[modeleng] Re: Electronic Speed Controllers


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Answers to FAQs on Battery Motors & Controllers
Part 1
Index - This page
  a.. 12v, 24v or 36v operation
  b.. 12v Motors on 24v
  c.. 12v systems
  d.. Amp and Volt meters
  e.. Applications
  f.. Batteries
  g.. Boats
  h.. Charging a 24v or 36v system from 12v
  i.. Choice of controller
  j.. Collision detection
  k.. Converters, voltage, step-up
  l.. Current requirements
  m.. Current monitoring

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12v, 24v or 36v operation
To give a particular vehicle an adequate performance takes a particular 
level of power. This required power level depends on the mass of the 
vehicle, the top speed of the vehicle, the acceleration rate you require and 
the gradients it must climb. If you think of how your car behaves the above 
seems to be common sense.

In an electric vehicle the motive power comes from the battery. Electrical 
power is volts multiplied by amps so that 40 amps drain from a 12v battery 
is 480 watts. But 480 watts is also given from a 24v battery by a current of 
only 20 amps. Therefore, for a particular power, the higher the voltage, the 
lower the current.

Now electrical current causes heating. Motor, wiring and controller will all 
get hot and waste power. The heat wasted is proportional to the square of 
the current multiplied by the resistance. Generally a 24v motor will have 
twice the resistance of a 12v one but even so a 24v motor would waste half 
as much power in heat as would a 12v motor (½ x ½ x 2). The controller and 
wiring will probably be the same for 12v or 24v, so they will waste only ¼ 
as much power on 24v as on 12v!

It is clear from this that a 24v system is always better than a 12v system - 
provided you can physically fit two batteries. By the same token 36 or 48v 
would be even better - but there is little practical advantage and 48v 
requires different controllers which are not so readily available. 
Nevertheless really heavy current systems (milk floats, electric cars, fork 
lift trucks) often use 72v or even 96v to reduce heating.

The amount of energy in the batteries is amps X hours X volts. Consider a 
12v 60 Ampere Hour battery. Clearly this is exactly the same as two smaller 
12v 30 AH batteries in parallel. But the total amount of energy in these two 
will not change whether we connect them in parallel or in series. So a 12v 
60 AH battery can store exactly the same energy as a 24v 30 AH battery.

There is another factor against 12v operation, except at low currents: 
MOSFETs need a good voltage to fully turn them on, so almost all of 4QD's 
controllers use an internal 9v supply rail, which is adequate to ensure 
proper turn-on. However, there is not much difference between 9v and 12v. It 
does not take much current to be drawn from the battery before it drops 2v 
at its terminals. A small mount of extra drop in and wiring - and the 9v 
supply drops. After that, the available current from the controller drops 
quite quickly! Remember that the battery current is actually a chopped 
version of the motor current, see our circuits archive for more detail, so 
the inductance and resistance of the batteries and battery wiring all 
contribute to any voltage drop.

For this reason, we would generally not advise 12v operation if the peak 
motor current is likely to be more than around 30-50 amps.

FAQ sheet index.


12v Motors on 24v
Motors are specified to run at a stated rpm at a particular applied voltage 
with a specified loading. The specified loading is usually that at which the 
motor takes its maximum continuous current. If you run the motor under a 
lighter load than this 'name plate rating' its current consumption will 
reduce and its speed will increase slightly. If you increase the load, then 
the motor's current consumption will increase and its speed will reduce. 
Obviously you are now exceeding the motor's continuous rating so it will 
start to get hotter than it should. The greater the overload, the quicker 
the motor will heat so there is a time limit on such an overload. However it 
is usually safe to run a motor at a 300%-400% over current for, perhaps, a 
minute - although this will vary from motor to motor.

If you run a 12v motor from 24v its current drain and speed will still 
depend on the mechanical loading. However under no load it will now run at 
twice the speed at which it ran with 12v. Heating in the motor is still 
related to the current so you can still run it at its full rated mechanical 
load/current. However if the motor is badly balanced you may expect noise 
and vibration as the general construction may be inadequate for the faster 
speed. There may also be a problem with brush wear since the brushes are 
being asked to switch the current twice as fast. These effects are, however 
not very likely and usually the speed increase is quite OK.

