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  • Hidden cable gaffes that can compromise electric motor performance

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Often overlooked in design planning, four preventive measures can help avoid overvoltage.

In industrial environments, connecting motors to variable speed drives (VSDs) via cables delivers flexible control, energy efficiency, and low-cost maintenance. But precautions are necessary to prevent overvoltage, which can compromise motor lifetime or cause a shutdown.

How cable length can create issues
For motors connected to VSDs, a combination of fast-switching transistors and long motor cables can cause a temporary overvoltage at the motor terminal connection. In extreme cases, high peak voltage can prematurely age the motor winding insulation, causing overall motor failure.

Consequences of overvoltage and overcurrent conditions
Effect on VSD:
The main risk of overcurrent in a VSD is a short circuit fault.Capacitive peak current can also cause power transistor temperature to rise, and this above-normal temperature can reduce VSD lifetime.

Effect on the electric cabinet: Disturbances that result from long cable and motor interactions create high frequency circulation current into the ground that can disturb appliances connected on the same network. High frequency currents also generate radiated emission, which can disturb electronic devices around the motor cable.

Effect on motor: Overvoltage at the motor terminal can occur between two motor windings. Depending on the class of winding insulation, partial discharge and insulation ageing can occur, leading to motor failure.

Side effects of a long motor cable: In standard applications, overvoltage occurs whenthe motor cable exceeds 32 feet (10 meters) in length. The longer the motor cable, the higher the overvoltage, an effect that amplifies with a shielded cable. Calculating the cable length correctly helps protect the VSD from any unexpected tripping.

Another side effect is motor bearing degradation, caused by common-mode voltage that the VSD inverter generates, which throws high frequency current into the motor bearings.

Four preventive measures
Often overlooked in design planning, these measures can help avoid overvoltage and related effects.

  1. 1. For more dependable results, preconfigured software protection is available in some VSDs. With this software, VSDs integrate a motor control that prevents “double transition,” and sets a minimum time between voltage pulses to avoid the “superimposition” of a voltage reflection situation.
  2. 2. Installed in motor-driven equipment to limit starting current, output reactors oppose rapid current changes by reducing the dV/dt (voltage rise time at the drive output) and peak voltage. The result depends on cable type and length. But this option requires careful device selection, as reactors can extend the duration of overshoot when electronic signal exceeds it target.
  1. 3. An output dV/dt filter is the most cost-effective solution to guarantee motor protection and reduce the overcurrent impact on VSDs. This filter minimizes the overvoltage effect and capacitive leakage between phases and phases to ground, and it works with most motors and any cable.
  2. 4. A differential sinus filter – a special type of low-pass filter suppresses the overvoltage effect and reduces electromagnetic compatibility (EMC) disturbance. If associated with a common-mode filter, this filter also suppresses bearing currents and reduces conducted EMC disturbances to the mains. It offers the most robust solution for avoiding VSD-to-motor connection issues and is cost-effective with a long (unshielded) motor cable.

Best practices for safeguarding VSDs and motors include:

  • Specify a motor designed for speed drive applications
  • Specify VSDs that integrate voltage reflection superimposition software suppression
  • Minimize the distance between the motor and VSD
  • Useunshielded cables if possible – shielded cables calculate as twice the length
  • Reduce the VSD switching frequency

By following these steps, a cable that is 984 feet (300 meters) or shorter will work without any additional options. For applications that involve longer cables, unknown motor insulation levels, or nonstandard motors, a dV/dt output filter or sinus filter is the best preventive measure.

Motor cable length
Motor conforming
to IEC60034-25
Motor NOT conforming
to IEC60034-25
1 m (66 ft) < Lm < 50 m (164 ft) Filter not required dVdt filter
50 m (66 ft) < Lm < 100 m (328 ft) Filter not required Sinus filter
100 m (328 ft) < Lm < 300 m (984 ft) Filter not required Sinus filter
300 m (328 ft) < Lm < 500 m (984 ft) dVdt filter Sinus filter
500 m (328 ft) < Lm < 1000 m (984 ft) Sinus filter Sinus filter

Selecting the right preventive measures depends on motor characteristics and cable length 

Protect motors and variable speed drives with this expert guide. Download the white paper, “An Improved Approach for Connecting VSD and Electric Motors.”
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