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Digging into motor application needs

01 November, 2018

In the second part of his series looking at the steps involved in choosing a low-voltage motor, Rob Wood, ABB’s UK sales manager for LV motors, continues to examine the “requirements” for such choices.

This month we continue to work through the acronym - R.I.S.E (Requirements, Infrastructure, Standards and Environment) – that helps to select the right motor for your application.

Let’s continue with “Requirements”. Before we can determine the required motor power, we must find out the speed and the load (torque) characteristics of the application. 

An induction motor is normally a single-speed machine, where its speed depends on the frequency of the electricity supply and the number of stator poles. The motor speed can be calculated using this formula: Speed (rpm) = Frequency (Hz) x 60 / Pole pairs

So for a four-pole motor (four-pole = two pole pairs) connected to a 50Hz electrical supply: Speed = 50 x 60 / 2 = 1,500 rpm.

It should be noted that this is the synchronous speed of the motor. The actual running speed at full load will typically be 2-4% lower due to slip.

For motors driven by a variable-speed drive (VSD), the minimum and maximum speeds must be taken into consideration as this can affect the cooling arrangements and even the bearings required. 

Knowing the behaviour of the load as a function of speed is extremely important. Three typical, yet different, load torque characteristics are:

• Applications that, after they start, accelerate and reach running speed and have a relatively fixed torque requirement. These are defined as constant-torque. Typical constant-torque applications include elevators, hoists, conveyors and positive displacement pumps. 

• Applications where the load torque varies as a function of speed are linear torque and normally sized based on the highest continuous load, which is usually at the highest speed. Applications where the load torque increases in proportion to speed include the rolling and processing of paper, textiles, and extruders.

• Applications where the load torque increases with the square of the speed are defined as variable or quadratic torque. This usually happens where there is gas or liquid friction, as found in many of the most common motor applications, such as blowers, fans and centrifugal pumps.

In applications with this relationship between power and speed, it is possible to make huge energy savings by adjusting the speed of the motor with a VSD, instead of controlling the amount of gas or liquid with a slide valve or throttle valve. To size a motor for this type of application, simply calculate the power required as a function of speed and torque using the formula:

Power (kW) = Speed (rpm) x Torque (Nm)/ 9550

…and then select a motor with a suitable output rating. 

It is also important to check the duty cycle. Will the motor be running continuously or in an intermittent duty cycle? 

The IEC60034-1 standard defines ten duty types from S1 to S10. To calculate the required power based on the duty cycle is not easy. Motor manufacturers can usually provide software tools to assist with this. 

Now that we have determined the required motor power, we should check the mechanical load forces. Axial and radial forces have an impact on the bearing lifetime, and in case of high radial forces also on the shaft dimensioning.

Speed, load torque and duty type form the basic information needed to determine the correct motor power for your application, but as we will see in the next column, network restrictions on starting current will often influence the choice of motor. 

For more information, you can view this video:

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