What is Motor Controllers and Drives

What is Motor Controllers?

Motor Controllers and Drives are electrical or electronic devices that regulate motor speed, torque, and position outputs. The drive modifies the power input to the motor to achieve the desired output. The controller circuits are commonly integrated with the drive circuits as one stand-alone unit, thus the terms motor drive and motor controller are frequently used interchangeably. There are four basic motor controller and drive types: AC, DC, servo, and stepper, each having an input power type modified to the desired output function to match with an application.

AC servo motor, a DC brush-less motor, and a stepper motor, electrical machines drives and power systems
From left to right, an AC servo motor, a DC brush-less motor, and a stepper motor.
Image credit: similis/Shutterstock.com

Motor Controller and Drive Types

AC
AC Motor Controllers and Drives are electronic devices that modify the input power to motors by typically adjusting the frequency of the power to the motor for the purpose of regulating the output speed and torque. Key specifications include the intended application, drive operating mode, motor type, inverter type, loop system voltage classification, power rating, communication interface, as well as input and output electrical specifications.

AC motor controllers and drives are used primarily in process applications to control the speed of pumps, fans, blowers, etc. They are known as variable speed drives, adjustable frequency drives, or AC inverters. The controller, commonly integrated with the drive circuits, supplies the control signals to the drive.

DC
DC Motor Controllers and Drives are electrical devices that modify the input power by adjusting the constant or alternating current source to a pulsed, direct current output of varying pulse duration or frequency. Key specifications include the intended application, drive operating mode, motor type, loop system, voltage classification, power rating, output signal type, communication interface, as well as input and output electrical specifications. DC motor controllers and drives are used primarily to control motor speeds and torques for machine tools, electric vehicles, pumps, etc. The controller, commonly integrated with the drive circuits, supplies the control signals to the drive.

Servomotor
Servo Motor Controllers and Drives are electronic devices that modify the input power by adjusting the constant or alternating current source to a pulsed, current output of varying pulse duration or frequency. Key specifications include the intended application, motor type, drive operating mode, loop system, power rating, output signal type, communication interface, as well as electrical specifications. Servo motor controllers and drives are used primarily in motion control applications in manufacturing and construction environments, among others, and used to control motor speeds, torques, and positions, and may be AC or DC driven. Servo motors are used in many applications including machine tools, micro-positioning, and robotics, among many other types of machinery, such as conveyors or spindle drive systems. The controller, commonly integrated with the drive circuits, supplies the control signals to the drive. Servo drives are also known as servomotor amplifiers.

Stepper
Stepper Motor Controllers and Drives are electronic devices that modify the input power by adjusting the constant or alternating current source to a pulsed, or “stepped,” current output.

Key specifications include the intended application, motor type, drive operating mode, loop system, power rating, output signal type, communication interface, as well as electrical specifications.

Stepper motor controllers and drives are used primarily in motion control applications in manufacturing and construction environments, among others, and used to control motor speeds, torques, and position. They are used in many applications including machine tools, micro-positioning, and robotics, among many other types of machinery, such as conveyors or OEM equipment. The controller, commonly integrated with the drive circuits, supplies the control signals to the drive. Stepper drives are also known as pulse drives and step amplifiers. Stepper controllers are also known as motor indexers.

Motor Controllers and Drives — Applications and Industries

Unlike servomotors and steppers, most AC and DC motors don’t require controllers or drives other than the simplest motor starters and similar protective devices. AC motor drives are used when speed control of an AC motor is desirable because controlling speed in an AC induction motor is not normally done—once a motor has been specified (by the number of poles) the operating speed is stated on the nameplate. Brushed DC motors, on the other hand, fundamentally speed-controllable by merely varying the voltage going to the motor rotor and field. This can be accomplished with a simple rheostat; no controller or drive is necessary. Newer brushless DC motors are not mechanically commutated and hence require controllers and drives to commutate the magnetic field electronically. Servomotors and steppers, because they are positioning devices as opposed to rotary motion machines, also require controllers and drivers for their operation.

AC motors drives are used to control the speeds of motors that drive pumps, fans, etc. where traditional valves or dampers might otherwise be used to throttle the flow. AC motor drives are used to increase efficiency by setting pump, fan, etc. speed to exactly match requirements.

DC motor drives are used to control permanent magnet DC motors running from AC sources. DC motors have very good low-speed torque, making them especially suited to winches, cranes, etc. where loads have to be lifted without a “running start.” Before the advent of electronic DC controls, DC motors we often coupled as motor-generator sets to produce direct current via AC induction motors.

Servomotor controllers and drives rely on feedback from the servomotors to control position, velocity, acceleration, etc. Servomotor manufacturers generally supply drives that work with their motors. While steppers don’t require a feedback loop, some use one. Stepper motor manufacturers also usually supply drives for their specific motors. Most servo- and stepper-motor manufacturers provide charts as guides to which motors will work with which drives.

Considerations
Choosing motor controllers and drives starts with knowing the motor type. Corresponding sub-categories then align with this basic information.

Deciding to use AC drives for induction motors is often an economic decision based on operating characteristics at specific installations: how often the pump or fan is running against throttled valves or louvered ducts. At least one manufacturer offers a calculator (see below) to help determine energy savings of AC drives based on specific operating scenarios. Motors designed for use with variable speed drives are typically rated for inverter duty.

Another consideration for AC drives is the nature of the application, with constant torque and variable torque being major divisions. For variable torque applications such as centrifugal fans, torque requirements vary with motor speed. For constant torque applications such as conveyors, torque requirements are the same regardless of motor speed. AC drives are usually designed for either variable or constant speed applications.

Brushed DC motors running off AC current usually are driven using thyristor-based bridge rectifiers which permit AC current to flow to the motor in one direction only, replicating a DC power source. More information can be found in the references below. Vendors such as Baldor provide DC controls for one way and regenerative applications for DC motor horsepowers up to 5, and through its parent company (ABB) offers DC drives up to 3000 HP. One-way drives usually require a brake for stopping the motor while regenerative drives can run the motor in either direction and so provide stopping force by reversing. The generated power is usually bled off through reversing resistors.

Brushed DC motors operating on DC systems such as found on electric pallet jacks also use controls for speed and directional changes. Brushless DC, or permanent magnet, motors also require controllers to electronically commutate their magnetic fields.

Servomotors can be of AC or DC types, with DC available as brush and brushless types. In all instances, they require controls because they are feedback devices. Linear motors are generally servo-based and require controls too.

Stepper motors generally do not require feedback but must be “homed” upon powering up so that the motor knows where it is. From there it counts steps to keep track of the position. Some steppers piggyback their drives directly on the motor frames.

Important Attributes
Semiconductor Device
Generally speaking, IGBTs and SCRs are used for medium and high voltage devices, while MOSFETs are used in low power applications.

Motor Input Phase
Motors are generally single or three-phase machines, referring to the phase of the alternating current that supplies them. Stepper motors are an exception in this regard because phase refers to the architecture of the stepper motor itself, generally described as two or five-phase. Oriental Motors has a nice write-up discussing the difference, cited below.

Enclosures
Electrical enclosures are specified in accordance with NEMA or IEC criteria for environmental and ingress protection.

Drive Operating Mode

As discussed above, AC drives are generally designed either as constant torque or variable torque depending on the application.

 

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