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 and working our way towards the grid supply.
The application domain is indeed vast. For starters, we will focus on the 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 torque delivered by the motor drives the machine. We will need to size the motor to provide enough continuous available torque to overcome the machine’s required torque over the required speed range.
The figure below shows the steady-state available motor torque versus the speed.
 You can directly view the relevant content on this video, or you can continue reading below the video.

Continuous operation up to the dotted green line on the left of “lowest speed at constant torque” is only possible in the 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 the base speed, in the zone often referred to as the 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, grinders,  …)

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.

Above relates to steady state operation, but in many applications, the drive-motor combination must also deliver short transient torques, even surpassing the rated motor torque. Consider a loaded large conveyor belt in the cement industry where up to 180% of rated torque can be required to start it up (often referred to as breakaway torque). Another example is a rubber mixer with regular, significant torque surges because of the operation in batch mode. New batches can impose up to 250% of rated torque. Proper sizing of the motor and drive needs to be taken into account.
In some cases, further investigation of the machine’s expected dynamic behaviour (e.g., torsional analysis, resonance speeds) can be required.
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 drives the machine, 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 the figure below for 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 drive’s configuration or topology.

	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 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 economically viable.
	Applications that impose important and consistent power flow from the machine to the motor will be equipped with an Active Front End (=AFE), allowing energy recovery 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 of this use case.

The requirement of our application’s uni- or consistent bidirectional power flow determines whether an Active Front End is required.
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, both on the input and output of the drive, is the big differentiator in these 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:

• Develop the project plan presentation.
• Define and manage the requirements of the power drive system PDS (converter duty transformer, variable frequency drive vfd and AC-motor).
• Evaluate the composition of the power drive system.
• Set-up and coordinate internal and external pre-bid meetings.
• Set-up the invitation to bid with Engineering, Procurement and Contracting companies and/or vendors.
• Assess preferred manufacturers of transformer, inverter and motor.
• Verify the offers and exceptions in regard to your specs of your PDS.
• Help to select the vendor of transformer, variable frequency drive and motor.
• Clarify the order of your PDS.
• Set-up witnessed and non-witnessed Factory Acceptance Tests (FAT) requirements and test plan.
• Define Site Acceptance Tests (SAT) requirements.
• Validate the site acceptance tests for the components of your power drive system PDS on-site.