There is one caveat on this. The motor is an inductive device and the 
commutator and brushes are a mechanical, switch. Such a mechanical switching 
system will have a limit on the maximum rate at which is can work and if 
this is approached, the commutation breaks down. Exactly what the limits 
are, I would not like to say but one effect is noise - and extreme noise 
can, on occasion, cause a controller to fail. The effect is quite rare - but 
beware of excessive over-revving.

However, limits on motor speed are not simply bearing quality. If you rev a 
motor hard enough - centrifugal force will take over and the rotor will fly 
to pieces. Also brush and commutator design is important. Depending on the 
design these will have a maximum switching rate and operating above this 
speed will cause tremendous brush arcing. In extreme circumstances this will 
generate severe noise transients which can destroy the controller. This is 
unlikely: we have only ever seen one customer do this: he was running 12v 
motors on 36v and blew two controllers! These motor limits are not things a 
controller manufacturer can really comment on: you need to consult the motor 
manufacturer.

If you overload the motor, its current rises in the same way whether the 
motor is running from 12v or 24v. However on stall the current from 24v 
could be twice that from 12v, so the motor could get four times as hot 
(heating is proportional to the square of the current). This however won't 
happen when you are using a good controller as the controller will limit the 
current to its designed value. Also the controller varies the voltage on the 
motor so you are probably not going to use the motor at full voltage in any 
case.

Another consideration is that, if you put too much current through a 
permanent magnet motor, it is possible to slightly demagnetise the magnets. 
This is cumulative: the motor's performance will drop slightly each time you 
do it. However, for battery motors, is is probably fairly safe to assume 
that, at the rated voltage, the current drawn when the motor is stalled will 
not reach this demagnetisation level. If you were to run a 12v motor off a 
24v battery the stall current could then be excessive if it weren't limited 
by the controller.

Therefore, provided you chose a controller suitable for the motor you use, 
you can usually run a motor 12v motor from a 24v battery with no effect 
except that full speed is doubled.

A simpler discussion of the above is in our Features - a Guided Tour.

Related topic: Speed Stability

FAQ sheet index.


12v systems
Operation at high current from 12v causes several problems, so many 
manufacturers do not offer 12v controllers. There is a list elsewhere of 
controllers that 4QD offer in 12v versions.

MOSFET gate voltage
Common MOSFETs require about 7 or 8 volts on their gates to properly turn 
them on. Because if this, most 4QD controllers have an internal supply of 
9v - which gives nearly 8v on the MOSFET gates.

Now if you view the terminal voltage of a 12v battery, with an oscilloscope, 
you will find that, when the controller draws chopped current from the 
battery, there is a squarewave of 2 volts amplitude shown. The battery may 
be 13v open circuit, but during the PWM periods when current is actually 
being drawn, the effective voltage is actually falling to 11 volts. If you 
want to know more about why there is a chopped current, see our circuits 
archive.

Consider also that a 12v battery may, when 80% discharged (a realistic level 
before recharging) has a terminal voltage (open circuit) of about 10.8v. So 
the PWM will be working from effectively 8.8 volts. So there is no way the 
9v internal rail of the controller can stay at 9v! And that's before we 
start to consider voltage drops in the battery wiring due to its resistance 
and also its inductance.

So it's pretty difficult to fully use a 12v battery at high currents and get 
the full rated current out of the controller, as the 9v rail will drop and, 
with it, the available current. See our service section for details of a 
modification to 12v version, Pro-120 because of this effect.

Motor stall current
Consider the stall current of a motor, for instance, the Sinclair C5 motor. 
On a freshly charged battery, its stall current can be 120 amps. This is 
limited by the motor resistance, the resistance of the leads supplying it 
and also on the internal resistance of the battery. Adding anything else 
into this loop will increase the loop's resistance. So, if you have a system 
that works nicely without a motor speed controller, adding a motor speed 
controller will inevitably reduce its peak performance. Many 12v systems are 
simply not designed for operation with a speed controller and adding this 
will greatly reduce the performance.

