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AQ: Frequency inverter failure analysis

Transistor frequency inverter has the following disadvantages: easy trip, difficult re-start, poor overload capacity. As the rapid development of IGBT and CPU, the inverter drive integrates perfect self-diagnosis and fault prevention features, improve the reliability greatly.

Vector control frequency inverter has “automatic torque compensation function” to overcome “starting torque inadequate” etc. This function is the inverter uses a high-speed microcomputer to calculate the torque required at current time, to modify and compensate the output voltage quickly to offset the frequency inverter output torque changed by external conditions.

In addition, because as the inverter software development more and more perfect, we can pre-set various failures parameters in the frequency inverter, to ensure continuous running after failure resolved. For example, re-start motor in free parking process; automatic reset internal failures and maintain continuous operation; adjust running curve if load torque is too high to detect the mechanical system abnormal.

AQ: Output torque of variable speed drive running above 50Hz

Generally, electric motors are designed according to 50Hz power supply, its rated torque also in this frequency. Therefore, the speed adjustment under rated frequency called constant torque speed adjustment. (T = Te, P <= Pe).

If the variable speed drive outputs frequency exceeds 50Hz, the motor torque is inversely proportional to the frequency in linear relationship decrease.
When the motor running in above 50Hz frequency, we should consider the motor loads to avoid motor lacks of torque.

For example, the motor torque is about a half in 100Hz running against 50Hz. Therefore, the speed adjustment in above rated frequency called constant power speed adjustment. (P = Ue * Ie).

As we know, for a specified motor, the rated voltage and rated current is constant.

For example, the variable speed drive and motor rated values are: 15kW/380V/30A, motors can operate at 50Hz or above.
When the frequency is 50Hz, the variable speed drive output voltage is 380V, current is 30A. Then if we increase the output frequency to 60Hz, the variable speed drive maximum voltage and current also is 380V/30A, it is obviously that the output power is fixed, so it called constant power speed adjustment, what’s the torque status now?

Since P = wT (w: angular speed, T: torque), as P keeps same, w increases, so the torque will decrease accordingly.

From another point: motor stator voltage U = E + I * R (I is the current, R is the electrical resistance, E is the EMF)
Then we can see, U and I are constant, E is constant.
And E = k * f * X, (k: constant, f: frequency, X: flux), when f changes from 50 to 60Hz, X will decrease accordingly.

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

And, if the frequency is less than 50Hz, as I * R is very small, so if the U/f = E/f is constant, the magnetic flux (X) is constant, the torque is proportional to the current, which is why use the variable speed drive overcurrent capability to describe its overload (torque) capability, and known as constant torque speed adjustment (rated current is constant -> Maximum torque is constant).

Conclusion: When the variable speed drive outputs frequency increases from 50Hz, the motor outputs torque will decrease.

AQ: What is the soft stop of an electric motor?

In electric motor stop, the traditional control ways are accomplished by momentary power cutting off. But in lots of applications, it’s not allowed the motor instant shutdown. For example: high-rise buildings, building’s water pump system, it will appear huge water hammer during instant shutdown, to damage the pipe, even the pumps. To reduce and avoid “water hammer” phenomenon, the pumps motor need be shut down gradually, that is soft stop. The soft starter can meet such requirements. In pumping station, soft stop technology can avoid the pump door damaged of the pumping station, to reduce maintenance costs and maintenance works. The soft stop function in soft starter is, when the thyristor gets stop instruction, decrease conduction angle gradually from full conduction, and achieve full closed after a certain time. Stopping time can be adjusted according to actual requirements within 0 – 120s.

AQ: Avoid variable frequency drive damaged in lightning

Sometimes

AQ: Motor output torque in rotation speed (frequency) changes

Frequency power: power supplied by the power grid (commercial power).
Start-up current: frequency inverter output current in motor starts.

The motor starting torque and maximum torque by frequency inverter driving is less than direct frequency power driving.
Motor accelerates in constant frequency power supply has high impact, which can be reduced by using frequency inverter. Because there is a big starting current in motor acceleration if it’s powered by constant frequency power supply; when using frequency inverter, the inverter output voltage and frequency is increased gradually, so the motor starting current and impact is much lower.

Generally, the motor torque is decreased with frequency decreases (speed reduction). By using vector control frequency inverter, to improve output torque during motor running in low speed, and even output sufficient torque at motor low speed zones.

AQ: Soft starter energy saving principle

Induction motor is inductive load, the current lags the voltage, most electrical appliances are the same. In order to improve the power factor we need to use capacitive load for compensation, parallel capacitors or with synchronous motor for compensation. Reduce motor excitation current also can improve the power factor (HPS2 saving function, reduce excitation current by reducing voltage at light loads, to increase COS∮). Energy-saving operation mode: decrease voltage in light loads to reduce excitation current, the motor current divides into the active component and reactive component (excitation component), to increased COS∮.

