Answers to FAQs on Battery Motors & Controllers:
Part 3
Contents, this page:
This is a simple section, mainly aimed at novice machine builders such as Robot Wars beginners, but it should also be useful to others.
A machine, for the purpose of this discussion is a mechanism of some sort, controlled by maybe a mixture of electronics, hydraulics and other systems. Machines of this sort can seem more complicated than they are, because the beginner has no clear idea of what they need to do.
So the first step is to get some idea of the mechanical task to be done. It does not matter whether is is a Robot, a welding machine or a golf buggy: if the basic mechanics is wrong, no amount of sophistication in the controls can correct this. So, before anything else, decide what movements the machine has to make and design a mechanism that will allow the machine do do those movements as simply and reliably as possible. Once this basic mechanism is designed, you will have a much better idea of the sort of controls you are looking for. There is no point in looking for a control system until you have a good idea of what it is you wish to control.
4QD's controllers can be used in fixed workshop type applications. However a battery is normally the best choice as the controllers all use use 20kHz chopping and a transformer fed full wave rectified mains power supply will have a 100Hz or (120Hz in USA) output so you will need very good smoothing. Except at quite low currents, a suitable transformer, rectifier and reservoir capacitor can prove very expensive so a small battery with a suitable charger is likely to be much more economical.
If you use a modern switch-mode controller, the switcher will probably be working at around 100kHz and you will have to consider its performance under full motor current, chopped at 20kHz and the fact that the inductive motor load of the controller can generate nasty over-voltage spikes back into the power supply.
Even our smaller controllers (e.g. Uni-4) are current limited at around 60 amps, so peak currents may be as high as this, unless they are limited by the motor. You must therefore consider what effect peak currents of this order may have on your supply.
The main capacitor on the controllers is intended to be sufficient when used from a battery - which itself acts like a large capacitance. For high current use of a power supply you are probably going to require a lot of (expensive) extra capacitance here.
However, if you do have a suitable mains power supply then you can use this but beware of the regenerative braking on many of the controllers since this feeds power back into the battery during braking. If there is no battery then the regenerated energy can pump up the power supply voltage to a high level if the motor is stopped too quickly. Although the controllers are protected against such over-voltage, this overvoltage could damage the power supply and anything else connected to it. A controller that cannot regenerate may be a better choice! See guided tour of controller features.
Most machine tool applications will benefit from using a tacho generator feedback system to close the loop. See the section on closed loop control elsewhere in this sheet and the diagram )on our sister site) of a tacho feedback circuit which also is available as a ready built and tested item.
12v at 25 amp is 300 (12 x 25) watts: about the same power as a small pistol drill. 24v will give double the power so 24v at 70 amps would give (24 x 70) 1680 watts. Don't forget that these controller ratings are intermittent.
Since the controllers use 20kHz chopping the current in the motor is essentially constant, not pulsed as in most mains operated controllers. This means that the motor will have better slow speed torque than with a lot of other controllers.
Micro-processor generated PWM.
Many microcontrollers can generate PWM signals. It is natural to ask whether these PWM controllers can be used to drive our motor speed controllers.
Commercial motor speed controllers all have their own internal pwm generator (either in hardware or software) - sample circuits are shown elsewhere on our sister site 4qdtec.com, as is the way the system works. To increase speed you increase pulse width, increasing motor voltage.
If the circuit current limits, it must decrease motor voltage to reduce motor current: the only way it can do this is to decrease the pulse width.
Similarly, if the regen current limit engages, it must increase motor speed - as the motor braking is excessive) so must increase motor voltage.
It should be obvious from this that both the current limits must be able to over-ride the modulator, so the current limits and modulator are an integral part of the protection circuitry.
So - if external PWM were to be applied, current sensing would also have to be fed into this external pwm source and processed there.
Since 4QD's motor current sensing is hardware based: it is very fast, operating 1-3µS after switching the MOSFETs. It is quite possible to do current limiting in software within a microcontroller, but it requires clever interrupt programming and is necessarily heavy on processor time.