24v systems
The overheads on a 24v system are nowhere near as critical. The 2v drop, 
even 4v, will still take the battery supply nowhere near to the 9v rail. 
Motor resistances are also higher, so the extra effect of controller and 
wiring is less noticeable.

FAQ sheet index.


Amp and Volt meters.
Generally an ammeter in a battery system is of little use: it can be 
interesting to know how much current you are taking, but once the system is 
set up - so what? If the motor takes 25 amps up a particular incline, then 
that is what it will always take - unless there is a mechanical fault such 
as a seized bearing. An ammeter might have been useful before you bought the 
controller, so you know which controller to get, but once the system is 
working OK, who needs one?

A battery voltmeter is much more useful - we would even say essential - 
since, as the battery discharges, its voltage drops, so this will tell you 
the charge state of the battery. Also, under heavy load, the battery voltage 
dips. If the voltage dips too far then either the load has increased or the 
battery is getting old.

4QD have LED meters available (3 LED for 12v systems, 5 LED for 24v and 36v, 
7 LED for 36v and 48v systems) which can be useful. They will show the 
voltage dips as you accelerate and will indicate the charge state. LED 
meters, working in steps, can never be as good an indication as an expensive 
voltmeter, but they can be very useful and better than most of the cheaper 
battery state indicators. They also give a nice display!

Or you can get a proper digital voltmeter: these can be bought for about £30 
from most electronic stores.

FAQ sheet index.


Applications
Our controllers get used for a very wide variety of purposes. We list a few 
below. Aerial rotators 2 off 2QDs in servo system or VTX with joystick board
      Agricultural equipment Uni, 1QD, 2QD, VTX series, Pro-120 and 4QD 
series
      Camera dollies VTX or Pro-120
      Caravan shifters VTX or Pro-120
      Carnival floats VTX or Pro-120
      Conveyors VTX or Pro-120, 2QD or 1QD
      Dog walking machines Uni, 1QD or 2QD
      Electric boats any!
      Electric bicycles Uni, 1QD, 2QD or Scoota
      Electric library trolleys Uni, 1QD, 2QD or VTX
      Electric wheelbarrows Uni, 1QD or 2QD
      Factory stores vehicles VTX or Pro-120
      Floor cleaning machines VTX or Pro-120
      Go Karts Pro-120 or 4QD
      Golf buggies Pro-120 or 4QD
      Golf caddies Porter
      Invalid vehicles Pro-120, VTX or 4QD
      Kiddie cars Pro-120, VTX
      Lathes & milling machines Uni, 1QD or 2QD
      Materials handling Pro-120, VTX, 2QD
      Miniature railways, 3", 5" and 7¼" VTX, Pro-120 or 4QD
      Mobile targets Pro-120, VTX
      Motorised storage racking VTX series, Pro-120 or 4QD
      Mountain rescue vehicles 4QD, Pro-120 or VTX
      Potter's wheels Uni, 2QD or 1QD
      Remote guided vehicles Pro-120, VTX
      Ride on golf buggies 4QD, Pro-120 or VTX
      Voltage dropper for battery lighting Uni, 1QD or 2QD
      Winches Pro-120 or VTX
      Window cleaning machines Pro-120 or VTX


FAQ sheet index.


Batteries
Car batteries are intended for sudden, heavy surges (i.e. starting currents) 
then to be recharged and kept fully charged. Their structure is such that 
they don't last very long if they are continuously discharged almost 
completely and then recharged. They will in fact be destroyed by over 
discharging.

The other type of battery is known as the 'traction' or 'deep discharge' 
battery. These are not designed for the 300 - 500 amp surge that can occur 
on starting, but they are designed to be continually discharged to near full 
discharge and then recharged on a cyclic basis. They are used to power golf 
vehicles and for caravan use. However, like car batteries, they also will 
will be destroyed by being left in a discharged state for any length of 
time.

