What is an Electric Drive?

What is an Electric Drive?

An electric drive (often referred to as an electric controller) is a device used to control the output of a motor used, for example, to produce linear motion in an electric actuator.  In our article on electric motors we referred to the feedback mechanism from a servo motor, shown schematically below.

feedback mechanism from a servo motor schematic

The drive will accurately control the motor output and the motor response against a controlling input (nset in the schematic above).


How does an electric drive work?

Electric drives require a three-phase AC supply. The incoming fixed frequency AC supply is rectified to provide a DC signal; this is then smoothed and circuitry provides a decoupling of the input and output stages as shown below.

The output stage uses software controlled Insulated Gate Bi-junction Transistors (IGBT’s) to switch and provide a variable frequency three phase AC supply to the motor. To understand the output stage in more detail, we can refer to the following equivalent circuit.

In the circuit, the IGBT’s are represented by the switches 1 to 6; a centre tap of each column of switches is connected to an individual phase of the motor windings. In the diagram switches 5 and 4 are closed which passes current through the W and V phases. In the next mode of operation, switches 5 and 4 may open and 3 and 2 close; this will pass current through the V and U phases. The software controls the successive opening and closing of the switches and the duration of each step; this creates a pulse width modulation.

The varying pulse width effectively simulates an AC waveform as seen by the load of the motor windings; for example, at A in the diagram above, a pulse width of ‘0’ will result in zero voltage across the windings, wheras at B, the maximum voltage will be applied.


How do I select an electric drive?

The drive must be capable of supplying and controlling the motor which, in turn, will be matched to the requirements of an application.

The motor voltage, power rating and full-load current will all need to be matched; any over-load requirements will need to be considered, for example if higher torque is required for start-up.

The feedback mechanism and any input/output (I/O) requirements should be accommodated.

Communication protocols– for example CANOpen, Profibus, etc, and the operating temperature should be considered and, hence, the need for any ventilation or forced cooling.


Types of electric drive

Drives will need to control either DC or AC motors, the latter single- or three-phase. Different variants will be required, dependent on the parameters mentioned above – for example, voltage.

Drives can also be classified into single-, group- and multi-motor drives. Single are the most basic and are often used in domestic appliances; group lend themselves to use in more complex systems and multi are used in heavy, or multiple motor applications.


Do I need anything else to make electric drives work?

Cabling will be required for power and control signals to the motor, along with suitable supply for operation of the drive itself.


cables required for an electric actuator

It is good practice to have input fuses for protection; if EMI is a potential problem, then filters can be built into the circuitry. A cooling fan, with suitable ventilation, will be required if high operating temperatures are encountered.

Depending on application requirements a dynamic brake is available.

Want to find out more?

Read our articles on electric actuators, motors and torque to further your understanding, or use our electric actuator configurator to design and order a custom cylinder today.