Also - if the controller is designed for external pwm inputs: then the device giving the PWM is itself an integral part pf the controller, so the bits left are simply MOSFETs and drivers - not a motor controller. So - if you wish to utilise the Microcontroller PWM outputs to switch the motor - you are designing your own controller and no commercial controller manufacture can do much for you!
For more information, see Digital control
Pavement 'scooters' nowadays operate at 24v, have a top speed of 4 mph (limited by law). Commonest motor is an EMD PM50 (about 250w rating). The controller will be capable of delivering about 50 to 70 amps peak. This gives them a hill climbing ability of about 1 in 4 and plenty of current for kerbs etc. If you do not require hill climbing, then less current is required. For road vehicles, which have a higher legally permissible top speed of 8 mph, proportionally more current will be required: theses usually double up the motors, one for each rear wheel. If the controller has a 'top speed switch' to restrict speed to 4 mph or 8 mph, then a dual purpose vehicle is possible. Some older chairs used to have 12v motors, with inferior performance.
Controllers for mobility aids have, historically, become very complex and sophisticated. In part this is because the earliest vehicles were designed for very disabled people with poor motor control. The controllers therefore became very sophisticated to cope with the user's disabilities. The second reason for this complexity is that early power semiconductors were temperamental and unreliable, so designs added more and more fail-safe protection circuitry. Then there are the commercial pressures of 'Specmanship' - selling a machine on specifications, to justify a higher price. This tends to lead to over-engineering.
4QD's controllers started life for the Golf industry, which has started from the other end - very low technology gradually improving with time but always very price conscious. Golf vehicles need 100 to 200 amps which is much more than most mobility aids, except for high performance dual purpose machines. For these either our new Pro-150 or our 4QD series is an ideal choice. The 4QD was originally designed for this market and it is so good that there was one multi-purpose vehicle on sale (made in Poland) which simply stole the complete design of our controller. As they say, imitation is the sincerest form of flattery! The 4QD series are economically very competitive with other available controllers!
The Pro-120 is another sensible choice, which is nearly as powerful as the 4QD-150, available for 12v, 24v, 36v and 48v.
The other choice is our VTX-75 controller. This is fine for 4mph use.
The 4QD, Pro and VTX series have fail safe mechanisms built in and, although no one can totally predict all possible failure mechanisms, an uncontrolled failure to high speed is extremely unlikely.
Most motors for D.C. operation are either of the permanent magnet (PM) type or the field energised type. Field energised motors are of two main types, shunt wound and series wound, but there are some motors where a shunt and a series field are both present (usually called compound wound motors).
Permanent Magnet Motors
In the field energised motor some of the motor's power is wasted, energising the field winding (hence field-energised) whereas in a permanent magnet motor there is no field winding as the magnetic field is provided by permanent magnets, not by electrical current. Therefore for battery operation permanent magnet motors are best as their efficiency is theoretically better than that of the field energised type. Modern PM motors are also generally smaller that field energised motors because modern magnets are smaller that the windings needed to give the equivalent field.
The Permanent Magnet motor has a fixed top speed which is dependent on the voltage fed to it (which is how a speed controller varies the speed - by altering the voltage fed to the motor). It the motor is rotated faster that this top speed then it tries to become a generator (if the controller will allow) and the generated power acts to brake the motor. PM motors are therefore ideal for invalid vehicles, kiddie cars, golf buggies, model locos and similar vehicles where the speed should be under control at all times and the vehicle's designed top speed should not be exceeded. D.C. motors may not be so suitable for open road vehicles, where the ideal is to gain maximum speed going down hill with the intention of using the momentum to get up the next hill, though road speed restrictions rather spoil that form of energy saving!
To reverse a PM motor, you must reverse the direction of the armature current either with a high current switch or relay, or with a suitable controller.
See also Reversing
Guided Features tour
In the shunt wound motor, the battery voltage is connected across the field winding. A steady current (equal to the battery voltage divided by the resistance of the field winding) flows in the field winding, causing it to become an electromagnet. These motors behave exactly like a PM motor, but with the permanent magnet replaced by an electromagnet. Usually the field winding is brought out separately from the armature, so the motor has four wires. With an ohm meter you can check that there are in fact two windings, a high resistance one (the field) and a low resistance one (the armature). To control the speed, connect the field winding directly across the battery and drive the armature from a controller as if you are using a PM motor.
