Blog – Page 4 – AC DRIVES SOLUTION.

AQ: 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.

AQ: Full load torque VS Rated torque

All motors have a “torque vs speed” characteristic.

DC machines are very simple: constant torque from zero speed to some “base speed”, and then a “constant power” ranging from base speed to top speed. In the constant torque range, acceleration is dependent on applied voltage, with the field under constant full current excitation. In the constant power range, voltage is held constant and the field current is reduced, thereby achieving an increase in speed (hence the term “field weakening”).

AC machines are somewhat more complex, since the curves are nowhere near as linear. The key points are:
– “starting torque”, which is the torque achieved at the locked rotor (zero speed) condition
– “pull in torque”, which is the available machine torque at the point where the machine pulls into synchronism (synchronous machines only)
– “pull out or breakdown torque”, which is the peak torque the machine can sustain momentarily before stalling
– “load torque”, which is the amount of torque actually required by the process at any operating point
– “accelerating torque”, which is the difference between what the machine is capable of producing and the load torque

A machine is rated for the “full load torque” condition which is the rated torque performance of the machine. In imperial (lb.ft) units, that would be 5252 * HP / RPM. It can produce this torque continuously, provided it has the rated conditions of applied terminal voltage and applied terminal current (for both rotor and stator, as applicable).

The time required to start a motor is dependent primarily on the accelerating torque available and the combined inertia (motor + remainder of drive train).

Note that available starting and pull-in torque during the transient operation of starting is proportional to the square of the applied voltage – if the voltage dips below 1.0 per unit, the available torque will be significantly reduced.

When operating an AC machine on a

AQ: Compensate electric motors effect of high altitude

Case: Two electrical motors that design for altitude <1000 m but now this two electrical motor have installed on altitude 1880 m and this electrical motors become very hot. The electrical machines power is 15300KW & 9700KW and they cooled by force air and water cooler.

First – machines designed for higher-than-normal altitude (i.e. in excess of 1000 m = 3300 ft above sea level) are designed with lower allowable temperature rises. The rule-of-thumb approximation is 1 degree C for every 100 m above 1000.

This means a typical Class B rise (max 80 C over 40 C ambient) will be designed for a max 71 C rise over ambient at 1880 m altitude.

Since temperature is more-or-less proportional to the square of the current, the design either reduced in output power to limit the current, or is “overdesigned” so that the resultant output power is the effective de-rate condition. In this case, the “sea level” rating of 15300 kW would become 15300 * (71/80)^2 = 15300 * 0.94 = 14382 kW. Likewise, the 9700 kW machine would be rated for 9118 kW.

The ability to cool the machine effectively is based on two things: the amount of coolant in direct contact with the heat source(s), and the pressure of the coolant flow. At altitude, the density of the coolant is reduced significantly, hence the requirement to operate at lower power ratings. The pressure of the airflow over the windings, etc is ALSO reduced at higher altitude, making the cooling more inefficient.

Speeding up the blower (i.e. going from 6 pole speed to 4 pole speed, for example) will overcome some of this by increasing both airflow and pressure. However, the power draw on the blower drive motor may also necessitate an increase in size to accommodate the new loading parameters (including the effects of high altitude on it!). Note that if the air movement within the machine enclosure is dependent solely on the MACHINE rotor speed (i.e. a shaft mounted fan), there will be a need to develop and apply a separately-powered fan to accommodate the required changes.

The probability of voltage breakdown / corona / flashover is increased above 1800 m as well, which means at least taking a cursory look at both creepage and strike distances.

And finally – if, after all this, the machine is still overheating … time to look at the cleanliness of the liquid side of the heat exchanger. This may mean cleaning or replacing the tubing and headers, determining liquid flow rates (and pressures) and ensuring they are within original design criteria (roughly 3.8 litres per minute for each kW of loss in the rotating machine).

AQ: Soft starter settings

Reference voltage adjustment
Reference voltage is the basic condition of the equipment is able to start or not. Reference voltage adjustment requires the electric motor rotates immediately after voltage applied and the load start up. If the motor does not rotate after voltage applied, we should increase the reference voltage setting value; if the motor start speed is too fast, then reduce the reference voltage setting value. Reference voltage adjustment should be repeated for several times until the load starts immediately after voltage applied. For example, a smoke blower has a 110kW motor in debugging process with soft starter, reference voltage adjusts to 75% rated voltage, the starting current is 500A, motor start up fast; reference voltage adjusts to 40% rated voltage, motor start up in slow speed, starting current rise from 200A to 600A smoothly, and current return back after motor start is completed, therefore, it’s fully meet the soft-start requirements.

