Speed Control of DC Motor

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. Additionally, the maximum motor speed may also be listed on the nameplate.

speed control of dc motor
Figure 1

Caution: Operating a motor above its rated maximum speed can cause damage to equipment and personnel. When only base speed is listed, check with the vendor before operating it above the specified speed.

Perhaps the greatest advantage of DC motors is speed control.

Speed Control of DC Motor by Armature

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 (Figure 2).

Figure 2. Armature-controlled DC motor.

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.

Speed Control of DC Motor by Field

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. Also, a reduction in field current will result in less generated counter EMF and a larger armature current flow for a given motor load.

A simple method for controlling field 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. An optional, more sophisticated method uses a variable-voltage field source.

Speed Control of DC Motor by Armature and Field

Coordinated armature and field voltage control for extended speed range is illustrated in Figure 3. First the motor is armature voltage–controlled for constant-torque, variable-horsepower operation up to base speed.

Figure 3. Armature- and field-controlled DC motor.

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

Caution: If a DC motor suffers a loss of field excitation current while operating, the motor will immediately begin to accelerate to the top speed that the loading will allow. This can result in the motor virtually flying apart if it is lightly loaded.

For this reason some form of field loss protection must be provided in the motor control circuit that will automatically stop the motor in the event that current to the field circuit is lost or drops below a safe value.

Speed Regulation of DC Motor

Motor speed regulation is a measure of a motor’s ability to maintain its speed from no-load to full-load without 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

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.

Additionally, many motor drive units are capable of high-speed communication with programmable logic controller (PLCs) and other industrial controllers.

A motor drive is essentially an electronic device that uses different types of solid-state control techniques.

Figure 4 shows the block diagram for a typical DC electronic variable-speed motor drive.

Figure 4. The block diagram for a typical DC motor drive.

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 shunt 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.

Thanks for reading about “speed control of dc motor”.

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