To reverse the motor, use a double pole changeover switch to reverse the connections to the field winding. Since the field winding uses much less current than the armature, only a low current switch is required. Of course, as with any motor, you should never change direction when the motor is running. You can of course use a normal reversing controller (e.g. our VTX series) and use armature control, with the field permanently wired to the battery, or you can use the parking brake driver to switch the field on and off to save battery power. Parking brake driver is suitable for field currents up to one ampere.
The magnetic field intensity of the field is proportional to the current flowing in it. If you reduce the field energising voltage, then you reduce the field current. Now, as the motor rotates, the armature cuts through the magnetic field, generating a voltage which tends to counteract the applied voltage (which is why it is called back EMF, or Electro Motive Force). The magnitude of the back EMF is proportional not only to the speed, but also to the intensity of the magnetic field. If you reduce the field voltage, then the armature must go faster to give the same back EMF with the reduced field. So you get the curious effect that if you reduce the armature voltage the motor goes slower (as you might expect) but that if you reduce the field, then the motor speeds up.
In a series wound motor, the armature current flows through the field winding, which is a high current winding in series with the armature. Both windings will be low resistance and, with an ohm meter, it can be difficult to tell them apart!
This series connection gives the series wound motor a performance much different to the PM and shunt wound types. In particular you cannot get regenerative braking with a series wound motor.
Regen Braking
To get regeneration, you need to maintain a magnetic field as the armature current passes through zero. Then, as the armature back emf decreases below the applied drive voltage, the armature current will reverse (if the controller allows this).
But you cannot do this with a simple series wound motor as the magnetic field collapses to zero as the armature (field) current reduces to zero. So there is no back emf at zero armature current, so the current can never reverse!
Plug Braking
If you switch the field coil's polarity, then the direction of drive reverses and you can get plug braking which is often jerky and always inefficient. But the current supplied by the controller is still positive (drive) and has not reversed as it needs to to get regeneration. All that happens is that the power supplied by the controller gets used reducing the load's kinetic energy - and all the power (supplied by the controller and supplied by the loss of KE) simply gets dissipated as heat in the motor.
Universal operation
However, because of this series connection, the series wound motor will therefore work equally from a.c. or d.c. In fact they are sometimes called 'universal' motors for this reason. To reverse such a motor you cannot simply reverse armature current, but you must use a high current double pole changeover switch or relay to reverse either armature or field connection. This is OK if the motor has been designed for reversing, but it is not practical with something such as a car starter motor since with these you cannot readily separate the field and armature.
Top Speed
One feature of series wound motors is that they don't have a theoretical top speed: as the armature speed increases, so the current reduces. As it does so, the magnetic field reduces. As the field reduces, the motor needs to rotate faster to give the same back EMF. As the current reduces to zero, the perfect motor would get faster and faster to give an infinite top speed under zero loading. Fortunately perfect motors don't exist and there is always some residual magnetic field and some friction. Nevertheless some early traction motors had a nasty habit or over revving to destruction if a drive shaft broke. This over-revving is also the rationale behind compound wound motors, where an extra shunt wound field provides a permanent field to restrict field reduction and to limit top speed.
This feature of the series wound motor makes it probably a good choice for open road vehicles, where a limited top speed isn't so desirable, and where you want to gain momentum down one hill to get up the next. However when you run a series wound motor at slow speed with a small mechanical load it becomes very inefficient. A car starter motor run like thus can draw 35 amps and do nothing at all. Low speed control can therefore be a problem.
Controllers
It is quite possible to use 4QD motor speed controllers with series wound motors: the 1QD, 2QD and Porter series are directly suitable but you will have to arrange your own reversing arrangement. The VTX series can be used but requires modification so the field can be correctly connected into the relay circuitry (see below).
- The 4QD series can also be used. We have an application note available which explains a method (and some problems).
- Uni, Porter, 1QD or 2QD series.