4QD don't actually make vehicles so we have no first hand experience of 
batteries. We know from our customers that lead acid batteries are the weak 
link in electric vehicles and they do cause trouble. The problem is that a 
battery's performance today will depend not only on its present state of 
charge, but also on how it has been treated during its life. All batteries 
can be damaged by overcharge, over discharge and by leaving too long in a 
discharged state. It also does no good to leave then unused for long, even 
though fully charged. Batteries that are used regularly (and properly) tend 
to last longest.

FAQ sheet index.


Boats
Most battery motor applications are land based and only draw high currents 
intermittently (when accelerating or climbing a gradient). Motor controllers 
are designed to cope with this market and will give high currents for short 
periods, ideally matching the demands of smaller terrestrial vehicles.

Boats are different from terrestrial vehicles in that the current drain is 
continuous and also increases as the propeller speed increases. So for a 
boats you generally need a larger controller that will deliver continuous 
current.

The subject is discussed in a separate article on electric boats

FAQ sheet index.


Charging a 24v or 36v system from 12v
One thing that sometimes puts people off 24v systems is the difficulty (and 
expense) in getting 24v chargers. Firstly, cheap 12v chargers are made for 
occasional use, for topping up car batteries. They do not properly care for 
the battery - this is done by the charging system in the car - so can easily 
overcharge the battery, and so shorten its life. 24v chargers are generally 
manufactured for small vehicle use so charge the battery properly without 
risk of overcharging.

However, it is quite possible to arrange switching so that two 12v batteries 
can be used connected in series as a 24v system yet they can be charged as 
two parallel connected batteries from a 12v charger.

The diagram shows the method.

Two 12v automobile relays are used for a 24v system. These relays are 
available with a 30 amp continuous rating. You could of course use a single 
double pole relay instead of two of single pole ones, but these are not 
generally available with more than a 10 amp rating. The 30 amp relays we 
suggest have contacts capable of carrying well over 100 amps for short 
periods so are fine for most controllers in this application

Consider the 24v system above. When the relays are not operated, the two 
batteries are connected is series through the normally closed contacts 
(solid black). When both relays are operated the batteries are in parallel. 
The relays are operated by a third contact, B, and are energised 
automatically by connecting the 12v charger.

It is tempting to connect the NC contacts effectively in parallel instead of 
series as here: this would give better current handling - but there is a 
danger that, is ever a relay contact stuck, one battery could be shorted 
out, destroying the other relay as well.

With this system you must make certain that the 24v (or 36v) which will, for 
an instant, be applied to the 12v charger, will not damage it. Alternatively 
you must arrange that contacts B and C make first, energising the relays 
before the charger is connected.

Other versions of this system are of course possible. The diagram below 
shows a 36v system which uses 4 relays.


FAQ sheet index.


Choice of controller
Most customers tend to buy controller larger than necessary. This is fine: 
our drives are so cheap you can do this. A larger controller will also stay 
cooler so will be more efficient. There is no such thing as 'too large a 
current' - the motor will only take what it requires. The only exception to 
this is that, if you run a 12v motor on 24v and stall the motor, then a 
current limited by the controller is a good idea to prevent damage to the 
motor.

Historically most controllers haven't included current limit so you have 
needed to use a larger controller than necessary for safety, mainly because 
stalling the motor could otherwise destroy the controller. 4QD's controllers 
have a current limit so you won't damage them by overloading them or by 
stalling the motor - unless you do this for so long the system overheats.

The current ratings on our drives are realistic ratings. The drives will, 
for short periods, give considerably more than we claim, thus the 70 amp VTX 
drive will, from cold, give around 115 amps. However if you run it at 70 
amps it starts to heat up. Internal circuitry detects this heating and 
reduces the output current to keep the drive safe.

So, if you chose too small a controller for your application, no damage will 
result, but the controller will get too hot. If this does happen you can 
easily and cheaply upgrade to a larger unit, or you can add extra 
heatsinking. The larger the heatsink, the longer the drive will take to heat 
AND reduce its output current. However a larger drive will also be more 
efficient so is a better choice.

