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The four steps to choosing the right motor

01 October, 2018

Rob Wood, ABB’s UK sales manager for low-voltage motors, kicks off a new series that will examine the steps you need to go through when choosing a low-voltage motor.

In the coming months, this column will look at steps to take when selecting a motor, which can be remembered using this handy acronym - R.I.S.E – or Requirements, Infrastructure, Standards and Environment.

Let’s start with Requirements.

Sizing the motor correctly is very important. A motor is correctly sized when it runs at the rated torque and speed, delivering the rated power output, while consuming the rated input current. To find out where this rated operation point is, we need to look at the torque curve of the machine.

Starting, or locked-rotor, torque is produced when the rotor is not yet turning. This must exceed the load torque for the motor to accelerate the load. Pull-up torque is produced as the load accelerates and motor speed increases. The motor will stall if the load exceeds this value. Breakdown or pull-out torque is the maximum torque that a motor can produce in the operating speed range. And full-load is the maximum torque that the motor can sustain in continuous operation without overheating.

For the starting phase, we need to check the moment of inertia of the load. High inertia loads at start-up generate extreme heat.

We also need to check whether the motor will be connected directly to the mains supply to run at fixed speed, or whether it will be controlled by a variable-speed drive (VSD).

This is important because direct-on-line motors, especially those driving heavy loads, need a high starting torque, whereas those connected to a VSD can have a controlled start with less impact on the electrical supply and less mechanical stress on driven equipment.

When calculating torque, it is important to assess the needs of the application, because under-sizing the motor will lead to overheating, potentially shortening the life of the motor.
Conversely, an oversized motor may be a waste of money. Not only are you purchasing more available power than you need, but the running costs can be higher as the duty point will be potentially lower on the motor efficiency curve.

For more information, you can view this video:

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