Electrical – Should I worry about EMC when building a PWM stepper motor driver with 1.5 meter cable

emcpwmstepper motorstepper-driver

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.

Best Answer

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.