FAQs - 4QD - Electric Motor Control


Choosing a controller

The quick answer is quality and support. For instance…..

Support- We design and build our products ourselves, we know what goes into them, how they work, and have many years’ experience in sorting out all the little issues that can occur in the wide variety of projects that our customers build. You just don’t get that from Ebay or Alibaba. Our customer support team never leave a man behind – we keep going until your installation is working properly.

Capacitors – Some of our competitors don’t put in much by way of main capacitors, you can get away with this on the test bench but in the real world that often involves long, thin battery leads, this can result in the capacitors and mosfets working a lot harder than they should, getting hot, and failing early. We put in a lot of capacitance and we spread it around the board by fitting many smaller capacitors, this helps the heat dissipation and gives better ripple performance. But it takes time to fit them.

Mosfets – These are the heart of any motor controller. As I write this a single seller [Mouser] lists 2713 different TO220 mosfets. The range of parameters is large. We take the time to sift through this and carefully evaluate the mosfets we use. We’re looking at current ratings, turn on / turn off times, capacitance values, figures of merit, and many other factors to get the best we can.

Metalwork – It’s a fact – all controllers generate heat. It’s no good having the best mosfets in there if you can’t get the heat out. Our controllers have the mosfets bolted directly to substantial aluminium heatsinks. That said, if you want to run high currents for long periods then you’ll need to think about how to manage that heat [heat management article coming soon].

Repairability – It’s also a fact that controllers can fail, and there’s a whole variety of reasons why [see this article]. If you are unlucky enough to suffer one of these there is a good chance we can repair your controller, again you just don’t get that from some other suppliers. We’ve also written an article on best installation practise which will help you to stop this happening in the first place.

If you know the current rating you need you can select directly from our range page

We have a comparison table, a flowchart, and some common application guidelines that will help  here.

We also have a page in our knowledge base that covers the basic physical principles.

If all else fails pick up the phone – we’re always happy to talk about your project.

Yes, some do. See the specification table for details. But if you are still designing your system you may want to read this page on why 24V is a better option.


Yes, most do. See a look at our specification table for details. Our new Pro-160 can go all the way to 84V.


At least not yet, we are looking into it though.

It is usually possible to tell the type of motor you have from the number of wires or terminals it has.

4QD controllers are designed to drive permanent magnet brushed motors, these generally have  2 external  wires or terminals which are usually black and red to indicate positive and negative.

Series wound and shunt wound motors generally have 4 terminals, 2 for the field winding, and 2 for the armature. Our controllers are not specifically designed to drive these types of motors but a number of our customers have managed to do so successfully. There are some articles in our knowledgebase on how to do it.

Brushless motors will have either 3 wires for a sensorless version, or 3 + 6 sensor wires if they are fitted with hall effect sensors. We do not currently have any controllers for brushless motors, but we are looking into it.

You need to consider the motors rated current, and also it’s maximum stall current. We have a flowchart here that can help you decide.


Installation questions

Yes. All 4QD controllers can be controlled by a voltage input.

You can use a DAC to provide a voltage output, or use a PWM output directly. We have tested 4QD controllers with PWM inputs between 10Hz and 10kHz.

Yes but be aware that with the high currents that our controllers can switch, come some constraints on mains power supplies. All our controllers 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 to have very good smoothing. Except at relatively low currents, a suitable transformer, rectifier and reservoir capacitor can prove very expensive, so a small battery with a suitable charger is a good [and often cheaper] alternative.

If you use a switch-mode mains supply, you should consider its operating frequency and how it will perform under full motor current chopped at 20kHz.

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.

One other issue to be aware of is the voltages developed by regenerative braking. Many of our controllers feed 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 our controllers are protected against such over-voltage, it may damage your power supply.

Yes,it is possible to have two or more controllers driving multiple motors. In the Loco worls it is called “double heading” and we have a number of diagrams in our knowledge base that show how this can be done.


Yes, our controllers don’t care how many motors they are connected to, so long as the maximum current rating is not exceeded. The current record is eight.


Fitting an ammeter is a much debated subject, from our perspective we see the following;


  • Can be interesting to see what you are using.
  • Can indicate mechanical faults.


  • Needs a shunt [adds losses], or Hall effect sensor [expensive].
  • What exactly is it measuring? [battery and motor current are usually different].
  • Most meters are not calibrated for measuring square wave PWM currents.


A battery voltmeter is 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 give as precise an indication as a more expensive digital voltmeter, but they can be very useful and better than most of the cheaper battery state indicators. They also give a nice display!





To give a vehicle a certain performance takes a particular level of power. This 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.

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

Now electrical current causes heating. The 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. Moving from 12V to 24V halves the current but halving the current reduces the heating losses to a quarter.

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 36V or 48V would be even better, heavy duty systems such as fork lift trucks often use 96V, electric cars are now using 600V!

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 the wiring and the internal 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.


Constant torque mode is the same as constant current control. The new Pro-160 has direct motor current sensing and uses this to implement a fully adjustable current limit. The software to provide a full torque control mode is being developed.

The 4QD-200 & 300 have an adjustable current limit which can be used to limit the torque from the motor.

Remember that the standard current limit is there solely to protect the MOSFETs from transient over-currents which could be destructive. If you run on the standard current limit for too long, then the controller will simply overheat and melt – quite literally, it is indeed possible to melt the solder on the MOSFET leads before the controller fails! So you may need to reduce the current limit quite significantly. The exact allowable current will of course depend on your heatsinking, the motor, and the controller you use.