The diagram shows how the armature and field can be connected with a double pole changeover reversing switch with these controllers. This can change over either the armature or the field winding but is here shown in the armature here.
- VTX series controllers.
These can be relatively easily modified so that the on-board relays control the reversing. An application note is available on site.
- Pro-120 and Pro-150 series
In theory, the modification to these is the same as to the VTX series but because of their construction, this is not a practical proposition.
Flywheel Diode
Note the extra diode connected across the field winding. You can usually get away without this diode but for best results it should be present. The reason is that armature and field windings have different inductances so the current in each will try to decay at different rates. The extra diode allows the field current to 'do its own thing' irrespective of the armature current. The current rating of this diode will depend on the motor but it will not get very hot as it is only conducting for a small part of the switching cycle.
Fortunately there is, in theory, a way of overcoming the problems with a series wound motor: use two controllers, one to operate the field and the second to operate the armature. The controller for the field must be set to deliver a current to the field. This controller is not varied but simply supplies a constant energising current to the field. The advantage is that the field current can be set for maximum motor efficiency. The second controller controls the armature and the motor will now perform exactly as if it were a permanent magnet motor, giving regenerative braking and reversing from the armature.
Sorry, we have no application note available on this and we have no more information to supply. If you wish to do it, you're really on your own, but if you'd like to share the details, maybe we can write an application note for others.
To many, this seems like an easily available motor. But in fact they are not good motors for conventional uses and we really cannot advise their use under any circumstances.
The problem is that they are designed for very high current intermittent use (very low duty cycle). They require a lot of current to create enough field even to make them work - just to start them rotating you have to pass around 35-40 amps through a starter motor. To get any sensible torque needs 80-100 amps, which will quickly overheat them. In a car, in normal use, they will take maybe 200 amps for the few seconds of starting. This is OK for intermittent use - but not for any length of time as the motor will rapidly get too hot and burn out. This design makes the starter motor impractical for any use as a conventional motor.
If you are an experienced engineer (but then why would you want to do so?) a starter motor can be optimised by complete overhaul: make sure the bearings are very free and the commutator and brushes are in optimum condition to minimise friction.
Starter motors are also series wound (although some of the more recent ones are permanent magnet, to which these comments won't apply) so they won't regenerate. It is also very difficult to re-wire them to split the field (necessary to reverse them) - so they cannot be reversed.
Names and addresses of low voltage PM DC motor manufacturers
Certain motors are very popular and we keep getting asked about them. Clearly it is not sensible for us to investigate every motor but this section gives our findings on a few of them. Clearly as we are not the manufacturers our opinions are just that and should not be relied upon as accurate specification. If you want accurate info on motors - ask the motor manufacturer.
Bosch 750w motor
The Bosch motor is a good motor at a good price. However Bosch make these in huge numbers and know accurately what they can do! The 750w rating is therefore accurate, rather than conservative! It applies strictly at full speed (maximum cooling) and the motor will still get pretty hot!
Because of this, do not expect the motor to be able to take a large overload percentage, or a long slow overload. In particular, if you stall the motor without a motor speed controller in circuit, the motor will probably burn out inside a few seconds. All 4QD motor speed controllers are protected and will limit the motor current to a level that is safe (for the controller).
The Bosch will take the output from controllers like the VTX-75, but should be used carefully with the Pro-120 or larger controllers - unless you only use the peak current for very short durations. The Pro-120 will supply quite enough current to get the Bosch uncomfortably hot, and if the stall is maintained for more than a few seconds, the motors will fry and smoke. Of course, for a robot, this may be what you want but for general use we can hardly advocate the use of a controller that has the capability of burning out motors!
Don't ask us for how long the Pro-120 will deliver full current into a stalled motor before it (the controller) overheats. We have never tried it as we did not want to fry the Bosch! But into an Ohio motor it takes the best part of a minute to cause the Pro-120 to cut out. We do not think the Bosch would survive that long!
