Iacdrive|Automation Solution

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.

Transformer Magnetic Design

Control Servo motor with a variable frequency drive

Looking at those AC drives they recommend an Induction motor. A servo motor with permanent magnets which is not quite an induction motor. So, if a servo with permanent magnets can be used instead an induction with these kinds of AC drives.

Actually, the term “Servo” makes a reference about “feedback”, it means, whether we need a control loop, we are talking in terms about Servo, in this case, we have, or we know, the “feedback” by an encoder. Typical variable frequency drive doesn’t have a input for an encoder, so, if you want to control a Servo Motor with a VFD, you can move the motor, but you can’t control it.

A servo motor can be an induction servo, a brushless servo, a reluctance servo a dc servo – each of these can be either linear or rotary and can come with a variety of feedback such as tachometer, resolver of various pole counts, incremental or absolute encoders discreet or serial interface with different bus options, laser feedback, halls etc.

Then you come to the term variable frequency drive. Brushless servo amplifiers are also vfds. Do standard inverters have proper control of induction, and brushless motors. Some allow for a software switch, some allow for a firmware download, some don’t. Will inverters accept feedback – some have it built in, most that allow it do so by option cards, many do not.

Normal input in a variable frequency drive is, digital to start or stop, and we could have an analogic input to control by potentiometer.

Using AC Drives for the servo application is quite possible, provided the application is less demanding in critical positioning purpose.
There are number of makes that showcases pinpoint positioning of motor shaft being driven by AC Drives like Hitachi SJ700 / Emerson Uni drive SP / Danfoss FC etc.

Its beneficial to opt for the AC Drives as it supports SLVC [ VFD gives almost servo-like torque at low rpms if you give it encoder feedback ], multiple motors can be accessed, torque requirement can be met if required, power dips can be sustained using VFD’s.