Iacdrive|Automation Solution

Electrical machine software

You can categorize the electrical machine software into 2 basic types:

1) FEA packages that may or may not have a front end for analyzing motors. These are available from companies like Vector Field (now Cobham), Infolytica and a few others.
2) Motor design specific software such as the SPEED software, RMxprt and MotorSolve from Infolytica.

In the first category, the FEA packages are expensive because they are general purpose modeling packages. The motor add-on is usually limited mostly to the building the model and perhaps some specialized post-processing for motors. Their main advantages are:

1) 2D and 3D versions.
2) The user is free to define what analysis he wants to perform since they have very advanced general post-processors.

Their main disadvantages are:
1) Cost, they can get very expensive depending on the options you require.In some cases, the motor design module is a cost option.
2) Although they have general post-processors, many users require a lot of training in order to be able to get useful information.
3) Geometry input can be a lot more complicated since the front-ends typically have a limited number of geometries available.

The second category, the motor design software, is specifically designed for motor analysis. It can be magnetic circuit based such as SPEED and RMXprt or full finite element based such as MotorSolve. The magnetic circuit type of software has been available for a long time but it has only been recently that full FEA based motor design packages have become available.

The general advantages of software of this type are:

1) Template based input so the user simply chooses the motor geometry, stator and rotor and sets the parameters for the geometry. The input is therefore very simple but limited to the templates that are implemented in the package.
2) Post-processing is specialized and presented in a form that a motor designer can use it.

The general disadvantages of this type of software is:

1) No specialized post-processing is available directly from these packages unless added by the software provider in a new release.
2) Geometries are limited to the templates and adding templates may be very difficult and has to be done by the software provider.

What factors cause Current unbalance

1. Voltage unbalance in supply side (1% volts could easily be 10% current).
2. Physical differences between individual stator coil shapes and connections causing small (but noticeable) resistance changes.
3. Unsymmetrical magnetic circuit – not as big a deal in the smaller “ring” lamination designs, unless highly saturated.
4. Lightly loaded machines will exhibit far higher unbalance than those loaded closer to the full nameplate rating (mostly due to the magnetizing current requirements and associated core/stray loss).

For quick solution measure the current in the three phases, then change the three supply terminals by shift the three terminal to rotate the motor in the same direction, and measure again the current, if the high current move with a certain phase (example: phase L1 of supply read high current in the two case above) the problem is from supply, you can then measure the voltage at motor terminal to be sure that the control circuit and cable are good.

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.

3 phase induction motor designs

For 3 phase motor designs, there is hardly any slot combination that will yield a perfectly smooth torque-speed curve. Keeping the following rules in mind will (mostly) avoid the combinations that tend to amplify magnetic noise, harmonics, and parasitic torques.

Let the number of stator slots be S, and the number of rotor slots be R, and the number of poles be P. Undesirable combinations occur when any of the following are true:

1. S – R = 0
2. S – R = +1 OR -1
3. S – R = +2 OR -2
4. S – R = +P or -P
5. S – R = +(P + 1) or -(P +1)
6. S – R = +(P + 2) or -(P + 2)
7. S – R = -(P * 2)
8. S – R = -(P * 5)
9. S – R = +(P * 3) or -(P * 3) .. or multiples of +/-(3 * P).

We know the stator should have an even number of slots to make winding easier – although for certain pole counts, it too can be an odd integer value. And except for a few cases, the number of rotor slots can be either even OR odd.

Then it comes down to the accuracy of the compound die or indexing die for the slot stamping.

Variable frequency drive installation requirements

Variable frequency drives are electronic devices, they have stringent requirements in installation environment which is specified in its user manual normally. In exceptional circumstances, if it does not meet these requirements, we must adopt appropriate suppression measures: vibration is the main reason to cause electronic devices mechanical damaged, for big shock and vibration occasions, we should use rubber anti-vibration measures; moisture, corrosion gas and dust will cause electronic devices such as corrosion, poor connection, insulation reduced and then cause short circuit, as a precautionary measure, we should do dust treatment and corrosion control for the control panel, and adopt closed structure; temperature is the key factor to affect electronic devices life and reliability, especially semiconductor devices, we should install the variable frequency drive according to its required installation environment or install additional air conditioning and avoid direct sunlight.

In addition to the above points, inspect the variable frequency drives air filter and cooling fan periodic is also very necessary. For special alpine occasions, to avoid the microprocessor can’t work properly due to temperature too low, we should take necessary measures such as setting the air heater.

Choose motors for electric vehicles

My experience with the types of motors in electric vehicle is the following. There are three choices for motors in EVs, permanent magnet PM, integral permanent magnet IPM, and induction motor IM. They each have their pros and cons. A PM has the highest power density; it was used on a military HEV on which I worked. A con for the PM is the back emf during a vehicle run-away. If the vehicle were to go down hill at a high rate of speed a large bemf would be generated that would damage the IGBTs due to excessive DC bus voltage. The integral permanent magnet motor has smaller power density because the magnets are smaller and interior to the rotor, but is a compromise on the excessive bemf during a run away. The IPM has “half” permanent magnet torque and “half” reluctance torque. The IM has the smallest power density, and thus the physically largest for the same power and torque. On the other hand, it does not have an excessive bemf condition during run-away. The IM is also less expensive, but this was not the main consideration on the HEV on which I worked.

The major reason for using PM or IPM motors is power density and efficiency. That results in better mileage, lower weight and additionally less cooling required.
The cost for PM is significantly higher and availability is lower. Especially in Hybrids PM seems to be standard (e.g. Prius) but they have their own motor design.
For run-away the solution Chip suggested is an option. The short circuit currents are not necessary to high for the inverter if the inductance is high enough. That obviously needs a special design for the motor and possibly a short circuit device between motor and drive. Additionally the transients for the short circuit currents can be twice as high as the steady state short circuit currents. Another option would be to disconnect the driveline from the motor mechanically.
Another motor type that has not been discussed here is the high speed switched reluctance motor. Inexpensive to build and high efficiency (although lower power density).

VFD external electromagnetic inductive interference

If there are interference sources around the variable frequency drive, they will invade into the filter on variable frequency drive input side to reduce high harmonics, thereby to reduce the noise impact from the power lines to the electronic equipment; and install radio noise filter on VFD output side to reduce its output line noise for the same.

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

Variable frequency drive cooling fan maintenance

Variable frequency drive cooling system mainly includes heat sinks and cooling fans, wherein the cooling fan service life is short. The fan generates vibration, noise increases and finally stops when approaching end-life, then the VFD drive tipped in IPM overheat. The cooling fan service life is limited by the bearing, which is about 10000 ~ 35000 hours. When the variable frequency drive continuous operation, we need to replace the fan or bearing in two to three years. To extend the cooling fan life, some VFD’s fan only operation when the VFD turn on, but not the power on.

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.