Closed loop control

The Pro, DNO, and Porter controllers do not incorporate internal armature voltage sensing. For most applications this is an unnecessary complication since the operator compensates for speed changes automatically. The 4QD series does incorporate armature voltage sensing. However this alone gives a limited improvement since the motor’s speed still changes because of the voltage loss in its internal resistance (IR loss). It is possible to compensate for motor IR loss, but the controller then must be set up for each different motor: this causes much confusion as setting up requires much skill, or a tachometer and a method of loading the motor whilst measuring the speed. The other problem is that the existing internal current sensing (which could be used for IR compensation, is sensitive to the MOSFETs internal temperature, so the IR compensation would drift.

The best method of closed loop control is via a tacho-generator. It is relatively simple to add on to the 4QD series as the internal circuitry used for armature voltage sensing can accept a tacho generator circuitry with minimal change.

It is also possible to add a tacho generator to the Pro 120 or the DNO with our optional tacho feedback board. Other controllers can also use this same board or, if you wish to make a circuit yourself, see Tacho generator motor speed feedback for the circuit of a suitable error amplifier to compare tacho feedback voltage with the demand speed.

It is also possible to use a pulse generator to measure motor speeds. If using analogue control this needs to be processed via a frequency to voltage converter to be used as feedback. The slowest motor speed will determine the maximum time between pulses and this will determine the minimum response speed of the F to V converter: if it responds too quickly the motor will accelerate between pulses! But if you chose a long time you will need slow response which can cause problems with motor speed stability. Pulse circuitry is not direction sensitive (unlike a tacho generator) so the error amplifier need only be uni-directional.

Because of these timing constraints, pulse generators are usually used with computer control systems: in this case the computer can give the controller a demand speed via a digital to analogue converter: the software can control response times and difference amplification (if the demand speed is input digitally), or the D-A can output to a conventional analogue error amplifier, working direct from the demand speed control.

The 4QD, Pro and DNO series have, at their inputs, sophisticated analogue ramps: ideally the error amplifier should be inserted after the ramping circuit and before the modulator: contact 4QD for more assistance.
See also Speed Stability

Disabling regen braking can be done on certain models

  • The following controllers can have regen braking disabled by an on-board link
    • 4QD series
    • Pro-120 Mk 2
    • VTX series


  • The following controllers can have regen braking disabled by cutting PCB tracks
    • DNO series


  • The following controllers have regen braking which cannot be disabled
    • Porter
    • Pro-150

A variety is the answer. Full details are in the comparison table on this page but all have adjustable acceleration and deceleration ramps.



Yes, we have a two channel interface [DMR-203] that connects a standard RC receiver to two 4QD controllers.



If you don’t think your battery life is all that it should be then;

  • Check the battery condition. The capacity of a battery reduces as it gets older. Also do a load test on it, we’ve seen batteries that gave 12.8V off load but that dropped to 11.1V as soon as a load was applied.
  • Check the condition of the power cabling. We’ve seen numerous cases where cable joints have degraded over time, overheated, and then caused a significant volt drop at the controller. Measure the voltage directly across the controller B+ & B- terminals whilst under load.
  • Is there mechanical drag in the system?
  • If all else fails, fit bigger batteries.  The controller can only do only one thing with the current it takes from the battery – pass it on to the motors. If the controller wasted any significant power – it would simply get hot and go up in smoke, so if the batteries don’t last – it’s a battery, a motor, a wiring, or a mechanical problem.

There are two types of lithium battery to think about here

1] The lithium polymer type, called LiPo in the radio control world. These are small, light, and have a large energy capacity. One significant issue with these is that they must not be discharged too deeply, if they are then they can be damaged. 4QD controllers were not originally designed to work with LiPo but the 4QD-200 / 300 have an adjustable low voltage cut-off, and the DNOs can have a simple modification done to work with them.

2] The lithium ion lead acid replacement type [LiFePO4]. These are usually a higher capacity replacement for a standard lead acid battery and very common in the golf buggy world. These usually work ok with 4QD controllers but some batteries have internal protection circuits that can cause unexpected results e.g. over current protection, and low voltage cut-off.

Our new BCM-5P1 battery meter has a battery protection feature which can be used to turn a controller off to prevent damage to expensive lithium batteries.


Why do speed controllers sometimes fail?

Here at 4QD we go to a lot of trouble to make sure our controllers are reliable. We make sure we have enough capacitance, we use the best mosfets we can find, we fit substantial heatsinks, we minimise the susceptibility to radiated and conducted noise, and we conduct extended life tests by running controllers on an automated test rig.

And yet very occasionally a customer will get a seemingly random failure. Actually there’s a clue in that sentence, in five years we’ve not had a single mosfet failure on our test rigs, the reason for that is that we take a lot of care with electrical noise suppression. Motors are electrically noisy things, just look at the back of an old electric drill to see the sparks. We’ve written a full article in our knowledgebase about motor noise and how to prevent it, it also describes other sources of noise, and some detail on mosfet failure modes. It’s well worth 10 minutes of your time reading this and taking the necessary steps to make sure that your controller doesn’t suffer an untimely end. If you just want the short “what do I need to do?” version have a look here.

We’re now shameless about plugging our suppression components. They don’t cost a lot but since we introduced them we have seen the field failure rate of our controllers drop significantly.

Increase the voltage. If you have a single 12V battery, consider fitting 2 smaller ones and connecting them in series to give 24V.  It is possible to arrange for a 24V system to charge from a 12V source, this page shows how.