The Bosch motor is available in 3 versions. Two of these have Offset Brushes. These are therefore less efficient in reverse. It is also probable that, even on the reversible motor, there are slight offsets due to manufacturing tolerances. So, on a robot, where one motor is left side and the other right, one motor will always be in reverse. The effect is that you may get a steering bias in one direction going forward and in the opposite direction in reverse. If you adjust out the imbalance in one direction, you will increase the reverse imbalance! Very difficult to set up properly, unless you fit tacho generator feedback to both controllers.
So, check the matching of the motor speeds. If there is imbalance, either use tank style steering, when you can probably compensate manually. Or fit an additional lay shaft to reverse the rotation of one motor. Then both will be rotating clockwise in one direction and anticlockwise in the other direction.
The three Bosch 750w motors are: 0 130 302 001 for left hand rotation, 0 130 302 013 for right and 0 130 302 014 which is bidirectional. The first two have threaded shafts (one RH thread, one LH thread) and the difference is not just the direction of the screw thread on the motor shaft!
The moral is: beware when using motors with threaded shafts... these almost certainly will have offset brushes!
EMD motors
EMD are probably the world's best seller of low voltage PM DC motors for small electric vehicles, though they have never made a significant impact on the US market. EMD make variations on all their motors so their motor numbers are 'families' rather than individual motors. They do three diameters of motors: PM44, PM 50 and PM63. PM stands, as you might expect, for Permanent Magnet. The 44, 50 and 63 are the armature diameters, in millimetres. The armatures are made in various lengths, so you get PM 44/25, /38 and /50. The PM50 comes in /25, /38, /50 and /70 whilst the PM63 comes in /50, and /100 mm armature lengths. Many of these sizes are uncommon!
EMD's ratings are reasonably conservative and they will all stand a good short-term overload, so a 24v PM 50/63, rated at 250w (12a) may stand 50 amps for nearly a minute.
EMD PM50-63
Probably their commonest motor, 12v versions are used on golf caddies whilst 24v versions are used on mobility aids (pavement scooters) and smaller golf buggies. Motor rating around 200w. PM63 is happy with 75-80 amps for short periods, mainly limited by the brushgear!
PM63-50
Also very common, used on larger golf buggies. (25v versions). Motor rating is commonly around 300-400 watts.
Ohio 13065
This is a 1HP motor with a C56 size frame. It is one of our favourite motors for test purposes. Although 'rated' at 41 amps, 24v, we regularly use it to test Pro-120 controllers and several US customers use it with our 4QD-150 and even 4QD-200 controllers, though we suspect the -230 amps (typical max current) from the 4QD-200 is possibly pushing even this motor a bit far! However it is an extremely conservatively rated motor. If Ohio's other motors are equally conservatively rated then they are to be thoroughly recommended.
Sinclair C5
This was once very popular but stocks are now low. It is name-plate rated at 29 amps - a continuous rating. The motor has a d.c. resistance of 0.1 ohms, so stalled, with no motor controller, you could expect the motor to draw up to 120 amps from a 12v battery. Off load it takes about 4 amps (with its own gearbox) so under actual working it will draw between 4 and 120 amps depending on the mechanical loading (see our motor current calculator). The motor, with gearbox but no mechanical loading, takes about 1/2 volt to overcome static friction, so a speed range of about 20:1 is attainable with our controllers. The motor is ideal with our VTX-75 or Uni-8 (which can give over 100 amps for short periods). The VTX-40 and Uni-4 controllers will however drive the motor quite satisfactorily, depending on the mechanical loading.
We have also tested the motor on 24v and it is quite happy (although the gear box is little noisy) so for best performance, try a VTX-75, Uni-8 (or even an Pro-120 or a Pro-150) with a 24v battery. Excellent value and highly recommended, but getting scarce.
Continuous current
The current and power ratings quoted on the motor's name plate are usually continuous ratings. Most motors will carry a 400% current overload for short periods. Most applications only use motor power very intermittently, so the motor current rating is often not a very useful guide.
The actual power/current you require is more a function of the mechanical loading than of the motor. In an electric vehicle the loading is a function of vehicle's mass (including passengers), its top speed, its maximum acceleration and the gradients it must negotiate.
