Why use a frequency inverter along with an electric motor?

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By Stefan Fassbinder (DKI)

An electric motor and an electricity generator are basically the same.

By principle, any electric motor can also generate electricity. Electric drives are way ahead of combustion engines, since, unfortunately, a car engine which sucks up exhaust fumes during braking and downhill rides and converts them into fuel and fresh air is still pending. The electric motor can deliver this, although during its first century of existence, its use has largely been hampered by two basic drawbacks:

• An electric motor has no accelerator pedal.
• A socket has no sort of “water tap”.

When an electric motor is running, it generates a voltage with a polarity opposite to the feeding voltage.

Therefore the current is excessively high at the first instance of switching on when the motor is not yet running. For big motors precautions have to be taken not to damage it or blow any fuses. As the motor speeds up, this induced voltage increases. In fact when exceeding the speed where the applied voltage and the mains voltage are equal the motor will generate a higher voltage than that found in the line. Current will flow the other way round, and the motor has inversed its function into that of a generator.

That is good, since it offers excellent energy efficiency advantages especially for cranes, elevators etc. which actually become power plants during downward motion.What is not so good is that the line always has approximately the same voltage, but with respect to other loads, e.g. lights, this has to be so. Hence, provisions have to be foreseen again if the motor speed shall be varied. In the old days, this used to be an onerous task. One had to use transformers with multiple taps, such as in locomotives, but which was a bulky and expensive solution, or limit the current with resistors, such as in trams, which was an inefficient solution.

And things become even more difficult when it comes to AC motors, be they single-phase or three-phase. The principle of an electric motor is always to create a rotary movement by attracting and repelling magnetic forces. In the strict terms of physics electric motors do not even exist, but all of these would need to be called magnetic motors from a purist point of view: An electric magnet attracts another – also electric, or permanent – magnet until it has come as close as can be. Then the polarity of the current in (one of the) electric magnet(s) is inversed, and the attracting force inverts into a repelling one. The motor’s mechanical design is made up so as to allow such motion only around in a circle because a rotating motion is desired. AC motors can be built simpler than DC motors because the periodic swaps of polarity occur anyway and do not have to be generated within the machine.

But it becomes obvious that the variation of rotational speed is difficult for DC motors, since it depends largely on the supplying voltage, which is approximately stable, and it is impossible for AC motors, whose speed coheres strictly with the frequency of the network, which is in technical terms totally stable.

Now either type of electric motor has to be designed in a way so that at the desired (rated) speed the voltage generated in the motor is about the same as the applied (rated) operating voltage. With DC motors the induced voltage has to be somewhat lower than that in the line. When loaded, the DC motor will lose a bit of speed, yielding a further drop of the induced voltage and hence a higher difference to the line voltage and a higher input current, matching the higher load. So it adapts (more or less) by its nature to varying load.

This is an advantage over a combustion engine and one of the substantial differences in operating behaviour to be discussed here. Imagine you disengage your car engine and place a brick on the accelerator pedal. You should not do that. An electric motor, however, will not mind running on full voltage with no load – exempting perhaps one particular type, the series connected commutator machine. Big units may actually get destroyed by centrifugal forces when fed with full voltage and no load applied. Small units, such as those used in kitchen appliances and e. g. for the windscreen wiper in a car, have sufficient friction losses to prevent this. But at a fixed feeding voltage applied a certain speed will always be linked to a fixed power output – and input. Since there is no plain and simple, straightforward thing like a water tap in the kitchen and in the bathroom which could be attached to a power socket to allow the control of electricity flow, regulating the power and / or speed of an electric motor has been a demanding task in the days before the invention of power electronics.

This applies all the more to AC motors. The speed of a synchronous machine is absolutely stable, be the machine used as a motor or as a generator. Well, it does lose a little bit of speed for a very limited time when, for instance, it shifts from neutral to motor operation, just until the phase angle between the electric phase and the position of the rotor are no longer “in phase”. After this short period of transition the motor’s speed and the mains frequency will be synchronous again. One could imagine the process like this:

When the machine is running under no-load conditions, the alternating voltage it generates is high when the line voltage is high, and it is low when the line voltage is low. They are in phase with each other, so practically no current flows either way (roughly speaking, ignoring the aspects of reactive power experts will highlight here).

Since the electrical power (also its instantaneous values) are calculated as voltage times current, the reversal of either voltage or current implies a reversal of sign and thereby a reversal of energy flow. Now when the machine is running as a motor the alternating voltage it generates lags behind the applied voltage. It is still somewhat lower when the line voltage already reaches its peak, so current will flow from the mains into the machine; so it acts as a motor. By the time the current finally swaps its polarity, the line voltage will also have swapped, so we are multiplying two times by -1 and get stuck with motor operation.

When we drive the shaft of the machine to run the machine as a generator the alternating voltage it generates is leading the applied voltage. It is already dropping again when the line voltage reaches its peak, so current will flow from the machine into the mains. By the time the current swaps its polarity… and so on.

