An improved approach for connecting VSDs and electric motors

November 6th, 2014, Published in Articles: Vector

 

Motor winding insulation can deteriorate quickly and the motor may fail when variable speed drives are connected to motors by long cable lengths.

Within the VSD, AC supply voltage is converted into DC by means of a rectifier. DC power contains voltage ripples which are smoothed using filter capacitors. This section of the VSD is often referred to as the DC link. This DC voltage is then converted back into AC (see Fig. 1). This conversion is typically achieved through the use of power electronic devices such as IGBT power transistors, by means of pulse width modulation (PWM). The output voltage is turned on and off at a high frequency, with the duration of “on-time”, or width of the pulse, controlled to approximate a sinusoidal waveform.

Fig.1: A basic schematic of a variable frequency drive linked to a motor.

Advancements in power create a temporary overvoltage at the motor terminal connection. In fact, the overvoltage can be more than twice the DC bus voltage of the drive (see Fig. 2). This can result in damage to the motor winding.

Fig. 2: The impact of long motor cable length can manifest itself as both overcurrent and overvoltage situations.

The overvoltage condition occurs at the motor terminal. Examples of how overvoltage and overcurrent conditions can have an impact in the system are:

  • Effect on variable speed drive: Short circuit fault (SCF) is the main risk of an overcurrent condition in the speed drive. The peak current can be read by the current sensors of the drive and lead to unexpected faults, due to the switching. Another consequence of the capacitive peak current is a rise in IGBT temperature in relation to the switching frequency. These higher-than-normal temperatures reduce the lifetime of the speed drive.
  • Effect on the electric cabinet: The disturbances that result from the long cable and motor interactions create high frequency circulation current into the ground and can disturb appliances connected on the same network. A frequent consequence is the triggering of differential protection relays located upstream of the drive. The high frequency currents which run through the cable motor also generate radiated emission, which can disturb electronic devices around the motor cable.
  • Effect on the motor: The overvoltage condition at the motor terminal can appear between two motor windings and can create partial discharge and premature ageing of winding insulation. This can then lead to complete motor failure. Note that the admissible overvoltage depends on the class of motor insulation (see Fig. 4).

Fig. 3:Longer cable lengths between drive and motor produce higher peak voltage at the motor terminal.

End-users and consultants often underestimate these types of phenomenon and do not specify adequate countermeasures. Even if short-term cost savings can be achieved by avoiding the extra cost of supplemental devices and options, this improper design approach can lead to motor breakdown or unexpected interruption of the applications in the long run. This level of risk is not acceptable in critical applications such as water supply plant or power stations.

Fig. 4: Limit lines for voltages of multiple motor types.

Fig. 4: Limit lines for voltages of multiple motor types.

Effects of long-length motor cable

Overvoltage occurs in standard applications where the motor cable exceeds 10 m. The longer the motor cable, the higher the overvoltage  (see Fig. 3). This effect is amplified when using a shielded cable. However, the overvoltage is limited to twice the DC bus voltage.

The voltage rise time at the output of the drive, often referred to as dV/dt, generates higher transient peak current at higher switching frequencies. This is the result of parasitic capacitance (occurs when two conductors at different potentials are in close physical proximity to one another. They are affected by each others’ electric field and store opposite electric charges, like a capacitor). These transient currents increase the drive, cable, and motor losses.

The most common IEC and National Environmental Management Act (NEMA) standards, technical specifications, and guidelines for admissible voltages and current for various motor types are highlighted in Fig. 4. The admissible overvoltage depends on the class of motor insulation. The relevant IEC and NEMA standards include:

  • IEC 60034-17 Limit line for general purpose motors when fed by frequency converters, 500 V motors.
  • IEC 60034-25 Limit for converter rated motors: Curve A is for  500 V motors and curve B for 690 V motors.
  • NEMA MG1 Definite purpose inverter-fed motors.

An IEC60034-25 class B or NEMA

600 V motor shall be specified for critical applications. IEC60034-25 B or NEMA 400 should be prescribed to reduce the risk of motor fault when used with a speed drive motor.

Another side effect of using a VSD is the degradation of motor bearings. This is caused by common mode voltage generated by the inverter of the VSD (noise induced into a cable by the switching) and it generates high frequency current into the bearings of the motor.

Three different common mode current loops could occur in the motor, depending upon the type of motor and whether or not the bearing is isolated:

  • A loop between stators, windings, and the motor shaft. In this case the induction current flows around the bearing twice.
  • A loop between parasitic capacitor windings and motor shaft connected to ground by the load. This can occur where the frame is not grounded adequately. The pulse capacitive current flows to the drive end bearing.
  • A loop between the parasitic capacitor of the stator, rotor windings, frame, and bearing. In this case, the frame is connected to ground correctly and the bearing current is a percentage of the common mode voltage. The bearing current occurs because of capacitive electrostatic discharge.

