High Frequency Measurements Web Page
Douglas C. Smith

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Technical Tidbit - December 2003
Troubleshooting Noise From Pulse Width Modulation Controlled 3 Phase Motors and Controls

  F40 and F90 probes
Figure 1. Probes Useful for Tracking PWM Motor Currents
(F-90 probe, left, and F-40 probe, right)

Abstract:  Pulse width modulation controlled 3 phase motors have become popular in numerous applications from small fractional hp motors to giant industrial motors. Common mode currents generated by the controller circuits and motors can cause signal interference to other circuits at relatively low frequencies. At these low switching frequencies used, some shielding and circuit design techniques do not work well. The result can be interference to low level higher frequency circuits. Troubleshooting techniques are presented.

Discussion: Pulse width modulation, PWM, controlled motors often use switching frequencies on the order of 10 kHz, much lower than what is normally considered "high frequency." However, the fundamental switching frequency and its harmonics can interfere with the operation of associated higher frequency circuits.

Common mode currents from the PWM controlled power fed to the motor windings can couple through the capacitance from the windings to the motor frame and result in significant currents that flow in the metal structures of the machine. These currents can cause enough voltage drop in "ground" to affect other circuits. In one case, such ground drops caused the common mode voltage on the balanced input to a high frequency device to exceed the device specifications. The result caused the circuit to malfunction.

The key to this problem is to either reduce the common mode currents or lower the ground impedance, which includes a significant resistive component at these low frequencies. One way to reduce common mode currents on the motor cables is to shield them. Normally, at high frequencies, the common mode current on the motor cables would be matched by a current in the opposite direction on the shield (if both ends of the shield are connected with low impedance connections) so the net common mode current on the cable would be significantly reduced as would currents in the machine's metal structure. However, at frequencies of a few kHz, shield resistance becomes an important effect that limits the shield effectiveness as it becomes comparable to the shield inductance. Another way of thinking about the effect is that the skin depth of the shield becomes comparable to its actual thickness. Click here for a skin depth calculator. Its accuracy is in question at the low frequencies considered here but it can give a general idea of skin depths at various frequencies.

How can these common mode cable and ground currents be measured and their paths traced? Figure 1 shows a pair of current probes from Fischer Custom Communications, an F-40 current probe and an F-90 surface current probe. Figures 2 and 3 below show the frequency response of these probes. They both have low frequency response to several kHz and enough high frequency response to cover many harmonics of the switching frequencies of PWM controlled motor circuits.  In addition, at frequencies above a few MHz, a good double braid shield can work well enough to significantly reduce ground currents, making them less of a problem. So these two probes cover the frequency range of interest for PWM controlled motors. Both probes can handle hundreds of Amperes or more of power current so that probe core saturation is generally not a problem, especially since common mode currents on the motor cables are of interest here (zero net power current).

  F40 response curve
Figure 2. F-40 Response

F90 response
Figure 3. F-90 Response

Tracing PWM currents in cables and the ground structure requires a pair of matched F-40 current probes (or similar) and an F-90 surface current probe (or similar). First, an F-40 is placed around the motor power cables (all conductors including a shield, if present) to measure the common mode current on the cables. Fischer Custom Communications indicates that special probes can be designed for large cables. A digital scope capable of displaying the fundamental and harmonics to several tens of MHz should be used. An analog scope will work, but is limited as to display and storage options. The current flowing in common mode on the power cables (including a shield, if present) is just the current that is flowing back in the machine structure.

One troubleshooting method would be to trigger the oscilloscope on the common mode motor current. Then, using the second F-40 and the F-90 probes, currents can be traced throughout the system. By triggering on the motor common mode current, the other probes can tell the direction of current flow as well as magnitude. For instance, an F-40 placed around another system cable can tell if some of the motor currents are present or an F-90 can find where in the metal structure that motor currents are flowing.  While triggering on the common mode current, a suitable voltage probe can be used to measure noise voltages in sensitive circuits that are due to the motor currents, such as the common mode voltage at a balanced input.

The key is to trigger the scope from the common mode currents on the motor power cable (all leads carrying power to the motor, including a shield if present). The technique is similar to that described in the paper on this site, A Method for Troubleshooting Noise Internal to an IC (~140K), where a scope is triggered from a loop positioned above a chip and a voltage probe is used to find where the noise picked up by the loop shows up on the chip outputs, although much higher frequencies are present in that case.

A word of caution: Be careful if you want to clamp a current probe around just one (really, less than all) of the motor power leads. The current probes described here can withstand a lot of power current (hundreds of Amperes or more) without saturating. However, the probes will be held closed with great force (you do not want your fingers in the way) by the magnetic field from the power current. The probes will not be able to be removed from the cable without turning off the power. Never clamp a current probe around a power conductor carrying many Amperes of current where you cannot break the power circuit. If you do, the current probe may be on the cable for a long, long time!

Summary and Conclusion: Low frequency common mode currents on PWM controlled motors can cause problems in other associated circuits. Frequencies on the order of tens of kHz (fundamental and harmonics) cause shields to be less effective, due to shield resistance, in reducing common mode PWM currents on motor power cables. Given a stable trigger from the common mode motor currrents, current probes can be used to trace these currents through the system and voltage probes can measure the resultant voltages in circuits due to the motor currents.


Interesting websites with information relating to this article and PWM controlled motors are:

Other articles on this website covering current probes include:

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Copyright © 2003 Douglas C. Smith