SERVOGENERATORS

The cinematic chain in a servomotor

Servomotors are one of the most technologically advanced applications in electrical machinery today.  The function to be performed, the mechanics, the transmission the servomotor and needless to say that the control electronics of the devices they are installed, in have usually been carefully studied.  A servomotor is no longer designed without electronic control and a new term which reflects this concept has even become widely used: mechatronics.

We technicians are proud of what is being done in this field and of its development possibilities because to an increasing extent we can move bigger components more precisely, more quickly and further.

There is a major paradox about electrical machinery.

Users who are ecperts in mechanical applications or transmissions see it as a mature, pedestrian and well –explored area with few development possibilities beyond improvements in materials.

We machine designers intuit a drastic and profound change in operating principles which will simplify mechanical transmissions and lower the price of units.

This change is not immediate and it is premature to hazard whether it will be evolutionary or revolutionary. But the process of putting the finishing touches to its ingredients has been ongoing since the last decade.  Never have we had the electronic control, calculation and materials capabilities which today are accessible today.

At this point in time it can safely be said that controls have increased their processing speeds to a far greater extent than drive systems have done which, by comparison go more slowly.  The market would value faster drive systems which reduced the drawbacks of mechanical transmissions.  It is worth taking technical risks to be first.

The infinite power network

The electricity power supply network for a present day servomotor is often seen as having infinite power.  This academic simplification, found in first year electrotechnology courses, often remains as a mental block especially in the case of technicians whose work centers around applications.

A theoretically infinite power supply network maintains its voltage in terminals although the electrical load absorbs a lot of current

Industrial power supply networks can often be deemed to have almost infinite power, but the increase in transient performance in servomotors and the reduction in investment in electrical installations would make checking advisable in each case.

This checking is very easy.  Connect up an oscilloscope or network voltage reader to measure the voltage during the most brusque start-up of our system.  If the voltage really does not drop unacceptable, then the supposition of infinite power is correct.  You cannot trust it that much as looks can be deceiving and it should not be taken for granted, above all if the power comes from a generator or another autonomous system.

Real networks powering servomotors

A servomotor that was even faster than present day ones would require much more instant performance from the electrical source which powers it.  In other words, another link would have to be added to the chain mentioned above-application, mechanics transmission, servomotor, electronic control-which included the energy source.  This would make it possible to control the impact of the energy sources absorbed current of the available voltage or current.  This cannot be considered a negligible factor however much it may.

Electricity supply networks usually have a short-circuit current noticeably higher than the rated voltage in all of their hierarchical protection levels.  Some of these levels belong to the electricity supply company while others form part of energy distribution in the factory or use.

This is because they are usually very over-sized and because connection between different voltages is made using electromagnetic transformers.  Because of the way it is built a transformer can supply short circuits or 20 times the rated voltage in around one second.  The protection systems on these networks are sized to prevent damage to the conductors and transformers in the event of overloads and short-circuit.  Protection setting when powering servomotors is not an unimportant issue.  Start up of the servomotor may look like a short-circuit in view of the transformer powering it, so the protection needs to allow the transient current to go through and only cut off if it lasts too long. 

If the servomotor is only powered using a very long cable then it is necessary to check for transient voltage drop in the cable, and not only in its resistive return component but also in its inductive component.  If more reliable information is not available, it is acceptable to assume that a round conductor has one microhenry per metre of length.

Supplying current through copper plates reduces the inductive reactance of conductors much more than using round conductors with the same cross-sectional area.  Moreover the elongated section increases the atmospheric cooling surface and promotes the dissipation, into the atmosphere, of the losses caused by the Joule effect.

As noted above, the electricity network can provide a transient current 20 times greater than the rated voltage before the protective devices are triggered, this taking around a second.

Consequently a servomotor will easily have enough transient power and energy to start up when connected to an industrial electricity network.  Something similar happens with regenerative braking in servomotors.  Returning kinetic energy to the network in the shape of electricity allows for very brusque braking which is of use in some processes.

Autonomous systems

An autonomous system is defined as one which is not directly connected to the electricity supply network.  Primary energy may be a fossil fuel which drives a rotary generator via a thermal motor.  A renewable energy sources is also often used to drive a rotary generator.

Another range of autonomous systems use a physicochemical reaction or charge different types of batteries.

