What Are the Effects of Excessively Long Cables on Motors?
The issues caused by long cables (including motor power cables and encoder cables) primarily stem from three factors: increased distributed capacitance, resistance, and inductance, as well as transmission line effects. Among these, the distributed capacitance of the cable is the main culprit behind signal quality degradation.
Common Effects on All Motor Types:
- PWM Waveform Distortion (The Primary Issue)
1.1. Cause:
The longer the motor power cable, the greater the distributed capacitance formed between the core wire and the shielding layer. The high-frequency PWM square wave output by the drive needs to frequently charge and discharge this capacitor.
1.2. Phenomenon:
The steep rising and falling edges of the square wave become slow and rounded.
1.3. Hazards:
- Dramatic Increase in Switching Losses: During switching, the drive's power transistors (MOSFETs/IGBTs) operate in a state where high voltage and high current overlap, causing abnormal heating of the drive and potentially leading to burnout.
- Motor Heating and Torque Reduction: The distorted waveform leads to impure motor current and increased harmonics, raising iron and copper losses in the motor. This causes significant heating and reduces effective torque.
- Voltage Overshoot and Ringing: The cable inductance and distributed capacitance form an LC resonant circuit, generating high-frequency oscillations (ringing) and voltage spikes (overshoot) at the waveform edges. These spike voltages can far exceed the bus voltage, threatening motor insulation and drive safety.
- Voltage Drop
2.1. Cause:
Wires have resistance, and longer cables mean greater resistance.
2.2. Hazard:
During high-current operation, the actual voltage at the motor terminals is lower than the drive's output voltage, leading to insufficient torque at high speeds and reduced performance.
- Electromagnetic Interference (EMI)
3.1. Cause:
Long cables act as efficient antennas, radiating strong electromagnetic noise from the distorted high-frequency PWM signals.
3.2. Hazard:
Interferes with other sensitive equipment in the system, such as sensors, encoders, and communication lines.
Specific Effects on Different Motor Types:
Motor Type | Specific Impacts and Risks |
|---|---|
Open-Loop Stepper Motors | Highest risk. Due to the lack of feedback, waveform distortion reduces the actual current, leading to a drop in motor output torque—unbeknownst to the drive. This makes the motor highly prone to losing steps during high-speed operation or sudden load changes, with no system capability for detection or correction. The impact is often catastrophic. |
Closed-Loop Stepper Motors | Moderately high risk. Although the encoder can detect and correct lost steps, this is a "reactive补救." Waveform distortion and torque fluctuations caused by long cables persist, resulting in unstable motor operation, vibration, and significant noise. The correction process itself can introduce speed jitter. Additionally, the encoder signal line is susceptible to EMI interference from the power cables. |
Servo Motors | Controllable risk, but still significant. Servo drives rely on precise motor current feedback (from encoders or resolvers) for closed-loop control. Long cables distort the current feedback signal, causing errors in the drive's calculated torque current, which compromises control accuracy and dynamic response. Furthermore, encoder signals are more prone to attenuation and interference over long distances, potentially triggering servo alarms or even shutdowns. |
In summary, the fundamental reason for avoiding excessively long cables is that they inherently degrade the drive's output waveform, triggering a chain reaction of issues ranging from performance degradation to equipment damage. Open-loop stepper systems, due to their lack of feedback, carry the highest risk.
Updated on: 30/09/2025
Thank you!