Overview

In a Levitronix pumping system, there are three main elements which convert the electrical power to obtain a combination of flow and pressure, which defines the hydraulic power. These elements are:

• Controller
• Motor
• Impeller

Each conversion step inevitably causes some losses. The relative weight of these losses over the power successfully transformed defines the efficiency of each component. The combination of the power drawn by the whole system and the hydraulic power defines the overall pumping system efficiency.

An example is brought in Figure 1: a Levitronix IPS-100 pump absorbing 76 W of electrical power from the grid will deliver 38% of it as hydraulic power. The main source of loss is within the pump head (hydraulic losses), but controller and motor contribute to the losses as well.

Overall, the system efficiency of Levitronix pumps can usually reach about 50%, at the BEP (Best Efficiency Point). The BEP is often used a design parameter for the pump developers, and as a pump selection criteria for end users.

In this chapter, for each component there will be a description of:

• Overview of its role
• How they perform the power conversion
• The sources of losses and efficiency.

Pump Controller

Overview

The pump controller acts as the brain of the pump. It has the critical role of receiving signals from the sensors, elaborate them, and regulate the amount of current directed towards each bearing coil in the motor to maintain the impeller levitating. Furthermore, the controller regulates how much current goes into the motor drive coils, to generate sufficient torque to rotate the impeller.

The controller can also receive external inputs (e.g. a speed setpoint, or a process setpoint) and use them as a reference to perform a PI control loop.

All these actions are performed simultaneously and continuously at a high frequency, to ensure proper functioning.

Power Transfer

The controller’s chip needs power to operate. Therefore, the controller absorbs a certain amount of electrical power from the grid, uses part of it to run its internal processes and calculations, and direct the majority of the electrical power in an ordered way to the motor.

Losses and Efficiency

The controller has an efficiency of about 90% (depending on model and working point). This means that 10% of the absorbed energy is ultimately dissipated in heat in the controller. As a result, the controller might heat up. The higher temperature achieved will cause a thermal dissipation due to convection with the environment. For this reason, it is important that the controller is placed in a ventilated cabinet. If this doesn’t happen, the temperature inside the controller could increase until the safety limit. This could cause premature failure of the electronic components in the controller.

Pump Drive (Motor)

Overview

Levitronix products operate with a variety of motors designs. Most of them are derived from 2- or 3-phase synchronous motors, whose rotor is a permanent magnet (with one or more polar pairs). Their specialty is that the motor’s stator has the additional task of creating a magnetic field to levitate the rotor. This is obtained by the magnetic bearing coils and currents.

Power Conversion

Their functioning principle is that the electric currents in the stator’s coils generate a magnetic field, which is picked up by the iron claws and directed towards the rotor. As the currents vary, the magnetic field will vary and the rotor will rotate to align itself with the magnetic field. The higher the current, the stronger the magnetic field, the higher the torque will be. Depending on the voltage, electronic components and resisting torque, the rotor will rotate at a different speed.

The mechanical power is defined as the product of Torque and Speed, therefore the motor converts electrical power in mechanical power.

Losses and Efficiency

The motor uses some of the electrical power to magnetize the motor ferromagnetic core. The continuous change of the magnetic field causes hysteresis in the material, and eddy currents. These two physical phenomena are commonly referred to as sources of “iron losses” (losses in the iron components of the motor). For practical purposes, the iron losses are generally considered to be almost constant for a certain rotational speed, and they grow with the square of the speed.

The other main source of heating in the motor is the “copper loss”. It refers to the “Joule heating” effect happening in the windings as the current flows through them. Since at high speeds the currents can be as high as a few Amperes, and the windings are long and thin, the copper losses can be decisive in for the heating of the motor. The copper loss grows with the square of the current flowing through the windings.

Levitronix motors generate two types of magnetic fields: one to levitate and stabilize the impeller (magnetic bearing field) and one to drive the rotation (drive field). To obtain them, the motor can have two separate sets of windings, or a combined winding. Either way, it is always possible to define the currents and copper losses associated with each magnetic field. The power associated with the active magnetic bearing is not converted into useful mechanical power, therefore the efficiency of a Levitronix motor is typically between 70% and 90%, depending on the working point and design.

The power dissipated as heat can increase the motor temperature, if the cooling is not sufficient to remove it. To maximize the cooling, most Levitronix motor have either an integrated liquid cooling loop, or external cooling modules (operated by compressed air, a fan, or water).

Impeller and Pump Head

Overview

The motor’s rotor has some fins directly mounted on, and acts as the pump’s impeller. Its purpose is to accelerate the process fluid in the pump head, and discharge it at the outlet. The pump casing (the static volute surrounding the impeller) decelerates the fluid to build up pressure.

Power Conversion

The details of the geometry, materials, and surfaces define how much flow rate and pressure the pump will generate, at a certain speed and torque.

Since the hydraulic power is calculated as the product of flow rate and pressure, it can be derived that the impeller converts the mechanical power into hydraulic power.

Losses and Efficiency

The impeller accelerates the fluid incoming through the inlet, while the volute decelerates it and converts the velocity in pressure. The fluid viscosity (a measure of the friction within the fluid) resists such accelerations, dissipating part of the hydraulic power in heat.

Levitronix pumpheads feature wide gaps around the impeller for functional reasons (cleanability, to avoid contact and particle accumulation, to reduce shear stress..). As a downside, these wide gaps can cause re-circulation of the fluid within the pump. As a result, part of the power impressed on the fluid is dissipated in fluid circulation within the pump head. In the case of a pump operating against a dead head, the fluid is exclusively re-circulated, generating no useful hydraulic power.

As a result, the pump head has a relatively low efficiency compared to conventional centrifugal pumps (which are optimized for efficiency, disregarding other valuable features). The hydraulic efficiency of Levitronix pump heads typically remains below 60%.

The hydraulic losses in the pump head result in fluid heat up. The temperature increase depends on the fluid properties (density, viscosity, specific heat), on the flow rate and on the pump head design.