FAQ: Permanent Magnet Motors
- What are the advantages of permanent magnet motors?
- What control system for permanent magnet motors is used in “Riteks” control stations?
- What is the difference between 8-pole, 6-pole, and 4-pole motor systems? Which is better?
- What is the rotational speed range of submersible permanent magnet motors?
- What control methods are used for permanent magnet motors?
- Will a permanent magnet motor rotate if directly connected to a three-phase power supply?
FAQ: Permanent Magnet and Synchronous Motors
- What is the difference between a synchronous machine and a permanent magnet motor?
- What is the difference between permanent magnet motors and stepper motors (in terms of application)?
- Why are universal brushed DC motors not used in the oil industry, since they do not require complex control systems like permanent magnet motors?
- What is the relationship between torque and the phase shift angle of rotor flux relative to stator flux?
FAQ: Permanent Magnet and Induction Motors
- What are the dimensional differences between permanent magnet and induction motors of the same power rating?
- How does shaft torque depend on current in permanent magnet and induction submersible motors?
- Starting currents of permanent magnet and induction motors.
- Methods of reducing starting currents in induction motors.
- What is the difference in the cost structure of permanent magnet and induction motors?
- Can a permanent magnet motor be operated using an induction motor control station?
- Can an induction motor be operated using a permanent magnet motor control station?
- What determines the efficiency difference between permanent magnet and induction motors?
- Is a 6–8% efficiency advantage of submersible permanent magnet motors significant?
FAQ: Permanent Magnet Motors in Submersible Systems
- What operating conditions are most harmful to submersible equipment?
- Does it always make sense to install equipment with permanent magnet motors?
FAQ: Permanent Magnet Motors
What are the advantages of permanent magnet motors?
See the article Comparison of Permanent Magnet and Induction Motors.
What control system for permanent magnet motors is used in “Riteks” control stations?
Six-pulse commutation (see the article Elements of Permanent Magnet Drive Theory).
What is the difference between 8-pole, 6-pole, and 4-pole motor systems? Which is better?
As the number of poles increases, the supply frequency increases at the same rotational speed (for an 8-pole motor: 3000 rpm — 200 Hz; for a 4-pole motor: 6000 rpm — 200 Hz). Thus, low-speed motors are designed with a higher number of poles, while high-speed motors use fewer poles.
What is the rotational speed range of submersible permanent magnet motors?
From 250 rpm (for progressing cavity pump drives) up to 6,000 (10,000) rpm.
What control methods are used for permanent magnet motors?
There are two main methods: commutation control (6-pulse control) and vector control (see the article Elements of Permanent Magnet Drive Theory).
Will a permanent magnet motor rotate if directly connected to a three-phase power supply?
It will not start from standstill. However, if it is accelerated by some external means up to the supply frequency speed (for an 8-pole motor — 750 rpm) and then connected to the grid, it will operate as an AC synchronous machine.
FAQ: Permanent Magnet and Synchronous Motors
What is the difference between a synchronous machine and a permanent magnet motor?
Structurally, a permanent magnet motor and a permanent magnet synchronous machine are identical. The difference lies only in the control system. In a synchronous machine, the rotor follows the rotating field, whereas in a permanent magnet motor the field adapts to rotor movement.
What is the difference between permanent magnet motors and stepper motors (in terms of application)?
A stepper motor is designed for discrete shaft positioning with rigid positional control. A permanent magnet motor is designed for continuous rotation.
Why are universal brushed DC motors not used in the oil industry, since they do not require complex control systems like permanent magnet motors?
The reliability of the brush assembly and its energy characteristics when operating in oil environments do not allow its use as a submersible motor. In addition, the specific energy characteristics (power-to-weight or power-to-volume ratio) of brushed motors are significantly lower than those of permanent magnet motors.
What is the relationship between torque and the phase shift angle of rotor flux relative to stator flux?
The relationship is practically sinusoidal.
FAQ: Permanent Magnet and Induction Motors
What are the dimensional differences between permanent magnet and induction motors of the same power rating?
The active section length of a permanent magnet motor is at least two times shorter than that of a comparable induction motor.
How does shaft torque depend on current in permanent magnet and induction submersible motors?
For permanent magnet motors — almost linearly. Induction motors have a significant no-load current, at least half of the rated current, which worsens their energy characteristics under partial load conditions.
Starting currents of permanent magnet and induction motors.
For permanent magnet motors (as well as induction motors operated with a VFD), the starting current does not exceed the operating current. For induction motors with direct-on-line starting, the starting current is typically 5–7 times higher than the operating current.
Methods of reducing starting currents in induction motors.
Design modifications aimed at reducing starting currents in induction motors simultaneously worsen their energy performance under rated operating conditions. Therefore, the preferred solution is the use of variable frequency drives, which allow motors with optimized energy characteristics to be used without concern for high starting currents.
(When directly connected to the power grid, the starting current of a motor designed for VFD operation may exceed the operating current by more than 10 times.)
What is the difference in the cost structure of permanent magnet and induction motors?
The cost of a permanent magnet motor equals the cost of an induction motor minus the cost of rotor windings plus the cost of permanent magnets.
(Permanent magnets are many times more expensive than copper, but the power density per unit length of a permanent magnet motor is approximately twice that of an induction motor.)
Can a permanent magnet motor be operated using an induction motor control station?
Yes, if the control station includes a variable frequency drive. However, in this case it will not be possible to fully realize the advantages of permanent magnet motors when operating with transformers and long cable lines (there is a high probability of instability during sudden load changes).
Can an induction motor be operated using a permanent magnet motor control station?
An algorithm for such operation can be developed, but the energy efficiency will be reduced.
What determines the efficiency difference between permanent magnet and induction motors?
In an induction motor, energy used to create the rotor magnetic field is continuously consumed from the power supply, while in a permanent magnet motor the magnetic field is created once during manufacturing.
Is a 6–8% efficiency advantage of submersible permanent magnet motors significant?
A 6–8% efficiency improvement corresponds to a 50–80% reduction in losses, resulting in lower motor overheating and, consequently, increased reliability.
(A 10-degree reduction in winding temperature increases mean time between failures by 2–4 times.)
However, economic benefits can only be achieved through proper selection and optimization of submersible equipment operating conditions, because the efficiency of a pump operating outside its optimal range may decrease by two or more times, offsetting the benefits of improved motor energy characteristics.
Therefore, the use of permanent magnet motors increases the qualification requirements for production engineers and field specialists.
FAQ: Permanent Magnet Motors in Submersible Systems
What operating conditions are most harmful to submersible equipment?
The most damaging operating mode for submersible systems is operation during flow interruption and initial startup, because there is no fluid circulation around the motor and through the pump.
Permanent magnet motors are significantly more tolerant of these operating conditions. There have been cases where they operated for several days in flow interruption mode at speeds close to maximum. For induction motors, such conditions inevitably lead to failure.
This is due to the significantly lower losses and correspondingly lower heat generation of permanent magnet motors.
Unlike induction motors, permanent magnet motors do not require shutdown for cooling during startup and stabilization.
Engineers with extensive operational experience using permanent magnet motors have formed the opinion that burning out a permanent magnet motor is practically impossible if surge protection systems are functioning properly. Therefore, they are often used in the most challenging wells, where multiple induction motors may have already failed.
Does it always make sense to install equipment with permanent magnet motors?
If well characteristics are well known and stable pump operation can be guaranteed using a direct-on-line induction motor, then the induction system may be economically preferable due to lower installation cost.
However, an induction motor with a variable frequency drive is almost always less efficient than a permanent magnet drive.