How does the inverter control the motor speed?

The frequency converter is mainly composed of rectification (AC to DC), filtering, re-rectification (DC to AC), braking unit, drive unit, and detection unit micro-processing unit.


1. Why is the rotational speed of the motor freely changeable?
Motor rotation speed unit: r / min rotations per minute, can also be expressed as rpm.
For example: 2-pole motor 50Hz 3000 [r/min]; 4-pole motor 50Hz 1500 [r/min]
Conclusion: The rotational speed of the motor is proportional to the frequency

The rotational speed of an inductive AC motor (hereinafter simply referred to as a motor) is approximately determined by the number of poles and frequency of the motor. The number of poles of the motor is fixed by the working principle of the motor. Since the pole value is not a continuous value (a multiple of 2, such as a pole number of 2, 4, 6), it is generally uncomfortable and the speed of the motor is adjusted by changing the value.
In addition, the frequency can be supplied to the motor after being adjusted outside the motor, so that the rotational speed of the motor can be freely controlled.
Therefore, the inverter for the purpose of controlling the frequency is the preferred device for the motor speed control device.
n = 60f/p
n: synchronization speed
f: power frequency
p: motor pole pairs
Conclusion: Changing frequency and voltage is the optimal motor control method

If the frequency is changed only without changing the voltage, the frequency will decrease and the motor will be over-voltage (overexcitation), causing the motor to be burned out. Therefore, the inverter must change the voltage at the same time 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 reduce the rotational speed of the motor by half, the output frequency of the inverter is changed from 50 Hz to 25 Hz, and the output voltage of the inverter needs to be changed from 400V to about 200V.

2. What is the output torque when the motor’s rotational speed (frequency) changes?
The starting torque and maximum torque when the inverter is driven are less than that of the direct-frequency power supply. When the motor is powered by the commercial power supply, the starting and acceleration shocks are large, and when the inverter is used for power supply, these impacts are weaker. A direct start of the power frequency produces a large starting current. When the inverter is used, the output voltage and frequency of the inverter are gradually added to the motor, so the starting current and impact of the motor are smaller.

Generally, the torque produced by the motor is reduced as the frequency decreases (the speed decreases). The reduced actual data is given in some of the drive manuals.

By using a flux vector controlled inverter, the torque of the motor at low speeds is improved, and even in the low speed range, the motor can output sufficient torque.

3. When the inverter is adjusted to a frequency greater than 50Hz, the output torque of the motor will decrease.
The usual motor is designed and manufactured at a voltage of 50 Hz, and its rated torque is also given in this voltage range. Therefore, the speed regulation below the rated frequency is called constant torque speed regulation. (T=Te, P<=Pe)
When the output frequency of the inverter is greater than 50Hz, the torque generated by the motor should decrease in a linear relationship inversely proportional to the frequency.

When the motor is running at a frequency greater than 50 Hz, the size of the motor load must be considered to prevent the motor from outputting insufficient torque.

For example, the torque generated by the motor at 100 Hz is reduced to approximately 1/2 of the torque at 50 Hz.

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

4. Application of inverter above 50Hz
As you know, for a particular motor, its rated voltage and current rating are constant.

If the inverter and motor are rated: 15kW/380V/30A, the motor can work above 50Hz.

When the speed is 50Hz, the output voltage of the inverter is 380V, and the current is 30A. At this time, if the output frequency is increased to 60Hz, the maximum output voltage of the inverter can only be 380V/30A. Obviously, the output power is unchanged. So we call it constant power speed regulation.

What is the torque situation at this time?
Because P = wT (w: angular velocity, T: torque). Since P does not change, w increases, so the torque will decrease accordingly.

We can also look at another angle:

The stator voltage of the motor U = E + I*R (I is the current, R is the electronic resistance, and E is the induced potential)

It can be seen that when U, I are unchanged, E does not change.

And E = k*f*X, (k: constant, f: frequency, X: flux), so when f is from 50–>60Hz, X will decrease accordingly.

For the motor, T = K * I * X, (K: constant, I: current, X: flux), so the torque T will decrease as the flux X decreases.

Meanwhile, when it is less than 50 Hz, since I*R is small, the magnetic flux (X) is constant when U/f=E/f is constant. The torque T is proportional to the current. This is why the overcurrent capability of the frequency converter is often used to describe its overload (torque) capability. Also known as constant torque speed regulation (rated current is not changed -> maximum torque is unchanged)

Conclusion: When the inverter output frequency increases from above 50 Hz, the output torque of the motor will decrease.

