4qd VTX Series Manuale di istruzioni - Pagina 13

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The rated current output of the controllers is with the

heatsink hot. When cold they will give considerably

more current. Thus the 40 amp version will in fact

give about 55 amps when cold. This is OK because

the MOSFETs used are rated at 60 amps continuous

with a case temperature of 25°C. As the MOSFETs

warm up their allowable current reduces so that at a

case temperature of 100°C they can (only!) handle

42 amps each continuously. The current limiting used

in 4QD's controllers senses the MOSFET

temperature and automatically adjusts as the

MOSFETs heat up. However, running the controllers

at full current will cause speedy heating so the

allowable continuous current will depend on the

external heatsinking.

All speed controllers should only be used with good

quality motors. Old, dirty motors can have damaged,

worn brushgear and this causes arcing. On occasion

arcs at the brushes can cause seemingly random

controller failure. Fortunately this effect is quite rare,

but it's best to be careful.

Motor capacitor: Ideally the motor should include

an internal suppression capacitor, a ceramic type of

10n value is suitable. If the motor does not include

this you are advised to fit one across the brushes as

close as possible to the motor body as shown on page

6. The controller will work without this capacitor, but

it can lengthen the life of the system.

A word of warning: many car type motors have the

chassis connected to one terminal. Take great care

with these as you could easily short the controller out

- which would be fatal. It is best to avoid these

motors. Otherwise either make sure the motor is

mounted on insulation (including the drive shaft), or

make certain that no other point of the control system

can be earthed to chassis. If in doubt contact 4QD.

Most modern d.c. motors use permanent magnets.

These are the best for battery operation. However,

other types can be used: at 4QD we regularly use a

12v car starter motor for testing (even with our 24v

150 amp drives) since these are a far worse load than

is ever likely to be met. Into such a motor (stalled)

the controllers simply deliver their maximum current

VTX series instructions

13: Heat & Heatsinking

Steel is not a good heatsink material: heat does not

flow easily in steel. Aluminium or copper is far

better. If you have a steel plate, sandwich an

aluminium sheet between the steel and the VTX to

spread the heat. For really arduous use we suggest

heatsink compound between the VTX's heatsink and

your own: this helps heat flow across the join. Make

sure both surfaces are flat and free of grit.

The VTX-75 incorporates a thermal sensor (optional

on other models) which cuts back the output current

if the controller gets too hot. The VTX-75 cuts back

to about 20 amps, when the MOSFETs will dissipate

about 20 watts. This is still enough to overheat the

controller if no external heatsink is used, so don't rely

on it!

14: Choice of motor

and get hot. It is virtually impossible to damage the

controllers by an unsuitable motor (the controller will

simply get hot quickly), so don't be afraid to

experiment.

There is no reason why you cannot use a 24v motor

from 12 volts - it will only go at half its design speed.

Also, if you use a 12 volt motor from 24v, it will go

at twice its rated speed. Since the VTX is current

limited you won't overload the 12v motor, provided

it can handle the available (limited) current.

Shunt wound motors can be used if the field

winding can be separated. Connect the field winding

permanently across the supply and control the

armature winding: the shunt would motor then

behaves like a permanent magnet motor.

Series wound motors may also be used but they

cannot give dynamic braking and are very inefficient

at low speeds so are not ideal. To use them with the

VTX series a modification must be made so the field

can be correctly connected into the relay circuitry:

contact 4QD.

You can reverse a field-energised motor by simply

swapping over the field windings. You should not do

this when the motor is rotating as armature currents

will then be very high. The field will draw much less

current than the armature so a much smaller switch

will suffice. A field energised motor, used on the

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