4QD's range is getting large enough to make choice difficult. The first 
choice is: do you want reversing? If so, then the choice is the Pro-120, the 
VTX series or the 4QD series - but don't forget that you can add reversing 
to a simple controller by a double pole switch to reverse the armature 
connections, so a 2QD is also a possible choice. However you must make sure 
that the switch cannot be operated whilst the motor is still rotating.


Uni or Scoota (or 1QD or 2QD)

Generally the choice is between the Uni or the Scoota, depending on power. 
There is little the 1QD or 2QD can do that the Uni cannot do equally well, 
which is why the 1QD and 2QD are now 'special' products with restricted 
availability.
When choosing between 1QD and 2QD, the choice is simply down to 'do I want 
regen braking?' and 'do I want reverse polarity protection?'. The 1QD series 
incorporates the same circuitry as the 2QD's braking since this, as a side 
effect, makes it far more efficient than the industry standard controllers 
for golf caddies etc. If you want regen braking, then the 2QD is indicated. 
If you definitely do not want regen braking, then the 1QD is indicated. In 
practise the choice is usually down to the style.

For higher currents, our Scoota 120 is indicated. This has far more features 
that either 1QD or 2QD. There is also a higher current controller - the 
Sco-180.


Porter series

As an alternative to Uni, we offer the Porter. This is an economy controller 
aimed at golf caddies (or golf trolleys), electric bicycles etc and it is 
available cased (or, for larger quantities, as a bare board). The

Pro-120, VTX or 4QD

The 4QD is designed for high current golf carts, 100 amps plus. The current 
ratings are shown in the specification sheets There is usually little to 
chose between the 4QD series and a pair of VTX controllers so the choice is 
down to individual preference and ease of wiring. The Pro-120 is very 
similar in performance to the 4QD but the Pro has reverse polarity 
protection built in, all the terminals are at one end and a cover is 
available, making it physically similar to other 'industry standard' 110 amp 
controllers, albeit giving considerably more power at a much better price!
FAQ sheet index.


Collision detection.
In a moving vehicle or on any moving machinery it is often useful to be able 
to take evasive action when the vehicle collides with an object. Naturally 
the action required is down to the vehicle's stopping distance - a car 
travelling at 60mph would need very sophisticated radar to be of any 
practical use! However a vehicle travelling at, say, 4mph may be able to 
stop sensibly within 10cm or so. The safe stopping distance is down to the 
vehicle's mass and speed and the load it is carrying and is therefore not 
something that we can completely control in the electronics.

However reversing controllers (such as VTX, VTX, Pro and the 4QD series) all 
have 'dual ramp' reversing. This means that, if the reverse switch is 
operated at speed the controller will automatically slow to zero speed 
(under control of the deceleration ramp), reverse and then start up again 
backwards. This means that if you have a sprung bumper at the front of the 
vehicle and place an auxiliary reversing switch so that it is operated when 
the springs of the bumper start to compress, the controller will go into 
reverse, slow down then back off until the switch opens again. The vehicle 
will now 'hover' at the switch's operating point. Naturally for complete 
safety the bumper's free travel should be greater than the vehicle's 
stopping distance or crushing would occur during braking.

The 'bumper' switch can just as easily be the top an bottom limits on, for 
instance, a lifting platform.

Left is a suitable circuit showing how to use two switches, one at front and 
one at rear for 'both end' collision detection. S1 is the normal 
forward/reverse switch. S2 stops forward motion by applying a reverse input 
when closed and S3 stops reverse motion by inhibiting forward movement.

With this system, if you drive into an end stop, the machine will hit the 
end stop and change direction, backing off the limit switch. When it 
releases the end switch it will change direction again, operating the limit 
switch. So the machine will hover at the point of operation of the switch a 
long as and movement (demand) speed is present.

This system will work with any 4QD reversing controller.