So if a particular motor is adequate for a particular job and draws, for instance, 15 amps drawing a certain load up a particular gradient then fitting a larger motor will do nothing - the system will still draw about 15 amps with the same load up the same hill. However, with the larger motor the vehicle will be able to draw a larger load up the gradient - or draw the load up a steeper gradient. It will then draw more current.
Ascending the hill slowly does nothing either - if the hill requires 20 amps at full speed, then at half speed it will require half the power. However halving the speed means that the motor voltage has been halved. Halving the voltage and keeping the current the same will itself halve the motor's output power. So you can see that the motor current depends only on the weight and the gradient of the hill and does not alter with speed.
Heating losses in the controller and motor vary with motor current, and are not affected by motor voltage, so going at half speed up the hill won't reduce the heating at all but will cause the heating to go on for twice the time. It is therefore better to go as fast as possible up hills.
Given the figures (top speed, vehicle weight, payload and hill climbing ability) we can calculate the current you require - or see current requirements.
If the motor is too small for the required load, then it will simply overheat if the loading is continuous. On short overloads you will loose top speed: if the motor has, for instance, a resistance of 0.24 Ohms and is operated off 24v, then, on stall, the motor may draw 24/0.25, or 96 amps (in practise it will be less because of extra resistance in wiring and controller). If you load the motor so it draws 60 amps, then the drop in the motor resistance (IR loss) will be 60*0.25, or 15v. With 24v supply, this only leaves 9v maximum for back EMF, so the motor cannot go faster than 9/24 full speed, i.e. slightly more than 1/3 full speed.
The larger the current rating of a particular motor, the lower its resistance, so the smaller the power loss in it and the faster it will be able to climb the gradient. Smaller resistance also means less heating, so a larger motor is usually more efficient! The exception is that larger motors usually have stronger springs on the brushes, so commutator friction is higher.
Stall current or Locked Rotor current
Many people thing that the controller must be capable of handling the full stall current of the motor. This is not true if the controller is properly current limited, as are all 4QD's controllers, so that they will never give more current than they are designed to. If the motor ever stalls, the current limiting will protect the controller.
Of course this means that the full stall torque will not be available, but most motors will not take full stall current for more than a few seconds without overheating, so the controller also helps protect the motor.
So you need to chose a controller that will give adequate torque under real conditions without giving enough current to fry the motor. That is not a simple matter of matching specifications, because the heating in the two will not be matched and because the specs are not always quoted!
For peak motor performance, the brushes need to be offset one direction for forward current, and in the other direction for reverse. If you try and reverse a motor with offset brushes, it will be very inefficient (and therefore slower speed) in reverse.
So some motors are designed with symmetrical brushed, giving equal performance in both directions. These will sacrifice a small amount of performance for the sake of symmetry. Most EMD motors are designed like this.
Be warned also that even a so-called symmetrical motor may have brushgear which is not perfectly centred so speeds may be fractionally different in forward and in reverse. For this reason the Robot Wars Hints and Tips sheet suggests using a lay shaft to reverse the rotation of one motor so that both motors always rotate in the same direction, rather than in counter directions. As far as we know the EMD motors are very good in this respect, the Bosch motors, not.
Other motors are specified by their manufacturer to have a preferred direction of rotation. These will have offset brushes. Reverse them with caution: check with the manufacturer about their reversing ability. They may have a reduced maximum permissible speed in reverse. The Bosch 750w motor is an example of one with brush offset.
The Lynch motor has user adjustable brushes which can be rotated to suit the proposed application. So for a motor which is not often reversed, and at slow speed, they can have some offset, but for a fully reversible motor, no offset.
There are two situations where different motors, of the same make, are mismatched. The first is when they are rotating in the same direction, the second is when they are rotating in opposite directions (Counter rotating).
Same Direction
If two motors are connected in parallel from a single controller, then at all times the two motors will have the same voltage across them. So will they rotate at the same speed under all conditions?
This is an impossible question for anyone but the motor manufacturer to answer properly but with most modern motors any two motors from the same batch will generally be close enough matched for most usual purposes.
With permanent magnet motors the speed of the motor (with a fixed voltage drive) is inversely proportional to the number of turns of wire on the armature and inversely proportional to the magnetic flux density.