Starting time adjustment
Motor acceleration torque and starting time has direct relationship. Electronic soft starter can make the motor with voltage ramp start from initial voltage to full voltage at the set time (0.5 to 2408). Like it can reduce water impact if we extend the time of water pump flow from 0 to 100%, increase the pump speed variation time means increase the starting time which can be achieved by adjusting the starting time of the soft starter. Starting time should be adjusted according to the specific loads and repeated tests, in order to achieve smooth acceleration within starting time.

Soft stop
Soft starter allows the output voltage decreases gradually to achieve soft stop, in order to protect the equipment. Such as the impact of the water pump, when the pump stops suddenly, the water flow inertia in the pipe will raise the pipe and valves pressure suddenly and cause pipeline damaged. Soft stop to extend parking time will solve such the impact.

AQ: 129 slot 48 Pole combination in motor design?

Koil can make the synthesis (i.e. design the winding layout from slot-pole combination) only for symmetrical windings. To have a symmetrical 3-phase winding the back EMFs must be equal and out of phase of 120 electrical degrees. Looking at the star of slots, this means that the spokes in the star (or phasors, one for each slot) must be equally spaced and the number of spokes must be multiple of the phase number.

Considering this example, the machine periodicity t is computed as:
t= HCF{Q,p}=HCF{129,24}=3.
Then the number of spokes in the star of slot is Q/t=129/3=43.

In order to have a balanced winding (assuming m=3 as number of phases) Q/t must be divisible by 3. Such condition can be written in general as Q/(m t) integer.

In this case we have Q/(mt)=129/(3 3)= 129/9=14.333 which is not integer, so that the winding is not symmetrical as here described.
Maybe there are some different/non standard arrangement of the winding.

AQ: Induction motor surge testing

I’d be very careful about surge testing motors in industrial environments. There is specific guidance from IEEE, NEMA and EASA that talks about surge testing being potentially destructive when done on motors in the field. More specifically, motors with unknown insulation conditions. Surge and hi pot testing are geared for shop testing on repaired or new motors. I’d recommend monitoring online impedance imbalance and current imbalance. We’ve seen many case studies where these two parameters were early indicators of stator faults. I agree that offline, phase to phase resistance and inductance can be great indicators of stator faults. The downside of offline testing is the fact the motor has to be shutdown.

We also recommend looking for faults conducive to stator failures. For example, if you have a high restive imbalance on the circuit this can increase heat inside the motor. The increased heat further stresses the insulation system and can lead to bigger insulation or stator failures. If we could have found the small problem, ie. resistance imbalance, then we could have prevented the stator fault.

Stator is a tricky fault zone because faults typically develop so quickly. With a good overall motor testing program you can find the faults that lead to stator issues and get them corrected early.
I was trying to point out that impedance imbalance and current imbalance can act as good indicators for stator issues. It seemed to me that most people in the discussion we’re focusing on offline tests and there wasn’t much mention of online stator testing.

I always think that these discussions are best if they focus on the technical aspects and remain fairly vendor neutral. That’s why I didn’t really bring up any vendors in my post. I think these discussions are a great way for people to gather a great deal of knowledge from a large sample of reliability professionals. I hope more threads like this pop up because I’m always interested in new technology and finding ways to better diagnose motor faults.

AQ: SCR broken failure in soft starter

SCR’s are limited to a maximum current rating, as well as a maximum voltage rating. In addition, the number of starts per hour is also limited. A combination of voltage spikes, too many starts per hour, or too much current during a start will destroy a soft starter. Phase imbalance for either voltage or current will cause an SCR to fail, as will a single phase condition on a 3-phase motor. What also needs to be considered is the load being started. If it is a high starting torque load it may require a heavy duty version of soft starter to get it going.

SCRs rarely “break” but they do short out, or rather, become full time conductors. The only thing that can cause this is excess tightening torque or clamping pressure. If on the other hand that the soft starter is giving an indication that one SCR is shorted, then that is where the comments from Terence Smith come to play. It will be either a voltage spike, a current spike, or excess heat caused by excessive starting current or starts per hour.

But reactors will not really help and will increase the throughput losses in the soft starter, I would not waste time on that. Starting a spinning motor is not an issue with soft starters either. Both of these are potential issues with VFD, totally different animal.

If the SCR fault covers the unbalanced starting current too, there is another possibility. At the motor connection box, on the side of the motor there are 6 bolts with screws, for connecting cable, star-delta cooper sheets, and motor coils. The lowest places on the bolt are the clamps of the motor coils, which is followed by a bolt. Over this bolt there are the star-delta sheet, bolt, cable connection clamp and upper the 3-rd bolt. In many cases the lowest screw, at the coil clamp is not tight enough. The maintenance electricians never check them, because it doesn’t belong to the cable installation. In many cases they occurred output phase fault in inverters and phase faults in soft starters.

