How does the hottest inverter control the motor sp

2022-08-17
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How does the frequency converter control the motor speed? <3 Ling wire industry completed a five month audit/p>

the rotation speed of induction AC motor (hereinafter referred to as motor) is approximately determined by the number of poles and frequency of the motor. The number of poles of the motor is fixed according to the working principle of the motor. Since the pole value is not a continuous value (a multiple of 2, for example, the number of poles is 2, 4, 6), it is generally inappropriate to adjust the speed of the motor by changing this value

in addition, the frequency can be adjusted outside the motor and then supplied to the motor, so that the rotation speed of the motor can be freely controlled

therefore, the frequency converter with the purpose of controlling frequency is the preferred equipment for motor speed regulation equipment

n=60f/p

n: synchronous speed

f: power frequency

p: motor pole pairs

conclusion: changing frequency and voltage is the best motor control method

if only changing frequency without changing voltage, the motor will be over-voltage (overexcitation) when the frequency is reduced, which may lead to motor burnout. Therefore, the frequency converter must change the voltage while changing the frequency. When the output frequency is above the rated frequency, the voltage cannot continue to increase, and the maximum can only be equal to the rated voltage of the motor

for example, in order to halve the rotation speed of the motor, change the output frequency of the frequency converter from 50Hz to 25Hz, and then the output voltage of the frequency converter needs to be changed from 400V to about 200V

2 When the rotation speed (frequency) of the motor changes, what will happen to its output torque

the starting torque and maximum torque when driven by the frequency converter are smaller than those when driven directly by the power frequency power supply.

the starting and acceleration impact of the motor when powered by the power frequency power supply is great, but when powered by the frequency converter, these impacts are weaker. Direct starting at power frequency will produce a large starting current. When using the frequency converter, the output voltage and frequency of the frequency converter are gradually added to the motor, so the starting current and impact of the motor should be smaller

generally, the torque generated by the motor decreases with the decrease of frequency (speed). The reduced actual data will be explained in some frequency converter manuals

by using the frequency converter with flux vector control, the insufficient torque of the motor at low speed will be improved, and even in the low-speed region, the motor can output enough torque

3. When the frequency converter is adjusted to a frequency greater than 50Hz, it can still maintain a good chemical stability rate, and the output torque of the motor will be reduced

generally, the motor is designed and manufactured according to 50Hz voltage, and its rated torque is also given within this voltage range. Therefore, the speed regulation under the rated frequency is called constant torque speed regulation (t=te, P =pe)

when the output frequency of the frequency converter is greater than 50Hz, the torque generated by the motor will decrease in a linear relationship inversely proportional to the frequency

when the motor operates at a frequency speed greater than 50Hz, the size of the motor load must be considered to prevent the insufficient output torque of the motor

for example, the torque generated by the motor at 100Hz should be reduced to about 1/2 of the torque generated at 50Hz

therefore, speed regulation above the rated frequency is called constant power speed regulation (P=Ue*Ie)

4. As we all know, for a specific motor, its rated voltage and rated current are unchanged

if the rated value of frequency converter and motor is 15kw/380v/30a, the motor can work above 50Hz

when the speed is 50Hz, the output voltage of the frequency converter is 380V and the current is 30a At this time, if the output frequency is increased to 60Hz, the maximum output voltage and current of the frequency converter can only be 380v/30a Obviously, the output power remains the same So we call it constant power speed regulation

how about the torque at this time

because p=wt (w: angular speed, t: torque) Because P is constant and W increases, the torque will decrease accordingly

we can also look at it from another angle:

stator voltage of motor u=e+i*r (I is current, R is electronic resistance, e is induced potential)

it can be seen that when u and I remain unchanged, e also remains unchanged

and e=k*f*x, (k: constant, f: frequency, x: magnetic flux), so when f is from 50 to 60Hz, x will decrease accordingly

for motors, t=k*i*x, (k: constant, i: current, x: magnetic flux), so torque T will decrease with the decrease of magnetic flux X

at the same time, when it is less than 50Hz, because i*r is very small, when u/f=e/f is constant, the magnetic flux (x) is constant Torque T is proportional to current This is why the overload (torque) capacity of a frequency converter is usually described by its overcurrent capacity It is also called constant torque speed regulation (constant rated current -- constant maximum torque)

conclusion: when the output frequency of the frequency converter increases from 50Hz or above, the output torque of the motor will decrease

5. Other factors related to output torque

heating and heat dissipation capacity determine the output current capacity of the frequency converter, thus affecting the output torque capacity of the frequency converter

carrier frequency: general frequency conversion. For example, Toyota Motor plans to realize the commercialization of solid-state lithium batteries in 2022. The rated current of the device is the value that can ensure continuous output at the highest carrier frequency and the highest ambient temperature Reduce the carrier frequency, and the current of the motor will not be affected. However, the heating of components will be reduced

ambient temperature: it seems that the protection current value of the frequency converter will not be increased because the ambient temperature is detected to be relatively low

create an innovation ecosystem

altitude: the increase of altitude has an impact on heat dissipation and insulation performance Generally, it is not considered below 1000m It is OK to reduce the capacity by 5% every 1000 meters

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