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I am building a board that would drive a stepper motor with PWM signal. Is it likely that it will pass the EMC test?

Here are the details: I am using Trinamic TMC5130 stepper motor driver chip. The PWM frequency is 78.0 kHz The board and the stepper are connected by 1.5 meter flat ribbon cable, (not shielded, not twisted). The current through the stepper motor coils is up to 900mA. Stepper coil resistance = 1.1 ohm, inductance = 2.6mH, Power supply voltage = 12V.

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  • \$\begingroup\$ radiated or conducted? \$\endgroup\$ – JonRB Sep 22 '17 at 17:38
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    \$\begingroup\$ If you are going to sell it, it's your responsibility not to guess but measure and make sure it's compliant. \$\endgroup\$ – winny Sep 22 '17 at 17:45
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Yes you should worry about it, if you care about EMC.

Home switch cables should be far away or else you have to use 0.1uF and 1k pullup. Any analog sensing will be worse.

Include twisted pairs (UTP) or better STP cables for stepper and sensor cables. Otherwise with current sensing, you may also be able to hear PWM noise modulating in motor as a high pitch squeel that changes with your hand over the cable, when in idle holding torque , due to EMI ingress from unbalanced and unshielded cables. Often many OEM's put large CM chokes in torroidal cores with wraps around it or clamp type with high mu to raise CM impedance and balance the lines at VHF/UHF range due to rise time rates. (ns)

Fig 3-10 also shows methods of reducing egress/ingress.

Choice of SMPS supplies and earth grounding is also important to avoid unintended ingress and egress.

The cables with pulse voltages from deadtime current act like great antenna unless balanced with CM chokes, shielded and shunted with RF Y caps.

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Should I worry about EMC when building..

ALWAYS. Any good engineer has EMC built in as a base level design criteria along with efficiency and cost. If you do not take it into account, you are setting yourself up for a bag of troubles later on.

Is it likely that it will pass the EMC test?

That really depends on the rest of the system. If the driver and the cables are all contained within a metal grounded case, it probably will. If it's a plastic box or the cables run outside the enclosure, it is much more likely to fail.

However, if you are designing properly, you will be endeavouring to reduce radiated electromagnetic noise within your system anyway. If you have any other electronics that are sensitive to noise, you really want to limit what you are generating in the first place so you don't trip yourself up.

Further EMC noise is cumulative. If this is part of a larger system, whatever noise this driver is creating gets added to the rest. As such it is important to take measures to ensure you contribute as little as possible.

Noise from the motor will depend on the quality of the driver as well as the chosen driving methods, but as a minimum you should be using shielded cables to the motor coils.

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    \$\begingroup\$ Both answers are great. I had to flip a coin to choose which one to accept. Thanks a lot! \$\endgroup\$ – Sergey Sep 22 '17 at 18:25
  • \$\begingroup\$ @Sergey, No worries.. :) \$\endgroup\$ – Trevor_G Sep 22 '17 at 18:32
  • \$\begingroup\$ If both answers are good, upvote both and accept the best. Especially if they contain complementary (or even complimentary) information. \$\endgroup\$ – Spehro Pefhany Sep 22 '17 at 19:18
  • \$\begingroup\$ @SpehroPefhany I have tried, but unfortunately I don't yet have enough reputation to upvote. It says that I need at least 15 reputation to vote. \$\endgroup\$ – Sergey Sep 23 '17 at 2:09
  • \$\begingroup\$ @Sergey Ah.. I would have thought you'd be able to upvote answers to your own question. \$\endgroup\$ – Spehro Pefhany Sep 23 '17 at 3:55

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