Advantages of Permanent Magnet Motors for Submersible ...

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IMAGE 1: Permanent magnet motors for water well pumps (Images courtesy of Grundfos)

Permanent magnet motors (PMMs) for use in water well pumps are routinely referred to as permanent magnet synchronous motors, electronically controlled motors, electronically commutated motors, brushless permanent magnet motors, brushless direct current motors and interior permanent magnet motors. 

These terms are not always correctly applied to PMMs of all configurations. Here is a simplified look at how electric motors function. In an induction motor, an electrical current is introduced into the stator. As this current rotates through the stator in the form of a magnetic field, it induces a magnetic field in the rotor. The stator and the rotor each have a separate magnetic field.

As the magnetic field of the stator rotates, it pulls the magnets of the rotor, causing the rotor to turn. Since the rotor is locked to the motor shaft, which is coupled to the pump shaft, the pump impellers rotate and water moves.

All motors work by virtue of the rotating magnetic field of the stator, causing the rotor to spin. In the case of both induction motors and PMMs, electricity must be introduced to the stator. The difference between the two is how the magnetic field of the rotor is created. As described above, with induction motors, the magnetic field of the stator induces a magnetic field in the rotor.

However, in the case of PMMs, the magnetic field of the rotor is created during the rotor&#;s manufacturing process. High-strength permanent magnets are built into the rotor, so there is no need for a magnetic field to be induced. In addition, there can be a considerable reduction in power consumption when using a large PMM, plus many more benefits.

The reduction of energy consumption not only means that PMMs are environmentally responsible, but it also results in lower power costs for the owner. While this may not be a major factor for most small residential systems that do not operate for long periods of time, other applications such as large irrigation and municipal systems with PMMs may gain large savings.

By their nature, all PMMs are operated with variable frequency drives (VFD). As with any VFD-controlled unit, this form of speed control enables the pump to adapt its performance to meet varying flow demands, which is the norm for most of these submersible pump applications. 

PMM Benefits

PMMs with small dimensions can run at a higher speed, potentially reducing the number of stages and size of the impellers while achieving target flow rates and heads. This can help 3-inch diameter pumps fit in boreholes that would be difficult for a nominal 4-inch submersible pump to accommodate.

IMAGE 2: Motors work by the rotating magnetic field of the stator, causing the rotor to spin.

When using small dimension PMMs, increased power density, speed capability and smaller overall size of the PMM results in easier installation and maintenance. Pumps with PMMs are smaller and lighter. While a 1.5-horsepower (hp) induction motor pump weighs approximately 35 pounds, a 1.5-hp PMM pump weighs approximately 16 pounds. PMMs provide a longer winding life by running cooler.

At low speeds, PMMs have a higher starting torque&#;a benefit in water wells with high mineral concentrations. The pump will have a reduced tendency to seize as minerals build up in tight-tolerance areas within the pump. Further, single-phase PMMs have starting torques that are comparable to that of three-phase induction motors.

Imagine a pump with an allowable speed performance range from 100% down to 28% of full-rated speed. To take advantage of this speed range, the pump is normally operated at a constant pressure setting. 

As shown by an example using this curve (Image 3), the pump and motor will maintain a pressure of 60 pounds per square inch (psi), which is 139 feet of head at varying flows from approximately 30 gallons per minute (gpm) down to zero flow. The unit is designed to shut down automatically when demand stops. The VFD-controlled PMM has built-in soft-start, which results in lower shock to the electrical and piping system.  

The integrated logic board that comes with the unit provides a wide range of monitoring, self-protection and diagnostics solutions. While the board monitors performance, the resulting self-protection includes automatic reaction to dry run, overload, over temperature and over/under voltage. 

Diagnostics include reporting on pressure, temperature, motor speed, power consumption, number of starts and hours of operation. 

Alarm functions include dry run, need for service, power supply failure, defective sensor, overload, over temperature, speed reduction, voltage alarm and no contact with the sensor.

IMAGE 3: Pump curve showing pump and motor maintaining pressure of 60 psi

In the future, the higher efficiency of larger PMMs may enable these pumps to meet evermore stringent mandates from the U.S. Department of Energy&#;mandates that are already in place for larger submersible groundwater pumps. Higher efficiency may also help the pump qualify for power use reduction incentives from utility companies. The higher power factor of PMMs may result in an overall lower utility cost for certain applications.