When using 4QD's joystick board with the VTX, a slightly different 
arrangement is required. The second diagram shows the direction output of 
the JSB (an NPN transistor with a pull-up resistor to +24v (or +12v) and the 
direction input to the VTX which senses at about 6v. The VTX's direction 
input is high to engage reverse. If S1 is closed, the VTX will always go in 
reverse, so if this is closed by forward travel, the machine will hit the 
switch, stop, reverse and back off to the switch's operating point where it 
will hover until the joystick is reversed. The extra 10K stops S1 'fighting' 
the JSB's output transistor. The 'daughter' version of the joystick 
interface (JSD-001) has connectors for such switches.

Similarly if S3 is closed, then the VTX will always travel forward. 
Alternatively S2 can be fitted. When this is open the joystick can never 
give a reverse signal to the VTX. Naturally in a machine you must consider 
what will happen if both end stops get operated simultaneously or if one 
switch sticks. In the the second diagram, S1 will always win. Note that the 
daughter version of the Joystick interface has this end stop circuitry 
included.

A magnetically operated reed switch (they are often used for detectors in 
burglar alarms) can be very useful for this purpose. Or you could arrange a 
rod straight through a vehicle (such as a kiddiecar), moved by the (sprung) 
bumpers. If a front collision occurs the rod moves backwards moving the 
magnet to close the reed switch so the vehicle automatically reverses. More 
information on Reed Switches


Second method
An alternative scheme is also possible with some controllers. The VTX series 
have ignition and reverse inputs that can be used as 'go forward' and 'go 
reverse' inputs. Used thus, a pot is connected and left set to the required 
speed and the two 'go' inputs are then used to enable motion. Clearly you 
can't easily do this on a controller with high pedal lockout fitted, but the 
VTX series do not have HPLO. See also an application note for the 4QD series 
controllers.

With these 'go' inputs, simply fit normally closed switches in series with 
the two go buttons so that the switch that opens for forward travel limit is 
in series with the 'go forward' input, This will stop the forward motion 
whilst still allowing the reverse input to be used.

The subject of end limits on machinery is clearly related, but can get a lot 
more complicated than you may expect. What do you really need the machine to 
do when it reaches the limit? Why has it reached the limit? One way of 
looking at things is that any system that has reached its limits is outside 
of normal operation - and systems outside of normal operation can behave 
erratically. It's a large subject on which a surprising amount can be 
written!

FAQ sheet index.


Converters, voltage, step-up
This question arises from several directions. First of all, as a separate 
stand alone device, for instance to supply 240v for other equipment. As 
such - it is indeed a separate device and has nothing whatsoever to do with 
motor control.

The other time it arises if in the form 'Can I get 36v to run a 36v motor 
from a 12v battery?'. No you cannot. See PWM motor speed control: how it 
works in our circuits archive. From that it is clear that an ordinary pwm 
chopper can never deliver to the motor a voltage higher than the battery 
voltage.

Such a voltage converting motor controller could be made. However any 
electronic process involves losses - there's no such thing as 100% 
efficiency in practise. Step-up conversion would be a separate process, 
however it was done and step-up converters are significantly less efficient 
than an ordinary pwm chopper, so the power losses would be too high for it 
to be useful. In fact - unless it resulted in better efficiency that a 
correctly designed motor running on the correct voltage, it is difficult to 
see how it could present any advantage at all!

Since 4QD do not design such voltage converters, I cannot give accurate 
facts on them but a PWM chopper can be 98-99% efficient. A step-up converter 
would be good at 85% efficiency. So it would get extremely hot at the sort 
of currents our controllers can give and would waste relatively huge amounts 
of power. So it is not a method ever likely to be used commercially.

FAQ sheet index.


Current requirements
A motor converts electrical energy into mechanical energy. However, in the 
conversion some of the electrical energy is wasted as heat. Some of this 
loss is because motors are not perfect, so if heavily loaded, they get hot, 
some of it is because mechanical systems are not perfect, they have friction 
and this also causes heating.

