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How is Vector Control improving motor output torque capability?

1: Torque boost: this function is the variable speed drive increases output voltage (mainly in low frequency) to compensate the torque loss due to voltage drop in the stator resistance, thereby improving the motor output torque.

2: Improve the motor insufficient output torque in low speed
“Vector control” can make the motor output torque at low speeds, such as (without speed sensor) 1Hz (for 4-pole motor, the speed is about 30r/min), same as the torque output at 50Hz power supply (maximum is approx 150% of rated torque).

For the V/F control variable speed drive, the motor voltage increases relatively as the motor speed decreases, which will result in lack of excitation, and make the motor can not get sufficient rotational force. To compensate this deficiency, the variable speed drive needs to raise voltage to compensate for the voltage drop in motor speed decreases. This feature called “torque boost”.

Torque boost function is to improve the variable speed drive output voltage. However, even if the drive increases voltage, the motor torque and current does not increase corresponding. Because the motor includes the torque and other components (such as the excitation) which generated by the motor.

“Vector Control” allocates the motor current value to determine the motor torque current component and other current component (such as the excitation component) values.

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.

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.

variable frequency drive

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.

Difference between ICCB, MCCB and MCB

The aforementioned types of Circuit Breakers are used in LV System and generally based on the same operating principle.
MCB and MCCB/ICCB have a bimetal heater for overload which releases the Contact s while for short circuit the trip / electromagnet hammers itself against moving contacts. The arc created by breaking contacts is extinguished in an arc chamber. Are defined as “Thermo-magnetic “ CBs , accordingly. It is operating characteristic addressing the overload by thermal action of the bimetal strip and instantaneously dealing with short circuit occurrences by electromagnetic action.

MCB – Miniature Circuit Breaker is suitable for domestic usage. Used to protect final circuits from O/C such as Overload & Short Circuit.
i- MCB is basically made in accordance to BS 3871, is now superseded by BS EN 60898 which recognizes type B, C & D.
Type B is suitable in domestic premises.
Type C is used in commercial & industrial applications.
Type D is suitable for application where a high in-rush current is expected.
ii- MCB is of low breaking / making capacity as well as low current rating compared with MCCB/ICCB. MCBs available in different number of poles (SP, DP, TP,,).

MCCB – Molded Case Circuit Breaker & Insulated Case Circuit breaker are also current limiting devices but with high making/ breaking capacity and current ratings compared with MCB. MCCB and ICCB are almost the same and both are manufactured in accordance to NEMA AB1/AB3 to suit industrial and commercial purposes.
The advent of electronic protection increased the use of them and the scope is widened like tolerances, range of time & current adjustment. By virtue of that a good discrimination can be achieved with accuracy about ±10%.
Eventually, MCCB/ICCB has advantages in the capability of accommodating further features which can be provided as
i. RCD.
ii. Under voltage device.
iii. A shunt trip coils that enabling remote tripping.
iv. Auxiliary switches for remote monitoring and/or control.

Why transformer rating is shown in KVA?

Transformers are rated in {VA, kVA, MVA etc.} due to flows of active and reactive power through transformer. In case of transformer we have active power losses as consequence of existence inside resistance of windings (primary and secondary) and existence of active losses of ferromagnetic core and other side we have reactive power losses as consequence of existence losses of magnetic flux (primary and secondary) and existence of reactive power losses of ferromagnetic core.

[VA]=sqrt(sqr[W]+sqr[VAr])

Transformer is rated in kVA by the manufacturer to inform users about the maximum power (voltage and current) that support it, the reason for not rating it in KW is that the active power (kW) is depend on the loads (lighting, machines..)

The simple answer is: It is because the kVA (or MVA) rating is only rating that matters to express a transformer’s “capacity” to allow the “passage” of power. That capacity is the thermal capacity dictated by the current it can carry at a given ambient temperature, regardless of the power factor. So combined with its voltage ratings, kVA (or MVA) is the value that matters. kW rating does not matter as transformer can handle unity power factor or in other words, a transformer can handle kW equal to its kVA rating at any time.

Remember that a transformer, as the name suggests, is only a transformation device or a pass through device and not a power producing device like a generator or an UPS, where their capacity to produce real power (kW) is an independent limit from the thermal ( kVA) limit.

To take it a step further, if you have an ability to cool the transformer further, you can augment the kVA (or MVA) rating of a transformer. This would explain having multiple kVA/ MVA ratings on transformers with forced cooling aids installed on them.

If you think of it, this is not different from a cable or a conductor’s capacity expression. Except that a transformer can have more than one voltage levels and different ampacities on primary and secondary, but the kVA rating remains the same on either side. So that makes kVA a more convenient way to express its thermal capacity vs. the amperes alone.