Preventive measures

Several solutions should be evaluated to limit the impact of overvoltage and current peaks. The viability of each solution depends on the environment of the application.

Fig. 5: Impact of software on the prevention of a double transition condition.

Software protection

Some VSDs are pre-configured with software which makes the solution more dependable. These VSDs always integrate motor controls preventing “double transition”, which occurs when one motor phase switches from minus to plus DC while another phase switches from positive to negative simultaneously (see Fig. 5).

A pulse which is too short with respect to the time constant of the cable can lead to the superimposition of two oscillations generating an overvoltage greater than twice the voltage bus DC condition.

The latest generation VSD technology avoids the superimposition of voltage reflection situation by setting a minimum time between two PWM voltage pulses (see Fig. 5). Although setting this time limit slightly degrades the speed drive performance (-3% of torque), overall system performance is not impacted under normal conditions.

Output reactor

Output reactors oppose rapid changes in current and are generally installed in motor driven equipment to limit starting current. They may be used to protect variable-frequency drives and motors. In essence, a motor choke associated with the parasitic capacitance of the motor cable reduces the dV/dt and peak voltage. The effect will depend on the cable type and length. However, care is needed as reactors can theoretically extend the duration of overshoot (when an electronic signal exceeds its target) if an improper output reactor is selected.

 

Table 1: Recommended preventive measures will depend on motor characteristics and cable length.
Motor cable length (unshielded cable)

Motor conforming to IEC60034-25 motor

Not-conforming to IEC60034-25

1 m < Lm < 50 m Filter not required dV/dt filter
50 m < Lm < 100 m Filter not required Sinus filter
100 m < Lm < 300 m Filter not required Sinus filter
300 m < Lm < 500 m dV/dt filter Sinus filter
500 m < Lm < 1000 m Sinus filter Sinus filter

Output dV/dt filter

Output dV/dt filters are the most cost effective solution to guarantee motor protection and reduction of overcurrent impact on speed drives. This filter reduces the dV/dt so the overvoltage effect and the capacitive leakage current between phases and phases to ground are minimised. Such filters offer flexibility because they can be used with most motors and any cable (independent of type and length) without problems. This method is recommended where the specifications of a particular motor are unknown.

Sinus filter

A special low pass filter (an electronic filter which passes low-frequency signals and reduces the amplitude of signals with frequencies higher than the cutoff frequency), called a sinus filter, allows high frequency currents to be shunted away. The result is that the voltage waveform at the motor terminal becomes pure sinusoidal. The differential sinus filter allows complete suppression of the overvoltage effect and reduces electromagnetic compatibility (EMC) disturbance.

Fig. 6: Diagram of how a sinus filter works.

The bearing currents and the reduction of the conducted EMC disturbances to the mains can be suppressed where the sinus filter is associated with a common mode filter (see Fig. 6). The combination of those two filters presents the most robust solution for avoiding VSD-to-motor connection issues. This solution is also very cost effective if a long motor cable is used, as long as a shielded cable is not required.

Application examples

Some applications are impacted more directly by the long cable length phenomenon than others. For example, in hoisting applications, motors spend a large part of their operating life in braking mode. The braking energy is transferred through freewheeling diodes back onto the intermediate DC link, thereby generating a 15 – 20% increase in the DC link voltage (and, therefore, an increase in the peak motor voltage).

The effect is similar to increasing the voltage supply by up to 20%. In such cases, a 400 V application should be treated as if it were being supplied with 480 V. In applications where motors are run in parallel, the appropriate cable length should be calculated based on the sum of all the cables. For example, if three motors in parallel are connected to a single VSD, each with a 20 m cable, the total length which should be calculated is not 20 m, but should be 60 m. Precautions must be taken to protect the VSD from any unexpected tripping.

Conclusion

When variable VSDs are used with motors, a combination of fast switching transistors and long motor cables can cause peak voltages up to twice the DC link voltage.
In extreme cases, this high peak voltage can cause premature ageing of motor winding insulation, which leads to overall motor failure.

Contact Ntombi Mhangwani, Schneider Electric, Tel 011 254 6539, ntombi.mhangwani@schneider-electric.com

Related Articles

  • Nine-year relationship with Malian gold mine
  • Background suppression laser sensor
  • Hotel’s energy usage reduced by 40%
  • Flexible condition monitoring system
  • Measuring digital inclination, precise degree