Iacdrive|Automation Solution

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.

Improve induction motor efficiency

The efficiency of an induction motor is determined by intrinsic losses that can be reduced only by changes in motor design. Intrinsic losses are of two types: fixed losses – independent of motor load, and variable losses – dependent on load. Fixed losses consist of magnetic core losses and friction and windage losses. Variable losses consist of resistance losses in the stator and in the rotor and miscellaneous stray losses. So by reducing these losses we can improve efficiency of induction motor.

Changing the rotation direction will not improve efficiency.
Core loss and copper, those are the dominant losses. Improve them and you will get better efficiency. Changing the slot shape etc will help considerably, as will using copper in the rotor. BUT, you can’t do either one without affecting the performance of the motor, specifically the starting torque and current as well as the maximum torque and current. In addition, if the motor is designed to have aluminum cage, then changing the cage material to copper won’t help the efficiency much since the rotor slot and end rings are not optimally designed.

Improving slot fill will help your copper loss, by putting bigger wires in the stator slot, the wire resistance will reduce and the copper loss will go down. Reducing the end turn height of the windings will also help reduce copper losses.
Stray losses are the only one which can improve efficiency without affecting size of the induction motor. This can be reduced by reducing harmonies in the machine, which can be controlled by selecting slot combination, winding layout, size of air gap, saturation, concentricity of air gap etc.

If an induction motor has to run in both direction and uses a bi directional fan it is inefficient. uni directional fans are used in higher ratings to improve efficiency. further direction of rotation is determined by the driven equipment and cannot be changed at will. Minimising losses both core and copper and stray losses, better cooling ,improvement in cooling fan design a combination of all this suitably balanced will improve efficiency but there is always a limitation on max value imposed by certain conditions of application, materials, willingness of customers to pay.

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.

VFD control loop circuit faults analysis

The affection on variable frequency drive life in the control loop circuit is the power part, the buffer capacitor in smoothing capacitor and IPM board. The ripple current pass the capacitor is a fixed value which won’t be affected by the main circuit, so its life is mainly determined by the temperature and power-on time. Since the capacitors are soldered to the circuit board, it difficult to determine the capacitor deterioration by measuring the electrostatic capacity. Generally, we calculate its life base on the ambient temperature and service time.

Power supply circuit provides power to the control circuit board, IPM drive circuit, operation display panel and cooling fan, the power is obtained from the main circuit DC voltage rectified by the switching power supply. Therefore, if one power short circuit, besides itself damaged, also affect other parts power supply, such as misoperation causes power source and the public ground short circuit, result in switching power supply circuit board damaged, the fans power supply short circuit etc. Generally it’s easy to find out by observing the power supply circuit board.

Logic control circuit board is the core of a variable frequency drive, it includes CPU, MPU, RAM, EEPROM etc large scale integrated circuits, the failure rate is very rare due to high reliability. But sometimes all control terminals closed simultaneously during startup which will cause the VFD drive appear EEPROM fault, in such case, just reset the EEPROM.

IPM circuit board contains drivers and buffer circuit, and over-voltage, phase loss protection circuits. Logic control panel PWM signal input to IPM module by voltage drive signal optical coupling, so, we should measure the IPM module optical coupling during module detection.