Why can't ordinary motor be used as
variable frequency motor?
General motors are designed according to
constant frequency and constant voltage, so it is impossible to completely get
used to the requirements of frequency converter speed regulation.
1. The influence of frequency converter on
the motor is mainly in the power and temperature rise of the motor
Inverter can produce different degree of
harmonic voltage and current in the operation, making the motor run in
non-sinusoidal voltage and current. The high order harmonic inside can cause
copper loss of stator, iron loss and copper loss of rotor, as well as
additional loss. The most significant is the rotor copper loss, which will
cause high temperature rise by 10% to 20%, and obvious power decrease.
2. Insulation strength of motor
The carrier frequency of converter ranges
from thousands to more than ten kHz, which makes the winding of the stator endure
a very high rate of voltage rise. This is equivalent to imposing a very steep
impulse voltage on the motor, and makes the inter-turn insulation of the motor endure
serious test.
3. Harmonic electromagnetic noise and vibration
When general motor uses frequency converter
power supply, the vibration and noise caused by electromagnetic, mechanical,
ventilation and other factors will become more chaotic. The harmonics contained
in the variable frequency power supply and the inherent space harmonics of the
electromagnetic part of the motor interfere with each other, and constitute a
variety of electromagnetic excitation force, and then increase the noise.
Because the operating frequency scale of the motor is wide and the speed change
scale is large, the frequency of various electromagnetic force waves is
difficult to avoid the natural oscillation frequency of each structural part of
the motor.
4. Cooling problem at low speed
When the frequency of the power supply is relatively
low, the loss caused by the higher harmonics in the power supply is large.
Secondly, when the speed of the variable frequency motor decreases, the cooling
air volume decreases in direct proportion to the cubic power of the speed,
which causes the heat of the motor to be emitted and the temperature rise to
increase sharply, making it difficult to achieve constant torque output.
Differences Between Ordinary Motor And Variable
Frequency Motor
1. Higher insulation grade requirements
The insulation grade of variable frequency motor
usually needs to reach F or higher. The ground insulation and turn insulation should
be strengthened. The resistance ability to impact of voltage of insulation
should be particularly considered.
2. Higher requirements of vibration and
noise of variable frequency motor
The design of variable frequency motor
should take the rigidity of the motor components and the whole into account. The
natural frequency should be improved to avoid resonance with various force
waves.
3. The cooling method of variable frequency
motor is different
Frequency conversion motors generally adopt
forced ventilation cooling, that is, the main motor cooling fan is driven by an
independent motor.
4. Different requirements for protection
The bearing insulation method should be adopted
for variable frequency motors with power exceeding 160kW. The main reason is that
magnetic circuit asymmetry is easy to occur, and the shaft current will also
occur. When the current generated by other high frequency weights is combined
with the action of the shaft current, the shaft current will be greatly added,
and then lead to bearing damage, so the insulation method is generally adopted.
For constant power variable frequency
motor, when the speed exceeds 3000r/min, special high-temperature resistant
grease should be selected to compensate the temperature rise of the bearing.
5. Different cooling systems
Independent power supply should be chosen
for variable frequency motor radiating fan to ensure continuous radiating
capacity.