Iacdrive|Automation Solution

What’s a variable frequency drive (VFD)?

Variable frequency drive is an electric device to change AC power frequency to control AC motor speed, In addition, it also can change the AC power voltage.

In the past, variable frequency drive was included in motor generators, rotating converters and other electrical equipment. With the emergence of semiconductor electronic devices, VFD can be completely manufactured independent.

Variable Frequency Drive allows the electric motor smooth start up, control startup current growing from zero to motor rated current, reduce impact to the power grid and avoid the motor being burned out, also provide protect in motor running process. Besides these functions, the main function of variable frequency drive is adjusting the motor running speed according to actual operation conditions, to achieve energy saving effect.

Generally, variable frequency drive contains two components: rectifier and inverter. The rectifier converts incoming AC power to DC power, then the inverter converts DC power to the desired frequency AC power. In addition to these two parts, variable frequency drive may also contain transformer and battery. Wherein the transformer changes the voltage and isolates input/output circuit, the battery compensates energy loss inside the VFD drive circuit.

The variable frequency drive not only changes the AC power frequency, but also can change electric AC motor rotation speed and torque. In such conditions, the most typical VFD structure is a three-phase two level source variable frequency drive. The VFD controls each phase voltage by the semiconductor switch and pulse width modulation (PWM).

In addition, variable frequency drive also can be used in aerospace industry. For example, the electrical equipment inside aircraft needs 400Hz AC power, but generally the power on ground is 50Hz or 60Hz. Therefore, when the aircraft is parked on ground, the variable frequency drive will convert 50Hz/60Hz to 400Hz AC power to suitable for the aircraft.

Motor testing and repairing

Are you having noticeable performance problems with these motors? The size and type of motor are critical as mentioned, a cast rotor with the right testing can pick up voids in the bar and resistance rings, not necessarily a problem as most mass produced cast bar rotors will have some sort of voids in the bars, and the motors are fine, the red flag comes up when using these black box tests, which picks up what appears to be a problem but is actually just a normal condition from the manufacturing process.

I have very little faith that any one test on an assembled motor, can tell the user everything about the condition of the internals, or health of the motor.

When you consider all the testing the health field can use, such as a full body scan, many times it leads to false alarms and more expensive testing.

I could ask a few dozen questions on the age, type, past testing, past history of the motors in question, but if you are basing the health or life expectancy of any motor by only the use of testing without a visual of the internals of a motor, those questions need to be addressed to the supplier of the testing equipment.

I believe in predictive maintenance, by vibration charting, insulation value testing, surge testing, all charted and plotted over time.

When you have insulation values at 100 megohms in March, and then 500 in July, it is likely the ambient conditions have effect on the readings. Dependent on the ambient conditions and area the motors are located, humidity in March is gone in July. So plotting the readings over time will give a plot to see if the trend is downward regardless, or it could be the readings in March are fairly constant, the readings in July are constant, but there is no downward plot of the insulation value.

When you get insulation values in March of 100 megohms, and again in July but the megger readings are now 60, then a user would want to decrease the time between testings, starting with say quarterly, once you develop a plot, if that plot changes downward, then it is time to test maybe even weekly as it may show some kind of insulation breakdown, or contaminates that would call for a visual inspection and possible cleaning/repair of the motor.

Same with plotting surge tests.

Same with plotting vibration testing.

But the answers to these questions where the test results are confusing at best, need to be addressed to the testing equipment provider.

I have yet to see any demonstration of a total motor health testing device, that did not have some caveat dependent on the speed or other design factors of particular motors.

Maybe these tests were not confusing prior to now, if so, I doubt two identical motors would fail/start to fail with the same exact type of problem.

Again I could ask a dozen questions such as are the motors new, is this the first time you have results that make no sense, and as much of the total history of the motors and testing programs you have in place.

When it comes to rotors, testing is critical, and often when problems are found with the motor, and all testing points to the rotor, often simply repairing the rotor will not resolve the problem.

In speaking with many engineers over several decades, a large manufacture of large electric motors, have decided once a rotor is identified as the problem, rebarring, or any single repair is usually unsuccessful, and their procedure is to scrap the rotor completely.

Which factors will affect VFD output torque?

Heating and cooling capacity to determine the variable frequency drive output current capability, thus affect its output torque capability.

Carrier Frequency: generally the variable frequency drive rated current is the continuous output value under the highest carrier frequency, the maximum ambient temperature. Reduce carrier frequency won’t affect the motor current, but will reduce electronic devices heating.

Ambient temperature: like will not increase VFD drive protection current when detect relative low ambient temperature.

Altitude: altitude increases will affect both heating and insulating property of the variable frequency drive. Generally it’s fine in below 1000m, and derate 5% per 1000meters for above.

Sensorless motor control with TI and Microchip

Question:
I need to learn about the sensorless control of permanent magnet AC (PMAC) motors. Can you recommend a tutorial and/or open source code for the sensorless motor control using the
a) TI TMS320 series processor, or
b) Microchip dsPIC33EP128 series processor?

