You have likely seen your amp readings higher than expected if you ever connected a rotary phase converter (RPC) to operate a three-phase machine. But don't worry! Your motor is most likely not working any harder than it should. The true scenario has to do with the RPC and load interaction, rather than an overload condition.

Let's understand this in easy terms.

A Simple Analogy: The Scale and the Dumbbell 

Try thinking about how it feels to step on a scale. 

A 200-lb person stands on a scale, and the scale reads 200 lb. 

Then, that same person picks up a 10-lb dumbbell, and the scale reads 210 lb.

Did the person actually gain 10 lbs? Of course not. The scale is measuring a new total system weight, the system being the person and whatever weight they are holding.

The same scenario happens when you connect a rotary phase converter idler motor to a 10 HP pump. The amp meter is not just "seeing" the motor operating; it is "seeing" the entire system operating, which is the pump motor and the rotary phase converter idler motor.

Why the Reading Appears “Too High” 

A rotary phase converter creates the third leg of a three-phase power supply from a single-phase source. It does this using an idler motor that draws its own magnetizing or overhead current, which is independent of the load connected to the converter.

After connecting a 10 HP motor (your load), both machines are now electrically tied together. Currents from the idler and the load motor are working together, and the amp meter you are using has no way of separating them. Regardless of where you connect your meter in the three-phase circuit, it will display a current value that is the sum of both the idler and load currents. 

That is why it is wrong to assume that you are measuring "just the motor amps." You are actually measuring and reporting the collective behavior of the system.

To add a little intrigue, the manufactured leg (one created by the RPC) usually carries a higher current or may vary from the other two legs. That is all normal and not an indication of your load motor being overloaded or inefficient.

What You Should Look at Instead 

In a remote powering system, trying to achieve perfectly balanced currents and voltages will leave you frustrated. Uneven phase currents are simply how these systems operate. On the other hand, try to concentrate on just a few items that are practical checks that will let you know if you are operating correctly: 

     1. Voltage Balance in the Motor 

While the motor is under load, measure the voltage on all three legs. You want the three voltages to be within ±2% of each other. Some differences are acceptable, but larger imbalances will result in heating issues and decreased performance. 

     2. Motor Temperature and Noise 

The motor is smoothly operational, within the normal temperature range, and sounds within normal limits. If the motor is not running hot or sound is not amplified, your remote powering system is likely sound in health; in fact, all your current readings don't need to be perfectly balanced. 

     3. Overload Protection Settings 

Set your overload relays to the nameplate full load amps (FLA) and service factor of the motor, not to the higher number you find on one phase. Properly adjusted overload protection will protect your equipment without nuisance trips. 

Bottom Line 

When taking current readings in a rotary phase converter system, it is important to note that you are reading the entire circuit instead of an isolated motor. Similar to how a bathroom scale cannot distinguish your weight from the weight of the dumbbell, the amp meter cannot distinguish the magnetizing current of the converter from the current doing work on the motor load.

So, do not be alarmed if one phase is getting more amps, or your readings are different from what you expect from the motor nameplate rating. This is the nature of RPC systems. If your voltages are balanced, your motor is running cool and smooth, and your overloads are set correctly, then all is well.