Iacdrive|Automation Solution

VFD overcurrent trip during acceleration/deceleration

First, we should know it’s caused by loads or itself. If it’s the variable frequency drive problem, we can check the trip current from the VFD operation history, to see if the current exceeds the VFDs rated current or electronic thermal relay settings value. If three-phase voltages and currents are balanced, we should consider overload or sudden change situations, such as motor stall. If the load inertia is big, we should extend the acceleration time appropriately, this is suitable for a good VFD. If the trip current is within the variable frequency drive rated current or electronic thermal relay setting range, then it maybe the IPM module or relevant parts failure. In this case, we can measure the variable frequency drive output terminals (U, V, W), and resistance of the P, N terminals on DC side to determine whether the IPM module damaged or not. If the module is good, then we can know it is the drive circuit trouble. If IPM module overcurrent or ground wire short circuit causes the VFD trip in deceleration, generally it’s the top half-bridge module or drive circuit fault; If IPM module overcurrent during acceleration, then it is the next half-bridge module or drive part fault. For such failures, mostly it’s the external dust entering the variable frequency drives or environment moisture.

Soft starter VS variable frequency drive

Soft Starter reduces electric motor starting current to 2-4 times during motor start up, reduces the impact to power grid during motor start up, avoid the motor being burned out, and provide protection in motors running process.

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.

So, from the function side, variable frequency drives are much better than soft starters.

One essential difference between a soft starter and a VFD in this regard is, that the VFD delivers “nearly” sinusoidal voltages (and currents) to the motor, which makes it possible to develop high starting torques during the acceleration, even higher than nominal full load torque, depending on the application, while a soft starter only supplies fractions of the basic waveform, which serves to reduce the current to the motor significantly, but still at the nominal frequency. This will reduce the available starting torque dramatically until the motor is up to around two-thirds of nominal speed, or maybe even higher.

Synchronous generators inter-turn faults

For the MW range of Synchronous generators, there is no terminology of “interturn fault” on the stator winding. There could only be coil to coil fault on the stator for such size of machine design.

There are possibilities of having inter-turn faults on the rotor winding: when the insulation positioned between adjacent conductors break (electrically) over time under certain mechanisms. These mechanisms can include; turn to turn movement caused by thermal expansions (during starts/stops cycles), rotor coil shortening, end strap elongation, inadequate end-turn blocking or conductive bridging formed by contamination. The protection of avoiding the interturn insulation is a function of how well the machine is designed, maintained and operated. The OEM of the generator usually provides recommendations to avoid any inter-turn fault during the lifecycle of the machine. Saying this, there are ways to monitor the interturn fault indication; such as data acquisition (air gap flux probe, air gap search coil), as supportive monitoring (RSO, Shaft voltage, shaft vibration levels, excitation current etc.). Ideally, you have to be knowledgeable with the machine design to interpret the acquired data to make valuable predictions.

If you start by contemplating what kind of symptoms inter-turn faults could give rise to, you will be part of the way.
While machine is at standstill, you could do some reflected-wave analysis. All phases should show (near) identical responses.
During operation, you could have non-identical current and voltage waveforms on the three phases (you must compensate for unequal load).
You may experience strange sounds, in the supersonic range. Changing for different locations around the stator. You can continue the list, and settle on systems that may be able to detect any anomalies, so you can react accordingly.

What is a soft starter?

Motor starter (also known as motor soft starter) is a electronic device integrates soft start, soft stop, light-load energy saving and various protection functions for motor controls. Its main components are the three phase reverse parallel thyristors between power supply and being controlled motor and related control circuits. Control the conduction angle of the three phase reverse parallel thyristors by different methods, to achieve different functions by the changeable of the input voltage on the controlled motors.

The difference between soft starter and frequency inverter

Soft starters and AC motor speed control, it can change output voltage and frequency at the same time; actually, soft starter is a regulator for motor starting, only changes output voltage but not the frequency. The frequency inverter has all the features of soft starter, but its price is much more expensive than the soft starter, and the structure is much more complex.

Frequency inverter allows the AC 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 protection in motor running process. Besides these functions, the main function of frequency inverter is adjusting the motor running speed according to actual operation conditions, to achieve energy saving effect.

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.