Last revised: October 2, 2024
Reviewed by: Scott Orlosky, consulting engineer

DC motor drives are the interface between a controller and a DC motor. Their job is to  convert speed and direction input from the controller to currents and voltages compatible with the motor.

There are lots of ways of doing this depending on the motor needed for the drive. These span the range from simple direct DC drive with a voltage control to sophisticated variable speed drives with speed and position feedback control.

Some manufacturers combine the controller and the drive as a 'drive controller'. The PLC control and motor drive are contained in a single panel. In some special cases a drive controller will be mounted directly on the motor.  This simplifies the wiring but this design is often limited by heat generation.

DC motors tend to be less complex than AC motors and are normally less expensive for smaller horsepower requirements. They are capable of providing large startup torques, for their size, exceeding 400% of the rated continuous torque. They have a long history of small horsepower applications with a wide range of options available for this purpose.

Types of DC Drives

DC motor drives can vary based on the type of DC motor being used. These types include brushed, brushless, servo, stepper, linear, and voice coil motors. Commutation is the process of driving the rotor by sequentially switching a magnetic field through a full 360 degrees.

Brushed motors commutate via physical contacts, often spring-loaded graphite brushes biased against the commutation bar. These contacts are subject to wear over time and are usually designed to be replaced. This commonly uses a trapezoidal commutation.

Brushless motors commutate electronically with no physical brush contact. One common technique to control commutation is the use of Hall effect sensors, small magnetic switches that detect rotor position in place of the brushes. Like the brushes version, this is commonly driven by a trapezoidal signal.

Servomotors typically use an internal sensor that tracks the rotor position.  This feedback sensor sends a rotor position signal to the controller and can use this feedback to control the rotational speed and/or rotor position very precisely. This configuration is often found in automation applications, and is driven sinusoidally.

Stepper motors have internal rotors that look like a flat gear with the teeth alternating in polarity. The stator consists of a series of electromagnetic “teeth” that match the gear tooth pattern. Coils on the stator can be successively energized to causing the rotor to “step” to an adjacent tooth in the rotor.  By energizing this arrangement the step speed and direction can be controlled up to the maximum torque of the motor.

Linear motors generate force only in the direction of travel. The motor technology resembles rotary motor technologies simply oriented in a linear fashion. Linear motors are capable of extremely high speeds, quick acceleration, and accurate positioning. Linear motor technologies include moving coil, moving magnet, AC switched reluctance design, AC synchronous design, AC induction or traction design, linear stepping design, DC brushed design, and DC brushless design.

Voice coil motors consist of a magnetic coil placed in a magnetic field. When current is applied to the coil, electromagnetic flux is generated that causes the coil to move. The motor's name is derived from its resemblance to audio speaker operation.

DC motor systems can also be classified based on types of control functions (e.g. integral motion controllers, variable speed drives, step and direction, motor speed controllers, etc.). 

Performance Specifications

There are a wide range of operating specifications to consider when searching for DC motor drives. The most important of these include:

• Continuous current — the highest current which can be supplied to the motor during continuous operation.
• Input frequency — the input frequency accepted by the controller
• Power — the rated power output of the drive motor system.
• Supply voltage — the voltage supplied to the drive.
• Operating temperature — the operating temperature of the power supply.
• Stall Torque – highest torque the motor can produce. The point at which the rotor no longer rotates when driven.

Other important characteristics include the mounting configuration and accompanying features.

Mounting Configuration

Drive mounting configuration is important for compatibility with the motor system. Drives can be mounted in various ways based on the design.

These include an IC PCB (printed circuit board), PC board, panel, DIN rail, or rack. Other drives may be stand-alone devices or designed to be incorporated into specific products by original equipment manufacturers (OEMs).

A critical feature of motor drives is their rated temperature rise. Since drives are converting electrical energy to mechanical energy there is a significant energy loss in the conversion.  Always check the manufacturer’s recommendations for heat management.  This may include the addition of heat sinks or extra layers of copper on a PCB design.

Features

DC motor drives may also include various features which may be important for certain applications. These include:

• Regeneration — Method of braking in which the motor is disconnected from the power supply and power generated from the rotating motor is sent back to the supply.
• Programmability and configuration — Device can be programmed with routine configurations and commands for greater functionality and process control.
• Autorestart — Drive is designed to automatically restart operation after a stall.
• Soft start – an incremental acceleration up to operating speed.

DC Motor Drive FAQs

What are the two main circuits in a DC drive unit?

The two main circuits in a DC drive unit are the armature supply and the field exciter.

What are the main power components used in each circuit, and what are the characteristics of each?

Silicon controlled rectifiers (SCRs) can be turned on with a small milliamp pulse but must be forced off with several amps. Insulated gate bipolar transistors (IGBTs) operate on the same principle, with milliamps required to gate them.

How is line notching corrected in a DC drive system?

Line notching is corrected by using line reactors or filters to smooth out the waveform.

What are some important performance specifications to consider for DC motor drives?

Important specifications include continuous current, input frequency, power, supply voltage, and operating temperature. Additionally, the mounting configuration is crucial for compatibility with the motor system.

What is dynamic braking and how is it accomplished?

Dynamic braking is a method of braking in which the motor is reconnected as a generator immediately after it is disconnected from the supply. The generated energy is dissipated in a resistor.

What is RFI and how is it controlled in a DC drive system?

Radio frequency interference (RFI) is unwanted noise generated by electrical devices. It is controlled using filters and proper grounding techniques.

What is the difference between single-, two-, and four-quadrant systems?

Single-quadrant systems can only drive the motor in one direction and provide braking in the same direction. Two-quadrant systems can drive the motor in one direction and provide braking in both directions. Four-quadrant systems can drive and brake the motor in both directions

What is motor cogging and what causes it?

Motor cogging is the jerky motion of a motor at low speeds, caused by the interaction between the permanent magnets of the rotor and the stator slots.  This can be especially pronounced in a stepper motor operating at a slow speed, due to the nature of how it is driven.

DC Motors Drive Media Gallery

References

GlobalSpec—Motors & Drives: A Practical Technology Guide    

GlobalSpec—Cutting edge motor and drive technologies from ElectroCraft Inc.

Joliet Technologies—Adjustable Speed Drives