Relation Between Motor And Battery Voltage

Introduction This is a JavaScript page that lets you calculate the motor current your vehicle will need. The calculator has been running for many weeks and the only reported problem is with Netscape 4.7, which is broken! In order to give some understanding, it is fairly long so may not fit on one screen: you may like to adjust your Browsers's choice of 'Base Font Size' to give smaller text!

Sorry, but without JavaScript enabled, you won't be able to use this calculator. The method is described in our FAQ sheet. You will need to enter some data for your machine. Some cells can be left at zero (or their default value). Others must be filled in for the calculation to make sense - these are tinted. • • •

A robot could enter a gradient of zero or some small number, with a fast acceleration, maybe 0.5 seconds, or even less. A golf buggy could use a steep hill, maybe 30%, with a longer acceleration time, maybe 5 seconds. A Loco would have a small gradient, 1% perhaps, a large overall weight and a long time to accelerate.

The page is here for you to experiment with to get the feel of how the values will affect your vehicle's performance. The cells to experiment with are: 1. Vehicle Speed. This is the speed the machine would go when unloaded and with full battery voltage on the motor, i.e. without any controller. Enter the value either in miles per hour, Km per hour or Metres per second. Make sure the calculator know which you have chosen! If you cannot easily measure your vehicle's speed, you should calculate it. See our Vehicle Road Speed Calculator. 2. Vehicle Weight. This should be the full laden weight under the worst condition you wish to use the vehicle. 3. Passengers. If the Vehicle Weight does not include passengers, enter the number of Adults and Children you wish to carry. If they are included in the overall weight, leave these set to zero.

4. Nominal Battery voltage. Usually 12, 24 36 or 48v, but enter whatever you wish, to see how it affects the answer. 5. Weight of one battery. This may be already included in the overall vehicle weight, if so leave it as zero. However, you may be toying with doubling the number of batteries, which will increase the overall weight (unless you use smaller batteries). It's here for you to experiment with. If you enter anything here, this must be the weigh of one 12v battery as that's the normal amount you will want to alter. 6. Motor current on level ground. This is a difficult one as you usually cannot guess or measure it until after you've finished making the vehicle. But it does affect the calculation, so is really required. If in doubt, make a guess or use our figure. It's likely to be a fairly small part of the total. 7. Hill climbing ability. Your machine must be able to climb some sort of gradients. For locos, 1% is usual, 2% quite extreme. For a golf buggy, 30% is about the worst. Try various values and see the results on the motor current. Gradient here is the sine of the angle of inclination, or the rise per length of travel along the surface. 8. Length of hill. You do not need to enter this but, combined with the vehicle's top speed, it tells you how long you will need the peak current for. This can give you good indication of the controller you need. 9. Acceleration. Time (in seconds) in which you want the controller to accelerate the machine to full speed. Data Entry area What is Vehicle Speed (at full motor speed)? What is the Vehicle Weight How many Adult Passengers? (Av'ge 12 stone) How many Child Passengers? (Av'ge 5 stone) What is the battery voltage? What is the weight of one 12v battery? Motor current on level ground? Hill climbing ability: Gradient in percent Length of hill. Acceleration. (Time to reach top speed) Seconds Output area: No need to fill in below! Total mass of all of the above is: Kilogrammes Vertical Speed is speed/gradient(Vv) metres per second At top speed, rate of climbing is: | Power for climbing. (Motor Current needed to climb)

Power= Mass x G x (Vv) where G is the acceleration due to gravity, 9.80 metres/sec2 So the power for climbing is: And as this is at full speed the motor current is:

Watts Amperes

Kinetic energy. (Motor Current needed to accelerate) KE = 1/2 x M x V2 So in this case, KE at full speed is: joules But this speed increase took place in the acceleration time you entered. So to get Joules/second, divide by the acceleration time. Now one Joule per second is a watt. We are accelerating from zero to full speed (zero to full battery volts) so divide by the average motor voltage (i.e. by half the battery voltage) to get the motor current needed for this acceleration: Amperes Total Motor Current Add the three currents: Amperes That on level ground + that for climbing + that for accelerating Time duration of Current You chose an acceleration time of Seconds You will climb the hill you specified in Seconds

Answers to FAQs on Battery Motors & Controllers Part 1

Index - This page • • • • • • • •

12v, 24v or 36v operation 12v Motors on 24v 12v systems Amp and Volt meters Applications Batteries Boats Charging a 24v or 36v system from 12v

• • • • •

Choice of controller Collision detection Converters, voltage, step-up Current requirements Current monitoring

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.