The mechanical energy out of the motor is used partly to accelerate the 
vehicle (it is turned into Kinetic energy) and partly in is used to climb 
hills (it is turned into Potential energy). If you are building a robot, you 
aren't too interested in the hill climbing ability, but an understanding of 
the principles can save mistakes.

This sounds quite complicated, but if you consider the electrical energy 
being used in five separate ways things start to get clearer.

  1.. Electrical inefficiency.
  shows up as heating in controller, wiring and motor.
  2.. Mechanical rolling losses.
  difficult to calculate!
  3.. Kinetic energy
  4.. Potential energy
  KE and PE are actually quite easy to calculate - if you have all units in 
Metres, Kilogrammes and seconds.
  5.. Windage
  difficult to calculate but not important at low speeds.
The other factor of importance to robots is of course torque. We'll get to 
that later. But you do first need to understand a bit about what happens to 
the electrical power you put into the motor.

There is a JavaScript Motor Current calculator available. Once you 
understand this section, you can plug in various performance values and try 
the effect on the motor current. Or how about opening the calculator in a 
second window alongside this one?


Electrical energy

Electrical energy = volts x amps x time. So a 12v battery giving 10 amps for 
one minute (60 seconds) will give 12 x 10 x 60, or 7200 Joules and a motor 
taking 20 amps at 10 volts for 60 seconds will deliver 20 x 10 x 60, or 
12000 Joules.

Electrical inefficiency
If the motor and controller and gear ratio are chosen correctly, electrical 
losses are small: motor efficiencies can be between 70 and 95%, controller 
efficiency much higher - we don't want them to get hot - in the region of 
97-99% range so, in a well designed system, nearly all the power taken from 
the battery goes to the motor. Remember that, with an efficient system, you 
can recover useful energy with regenerative braking.

Generally electrical inefficiency shows up as heating. Heating is 
proportional to the square of the current, so it pays to keep the current 
down and go for a higher voltage. See Heating.

Remember motor and battery current are not the same: because our controllers 
use high frequency chopping, the motor's inductance sustains and smoothes 
the current so that it is pure d.c. with very little ripple. However the 
battery current is chopped on and off, only flowing when the motor is 
connected to the battery. So at 50% modulation (i.e. at half full speed) 
battery current will flow 50% of the time, so you will measure a battery 
current equal to half the motor current.


Battery current X Battery Voltage = Motor Current X Motor speed.

Mechanical rolling losses
These you will have to measure. Go for an efficient gear train (worm gears 
tend to be bad). Keep all bearings well lubricated.


Kinetic energy

Kinetic energy is defined as 1/2 x mass x velocity². Mass should be in 
kilograms, velocity in metres/second. So a train weighing 250kg travelling 
at 4.47 metres/sec (which is 10mph) would have an energy of 0.5 x 250 x 4.47 
x 4.47, or 2497 Joules
If we require our vehicle to accelerate smoothly to top speed in, say, 60 
seconds then current must flow for this full 60 seconds and the electrical 
energy used in accelerating will equal the kinetic energy gained.


So: volts x amps x time = 12 x amps x 60 = 2497 (the K.E. gained).
Therefore amps = 2497/60/12 = 3.47 amps
So we only need less than 4 amps of motor current for this acceleration.


Potential energy

Potential energy is Mass x g x height, where g is 9.80 metres/second/second. 
the acceleration due to gravity. If we have a gradient of 1 in 50, 30 metres 
long, then the height gained on this incline will be 1/50 x 30 or 0.66 
metres. In ascending this incline our vehicle will have gained a potential 
energy of
250 x 9.8 x .66, or 1617 Joules
At top speed the train will travel at 4.47 metres/second so it will take 
30/4.47 seconds to travel the 30 metre incline, i.e. 6.71 seconds. The 
current must flow for this time so

amps = 1617/12/6.71 = 20.08 amps
So we need 20 amps of motor current for this incline.