PMMs also lend themselves for use with submersible pumps which use alternative energy sources (solar, wind, batteries, generators and fuel cells) and can achieve flows from 3 to 60 gpm and heads up to 820 feet. These alternative energy pumps have many of the same control, monitoring and diagnostic features mentioned above.  

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In an electrical system, synchronous motors are the most widely used steady-state 3-phase AC motors, which convert electrical energy into mechanical energy. This type of motor operates at synchronous speed, which is constant and it is synchronous with the supply frequency and the period of rotation is equal to the integral no. of AC cycles. That means the speed of the motor is equal to the rotating magnetic field. This type of motor mainly used in power systems to improve the power factor. There are non-excited and DC excited synchronous motors, which operate according to the magnetic power of the motor. Reluctance motors, hysteresis motors, and permanent magnet motors are the non-excited synchronous motors. This article is all about the working of a permanent magnet synchronous motor.

What is a Permanent Magnet Synchronous Motor?

The permanent magnet synchronous motors are one of the types of AC synchronous motors, where the field is excited by permanent magnets that generate sinusoidal back EMF. It contains a rotor and stator same as that of an induction motor, but a permanent magnet is used as a rotor to create a magnetic field. Hence there is no need to wound field winding on the rotor. It is also known as a 3-phase brushless permanent sine wave motor. The permanent magnet synchronous motor diagram is shown below.

Permanent Magnet Synchronous Motor Theory

The permanent magnet synchronous motors are very efficient, brushless, very fast, safe, and give high dynamic performance when compared to the conventional motors. It produces smooth torque, low noise and mainly used for high-speed applications like robotics. It is a 3-phase AC synchronous motor that runs at synchronous speed with the applied AC source.

Instead of using winding for the rotor, permanent magnets are mounted to create a rotating magnetic field. As there is no supply of DC source, these types of motors are very simple and less cost. It contains a stator with 3 windings installed on it and a rotor with a permanent magnet mounted to create field poles. The 3-phase input ac supply is given to the stator to start working.

Working Principle

The permanent magnet synchronous motor working principle is similar to the synchronous motor. It depends on the rotating magnetic field that generates electromotive force at synchronous speed. When the stator winding is energized by giving the 3-phase supply, a rotating magnetic field is created in between the air gaps.

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This produces the torque when the rotor field poles hold the rotating magnetic field at synchronous speed and the rotor rotates continuously. As these motors are not self-starting motors, it is necessary to provide a variable frequency power supply.

EMF and Torque Equation

In a synchronous machine, the average EMF induced per phase is called dynamic induces EMF in a synchronous motor, the flux cut by each conductor per revolution is Pϕ Weber
Then the time taken to complete one revolution is 60/N sec

The average EMF induced per conductor can be calculated by using

( PϕN / 60 ) x Zph = ( PϕN / 60 ) x 2Tph

Where Tph = Zph / 2

Therefore, the average EMF per phase is,

= 4 x ϕ x Tph x PN/120 = 4ϕfTph

Where Tph = no. Of turns connected in series per phase

ϕ = flux/pole in weber

P= no. Of poles

F= frequency in Hz

Zph= no. Of conductors connected in series per phase. = Zph/3

The EMF equation depends on the coils and the conductors on the stator. For this motor, distribution factor Kd and pitch factor Kp is also considered.

Hence, E = 4 x ϕ x f x Tph xKd x Kp

The torque equation of a permanent magnet synchronous motor is given as,

T = (3 x Eph x Iph x sinβ) / ωm

Direct Torque Control of Permanent Magnet Synchronous Motor

To control the permanent magnet synchronous motor, we use different types of control systems. Depending on the task, the necessary controlling technique is used. The different controlling methods of permanent magnet synchronous motor are,

Sinusoidal Category

• Scalar
• Vector: Field oriented control (FOC) (with and without position sensor)
• Direct torque control

Trapezoidal Category

• Open-loop
• Closed-loop (with and without position sensor)

Direct torque control technology of this motor is a very simple control circuit with effective dynamic performance and good control range. It doesn&#;t require any position sensor for the rotor. The main disadvantage of using this control method is, it produces high torque and a current ripple.