5. Other factors related to output torque
The heat and heat dissipation capacity determine the output current capability of the inverter, which affects the output torque capability of the inverter.

Carrier frequency: Generally, the rated current of the inverter is the highest carrier frequency, and the value of continuous output can be guaranteed at the highest ambient temperature. When the carrier frequency is reduced, the current of the motor will not be affected. However, the heat of the components will decrease.

Ambient temperature: It is not like increasing the protection current value of the inverter because it detects that the ambient temperature is low.

Altitude: Increased altitude, which has an impact on heat dissipation and insulation performance. Generally less than 1000m can be ignored. It is enough to derate 5% per 1000 meters above.

Causes and treatment methods of inverter overcurrent

(1) Overcurrent in operation means that the drag system has an overcurrent during the working process. The reason is mainly from the following aspects: 1 The motor encounters an impact load, or the transmission mechanism is “stuck”, causing a sudden increase in the motor current.

2 The output side of the inverter is short-circuited, such as a short circuit between the output terminals and the motor, or a short circuit inside the motor.

3 The inverter itself is not working properly. For example, two inverter devices of the same bridge arm in the inverter bridge are abnormal during the alternate work. For example, due to the high ambient temperature or the aging of the inverter device itself, the parameters of the inverter device are changed, resulting in one device being turned on during the alternation process, and the other device is still turned off in the future, causing the same The “straight through” of the upper and lower devices of one bridge arm makes the short circuit between the positive and negative terminals of the DC voltage.

(2) Over-current when the speed is increased When the inertia of the load is large and the speed-up time is set too short, it means that during the speed-up process, the working efficiency of the inverter rises too fast, and the synchronous speed of the motor rises rapidly. However, the rotation speed of the motor rotor cannot be kept up due to the large inertia of the load, and as a result, the speed increase current is too large.

(3) Overcurrent in the deceleration When the inertia of the load is large and the deceleration time is set too short, it will cause overcurrent. Because the speed reduction time is too short, the synchronous speed decreases rapidly, and the rotor of the motor maintains a high speed due to the inertia of the load. At this time, the speed at which the rotor winding cuts the magnetic line is too large to generate an overcurrent.

Inverter overcurrent processing method

(1) Tripping at a speed up at start-up, this is a very serious over-current phenomenon. The main check: whether the working machine is stuck; whether there is a short circuit on the load side, check whether there is a short circuit to the ground with a megohmmeter; The module is not damaged; the starting torque of the motor is too small, and the drag system cannot be turned.

(2) Do not trip immediately when starting, but trip during operation, the main check: the speed increase time setting is too short, lengthen the acceleration time; the deceleration time setting is too short, lengthening the deceleration time; torque compensation (U/f ratio The setting is too large, causing too much no-load current at low frequencies: the electronic thermal relay is improperly set, and the operating current is set too small, causing the inverter to malfunction.

How inverter works on air compressor

The use of the frequency converter in the air compressor system is precisely to enable the air compressor system to accurately control the electrical control and frequency conversion control, and immediately control the air compressor motor torque (ie, the ability to drag the load) without changing the air compressor motor torque. The motor speed (ie, output power) responds to system pressure changes by changing the compressor speed and maintains a stable system pressure (setpoint) to achieve on-demand output of high quality compressed air. When the air consumption of the system decreases, the compressed air provided by the compressor is greater than the system consumption. The compressor will reduce the speed and reduce the output compressed air volume; otherwise, it will increase the motor speed and increase the compressed air volume to maintain a stable system pressure value. . The function of the frequency converter in its system is the same as the power saving of the fan motor water pump. According to the load change, the frequency of the input voltage is controlled.

Its energy-saving principle and effect are as follows:

1. After the inverter is used, the pressure setting of the air compressor can be one point, that is, the minimum pressure that meets the requirements of the production equipment can be set pressure, and the inverter will adjust the speed of the air compressor according to the fluctuation of the pipe network pressure. The speed of the air compressor even eliminates the unloading operation of the air compressor and saves energy.

2. Because the inverter in the system makes the pipe network down pressure stable, it can reduce or even eliminate the pressure fluctuations, so that all the operating air compressors in the system are operated under a lower pressure that meets the production requirements, reducing the pressure. Power loss caused by upward fluctuations.

3, because the air compressor can not rule out the possibility of long-term operation under full load, it can only determine the capacity of the motor according to the maximum demand, so the design capacity is generally larger. In actual operation, the proportion of light-load operation time is very high. If frequency conversion speed regulation is adopted, the operating efficiency during operation can be greatly improved. Therefore, the inverter has a great potential for energy saving in the air compressor system.