Autonomous systems are optimized for the average consumption, response time or length of supply between charging of the primary energy source.  Rarely are they sized beyond what is strictly required for maximum and transient performance.

If the generator is a rotary machine its short-circuit current will not exceed three time the rated voltage at best.  In addition the electronic voltage control has a transient response time and mode which make significantly different from that of an electromagnetic transformer. 

The result, as is well known by NGO personnel, is that a simple 200 W single-phase electric refrigerator in a field hospital usually cannot cope with the start up transient of the single-phase induction motor in its compressor if it is powered by a 1500 W generator.  These types of cheap motors need comparatively large amounts of transient current, and the generator usually falls short in this respect.  Surprises in the field can not only be disagreeable but can also a show a purchasing decision that was apparently sound but proves inadequate during transient and makes the unit unusable.  There is an easy solution to this: oversize the generator to 2200W and keeping the same power in the thermal motor which drives it. 

In the case of lead-gel batteries available transient current is very high.  In 17 AH batteries, 20 hour discharge current is 0.8A.

They can continuously supply 8A and even 80A with 250A maximum peaks.  The problem lies in the fact that from 20 A upwards the discharge current lasts a lot less than expected in the capacity at 20 hours calculation.  Under these circumstances the batterys energy output dip below 20%.  Its life cycle can also be affected by the discharge conditions.

In the case of the battery in the example above, the manufacturer recommends not exceeding 5.1Aduring charging.

The outlook in other battery technologies may be worse form the point of view of transient performance.  Moreover, battery charging conditions in some cases are extremely delicate.  Thus it seems fair to say the greater the energy output of an accumulator battery technology, the worse is transient performance.

Fuel cells are an extreme version of this statement.  They may be very efficient but their transient performance is frankly limited.  Some of these technologies require 4 hours to stabilize the rated speed.  Variations in the current supplied may affect the systems stability and output.

The servo generator

This is a device which is only appropriate for powering servomotors with autonomous electricity supplies or when electricity networks are poor in terms of required transient performance.

An electrical energy source is supposed to have the capacity to supply the rated power for a given application with the technologies mentioned above.  The problem usually occurs when powering the transients as requested by the servomotor at the required current without letting the voltage drop or the response take too long.  As in many industrial applications a compromise solution between servo generator and servomotor control is acceptable.  For this to be possible the generator or power supply has to become part of the design chain of the servomotor mentioned above.

At this point it is time to look into the utility of kinetic electricity accumulators, ultra-condensers and other technologies capable of storing and supplying high energy levels with high instant outputs and power at reasonable costs.

The control capability has been available for years.  What is lacking is the development of the hardware required for this transient energy supply.  The author’s personal view is that the solution involves the development of rotary machines with very low internal impedances connected with adequate inertia which act as an energy buffer.

The reliability of the electricity supply network has delayed these developments.  The needs of autonomous applications and the increase in transient power required in powering servomotors make the development of these energy buffers advisable, and which in combination with available generators make up the concept of the servo generator as the electricity supplier for a servomotor.

An electrical or hybrid vehicle is an autonomous energy system.  The performance of its tractor engine under urban conditions is practically the same as that of a servomotor, with long, frequent and brusque start ups and stops.  The need for a servo generator in this application to optimize the powering of the tractor servomotors is quite clear.

The standard hybrid vehicle is one which moves exclusively with electrical servomotors.  A generator driven by a thermal motor working at optimum output speed can supply the average electricity needs of the vehicle.

If this is complemented by a set of batteries, night charging can be carried out using the electricity network and the mild, continuous transient deviations with respect to average power supplied by the generator can be managed.

Urban driving and the potential of the vehicles tractor servomotors show that this is not enough.  A new hardware device needs to be developed which acts as an efficient energy buffer with high instant power and flexible transient response.  Only in this way shall we be able to talk of servogenerator units for powering servomotors.

The technologies to be developed may be kinetic accumulation or ultra-condenser technologies amongst other types.  Devices have to be assembled, tried out and their reliability and performance demonstrated.  Only in this way can the viability and utility of these devices be demonstrated.

That is why DEE-ESTEIB UPC and Mavilor Motors are working together to develop devices along these lines.

For more information vist. www.automotioninc.com