Construction

The permanent magnet synchronous motor construction is similar to the basic synchronous motor, but the only difference is with the rotor. The rotor doesn&#;t have any field winding, but the permanent magnets are used to create field poles. The permanent magnets used in the  PMSM are made up of samarium-cobalt and medium, iron, and boron because of their higher permeability.

The most widely used permanent magnet is neodymium-boron-iron because of its effective cost and ease of availability. In this type, the permanent magnets are mounted on the rotor. Based on the mounting of the permanent magnet on the rotor, the construction of a permanent magnet synchronous motor is divided into two types. They are,

Surface-mounted PMSM

In this construction, the magnet is mounted on the surface of the rotor. It is suited for high-speed applications, as it is not robust. It provides a uniform air gap because the permeability of the permanent magnet and the air gap is the same. No reluctance torque, high dynamic performance, and suitable for high-speed devices like robotics and tool drives.

Buried PMSM or Interior PMSM

In this type of construction, the permanent magnet is embedded into the rotor as shown in the figure below. It is suitable for high-speed applications and gets robustness. Reluctance torque is due to the saliency of the motor.

Working of Permanent Magnet Synchronous Motor

The working of the permanent magnet synchronous motor is very simple, fast, and effective when compared to conventional motors. The working of PMSM depends on the rotating magnetic field of the stator and the constant magnetic field of the rotor. The permanent magnets are used as the rotor to create constant magnetic flux, operates and locks at synchronous speed. These types of motors are similar to brushless DC motors.

The phasor groups are formed by joining the windings of the stator with one another. These phasor groups are joined together to form different connections like a star, Delta, double and single phases. To reduce harmonic voltages, the windings should be wound shortly with each other.

When the 3-phase AC supply is given to the stator, it creates a rotating magnetic field and the constant magnetic field is induced due to the permanent magnet of the rotor. This rotor operates in synchronism with the synchronous speed. The whole working of the PMSM depends on the air gap between the stator and rotor with no load.

If the air gap is large, then the windage losses of the motor will be reduced. The field poles created by the permanent magnet are salient. The permanent magnet synchronous motors are not self-starting motors. So, it is necessary to control the variable frequency of the stator electronically.

Permanent Magnet Synchronous Motor vs BLDC

The differences between permanent magnet synchronous motor (PMSM) and BLDC (brushless DC motors) include the following.

Permanent Magnet Synchronous Motor

BLDC

These are brushless AC synchronous motors These are brushless DC motors Torque ripples are absent Torque ripples are present Performance efficiency is high Performance efficiency is low More efficient Less efficient Used in industrial applications, automobiles, servo motors, robotics, train drives, etc Used in electronic steering power systems, HVAC systems, hybrid train drives (electrical), etc Produces low noise Produces high noise.

 Advantages

The advantages of permanent magnet synchronous motor include,

• provides higher efficiency at high speeds
• available in small sizes at different packages
• maintenance and installation is very easy than an induction motor
• capable of maintaining full torque at low speeds.
• high efficiency and reliability
• gives smooth torque and dynamic performance

Disadvantages

The disadvantages of permanent magnet synchronous motors are,

• These type of motors are very expensive when compared to induction motors
• Somehow difficult to start-up because they are not self-starting motors.

Applications

The permanent magnet synchronous motors applications are,

• Air conditioners
• Refrigerators
•  AC compressors
• Washing machines, which are direct-drive
• Automotive electrical power steering
• Machine tools
• Large power systems to improve leading, and lagging power factor
• Control of traction
•  Data storage units.
• Servo drives
• Industrial applications like robotics, aerospace, and many more.

Thus, this is all about an overview of the permanent magnet synchronous motor &#; definition, working, working principle, diagram, construction, advantages, disadvantages, applications, emf, and torque equation. Here is a question for you, &#; What is the purpose of using a permanent magnet in synchronous motors?

For more information, please visit Electric Motors Are Rated at Synchronous Speed.