Now things become difficult when we come to discuss the most widely used electric machine, the asynchronous motor, since the processes that drive it are hard to imagine in an illustrative manner. It has electric magnets on either side, in the stator and in the rotor. The rotor windings are shorted and act like the secondary windings of a transformer. The magnetic field rotating in the stator induces a current in the shorted rotor windings, which then generates its own magnetic field. Like in a synchronous machine, the poles of the stator fields, driven by the mains frequency, run around in a circle and chase the poles of the rotor field ahead of them, so to say. So the rotor starts to spin. An asynchronous motor will always spin a little bit slower than the magnetic poles in the stator do. This little difference, the slip, is necessary to sustain the current in the rotor windings and thus keep the rotor magnetic. The slip frequency may be as low as 1 Hz or even less in a big machine, so if in a 2-pole asynchronous motor fed with 50 Hz the stator poles spin around at 3000/min, the rotor will rotate at 2940/min. When you speed it up it will act as a generator. At 3060/min, say at the same slip with inverse sign, the current output will be the same as was the current input at 2940/min.

Together with the DC motors including the series connected commutator motors that can be operated on both AC and DC, the asynchronous three-phase motor will start up alone as soon as the mains voltage is applied. More than that: It will do so very abruptly with several times the rated torque and current intake, as explained above. This is the next distinction against the combustion engine which requires a small DC motor to start it up.

A synchronous machine as such cannot start up alone. For this and other reasons it is usually used as a generator only.

As an aside, the serial commutator machine is by principle a DC machine, but because its stator and rotor are connected in series they will both swap polarity when the current does, so the sense of rotation remains the same. Therefore it can also be operated as an AC motor, but when used as a generator it will generate DC, the polarity depending on some accidental residual magnetism, if not defined by a dedicated supplementary spool.

Now while it is a piece of cake to control the power and speed of a combustion engine simply by throttling the fuel supply, which on the other side is a dreadful necessity while the electric motor more or less regulates itself, the “water tap” for electricity was finally invented in the seventies: now there are inverters available which convert AC to DC and the DC back to AC again with electronic components (and very low additional losses). The AC output can be controlled both in amplitude and frequency as to adapt it to the requirements of any motor at any desired point of operation. Speed and torque can now be controlled independently of each other. So the inverter overcomes virtually all disadvantages of the electric motor against any combustion engine, while the advantages remain as outstanding as they are, including the power feedback (inversion of energy flow) if a 4-quadrant inverter is used (2 rotational directions, 2 energy flow directions).

Broken down into very simple terms, such inverters build up a connection between the direct voltage in the DC link when the momentary alternating voltage in the line is higher than the DC voltage in the link, thus allowing energy intake, and disconnects both from each other when the voltage “out there” is lower. This is the principle of motor operation. For feeding back energy in generator mode the inverter, justifying its name, does the inverse thing: Connect when line voltage is low and disconnect when it is high. In this way the energy can go either way even though the line voltage is constant – and the DC voltage in the link circuit may also be kept at a constant level, depending on design.

The other end, the motor side of the power electronic inverter, is somewhat more sophisticated. Simplifying again, the principle is to switch the motor on and off very rapidly, much more rapidly than any mechanical switch could do. By varying the on / off time ratio the average motor current can be continuously varied, even if the DC voltage in the link circuit is kept at a constant amplitude. The principle is much more sophisticated and pretty much more expensive than controlling the water flow in the bath with a water tap, but the advantages are so salient that this principle is steadily making its way all through the realm of electric drives.

Inverters can also be used on DC networks.

While old trams – and many of them are still around – could very well use their motors for braking, the electric power could not be fed back into the lines because the voltage the motor generates is, roughly speaking, a bit lower than the voltage on the line, so an inversion of the power flow was not possible. The electricity generated during braking was absorbed in resistors and went lost as heat. Nowadays inverters can chop the DC into AC, AC can be transformed (the transformer being the smaller, the higher the chopping frequency is chosen to be), rectified back to DC and fed back into the overhead line.

Now a combustion engine has a certain power output rating, and that’s it. If you try to get a little bit more torque out of it than what the rating plate offers you, you just stifle the engine.

What a difference to the behaviour of an electric motor! It also has a certain maximum power and maximum torque, but what does it do if you want more? It gives you more!

The speed of a DC or asynchronous motor drops a bit, while in a synchronous motor the angle between applied and induced voltage becomes a bit greater. Both leads to a higher current intake, which facilitates a higher output torque at approximately or exactly the same speed, respectively. The motor will offer you double the rated torque if you want it. Depending on type of design and size of the motor it may be more than 5 times as high as the rating. The only problem is that it allows for this only for a limited time because the excessive current generates excess heat in the motor, and in the long run the motor would burn out. Special motor protection switches which are adjustable to the current rating break the motor current if the rating is exceeded for too long. The better approach is monitoring the actual motor temperature. Or to use an inverter. Its electronic control offers unlimited programming options.

So here we go:

• Since an electric motor starts up alone and many types even offer their greatest torque (breaking torque) in standstill mode, no clutch is needed in an electric vehicle.
• Since an electric motor offers a much greater torque for a limited time than for continuous operation, no gear change is needed in an electric vehicle because vehicles always need their maximum traction force only for the limited periods of acceleration and uphill ride.

So, an electric motor is a much better and more sustainable option for vehicle operation than combustion engines of any kind. Together with a power electronic inverter they are close to ideal, while the combustion drive is more or less a makeshift manner to move a vehicle which only on account of more than 100 years of experience together with a huge and powerful market could be optimised by and large to the state we observe today. There is no further progress in sight.

All that is now still lacking is a usable battery. When it comes all land transportation will immediately go to electric drives. Wherever a catenary wire is available the electric drive is already demonstrating its superiority, and there are still some potentials left.

Morals: a combustion engine and an electric drive could not be any less similar. If you want to understand electric drives the first thing you must do is to forget everything about your car engine.