Energy saving operation mode: when the motor load is light, the soft starter working at energy-saving conditions, PF switch to Y position, under the current feedback action, the soft starter reduces the motor voltage automatically, to reduce excitation component of the motor current. Thereby improving the power factor of the motor (COS∮). If the contactor in bypass state, this feature cannot works. TPF switch provides energy saving features with two reaction times: normal speed and slow speed. The soft starter operation in energy saving state automatic (In normal and slow speed), saving 40% energy in no-load and 5% with load.

AQ: Why the motor rotation speed is changeable?

r/min: motor rotation speed unit, the number of revolutions per minute, also can be expressed in rpm.
For example: 2-pole motor 50Hz 3000 r/min, 4-pole motor 50Hz 1500 r/min
Conclusion: The motor rotation speed is proportional to the frequency

Herein, the motor is induction AC motor which is used in most industries. AC induction motor rotation speed is approximate depend on the motor’s poles and frequency. As we know, the motor poles constant. Since motor poles are not continuous numbers (multiples of 2, for example, the number of poles is 2,4,6), so it’s not suitable to change this value to adjust the motor speed.

The frequency can be adjusted before supplying to the AC motor, then the motor rotation speed can be controlled freely. Therefore, motor speed controls.

n = 60f / p
n: synchronous speed
f: power frequency
p: number poles

Conclusion: change frequency and voltage is the best motor control method.

But, if just change the frequency without changing the voltage, it will occur overvoltage (over-excitation) when frequency decreases and may cause AC motor damaged. So, the voltage should be changed while the frequency inverter outputs different frequency. If the inverter output frequency exceeds rated frequency, the voltage can not continue to increase, the maximum voltage only can be equal to the motor rated voltage.
For example: In order to reduce the motor rotational speed by half, the inverter output frequency change from 50Hz to 25Hz, then the inverter output voltage should change from 400V to about 200V.

AQ: Change 230V to 460V for operating an Electric Motor

I have a generator of 3 hp, and it outputs 230 V, and I have a submersible Electric Pump, the motor of which is rated to operate at 460 V, Can I use a step up transformer to increase the voltage output from my generator and power the pump? What more parameters do I need to know of in this case?

Check to see if the generator has 3 phase power output. A typical home generator will provide 230 volt single phase output. You will not be able to step up to 460 volt and start a 3 phase motor with single phase. The only way at that point to generate 3 phase would be to use a VFD with single phase input capability and use the drive to generate 3 phase. You will still need to use a transformer. Variable frequency drives won’t normally behave well on generator power but may for an intermittent load like a submersible pump.

AQ: Transformer Magnetic Design

AQ: Why industrial induction motor star point not grounded?

In any electrical system, we limit the neutral grounding to 1 or 2 locations at the power source, eg, the star-points of generators or transformers. By keeping the grounded neutrals at the power source, earth leakage current will be flowing radially from the power source to the point of short-circuit at downstream. In this way the direction of earth fault current flow can be easily identified and the earth fault protection relays in the distribution system can easily be coordinated.

Grounding a motor star point will create an earth path for earth leakage current to flow through that motor’s star point. If there are 10 motors in a process plant and their star points are all grounded, there are 10 additional paths for earth leakage currents to flow through.
If all the motors’ star points are grounded in this way the earth fault current detections by the protection relays will be complicated and likely they will trip at the incorrect locations because earth fault currents are flowing in many directions toward multiple grounded neutral points.

Therefore the electrical consumers (ie the load, including the capacitor banks), even if they are star connected, are not to be grounded.

Grounding of neutral point is not being decided base on the presence of unbalance loads. It is decided for safety reason and for earth fault protection requirement. Unbalance 3-phase load will result in some current flowing through the neutral conductor but it doesn’t result in a (residual) current flowing through the neutral-ground connection.

Motor is a balanced 3-phase load, this I agree. However when the system supply voltage is unbalanced caused by unbalanced loads somewhere else or due to network conductors problem, the motor operating under unbalance voltage will result in unbalance current in the 3 windings. The same is true for the generator windings under that condition. The design engineer may then decide that individual machines should be fixed with negative phase sequence current protection.

Even if there is a neutral voltage shift in the induction motor, we should not ground the motor’s neutral point. If you ground it, it may create nuisance trip of earth fault protection relays (the motor’s EF relay, upstream EF relays, or the EF relay connected to transformer’s neutral-ground CT).

I am sure in reality, there is some neutral voltage shift in motor’s star point. However, there is no harm with that.

If you ground the star point, you still will not get rid of the unbalance current/voltage from the motor windings. There the negative sequence current is still present in the motor winding.
If you think an unbalance voltage supply is causing problem to the motors, you should solve the unbalance voltage problem elsewhere, not by grounding the motor’s star point.