how to define your motor inside your drive train

Your machine determines your power drive system.

When defining your power drive system we should always rely on clarified knowledge of your machine. Your application data is indeed paramount. Therefore, we will work our way through the drive train, starting from your application towards the grid supply.
The application domain is indeed very wide. For starters, we will focus on mechanical requirements of your critical machine.
Let’s get into some basic questions first:

What is the available motor torque over its speed range?

The machine is driven by the torque delivered by the motor. We will need to size the motor to provide enough continuous available torque to overcome the required torque of the machine over the required speed range.
The figure below shows the steady state available motor torque versus the speed.

    continuous available motor torque versus speed

    Figure 1: Continuous available motor torque over the whole speed range

    The dotted green line on the left of “lowest speed at constant torque” is only possible in case of forced cooling (i.e. separately ventilated, surface -cooled (TEFV or IC416)). Rated torque is available up to base speed (i.e. for a 4-pole motor at 50Hz base speed is 1500rpm, at 60Hz base speed is 1800rpm). Above base speed, in the zone often referred to as constant power zone, the continuous available torque decreases inversely.

    The loads of many applications can be defined by one of three primary types:

      • variable torque (centrifugal pumps, fans, centrifuges, …)
      • constant torque (conveyors, augers, reciprocating compressors, crushers, positive displacement pumps, kiln, …)
      • constant horsepower (center driven winders, machine tools, …)

    Just click on each of the load types below to check your self how the load curves of the different primary types will map onto the torque vs speed diagram of the motor.

    TIP:
    Ask your machine manufacturer to provide the specific torque versus speed curve of the machine.
    The motor-drive combination should deliver a torque versus speed that covers the required machine torque over the whole speed range.
    Your variable speed drive-motor-manufacturer or solution provider should deliver this mapped diagram in his quotation.
    Reach out to us for expert assistance for your project team in organizing pre-bid meetings and setting up invitations to bid.

    Above relates to steady state operation, but in lots of applications the drive-motor combination must also be able to deliver short transient torques even surpassing the rated motor torque. Think about a loaded large conveyor belt in the cement industry where up to 180% of rated torque can be required for starting it up (often referred to as breakaway torque). Another example is a rubber mixer with regular important torque surges because of the operation in batch mode. New batches can impose up to 250% of rated torque. Obviously, proper sizing of motor and drive needs to take this into account.
    In some cases further investigation of the expected dynamic behaviour of the machine can be required (i.e. torsional analysis, resonance speeds, …).
    Close collaboration of your project team with your machine manufacturer is preeminent.

    How does the power flow influence the power drive system?

    For now, we focused only on the steady and transient torque. What about the direction of the torque?
    When the motor is driving the machine, the power flows from the source to the motor where it is converted into mechanical power for the load.
    When braking the machine, the power flows in the reverse direction from the load to the motor. This power is fed back electrically to the source (4 Quadrants-operation) or absorbed in a braking resistor and dissipated as heat.
    Check your self in the figure below how the power flow changes for the different operation modes.

    Without going into detail about the 4 quadrants, we can simply focus on the direction of the power flow and its consequences for the configuration or topology of the drive.

    General purpose unidirectional applications such as centrifugal pumps, fans, compressors and extruders require only one quadrant operation and the power flow is always unidirectional, that is from the grid to the motor to the machine. So the front-end of the drive can simply be considered as a one way rectifier often referred to as a Direct Front-End (=DFE).
    If the power flow reverses intermittently, that is from the machine to the motor, an electronic braking unit can dissipate that energy into a braking resistor. Enforcing a deceleration ramp to a high inertia machine is a typical use case: occasional recovery of the braking energy is in most cases not economical viable.
    Applications that do impose important and consistent power flow from the machine to the motor will be equipped with an Active Front-End (=AFE) allowing recovery of energy to the grid. Large inclined conveyor belt machines and test stands with the power-drive-system simulating the load of the tested machine are typical examples for this use case.

    The requirement of uni or consitent bi-directional power flow of our application determines whether an Active Front-End is required or not.
    This is only a fraction of the parameters that determine the configuration or topology of your drive.
    In our chapter on the different topologies of Medium Voltage drives you will notice that power quality as well on the input as on the output of the drive is the big differentiator in Medium Voltage drives configurations.

    We are ready to assist you. All it takes is that you contact us for a free consultation.
    Further down the road, we can enable informed decision-making by providing you our independent advice and support for any of following tasks:

    • Setting up the project plan presentation.
    • Definition of the requirements of the power drive system(PDS).
    • Evaluation of the composition of the power control system (transformer, variable frequency drive and motor).
    • Internal and external pre-bid meetings with vendors and/or EPC’s.
    • Seting up an invitation to bid for EPC’s.
    • Screening of preferred vendors of inverters, motors and transformers.
    • Verification of the received quotations and exceptions versus  your specs of the converter duty transformer, variable frequency drive vfd and motor.
    • Selection of the vendor of the power control system.
    • Order clarification of the components of the power control system.
    • Factory acceptance tests requirements.
    • Witnessed factory acceptance tests requirements and test plan.
    • Site acceptance tests requirements of your power drive system.
    • Validation of the site acceptance tests for your power drive system in the field.

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