Variable Speed Drives – Understanding Your Application – Part III

In Part II, we discussed several electrical line-side issues which should be factored in when selecting a drive. Life would be too easy if this were the whole story. There are several critical load-side (i.e. from the drive output to the motor) concerns which should be examined carefully because they can impact equipment life and your prime mover’s ability to perform the work necessary for the process.

Load Side Considerations:

• Motor amperage: Drives are properly sized by amperage, not horsepower. In order to ensure proper output capacity, the driven motor nameplate full load amperage (FLA) should be known. It is important to note that sizing the drive based on FLA is not merely being conservative. Under the working assumption that the motor is sized correctly for the load torque needed, sizing a drive for only what the motor draws under “normal” (i.e. non-peak) load conditions may not provide sufficient torque to drive the process under heavy load conditions. Also, sizing a drive by horsepower alone ignores the amount of overload the drive can provide. For example, a 460-volt drive suited for a 75hp motor under variable torque conditions may be capable of putting out 96 amps continuously; under constant torque conditions (a.k.a. heavy duty) that same drive would only be suitable for 60hp and 77 amps. This is because under heavy load conditions the output electronics (typically IGBT’s or insulated gate bipolar transistors) are asked to fire for longer periods and are more subject to over-heating, so the ratings are backed down to protect them.

• Voltage frequency and magnitude: These same IGBT’s are controlled by the drive circuitry to fire (switch on and off) at a high frequency, typically from 2kHz to 16kHz, creating high frequency voltage transients at the drive output. And unlike a pure, nicely balanced three-phase sine wave, the transients do not cancel each other out.  As a result, they can result in voltages at the motor terminals of 2-3x or more of the incoming supply voltage. This effect is greatly exacerbated by long motor lead length. Modern premium efficiency motors, particularly those in compliance with NEMA MG-1 standards, are built to withstand these transients, to a point. Once the motor lead lengths become excessive (depending on manufacturer and testing agency, anywhere from perhaps 30 meters and up), output filters are recommended.

• Motor age/condition: In large part due to the factors mentioned above, care must be taken when attempting to control an older motor or one with a marginal insulation system with a variable frequency drive. The high frequency voltage transients can place a lot of stress on motor winding insulation, eventually causing breakdown of the dielectric and shorting out the windings. Also, common mode (i.e. line to “earth”) noise generated by the drive electronics can cause currents to flow in the motor frame, shaft and bearings; these currents will seek a path to ground, often resulting in pitting of bearings and races. This is not typically a significant factor for smaller frame motors (say, less than 500 NEMA/315 IEC), but larger motors often require insulated bearings and shaft grounding to prevent premature wear of the bearing components. And again, the problem is exacerbated by lead length, so even smaller motors may need protection if a sufficient distance from the drive.

• Wiring: For the drive to operate cleanly and with minimal problems, good wiring practices must be followed – segregate power connections (line and load) from each other and from control wiring; use shielded cable, correctly grounded, where susceptible to common mode noise; provide metallic raceway where possible; and ensure the conductors are sized for the drive output current, with a properly sized grounding conductor. Consult the local/regional governing electrical codes for additional information.

So with all of the issues and concerns expressed in the first three parts of this series, why choose a drive in the first place? There are two primary reasons: energy savings, and reduction of electrical and mechanical stresses on driven components. Next week, we will discuss the first of these, and look at ways that drives can increase overall operating efficiency by effectively controlling the process for optimum output.

In the meantime, please feel free to jot down some thoughts in the Comments section. And as always, you can contact me with any questions, comments or application needs you may have at simon.fan@vtdrive.com. Thanks for reading!