4, some adjustment methods (such as adjusting the valve opening and changing the angle of the blade, etc.), even in the case of a small amount of demand, it can not reduce the operating power of the motor. After adopting the frequency conversion and speed regulation, when the demand is small, the rotation speed of the motor can be reduced and the operating power of the motor can be reduced, thereby further achieving energy saving.

5, single-motor drive system can not be continuously adjusted according to the severity of the load. The use of variable frequency speed regulation, it can be very convenient for continuous adjustment, can maintain the stability of pressure, flow, temperature and other parameters, thereby greatly improving the performance of the compressor.

How to deal with the aging of the inverter

Inverter components performance deterioration, not a qualitative change phenomenon, but a quantitative phenomenon, into the machine for many years to use, like the capacitor electrolyte dried up, the transistor to reduce the ability to enlarge, the pin oxide components, etc., with the With the passage of time and gradually changing, so overhaul “old machine”, it is necessary to pay attention to this issue.

The definition of damage to such components, with aging, inefficient, failure, worse performance, with breakdown, open circuit, etc. is not appropriate. Components of the performance deterioration, the extent of its deterioration often vary greatly The phenomenon of failure and detection are also ever-changing, difficult to grasp, and often manifested as a difficult fault, or as a “soft fault”, people scratching his head – check No bad parts, but the circuit is clearly not the normal state! Overhaul of such failures, need to repair the basic skills of electronic circuits solid, years of accumulated experience, and even the psychic quality of maintenance, but also a test.


Fortunately, after all, this type of fault is a minority, generally or components “hard damage” as much. If the maintenance person is willing to accept such a challenge, the overhaul of such a soft fault will also translate into a fun, overhauling process can even become a enjoyable process (ordinary fault on the change, what fun at all It?), People are very sense of accomplishment. In our long-term overhaul work, we always encounter such a failure. We can accept this kind of machine without any modification and enjoy it. What’s wrong with it?


Some components, manufacturers have been given the useful life, such as the inverter cooling fan and electrolytic capacitors, manufacturers given the replacement period of 8-10 years. Fan is a rotating part, such as bearings, rotating parts, there is always wear and tear; In order to enhance the capacitance, the electrolytic capacitor with electrolyte inside, so there is the reverse leakage current production, installation and use should pay attention to its polarity. At the same time, as the service life increases, the electrolyte will inevitably dry up, so that the capacity decreased. After reaching the service life, in principle, the fan and electrolytic capacitor should be replaced, even if the inverter is not damaged.

Solar / photovoltaic DC- AC inverter

1、Product introduction

  • Product Type: D type (for photovoltaic type)
  • Product brand: Ecogoo
  •  Voltage rating: three-phase 220V / 380v
  • Power rating: 0.75KW-22KW

2、Product features

  • Built-in MPPT controller, excellent MPPT algorithm for a variety of solar panels, the main circuit using intelligent power module, high reliability, conversion efficiency of about 95%
  •  Support less than 2KW boost module, increase the PV output voltage;
  •  Can be based on the actual system conditions free to set the pump speed range;
  •    Soft start, reduce the top of the voltage spikes, with overvoltage, undervoltage, overcurrent, overload and other protection functions
  • Support for PV input and AC grid input;

3、Application

  • Photovoltaic pumps
  • Wind power pumps

4、Notes

  • Do not overload the use of inverter, such as the inverter power than the motor power is small, or ordinary motor is greater than 50HZ run, otherwise the inverter will frequently jump overload protection failure, and even blowers, the motor may also be too hot and burned;
  •  The default frequency of the inverter 50Hz, the default off anyway function, if you want to change, please contact the manufacturer after the sale or by the correct operation of the manual;
  • The inverter output U, V, W and the motor can not be connected in series between any switches, including the contactor, can not use the inverter as a power supply, the inverter can not run through the switch suddenly connected to any equipment;
  •  Inverter operation panel outside the lead distance should not be too long, such as more than 5M, it is recommended to use external potentiometer speed and shielded cable connection, start and stop control can use the terminal control mode;
  •  Panel cable is too long susceptible to harmonic interference, so that the inverter frequency control is not accurate, the frequency fluctuations
  •   DC input voltage should try to meet the recommended voltage, otherwise it will reduce the solar energy conversion rate;
  • AC input and DC input switch can not be closed at the same time, otherwise it will damage the inverter or solar panels.

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