Or….why should I fit motor noise suppression? In this article we will talk about what is EMC, why it can cause problems, and what can be done about it. We won’t go into the technical details – there are many excellent books available for that. This is a brief canter through the main concepts and how they relate to motor controllers.
So what is EMC? One definition is the study of the compatibility of systems that transmit and receive electromagnetic radiation. Another term often used interchangeably with EMC is radio frequency interference or RFI.
For RFI to occur there must be both a transmitter of radio energy and a receiver that suffers the interference. It is easy to say but if there is no transmission then there will be no RFI. Equally if the receiver does not “hear” the transmission then there is no RFI.
RFI becomes a problem when the receiver “hears” a transmission that is loud enough to cause some sort of incorrect operation. This could be a crackle in the case of a car stereo system, or something much worse if the receiver is part of a safety critical system such as electronics in a hospital.
We also need to say that the receiver does not have to be a “radio” receiver. Almost any electronic equipment can act as receiver for RFI if it is not designed correctly. As the number of transistors in a microprocessor has grown over the years, so the typical operating voltage has come down, from 5V ten years ago to 1.3V now. This means that an interference signal that had no effect a few years ago can cause problems today.
Lets now think about what makes a radio transmitter.
In the early days of radio Guglielmo Marconi used a spark gap transmitter to generate radio waves that could travel hundreds of km across the Atlantic. All it took was a spark and two wires like this….
So how does all this relate to electric motors?
Here’s a picture of an electric motor and the sort of sparks that get made between the brushes and the commutator….
[There is a slow motion video of this sparking at www.youtube.com/watch?v=gc4l1eooPKM&t=30s ]
If we draw this as a picture and add some motor wires we can start to see the similarity…
An electric motor making sparks is just the same as an early radio transmitter.
So that’s the transmitting part, now let’s have a look at how this radio energy can cause problems.
In the early days of electronics, a motor controller looked like this…
They were bullet proof – if rather heavy, and used mechanical contacts to switch large resistor coils in the circuit.
Nowadays we use a special type of transistor called a MOSFET, these are about the size of a thumbnail and each one can switch 100 amps 20,000 times a second using a very small drive signal. This is called pulse width modulation or PWM.
In fact, that small drive signal can be part of the problem, if the RFI noise signal from our spark transmitter [motor] somehow finds its way onto the drive section [or gate] of the mosfet it can make the mosfet switch on when maybe we did not want it to.
Consider this simple half bridge motor control circuit, normally either the drive mosfet is on or the flywheel mosfet is on but never both at the same time.
If the RFI noise signal is large enough it can make a mosfet turn on when it should be off. If this happens it will create a short circuit straight across the battery which will make both mosfets self-destruct.
So what can we do to prevent this?
RFI can get into a system by either conduction along the wiring, or by radiation through the air and there are three aspects to consider.
The first aspect is to reduce the generation of RFI.
- Keep the motor and brushes in good condition. As the brushes get worn away they have less spring pressure pushing them onto the commutator, this leads to bigger sparks. Also clean the commutator periodically.
- Make sure to keep metallic swarf away from the motor.
The second aspect is to reduce the transmission of RFI.
- Fit a suppression capacitor as close to the brushes as you can. This will short circuit some of the high frequency radio signal right where it is created.
- Fit ferrite beads to the motor wires near the motor. These help stop any noise that gets past the capacitor from moving along the motor wires.
- Twist the motor wires together, this stops any remaining noise from radiating out from what would otherwise be a loop antenna.
The third aspect is to make it harder for the RFI to get back into your system.
- Route the motor wires away from the control wires. The motor wires carry high currents which are being switched very quickly, this means they create large magnetic fields which, just like a transformer, can be coupled to control wires and induce unwanted signals.
- Fit a transient suppressor across the battery terminals of the controller. This helps kill any remaining noise spikes that do get through.
It is also worth thinking about other sources of RFI. These include;
- Horns – especially inductive car or motorcycle types, and relays. These should have a catching diode fitted across the coil like this…..
- Compressors or vacuum pumps operated by a switch.
- Anywhere where a significant current is switched on or off.
Many systems have been built with no consideration to RFI. Many of these systems have functioned perfectly for years with no problem.
But the nature of RFI follows a Gaussian distribution curve with the largest spikes occurring only infrequently.
So you have to be a bit unlucky for a large spike to arrive at exactly the wrong time in the mosfet on / off cycle. But if this does happen, then the result can be a catastrophic failure of the controller.
The suppression components are cheap.
You may not need them [at least to start with].
But in the words of Inspector [Dirty] Harry Callahan
“You’ve got to ask yourself one question….do I feel lucky?”
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