AQ: Voltage transient/inrush current in induction motors

The voltage transient which occurs whenever there is a sudden change in current in an inductive device. Inductors resist a sudden current change.

V=L di/dt

In electric motors this occurs at start up when the contactors close and shut down when the contactors open. Soft starters reduce the start up transient, but not the shutdown transient.

This also occurs with variable frequency drives which switch the current rapidly and repeatedly.

Voltage transients of 2 to 5 times line voltage are common. This is a primary reason for failure of weakened motor insulation systems. Test standards require high voltage Hipot and Impulse testing of insulation systems in order to ensure that a motor can withstand these transients.
Inrush is something we have always had to deal with, especially with motors that are direct on the line start. The inrush can be as high as seven times the nameplate current. The damage created can be minimal if the motor is started up in the morning and them runs all day.

A motor that runs on a There is one situation that creates a huge inductive spike. Take a motor, lets say it is driving a fan, and it is coasting to a stop. The operator decides to push the start button while it is still coasting. It is a misconception that because the motor is already in motion that you will reduce the starting inrush. You will cause more damage to the insulation system by doing this than you could ever imagine.

The inrush current at start-up for a motor is not an inductive spike. In fact, the small inductance in a motor winding is a slight impedance to the inrush (hence the term), though very slight unless it is a high inductance winding.
An inductive spike is the spike that occurs when voltage is quickly switched between windings. The inductance will not allow current to change instantaneously and must go somewhere.
Changing voltages when the motor is moving because the inductance is an energy storage device. If you reverse voltage on a winding in a permanent magnet motor while the motor is active, the voltage on the winding is momentarily doubled, in theory, but the released energy in the winding can cause huge spikes when the back EMF is no longer opposed by the applied voltage, etc.

AQ: Variable frequency drive power anomalies

Variable frequency drive power anomalies can be divided into following three types: phase loss, low voltage and power off, sometimes they maybe appear mixed. The main reasons for these anomalies are transmission line impact by wind, snow and lightning, sometimes it’s the power supply system appear ground wire and phase short circuit. The lightning is very different due to geographical and seasonal factors. In addition to voltage fluctuations, some power grid or self-generation units will have frequency fluctuations, and these phenomena maybe appear repeated in short times, in order to ensure normal operation, the variable frequency drive power supply also need to make corresponding requirements.

If there is a direct-start motor or cooker or other equipment near the variable frequency drive, to avoid voltage decrease when these devices power on, those devices power supply should be separated with the VFD power supply to reduce influence each other.

For the applications require continues operation in instantaneous power off, in addition to select appropriate VFD drives, we also need to consider the motor load deceleration ratio. When the variable frequency drive and external control loop are adopted instantaneous power off compensation, we need to prevent over current during acceleration by detect motor speed when power on.

For the application requires continuous operation, it’s better to install additional automatic switching uninterrupted power supply devices. Like adopt diode input and single-phase control power variable frequency drives, it can continue work even if in phase loss status, but individual rectifier device current is too high, and the capacitor pulse current also high, it’s not good for the variable frequency drives reliability and service life in long time running, so we should handle it the early the better.

AQ: Variable frequency drive main circuit failure analysis

Variable frequency drive includes main circuit, power circuit, IPM drive and protection circuits, cooling fan and other several parts. The structure is mostly unitized or modular. Incorrect or unreasonable setting will cause the VFD malfunction and failure easily, or can’t meet anticipated operation effect. As a precaution, careful analysis before the failure is particularly important.

Variable frequency drives main circuit mainly consists of three-phase or single-phase bridge rectifier, smoothing capacitor, filter capacitor, IPM inverter bridge, current limitation resistors, contactors and other components. Many common failures are caused by the electrolytic capacitors. The electrolytic capacitor life is determined by the DC voltage and the internal temperature on the capacitor both sides, the capacitor type is confirmed during the circuit design, so, internal temperature inside the electrolytic capacitor is critical important. Electrolytic capacitor will affect the variable frequency drive life directly, generally, temperature increase 10 ℃, VFD life reduce a half. Therefore, on one hand, considering proper ambient temperature in installing, on the other hand, reduce ripple current by taking some measures. Adopt power factor improved AC/DC reactors can reduce ripple current, thereby extend the electrolytic capacitor life.

During variable frequency drive maintenance, usually it’s relative easy to measure the electrostatic capacity of to determine the capacitor deterioration, when the electrostatic capacity is less than rated 80%, insulation impedance is below 5 MΩ, it needs to replace the electrolytic capacitors.