Sizing and Selecting of Variable Speed Drives

Sizing and Selecting of Variable Speed Drives banner

You’ve got to get the drive rating right!

The key aspect of designing a variable speed drive system is selecting the right VSD. If you choose a unit that is too small it will be unable to control the connected motor optimally at all necessary operating points. However, if the unit it too large there is a risk that the motor will not be controlled properly, the design may also not be cost-effective.

It is important to have knowledge of the following basic parameters:

  • Central versus decentralisation
  • Environment
  • Motor cable length
  • Regenerative energy
  • Derating of VSD
  • Control range and field weakening
  • Overload capacity
  • Current distribution in the VSD
  • Rating of VSD from motor specifications

After clarifying the basic design parameters for an application, design and analysis of the mechanical components is carried out. It is vital that you determine which motor is used before you select a suitable VSD. For example, in facility service systems, final selection is often done shortly before the building is completed.

In order for an optimised analysis of flow conditions to be carried out reliably most of the components are defined at this time.

The more complex and challenging the application is, the more things need to be taken into account in the design. It is important to check that the VSD has the features needed for the drive, some manufacturers can save costs by restricting technical features.

Rating of the Variable Speed Drive from Motor Specifications

A common method engineers use when selecting VSDs is based on the power rating of the motor to be used. Even though manufacturers specify the power ratings of their VSDs, this data normally relates to standard four-pole motors.However, the rated currents of motors differ a lot at the same power depending on the construction of the motor (e.g. standard motor and geared motor) and it’s number of pole pairs, meaning this method is only suitable for providing a rough estimate of the proper VSD size.

It should also be noted that the current drawn by a motor depends on whether it is connected in star or delta configuration. For this reason VSDs should be selected based on the rated current for the type of configuration selected (star or delta).

In addition to the motor current, the required motor voltage must be taken into account. Many VSDs can operate over a wide mains supply voltage range (e.g. 3 x 380 - 500V) and thus provide a wide output voltage range. Thus the maximum output current is higher at a lower mains voltage and, correspondingly, lower at a higher voltage.

Overload capacity

When selecting a VSD, the load conditions of the application should always be taken into account first. A fundamental distinction is made between quadratic and constant load load characteristics, which are the most common in practice.

When a VSD controls a motor, torque limits can be set for that motor. Selecting a VSD with an apparent power rating that matches the rated current or power of the motor ensures that the required load can be driven reliably. However, an additional reserve is necessary in order to enable smooth acceleration of the load and also cater for occasional peak loads.

Below are examples of consistent load of torque characteristic. If a load is placed on a conveyor belt, the torque that must be applied to transport the load is constant over the entire speed range.

Application Excess load
Lifting equipment 160%
Conveyor belt 160%
Stirrer / Mixer / centrifuge 160%
Rotary piston compressor / piston compressor 150%
Spiral pump (thick sludge) 150%
Sludge dehydration press 150%
Piston pump 150%
Rotary gate valve 150%
Rotary piston blower 110%
Surface aerator 110%
Metering pump 110%
Booster pumps (2-stage) 110%
Recirculation pump 110%
Side channel blower for pool aeration 110%

With a constant load, an overload reserve of approximately 50 to 60% for 60 seconds is typically used. If the maximum overload limit is reached, the response depends on the VSD used. Some types switch off their output and lose control of the load. Others are able to control the motor at the maximum overload limit until they trip for thermal reasons.

A quadratic load of characteristics usually occurs in applications where the increasing speed leads to an increasing quadratic load torque. Fans and centrifugal pumps are amongst the types of equipment that display behaviour of this kind. Furthermore, most applications with quadratic torque characteristic, such as centrifugal pumps or fans, do not require rapid acceleration phases. For this reason excess load reserves of 10% are usua;;u chosen for quadratic torques.

Here are some examples of quadratic load torque characteristics.

Application Excess load
Fan 110%
Well pump 110%
Booster pump / centrifugal pump 110%
Filter infeed pump 110%
Groundwater pump 110%
Hot water pump 110%
Non-clogging pump (solid materials) 110%
Centrifugal pump / fan 110%
Primary and secondary heating pump 110%
Primary and secondary cooling water pump 110%
Rainwater basin evacuation pump 110%
Recycling sludge pump 110%
Spiral pump (thin sludge) 110%
Submerged motor pump 110%
Excess sludge pump 110%

Even with quadratic load and an overload capacity of 10% modern VSDs can be set up to have a higher break-away torque at start to ensure the proper start of the application. Remember to consider whether the application will always require quadratic torque. For example, a mixer has a quadratic torque requirement when it is used to mix a very fluid medium, but if the medium becomes highly vicious during processing, the torque requirement changes to constant.

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