How Variable Frequency Drive Work
There are a few technologies to control variable frequency drive. One of the simplest control-method among them is linear voltage/frequency or scalar method, which is suitable for general purpose applications.
Before we go ahead, we will go back to refresh some basic about ac motors.
When AC voltage is applied to stator winding, magnetic flux is developed in winding.
The amount of flux lines the magnetic field produce is proportional to the voltage divided by frequency as per below formula.
f~E/f That means increasing the supply voltage increases the flux of the magnetic field and decreasing the frequency increases the flux.
The following diagram represents a simplified equivalent circuit of an AC motor.
In above diagram,
IM stands for magnetizing current and is responsible for producing magnetic line of flux which magnetically links with rotor. IM is typically about 30% of rated current.
Iw is working current that flows into the rotor and produces torque. Iw depends on load. An increase in load causes Iw to increase and an decrease in load causes Iw to decrease.
IM = E
2pfLm
Take note E/f is proportional to flux f.
Is is referred to stator current, which flows through the stator and is known as line current.
Typically magnetizing current IM remains constant. Iw will vary with the applied load which causes the corresponding changes in Is.
Is= sq of IM2+Iw2
Now we will study how scalar control method of VFD works.
Every motor has it’s rated voltage and frequency. The ratio between voltage and frequency is referred to as volt per hz.
For example: 460V and 60Hz rated motor’s volt per hz ratio is 460/60=7.67 V/Hz
Let’s see some formulas related to motor here.
ns=120f/p ………………………………………………..1
f ~ E/f ………………………………………………........2
T=kfIw ………………………………………………......3
IM= E …………………………………………………...4
2pfLm
Is= sq of IM2+Iw2 ………………………………………5
As per above formulas, we can see that flux, torque and magnetizing current are dependent on the ratio.
Without increasing voltage (E), increasing frequency will cause a corresponding increases in speed and flux will decrease causing motor torque to decrease.
Magnetizing current (IM) will also decrease. A decrease in magnetizing current will cause a corresponding decrease in stator or line (IS) current.
These decreases are all related and greatly affect the motor’s ability to handle a given load.
The basic principle of inverter is when frequency is reduced in order to have lower speed, VSD reduces supply voltage to motor correspondingly so that flux remains constant. When flux is constant as per formula 2, torque remains constant too.
The motor running constant flux has constant torque and known as working in the constant torque region.
Now frequency is increased more than it’s rated frequency in order to have higher speed. Since supply voltage is 460V, VSD can’t increase supply voltage more than 460V, causing a corresponding flux and torque decrease.
Normally VSD can generate 600Hz.
Motor running at rated voltage and above rated frequency is in constant speed region.
T~f2
Some application requires to run motor above its’ rated frequency.
In this case, self-cooled motor may not develop enough air-flow for cooling at reduced speed and full load. In self-cooled motor, fan is attached to rotor. When motor’s speed is reduced, the rotation of rotor and fan decreases and can’t provide enough air to cool down motor.
For self-cooled motor, recommended minimum frequency is 1/3 rated frequency as rule of thumb. But for safe side, engineer normally reduces half of motor’s rated frequency.
If application is required to reduce more less than 1/3, non self- cooled motor must be used. In non self-cooled motor,fan isn’t attached to rotor and then fan speed doesn’t depend on rotor. Fan run itself with separated power supply so that even motor speed is reduced, fan speed remains constant.
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