Challenge of too long motor cable – and how the problem can be solved elegantly with a DC link filter

Karlheinz Wirsching, Baumüller Nürnberg GmbH

Read min.

If control drives are supplied through cables that are too long, impermissible voltage stresses can occur in the motor. In the worst case, if it occurs continuously, it can irreparably damage the motor insulation and lead to total failure of the motor. This article explains how this can be prevented.

Basic question: When are motor cables too long?

The feeding converter is usually attached near the electrical machine. The cable length is then absolutely within the allowable range. Yet this is not always the case. There are application cases in which the length of the motor cable is disproportionately long. Typical examples are:

  1. Central control rooms for large machines: All electrical machines are fed with electricity from a central control room. The cable length can then easily grow to 100 m or more.
  2. Large factories: Electrical machines are accommodated spaciously on the company site. The consequence is long motor cables.
  3. In the case of offshore underwater drives, which are supplied from a main platform.

In all cases with motor cables that are obviously significantly longer than usual, it is necessary to check whether the allowable length is exceeded. If yes, appropriate additional measures must be taken to prevent dangerous voltage peaks, which could irreparably damage the motor.

The motor cable lengths are usually divided into the following categories:

  1. Short cable: Cable length less than 5 m
  2. Long cable: Cable length from 5 to 500 m
  3. Very long cable: Cable length from 0.5 to 500 km

Interestingly, the relevant standards contain enough notes on allowable voltage peaks at the motor terminals with short cables. But when it comes to the influence of long motor cables, any search for detailed information in the standards is in vain. 

Many applications fall within the “long cable” range. This is why we will now take a closer look at this case.

What happens when the motor cables are too long?

Basically: If electric motors are supplied through cables that are too long, impermissible voltage peaks can occur at the motor. What precisely happens then? If the cables are too long, the cable impedances change the output voltage of the converter. As a result, harmful voltage peaks occur at the input terminals of the motor. Specifically, the limit values of the voltage rise and voltage level are exceeded. The consequences can be serious:

Premature winding damage and, particularly in flameproof (i.e. explosion protected) machines, with considerable consequential damage.

The two critical variables are:

  1. The rate of the voltage rise.
  2. The maximum motor terminal voltage.

In the case of long cables, a rectangular signal is applied to the converter, but voltage peaks arrive at the motor. If the voltage rise is too large, this leads to nonlinear distribution of the voltage along the winding with values in the input coil that are too high.

If the motor terminal voltage Umax is continuously or frequently too high, the insulation between the conductors is exposed to very high loads. This is why the standard specifies maximum voltage values for standard motors.

What are the specific consequences of the voltage peaks?

If continuous voltage peaks occur at the motor terminals, this can lead to total failure of the  motor, because these voltage peaks irreparably damage the winding and lead to sparking between the conductors or to ground. In flameproof environments protected against explosions, this can cause a fire.

What options are available for protecting motors from these dangerous voltage peaks?

There are basically four options:

  1. Improving the winding insulation and optimizing the terminal panels
  2. Optimization of the pulse program
  3. Installation of a link filter
  4. Multi-level converters

How does Baumüller protect the motor if the motor cables are too long?

One thing is clear: The harmful voltage peaks must not reach the motor. They must therefore be stopped or removed between the converter and the motor or even better, they must be prevented from occurring in the first place. How does Baumüller do that? By connecting an additional link filter to the link filter of the converter. This changes the capacitive system of the converter-motor combination so that the harmful voltage peaks are largely weakened. The link filter is positioned as close as possible to the supply infeed.

DC link of the converter
The DC link filter must be placed as near as possible to the supply infeed

The following must be determined before the final installation:

  1. How long is the complete motor cable length? That means, what is the total length of all motor cables in the system, which are connected to a joint link via the inverters.
  2. If there are side-by-side converters, the sum of the individual cable lengths must be taken into consideration.

Once the motor cable length has been determined, the number of link filters to be installed must then be determined.

To summarize

If the motor cables are too long, voltage peaks that are dangerous for the motor can occur. If these are frequently or continuously above the limit value, motor damage can occur, and in the worst case, total failure of the motor. This can be prevented if link filters are installed at the converters or link involved, because these filters prevent or remove the harmful overvoltages before they reach the motor.

Do you have further questions?

Karlheinz Wirsching
Product manager drives I Baumüller Nürnberg GmbH

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