It doesn't help at all to go slowly up the incline (unless you have 
mechanical gear change): if it takes 20 amps of motor current at full speed, 
then the motor current will still be 20 amps at half speed, because full 
speed corresponds to 12v on the motor (which we used in our calculation) so 
half speed will be 6v on the motor. Halving the motor voltage halves the 
power, so the motor current won't change. Yes - the battery current will 
halve, but it will flow for twice the time since the slower machine will 
take twice as long to climb the hill, so there is no overall benefit. At 
high motor currents the motors and controller will get hot (wasting power). 
The power wasted is only down to the motor current: the quicker you get up 
the incline therefore the shorter the time for which you will be wasting 
power, so the smaller the overall losses.


K.E. (alternative)

Kinetic energy is defined as : ½ x mass x velocity².
Electrical energy = volts x amps x time.
Equating the two and rearranging to get current,
½ x mass x velocity² = volts x amps x time.
Amps = ½ x mass x velocity² / volts / time.

Current = ½ x (vehicle laden weight) x (max vehicle speed)² / battery volts 
/ (time to top speed)


P.E. (alternative)

Potential energy is Mass x g x height,
where the vehicle's mass is measured in Kilograms,
height is in metres and
g is 9.80 metres/second/second, the acceleration due to gravity.
If we have a gradient of T%, then the height gained will be
T/100 x Length (the length of the incline in metres)
the potential energy will be Mass x g x T/100 x Length
Our vehicle will traverse the incline in Length / Speed seconds.
Current must flow for this time so electrical energy will be: Volts x Amps x 
Length/Speed
Equating electrical to mechanical energy we get
Mass x g x T/100 x Length = Volts x Amps x Length/ Speed
So the motor current must be
Mass x g x T/100 x Speed/Volts

Volts and amps in the calculation must be the motor volts and amps, not the 
battery volts and amps but, at top speed, motor volts and amps are equal to 
battery volts and amps and the calculation approximates to
Current =1/10 x (Vehicle laden weight) x (gradient) x (Top vehicle speed) / 
(Battery voltage)


Windage

This is difficult to calculate and I am not the right person to ask but it 
is proportional to something like the fourth power of the speed. It is 
generally not important at low speeds.
FAQ sheet index.


Current monitoring
Many people think that it is desirable to monitor the battery current. 
However - unless you use expensive Hall effect current monitoring, you are 
going to add extra resistance and inductance in the battery by doing this. 
See our circuits archive for why this is undesirable.

And what do you expect to gain from a measurement of battery current. 
Remember, battery current and motor current are not the same thing (see 
current requirements). The actual load affects motor current. Battery 
current depend on speed as well. Also, the battery current is not easy to 
use to tell how well charged the battery is: battery voltage is a better 
indication of battery charge level - and gives a better idea also of the 
state of health of the battery.

So generally fitting an ammeter in the battery is harmful but is not useful.

FAQ sheet index.


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Page Information

Document URI: www.4qd.co.uk/faq/bmnc1.html
Last modified: Thursday, 10-Jul-2008 10:45:50 BST
Page's Author: Richard Torrens
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----- Original Message ----- 
From: "Peter Sheppard" <peter@xxxxxxxxxxxxxxxxxxx>
To: <modeleng@xxxxxxxxxxxxx>
Sent: Tuesday, August 26, 2008 9:27 AM
Subject: [modeleng] Electronic Speed Controllers


> We had a recent discussion on this list about suppliers of electronic 
> speed
> controllers and methods of getting in touch.
>
> I had a question about a 24v speed controller supplied by one particular
> company and had to use their e-mail system.  The question was "I have a 
> Uni
> 8, 24 volt controller, will this work on a 12v system?"  The e-mail
> response was "The 12v and 24v are built differently".  Now I can assume
> that this means "no", but I am really none the wiser!
>
> So, on a related subject! Anybody got a 12 volt controller they want to
> swap for a 24 volt controller?
>
> Cheers
>
> Peter
>
>
>
> MODEL ENGINEERING DISCUSSION LIST.
>
> To UNSUBSCRIBE from this list, send a blank email to,
> modeleng-request@xxxxxxxxxxxxx with the word "unsubscribe" in the subject 
> line.
> 


MODEL ENGINEERING DISCUSSION LIST.

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