How to Reduce EMI

How to Reduce EMI, Voltage Spiking and Arcing

DC Motor and Switch Applications

In applications that involve motors, switches, or relays, there are common problems that can arise in the design phase. The cost of designing filters to deal with electromagnetic interference, arcing, and electromotive force from an inductor can add up drastically. Most components will need to be replaced eventually but being able to prolong the life of an element is crucial for any application.

AC or DC for Switches - What difference does it make?

In any application, it is good to know if your components are rated for AC or DC voltage. Switches are no exception, as they typically have both an AC and DC voltage rating. Arcing is an issue when it comes to switches since the circuit is being broken and the current cannot instantaneously drop to zero. Therefore, a transient arc will form across the contacts. If ignored, this will significantly decrease the lifecycle of a switch because the contact’s material will erode from the constant arcing to the point where the switch becomes defective.

How do we prevent arcing from a switch?

One may believe that buying a switch with a higher contact rating would be the solution to the problem. The best solution is to use an RC network to reduce the arcing of the switch.

Relay Contacts
Figure 1 – Relay Contacts before and after arcing
(Source: Wikipedia)

The photo above shows relay contacts after 100,000 cycles with no arc suppression. By reducing arcing, it minimizes the contact damage, reduces electromagnetic interference, and heat generation.

Arc Suppression Examples
Figure 2 – Two Arc Suppression Examples
(Source: Illinois Capacitor Inc.)

Once the switch is open, the applied voltage is soaked up by the capacitor and avoids damaging the contacts, preventing an arc from occurring, thus prolonging the use of the switch. Then when the switch closes again, the charged capacitor begins to discharge and the resistor current limits the inrush current. This RC circuit is known as a snubber. The snubber on the left can be used for both AC and DC while the snubber on the right is designed for DC.

How to reduce EMI and inductive voltage spiking caused by DC motors?

Simple Circuit
Figure 3 – Simple Circuit

The AC circuit above shows a simple application where a switch is connected to an inductor. Inductors are designed to resist change in current and generate electromagnetic fields when there is current flowing through an inductor.

Inside of a Motor
Figure 4 – Inside a Motor
(Source: HowStuffWorks)

The photo above shows what a motor looks like internally and how it relates to an inductor. Motors generate voltage spikes, EMI, and radio frequency interference (RFI) which can damage or disrupt circuits nearby. Arcing initially occurs once the motor rotates, then the generated EMI from the commutator flows back towards the power supply, RFI goes into the air, and finally as the electromagnetic field from the inductor collapses, a huge voltage spike occurs. The EMI may cease a circuit from functioning or degrade the performance dramatically while the RFI would disrupt any equipment relying on RF. Luckily, there are methods to suppress or at least lower the intensity of the disruptions.

Inductive Load Solutions

There are a few solutions when it comes to dealing with inductive voltage spikes going back towards a power supply or switch.

TVS Method
Figure 5 – TVS Method
(Source: Littlefuse Application Note)

To deal with a large voltage spike in an AC circuit, a metal-oxide varistor (MOV) or a bidirectional transient voltage suppressor (TVS) diode is used. In this case, a TVS diode is used because the TVS diode blocks the current from flowing during both cycles when the switch is open.

Suppression Diode Method
Figure 6 – Suppression Diode Method
(Source: Littelfuse Application Note)

The suppression diode or flyback diode, in the example above, helps prevent a voltage spike since it provides a path for the current once the switch is open. The diode needs to have a high reverse voltage rating to protect the circuit. Snubber networks can also be used to reduce the voltage spikes from the inductor, but each method has their benefits and deterrents.

Dealing with EMI & RFI

DC Motors generally produce a lot of noise in an electrical system due to the excessive amount of EMI and RFI they generate. Some DC motors are designed with EMI in mind by selecting certain overlapping case materials to provide additional shielding. There are many different EMI filtering elements, but each has their advantages and disadvantages.

EMI Filtering Elements
Figure 7 – EMI Filtering Elements
(Source: X2Y Attenuators, LLC DC Motors)

Understanding how each element works is extremely advantageous when it comes to designing the proper EMI suppression filter or RFI for an application.

These were a few methods to control all the noise and voltage spikes when it comes to working with motors, along with protecting switches/relays if they are switching an inductive load. The most challenging part is coming up with a reasonable solution that is cost effective and makes the application operate reliably.