You may have heard the term drive isolation transformer before. What sets it apart from a standard isolation transformer? Is there a difference between drive isolation and isolation?
Yes there is.
In the table below you can see how Drive Isolation transformers compare to standard isolation and k-rated units. Let's break down what’s special about drive isolation transformers.
What is a Drive Isolation Transformer?
Drive isolation transformers serve applications with motor drives (VFDs or VSDs). Variable speed and variable frequency drives control the speed of motors. They do this by changing the voltage or frequency supplied to the motor. This improves the overall efficiency and performance of the motor. But, this process also creates unique stress on transformers. Drive isolation units are designed to handle this stress.
The term drive isolation is key. DITs separate (or isolate) the motor drive circuit from the circuit supplying power to the transformer. As shown below, there is no electrical connection between the primary and secondary of the transformer.
Why Isolation/Separation?
Both VFDs and VSDs use rectifiers and inverters to control motor loads. The rectifier turns AC to DC, and the inverter turns DC back into AC. The process of changing AC and DC power back and forth can yield certain power quality issues, such as voltage swells and sags.
How does this happen?
Unwanted Ground Currents
Oftentimes, capacitive coupling occurs between the drive’s components and earth ground. An isolation transformer keeps any induced ground currents on the load side, blocking them from circulating upstream to the building’s main source.
The solidly grounded neutral of the wye winding, feeding the drive, isolates the ground currents from the source upstream of the DIT.
An isolation transformer separates the primary side of the transformer from the motor drive.
Common-Mode Noise
The DC component within the drive often creates “mode noise”. This mode noise becomes superimposed on the AC power supply–creating power quality issues if not accounted for. The two-winding isolation scheme helps with this problem. By creating a separately derived system for the motor drive, common mode-noise is contained in the drive circuit. An optional electrostatic shield may be included between the HV and LV windings for added attenuation.
Other than an E-shield, there is nothing unique in the design we have described so far. A regular isolation transformer can offer all these characteristics. The differences in drive isolation units are more apparent in the loading design and kVA denominations.
Standard Drive Isolation kVA Ratings
You may notice unusual kVA ratings for DITs. Unlike regular isolation transformers, DIT kVAs are sized to standard motor horsepower ratings. For example 7.5 hp equates to 11 kVA, 50 hp to 63 kVA, and so on. The table below lists the standard sizes we stock for our 3-Phase 480 D - 480 Y 277 Drive Isolation Transformers.
Loading Profile
Due to the unique loading profile of drive circuits, DITs include a loading capacity for 150% overload for 60 seconds, and 200% overload for 30 seconds. This is one of the main reasons a standard isolation transformer could fail prematurely in this application. DITs sometimes include heat sensors for monitoring winding temperature. These sensors are usually wired to a terminal block mounted at the top of the core.
What a DIT is Not
A DIT is not a k-factor rated transformer. DITs come with 100% rated neutrals, whereas k-rated units have 200% rated neutrals (to account for triplen harmonics). The inclusion of an E-shield is optional with DITs. But, it is a standard provision with a k-rated design.
Conclusion
While drive isolation transformers can function in standard step-up or step-down applications, the reverse is rarely true. While a regular delta-wye isolation transformer isolates a drive circuit, it does not account for the unique loading profile of motor drives. It could overheat or fail prematurely. This is why Maddox keeps standard drive isolation transformers on the shelf, ready to go. Fill out the form below to get started.
