In a previous post about VFD, we have described some common know facts. What are they, what elements are they consist of and what are their purpose. It’s time to look at VFDs in greater detail. Let’s focus on scalar control of VFD!
The introduction of VFD control modes
Variable frequency drives are coming with various names, shapes, and control algorithms. Although Drives OEM’s are racing in implementation on new functions and algorithms, principles are always the same and thus we can divide them into two groups of control algorithms:
– Scalar Control – which is the easiest control mode of VFD. It is widely used in dynamically non-demanding applications like HVAC or pump control. Its principle is based on controlling the motor steady-state values, which are amplitudes and pulsations of supply voltage – as a result of its voltage and frequency. Position of current and flux vectors are not considered, thus there is no control over motor transition states.
– Vector Control – which utilizes complex algorithms of decoupling torque and flux vectors to provide high dynamic control over machine steady and transient states. It is used in highly demanding dynamic applications where precise control over speed and torque is crucial. Good examples are cranes or winches.
Both have their advantages and limitations. Let’s talk about scalar control or so called u/f control.
The principle of the scalar control
The purpose for speed control is to vary motor’s speed over a wide range for a specific load. But how we can vary the motor speed without changing the load or torque?
One’s idea might be to vary the supply voltage, like it is done in soft start applications.
Unfortunately decreasing the voltage force the motor to draw excess current to deliver expected power and in result overheats motor’s windings. Increasing the voltage in the other hand, by exceeding the insulation limit will damage the windings as well. After all we would be able to vary the motor speed around 10% only.
Another idea might be to vary frequency of supplied voltage. Besides the motor poles, frequency is a component used to calculate motors speed.
By reducing the frequency we will decrease stator and rotor impedances and as a result increase, od magnetizing current shifting motor in deep saturation which increases losses and causes degradation of efficiency. Increasing frequency above nominal value affects in loss of torque with inverse proportion to frequency.
For a given load, motor torque is expressed by the following equation:
Which result is approximated relation:
The U/f ratio determines the magnetic flux density. Keeping constant magnetic flux density, thus constant U/f ratio we ensures optimum and constant torque for the motor load. That means, if we want to reduce motor speed, we need to reduce frequency together with voltage. By keeping the constant ratio we will overcome mentioned problems.
The linear characteristic of U/f is defined in few points (very often in 2 endpoints). Often there is a possibility of boosting starting point voltage to compensate for voltage losses on stator resistance on low frequencies. If there would be no voltage boost, selecting the frequency below the minimum frequency would result in stopping the motor shaft.
In addition to linear characteristics, VFD manufacturers often implementing quadratic (U/f2) and custom (user-defined multi-point) characteristics to match various load curves.
The stator of the motor is consists of a bunch of coils using a considerable amount of copper wires and all copper wires have their resistance. Due to voltage loss on this resistance, we can see one limitation of scalar control. On the lower part of the U/f characteristic, when we operate the motor at a very low speed, voltage fed into the stator may be just “swallowed up” by stator voltage losses, and as a result, the motor will stop. To overcome this issue voltage boost may be applied to the lower part of the characteristic to generate the necessary torque to run the motor. Above the threshold, VFD is running with its linear characteristic.
Slip of induction motor is defined as difference between speed of rotation of magnetic field (synchronous speed) and rotation speed of rotor and is presented by the following relation:
An increase of the shaft torque (load) is causing an increase in slip which results in a decrease in rotation speed. If speed control is necessary for application and there is no encoder installed on the motor, slip compensation function may be handy. The current on the output of VFD is measured and actual torque is calculated. This calculation is used to increase frequency proportional to the torque. This function is very useful when the motor is operated on low frequencies, but with high load.
Let assume we have a hypothetical 2 poles motor with a synchronous speed of 3000 rpm and slip of 30 rpm. When we operate a motor with a high load on the nominal speed we are getting a 1% of speed error. When we operate the same motor with the same load at speed of 300 rpm we get a 10% error. Here slip-compensation may become handy.
Yes! Most of the VFD on the market allow you to control few another VFDs in various master-slave configuration, but that’s not all. Using VFD which scalar control you may “trick” your VFDs to see few smaller motor connected to it as a one bigger induction motor and operate all of them simultaneously with the same speed. There are some important limitations though:
- The total sum of all motors currents cannot exceed VFD current rating,
- All motors must be identical,
- Each motor have to be protected separately against overcurrent,
- Running few motors on one VFD may require installation of additional filters,
- Motor circuit breakers and contactors must be closed before starting VFD and cannot be opened until motors stop turning after stopping the VFD.
Advantages and Disadvantages in scalar control of VFD
As I have described before, the U/f control You can find in non-demanding applications. Due to its simplicity, this control mode has the following disadvantages in comparison with vector control:
- No control over transient states of machine,
- No torque control,
- Oscillations of speed around its set point,
- Low torque on low frequencies,
Which are in oppose to the following benefits:
- Low cost and high simplicity,
- Ease of commissioning,
- Possibility to use 1 VFD to control few motors,
This make scalar VFDS a perfect choice for pumps, fans, compressors, blowers, conveyors and all another type of loads where there is no need for precise speed or torque control. Good candidates for implementing VFDS onboard would be Sea Water pumps, ventilation fans or refrigeration fans – flow or temperature loops where VFD scalar control may bring energy saving.
A short comparison to Vector control
|Scalar control||Vector control|
|Low cost||High cost|
|Excellent for non-demanding applications||Excellent for high dynamic applications|
|Low complexity||High complexity|
|Low torque on low frequencies||Optimum torque on whole speed range|
|Ease of use||Require more preparations|
|Multi Motor Control||Controlling 1 motor at the time|
|Lower precision with speed control||Excellent precision of speed and torque|
Do You want to learn more about VFDs? Look for application notes and technical guides from biggest AC drives manufacturers or stay tuned for the next posts. Next we should look at what happen with VFD in scalar control above its rated frequency.