Switched Reluctance Motor Drive: Definition and Function

The technology behind the switched reluctance motor (SRM) has been in existence since the mid-nineteenth century. These early examples never reached their potential because they were difficult to control, noisy and unreliable. However, with recent developments in electronic switching, using insulated-gate bipolar transistor (IGBT) technology, the SRM has become an excellent solution for a wide variety of different applications. The latest designs of SRM allow users to take advantage of the higher efficiency, low starting currents, and robust construction that characterizes this type of motor.

What is a switched reluctance motor drive?

The switched reluctance drive (SRD) is a combination of the motor and its electronic control system. It creates a low-cost, reliable, highly efficient, and flexible electric drive system. Its ability to use potential energy stored in one coil to energize the next takes this technology to a whole new level of efficiency and performance.

A switch reluctance motor (SRM) does not have magnetic “slip”, so there is virtually no heating of the armature (which is a weakness of standard induction motors). The losses that do occur are in the magnetic coils connected to the stator, where heat is more readily dissipated. As SRDs do not have bi-directional, three-phase, high-frequency switching, they do not have the eddy and bearing current issues commonly associated with squirrel cage (induction) motors.

The technologies that make up SRDs are well established and use commonly available parts and materials; the difference is how they are combined and applied. Because of this, SRDs are not accompanied by a “leap in technology” risk that affects many new solutions.

Why SRD instead of a standard induction motor and VFD?

For an application that requires a relatively low torque start, which will then run at nearly full speed and a fairly constant torque for many hours at a time, a VFD system may make sense. However, for high torque starts (200-300%), operation at 10-150% speed and torque, wide operational load environments, or any number of scenarios that fall outside of the VFD comfort zone, SRD is the easy and obvious solution.

Benefits of SRD

• Low starting current
• High starting torque
• Robustness
• Simplicity and reliability
• High Power
• Frequent rotational direction changes
• No need for magnets or rare earth metals
• High overload capacity

Why is the SRM system so robust?

The rotor of SRM is made of laminated silicon steel sheets with no windings or permanent magnetic material. This makes it extremely strong, such that it can withstand frequent shocks received in applications such as press machines.

The SRM drive system can reach up to (IE4) “Super Premium Efficiency” limits (in accordance with IEC/EN 60034-30-1:2014). This efficiency is especially evident at low speeds and light loads and is maintained across a wide speed range.

How does a switched reluctance drive work?

The switched reluctance drive (SRD) system consists of an SRM and its associated control system. The electrical power is delivered to the SRM windings positioned around the stator, which creates a magnetic field. The magnetic reluctance of the rotor results in its poles aligning with an energized stator winding. By using an electronic switching circuit to sequentially energize the windings around the stator, the rotor is attracted to the magnetic field on each stator pole in turn and hence, rotates until it is aligned.

If the relative positions of the stator and the rotor in the diagram are taken as the starting position energizing the B phase winding will result in the rotor rotating anti-clockwise against the excitation sequence. The direction of rotation of the motor is independent of the direction of the current in the phase winding and the current of the stator winding of the switched reluctance machine is uni-directional.

Applications of switched reluctance drive

Electric screw press motors

The electric screw press is a major challenge for motor drive technology. It is required to manage heavy loads, frequent starts, rapid braking, and impact resistance. The SRD motor has a high starting torque coupled with a low starting current and can be frequently reversed while providing the required fast braking. The rotor has no coil or permanent magnet and is immune to the load’s shock and vibration. Additionally, there is a direct drive which makes gear chains and clutches unnecessary, therefore increasing the efficiency of the system.

Machine tools

SRD motors have an excellent dynamic response, fast, closed-loop speed control, and constant speed even at speed full load. The machine tool motor is used to digitally control the operation of the planer, which can be moved forward and backward, as well as jog forward, move back, lift and drop the bit, and other functions. The high and low speed operating parameters are self-adaptive, such that there is no need to adjust the parameters when the speed of the table changes.

Oil pumping motors

Many oil fields are still using beam pumping units, otherwise known as Nodding Donkeys. With these, electricity costs can account for more than 25% of the cost of oil production, and the stroke adjustment is not optimal. Due to increasing costs and lower revenue, reducing energy consumption is a high priority, so an energy-efficient pumping unit can be very beneficial. The SRD can facilitate a long-stroke pumping unit. The motor directly drives the oil rod moving it up and down by using its ability to readily reverse its direction, simplifying the transmission. The overall efficiency is greatly improved compared to the original pumping unit. This includes a maintenance-free motor, ideal for the harsh environment in many oil fields and which is simple to commission.

Textile machinery motors

The SRD motor features a low starting current and high torque, with a fast-dynamic response. It can deliver variable-speed weaving and other enhanced weaving processes. The textile motor starting torque is up to 5 times its rated torque, with fast starting acceleration from standstill to a rated speed of only 200ms.

Mining machinery motors

Switched reluctance motors are especially suitable for a variety of coal mining equipment, such as cutters, and coal plows. This type of motor provides a low starting current and high torque (30% of rated current gives starting torque up to 150%). The simple winding-free rotor is ideal for high speeds and its mechanical strength is resistant to impacts, the stator has no overlapping windings, making short circuits unlikely.

Electric vehicle motors

The low starting current, quick acceleration, and extremely efficient direct drive associated with the switched reluctance motor make it ideal for electric vehicles as it minimizes the load on the battery and extends the driving range from a single battery charge. Its ability to cope with frequent starting, stopping, and reversal is also beneficial for transport applications.

Conveyor systems

Conveyor systems have attributes that make them ideal for the application of SRD systems. This is especially true where the load on the conveyor has one or more of the following attributes; high capacity; regular starting and stopping; low-speed operation. These requirements are often found on coal and mineral transport systems, or for use in airports.