power section and the control section. The operation of the drive system can be summarized as follows:

• Controlled power to the DC motor is supplied from the power section, consisting of the circuit breaker, converter, armature shunt, and DC contactor.
• The converter rectifies the three-phase AC power, converting it to DC for the DC motor.
• Attaining precise control of the motor requires a means of evaluating the motor’s performance and automatically compensating for any variations from the desired levels. This is the job of the control section, which is made up of the speed command input signal as well as various feedback and error signals that are used to control the output of the power section.

DC motor drives use a separately excited field because of the need to vary the armature voltage or the field current. When you vary the armature voltage, the motor produces full torque but the speed is varied. However, when the field current is varied, both the motor speed and the torque will vary. In addition to managing motor speed and torque, it provides controlled acceleration and deceleration as well as forward and reverse motor operation.