Answer:
I have used Microchip and TMS320 to develop VFD. They provide you with a demo kit, PCB and a motor. It take me half a day to get the demo PCB running with my PMSM. Then I copy their design to my own.

The Microchip solution provides you with demo code. I used that before, but it require quite a bit of C programming, and motor tuning take even longer. The demo code and application note are no where near the performance of the Ti solution (I do not work for Ti -so I am not advertising). I take me a week to get my motor spinning with the demo kit from Microchip.

Then there are the International Rectifier solution that is available from many years. The IR sensorless motion control solution have implemented a FOC motor control in FPGA. So you don’t need to write code for motor control. In the chip, it also has a 8051 cpu. You write the program in C; 1 page of code will get a washing machine working. It takes me 1 day to get a PMSM motor running with this solution.

I will use the TI solution for high end motor control – such as a US$40,000 dollar, 100HP direct drive PCP used in the oil field.
I will use the IR solution for a water pump, washing machine – things that is a few kw.
I will use the microchip for solution for toys, because Microchip is so much fun to play with.

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.

ACS800-104-0105-3 (ABB VFD Drives)

Question:
I have a problem with ABB ACS800-104-0105-3 drive model, the output current reading on the VFD is always double the reading of the clamp ampere(i.e. drive reading= 40 A, clamp ampere reading=20 A), what is the procedure that i can follow to detect the cause of this error?

Answer:
I don’t know about ABB drives, but hope this thing will help you.
1. The variable frequency drive may have problem with current sensor, just replace with another drive for comparison.
2. Make sure you use, true RMS type clamp meter.
3. If there is leakage current (through cable insulation and air) between each phase. This normally because of the cable insulation already degraded. Add output reactor and replace the cable with suitable insulation can fix this kind of problem.
4. If there is leakage current between this VFD drive and the other drives, that both motor cable is quiet long and run in parallel together.

To Collect more data and get more idea, you can do this:
1. Clamp all the 3 phase motor cable together using clamp. The reading will show you the leakage current. Normally about 10% of motor rated current at full load.
2. Check the current on each phase, and see if the current is balance for each phase.
3. Run the variable frequency drive without the motor cable, check the current reading and clamp meter.
4. Run the AC drive with the motor cable but without the motor, check again the reading and clamp meter.
5. Run the drive with motor, check if any oscillation in motor current.
6. Check current input to the AC drive inverter.
7. Turn of the other drive (if the motor cable run parallel together with other VFDs), and see if any change in current.

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.

Figure out variable speed drives failures

If there is frequent current-limitation or overcurrent alarm during the variable speed drive running, we should check the loads and inverter IGBT module is normal or not, if its good, then the failure is the Hall magnetic compensation current sensor damaged on the control circuit of the variable speed drive. Hall magnetic compensation current sensor is a device to measure the current value of sinusoidal and non-sinusoidal periodic, which can truly reflect the real current waveform, to provide a control and protection signal to the variable speed drive. Generally, this device in variable frequency drive mostly is Swiss company LEM LA series components, LA Series Hall current sensor magnetic compensation can be divided into three and five terminals, for different variable speed drives capacity, the Hall current sensor magnetic compensation also is difference.

Electronic components are very sensitive to static electricity, it will cause electronic components soft breakdown and then cause the circuit board cannot work. So we should be careful when we replace the circuit board, and ensure wearing grounding wrist strap before working, make sure the strap ground directly and human body is at zero potential, in order to prevent body’s electrostatic damage to the circuit board. If there is no grounding wrist strap, we should touch the variable speed drive metal cabinet before replacing the circuit board, to ease static electricity through the variable frequency drive enclosure.

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.

AC drive faults analysis

It will cause a series problems during AC drive operation in various environmental conditions, take an example as: when failure occurs, AC drives protective function is activated, and the AC drive tripped immediately, the electric motor stop slowly, the red LED alarm indication turns on, the display panel shows alarm message code or fault content. Then we can analyze the variable frequency AC drive fault reasons base on the display information, if it is soft failures, we can cut of the AC drive and reset it. If the drive still not works, we need to check it manually or automatic initialization, and input the parameter values after the initialization finished. In this way, the AC drive can work if the failure is not critical. If the AC drive still can’t work after above detection, then we need to check the variable frequency drive damaged parts according to the fault phenomena, to replace components or circuit boards. Troubleshooting should follow the drives failure sequence. Like:

(1) Fault code 36, its main power failure, then the three-phase rectifier bridge modules may be breakdown shorted or opened.

(2) Fault code 14, its ground failure, check the motor windings and insulation with megger to see if it’s damaged or not.

(3) Fault code 37, its the inverter failure, the IGBT module may short-circuit breakdown. If the IGBT module short circuit, the main circuit fuse will burnout too. When a phase gate damaged, the variable frequency AC drive will appear overcurrent phenomenon, then it’s time to check the IGBT modules.