The motor manufacturer is not going to vary the number of turns of wire, so the variable factor is the magnetic flux. This is down to the actual magnets and to the magnetic paths - determined by the geometry of the motor, so very closely matched and reproducible in a batch of motors.
So the only real variable is the strength of the magnets. These are not produced by the motor manufacturer but by a magnetic materials supplier and the motor manufacturer will probably know what the variations in strength are. A motor's speed can be modified by choosing slightly weaker or stronger magnets and some manufacturers do select magnets.
It is also possible to demagnetise a motor by exceeding the 'demagnetisation current'. See 12v motors on 24v. Permanent magnets are also susceptible to degradation from mechanical shock and from being subject to high external fields - although modern magnet technology has improved to a situation where theses effects are minimal.
To check the matching on two motors, connect their shafts together so they must rotate at the same speed in the same direction and apply a voltage. Now measure the motor currents. They should be very nearly the same. Any slight difference in current will indicate a slight mismatch. If mismatching is severe, one motor can even generate a current.
Counter rotating
Many motors have offset brushes. This is discussed Brush Offset elsewhere. If the brushes are offset, then a pair of motors that are well matched in forward direction will not be matched if one is reversed.
Since the only way a motor's off load speed can vary between forward and reverse is if the efficiency varies, you should be able to detect offset brushes by measuring the motor current with the motor unloaded. Compare the current between forward and reverse. Any difference indicates asymmetry.
Compensation
Badly matched motors can be compensated for electronically by 'closing the loop' with tacho generator feedback.
The word 'quadrant' comes up, as in 2 quadrant controller or 4 quadrant controller. The word 'quadrant' comes from a graph. The graph below shows the blank on which you might start to draw motor voltage and motor current.
At the top right, both motor voltage and current are positive: the motor is driving in the forward direction. To brake the motor, the current must be reversed but the voltage remains positive as the motor is still rotating in the same direction. so forward braking is the top left quadrant.
The bottom two quadrants are for reversing.
So a '1 quadrant' drive gives forward drive but no regen braking and no reversing. A 2 quadrant gives forward drive and braking bit np reversing - yes one which gave drive but no braking , with reversing, would also be 2 quadrant, but this is not usually used.
A 4 quadrant drive gives reversing with regenerative braking, thus operating in all 4 quadrants.
When you apply a battery direct to a motor, without a controller, there is a large surge as the motor gets up to speed and the vehicle is thrown into rapid acceleration as it immediately tries to reach full speed. When a motor speed controller is fitted, this sudden surge does not occur as the motor speed controller winds the motor up at a controlled rate. The 'winding up' of the speed is not only done by the operator slowly turning up the demand motor speed control, but there is also a wind-up rate built into the controller, so even if the rider turns up the demand speed suddenly, the controller will still wind up the motor at a more sedate rate. This 'winding up rate' is called an 'acceleration ramp'. There is also in the controller a 'deceleration ramp' (which may or may not be independent of the acceleration ramp) to control the winding down.
The easiest ramp to put into a circuit is a simple C-R ramp (so called because it is done by means of a capacitor charging up through a resistor). A better ramp is a 'linear' ramp. The diagram shows the two.
At point A the demand speed control is suddenly turned to full speed. Initially the motor speed starts increasing fast. However as it gets faster, its rate of speed increase slows and as it reaches full speed the rate is quite slow. At point B, the demand speed is suddenly reduced to zero. With the CR ramp the speed reduces quickly at first but the deceleration decreases as the motor slows down so that the speed finally fades slowly to nothing.
The second curve shows the effect of the more sophisticated linear ramp. Here the acceleration and deceleration rates are constant throughout, with no 'fading'.
The CR rate is fitted to the 2QD range whilst the more sophisticated linear ramp is used on the 4QD, Pro 120 and VTX ranges where the ramps are also separately adjustable. 4QD's ramp circuitry is probably the best available in any comparable controller!
FAQ sheet index.
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Last modified: Wednesday, 04-Jun-2008 11:21:57 BST
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