If you think choosing the right VSD for your application is more difficult than it appears, read our 6-point guide to ensure you get it right every time.

Basically, a variable speed drive (VSD) controls the flow of energy from the power supply to the process. Variable speed drives are located between the power supply and the motor. Power from the power supply is fed into a drive, which then regulates the power fed to the motor. IPD offers ABB drives that are used to improve energy efficiency in a wide range of industries and applications.

1. Keep in mind that the motor is a source of torque

Although any AC motor produces torque, the power rating is the most commonly used when examining motor catalogue ratings.

When connected to the network, these ratings are based on the fixed speed output. Torque is proportional to both power and speed, as T (Nm) = P (kW) x (9550)/ n (r/min). When dimensioning any variable speed drive, it is critical to consider torque.

2. Keep in mind that motor speed is not limited by network frequency

A motor with a speed range of 0 - 1500 rpm can have a frequency range of 0 - 50 Hz for a 4-pole motor and 0 - 75 Hz for a 6-pole motor. This has some advantages, such as the fact that a specific motor frame is capable of a specific output torque - not power.

For example, if a constant torque of 750 Nm is required from 150 to 1500 rpm, which is equivalent to 115 kW at 1500 rpm, a 160 kW 4-pole motor with shaft mounted fan could be used. A 110 kW 6-pole motor could also be used. If a higher starting torque than nominal is required for these combinations - for example, at 1000 Nm, the current equivalent to 160 kW for the 4-pole option and 110 kW for the 6-pole option - this could result in a frame size savings for the converter.

3. Correct motor selection based on load type

The motor is a necessary component of an AC drive system; the motor and converter cannot be considered separately. Before making any decisions, it is critical to understand the type of load to be controlled. There are three types of loads:

1. i) Quadratic load This is the most common load type, and it includes centrifugal pumps, fans, and the majority of mixers.
2. ii) Constant torque This happens when fixed volumes are handled, such as in screw compressors, extruders, or straight line web handling.
3. iii) Constant power This is typically used when a material is being rolled up and the diameter changes.

4. Know the operating speed

It is critical to understand the true operating speed range. This is not normally a problem with a centrifugal load, but with a constant torque load, it is quite possible to over-frame the motor if the true speed range is 10% to 100% (say 5 - 50 Hz) and the motor is specified for 0 - 100%. (0 – 50 Hz).

The ventilation capability of the motor fan is critical here; at low speeds, the cooling capability is clearly reduced, but this reduction is not linear.

5. Forced ventilation

When dealing with constant torque loads, selecting a fixed-speed fan on the motor can be advantageous. Extra connections to the motor are required with a fixed-speed cooling fan. It should, however, be able to produce constant torque output from a standstill or a very low speed up to the base speed.

6. Ensure the duty cycle is correct

Even a simple drive application, such as a pump or fan, will have a duty cycle. To achieve a specific rate of acceleration, an initial start-up may necessitate a high torque. A drive converter's thermal time constant is relatively short, possibly 4 - 6 minutes and it heats up and cools down quickly, whereas the thermal time constant of a motor can range from 15 minutes for a small motor to a couple of hours for a large motor.

This means that the overload requirements for converters are generally far more critical than for motors.