Not too long ago, when energy was still cheap, the majority of electric motors in industry, e.g. used to drive conveyor belts, compressors, HVAC, and pumps, were used at constant speed. Now the cost of the motor is only one or two percent of the total cost of ownership due to the energy it consumes in its lifetime, so variable frequency drives have become fairly mainstream due to the major advantages of better process control and the energy savings they offer.
An additional advantage over driveless motors is that they lower the maintenance costs due to much lower stresses on the mechanical parts, e.g. when starting up an application. With the numerous suppliers currently offering wide ranges of standard VFD’s it is safe to say that they have become a commodity.
Or are they all? Most drives on the market cater to IE3 and IE4 motors, as these will be required by legislators around the world at some point in time. These drives are often controlled by decreasing both frequency and voltage, keeping them at more or less the same ratio, to decrease motor speed.
However, to get the maximum savings some major OEM’s of applications where VFD’s are used in are looking towards IE5 motors, i.e. permanent magnet (PM) and synchronous reluctance motors.
To be able to drive these motors the controls need to be rather advanced as it needs to know the motor model, on which it can then base the torque and magnetic flux that is needed to maintain the speed that is being requested. With this capability, not only can it control these more efficient motors, but it also will be able to respond better to sudden loads and have better speed regulation, in turn improving process control!
We had several of these OEM’s come to us with the request to develop a drive for such motors for them, and they had several very good reasons:
- Decentralization. They wanted to have motor with the drive built directly on top of it, with just a communications line to the outside world. In this approach, cost can be saved by reducing shielded cables and / or filter costs.
- Ease of installation. These OEM’s handle a LOT of drives. Standard drives all have to be programmed, often with the same program. We can pre-load their drives during production with the most common settings used by the OEM, saving them a lot of time.
- Standard-plus configuration. Standard drive manufacturers try to cover a wide range of possible configurations, with often the result that the OEM ends up with a configuration that is almost but not quite right, with still some features the OEM did not need. We offer a standard power PCB, with everything else either standard or standard-plus, e.g. a different IP rating or a different interface.
However, what our customers did not realize when they came to us is that Prodrive is able to integrate more features or requirements into the drive due to the customizable character of our drive. Let’s look at two examples. First of all, these advanced drives have a lot of computational power inside. This offers in many cases, e.g. in heat pumps, an opportunity to integrate the controls of the OEM’s application into the drive by making use of the computational power already there.
Another example is that in many OEM applications fans are used that are typically running at continuous (maximum) speed. However, as we can derive an additional drive for such a fan (by using the DC Bus already present) a lower or variable speed for the fan can be implemented, at the same time reducing the amount of different types of fan motors normally required to cover worldwide different voltages!
Sensorless vector control
field oriented control
Due to the increased computational power in modern microcontrollers, field oriented control or FOC is gaining popularity compared to the more traditional V/f control.
In FOC, the (estimated) angle between the fields generated by rotor and stator is used to decompose the phase currents into a magnetizing and a torque producing component.
By separating these components, a more direct control over the torque becomes possible. Compared to a V/f based control, this leads to following benefits:
- More accurate speed control
- Torque control and/or limitation
- Power limitation
- Improved dynamic behavior
Furthermore by optimizing the magnetizing current, efficiency improvements can be obtained.