In the oil industry, both submersible induction motors and permanent magnet motors are used. Induction motors have been applied in this field for many years, while permanent magnet motors have appeared relatively recently. In this article, we will attempt to explain in simple terms the basic principles underlying permanent magnet drives, as well as their similarities and differences compared to induction motors.
Definition of a Permanent Magnet Motor
Permanent Magnet Motor (PMM) is a synchronous brushless direct current motor used together with an electronic control system that synchronizes the rotation of the electromagnetic field with the rotor rotation.
Terminology in International Literature
There is no direct equivalent of the term “Permanent Magnet Motor” as used in Russian technical literature. Internationally, the common abbreviations used for synchronous brushless DC motors are BLDC and PMSM:
- BLDC — Brushless DC electric motor.
- PMSM — Permanent Magnet Synchronous Motor.
Essentially, these definitions describe the same type of machine. However, in Russian engineering practice, the term “Permanent Magnet Motor” also implies the specific method of motor control. In other words, it is a broader concept describing a brushless DC motor operating together with a dedicated control system.
Design
Structurally, a permanent magnet motor, like any electric motor, consists of a stator, a rotor, and rotor support assemblies (bearings).
Stator
The stator is the stationary part of the motor containing the armature windings and is designed to create a rotating electromagnetic field inside the motor.
Rotor
The rotor is the rotating part of the motor and consists of a shaft with either an electromagnet or permanent magnets. In the first case, the rotor windings are supplied with direct current.
Depending on the number of magnets located in the rotor cross-section, the motor will have a corresponding number of poles.
In submersible permanent magnet motors used in the oil industry, the rotor contains permanent magnets. As a rule, submersible PMMs are manufactured mainly in 8-pole and 4-pole configurations.
At the same rotational frequency of the stator electromagnetic field, a permanent magnet motor with a greater number of pole pairs will rotate at a lower speed. For example, if the rotational speed of an 8-pole motor is 1500 rpm, then a 4-pole motor operating at the same field frequency will rotate at 3000 rpm.
Operating Principle
Operating Principle of a Permanent Magnet Motor
A permanent magnet motor (unlike induction or brushed motors) is not self-sufficient, and its operation is impossible without a dedicated electronic control system. Depending on the rotor position, the control system supplies electric current to the stator windings in such a way that the generated electromagnetic field attracts the rotor magnets, thereby rotating the rotor by a certain angle. The control system then supplies current to adjacent stator windings, and the rotor continues rotating, attracted by the newly generated magnetic field.
Thus, the stator creates a rotating electromagnetic field, while the rotor follows it synchronously.
This is one of the key differences between permanent magnet (synchronous) and induction motors.
Operating Principle of an Induction Motor
In an induction motor, the rotor also follows the rotating electromagnetic field, but at a lower speed. This effect is called “slip,” and it is an inherent property of induction motors. It is precisely this phenomenon that enables induction motor operation.
The rotor consists of windings with short-circuited turns resembling a squirrel cage, where the bars are connected by rings at both ends.
The rotating electromagnetic field of the stator induces electromotive force (EMF) in the rotor. Current begins flowing through the rotor bars, generating a magnetic field that causes the rotor to rotate following the magnetic field created by the stator. The relative motion between the stator field and the rotor is the condition required for inducing EMF in the rotor.
If the rotor is forced to rotate at exactly the same speed as the stator magnetic field, no EMF will be induced in the rotor. Therefore, the rotational speed of an induction motor rotor is always lower than the rotational speed of the field.
A synchronous motor, on the other hand, does not require rotor EMF excitation from the stator magnetic field because the magnetic field is already present due to the permanent magnets. Therefore, the rotor of a synchronous motor rotates strictly at the same frequency as the field generated by the stator windings.
If the rotational speed of a synchronous motor is set to 3000 rpm, this means that the magnetic field created by the stator windings also rotates at 3000 rpm. Under the same conditions, the rotational speed of an induction motor would be approximately 2910 rpm.
Permanent Magnet Motor Control System
As mentioned above, the rotation of a permanent magnet motor is provided by a dedicated electronic control system generally referred to as an inverter (converter).
In this case, an inverter is an electronic system that supplies voltage to the motor windings at a frequency independent of the supply frequency.
Inverters are used not only with synchronous (permanent magnet) motors, but also with induction motors where rotational speed control is required.
There are two main approaches to permanent magnet motor control:
- 1. Commutation control (6-pulse control).
- 2. Vector control.
Commutation Control
With commutation control (in a three-phase system), direct voltage is simultaneously applied to only two stator winding phases, while the third phase remains disconnected from the power source.
During motor operation, the control system monitors rotor position and applies voltage of the required polarity to the appropriate winding pair so that the magnetic field generated in the stator pulls the rotor along, forcing it to rotate.
Rotor speed is controlled by the magnitude of the DC voltage commutated to the stator windings. At the same time, the commutation frequency is adjusted according to rotor speed changes.
And once again we would like to emphasize: the rotor speed does not change because of the field frequency — instead, the field adapts to the rotor speed. The inverter regulates rotor speed by changing the current and/or voltage commutated to the windings.
Rotor Position Detection
Various methods are used to determine rotor position:
- Using sensors (for example, Hall sensors).
- Sensorless methods.
Sensorless rotor position detection is used in submersible permanent magnet motors because the use of physical sensors is impractical due to operating conditions.
In sensorless systems, rotor position is determined by the EMF induced in the free stator phase (the phase not currently supplied with voltage). As the rotor moves, the EMF in the free phase changes, and its zero crossing serves as a reference point for rotor position.
With this control method, the current flowing through the stator windings has a waveform close to trapezoidal.
This control approach is characterized by simplicity and reliability, making it possible to operate permanent magnet motors not only over short distances but also through long cable lines — hundreds of meters and even kilometers — which is particularly important for submersible oilfield equipment.
Vector Control
The essence of vector control lies in creating an accurate mathematical model of the motor and controlling it by supplying frequency-modulated voltage simultaneously to all motor phases. The frequency and magnitude of the voltage applied to each phase are individually adjusted at every moment so that the sinusoidal current generated in the stator windings ensures rotor rotation under the most electrically efficient conditions, where the phase shift between the rotor and stator magnetic fluxes is close to 90 degrees.
This method allows extremely precise motor control. It is computationally demanding; however, advances in microprocessor technology compensate for this complexity.
Its application on short cable lines is highly efficient, but operation over long lines presents many challenges for developers, and solving these issues is a non-trivial task.
Vector Control or Commutation Control?
From a mathematical perspective, vector control is the most natural control method for permanent magnet motors. From the standpoint of technology and operational experience in the oil industry, commutation control systems remain highly practical.
There is no universal answer as to which method is more effective. In every specific case, many factors must be considered: mathematical efficiency, technological maturity, field of application, economic feasibility, and the qualification level of personnel operating the equipment.