An Improved Approach for Connecting VSDs and Electric Motors

When electric motors are connected with Variable Speed Drives, some precautions must be taken. Motor cable length can create short time over-voltage at the motor terminal. This phenomenon results in premature ageing of the motor winding insulation, and, if not properly designed, will lead to motor failures. End users and consultants often underestimate this problem and neglect to apply best practices or to specify adequate countermeasures. This white paper provides guidance on how to avoid this potentially problematic situation.

In industrial environments, flexible control, energy efficiency, and low cost maintenance are all well documented benefits of variable speed drives (VSDs). However, when long cable lengths are utilized to connect VSDs to motors, motor winding insulation can deteriorate more quickly, and eventually the motor itself may fail. To understand the risks involved around this issue and the solution options available to address the situation, the technical characteristics of the drive and the motor first need to be understood.

Within the VSD, the AC supply voltage is converted into DC by the use 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 Figure 1). This conversion is typically achieved through the use of power electronic devices such as IGBT power transistors using a technique called 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.

Figure 1. A basic schematic of a variable speed drive linked to a motor

Advancements in power component technology have enabled both increases in switching frequency and reductions in losses. IGBTs are commonly used in scenarios characterized by switching frequencies of up to16 kHz and motor output frequencies of up to 1kHz….

 Continue reading  Schneider white paper

See Schneider Electric’s range of variable speed inverter drives here, including ATV12, ATV212, ATV31C, ATV32, ATV312, ATV61, ATV71