You're right to be cautious and ask questions! RPCs (Rotary Phase Converters) are a bit of a black art, and understanding how they work is crucial for ensuring the longevity of the motor and the reliability of the system.Here's a breakdown of the key considerations and the "real answer"

Understanding RPC Basics
Not Perfect Phase Conversion

An RPC

does not create perfect 3-phase power. It's an approximation. You're taking single-phase power and using a rotating inductor (the idler motor) to shift the phase of one leg to mimic a third phase. Voltage Imbalance

The manufactured leg (the one generated by the RPC) will almost always have a lower voltage than the incoming utility legs. This is because the idler motor has losses and the phase shift isn't perfect.

Current Imbalance

The currents in each phase will also likely be unequal. This is a consequence of the voltage imbalance.

Phase Angle

While you mentioned the phase angles, the critical thing is that they won't be perfectly 120 degrees apart. The manufactured leg's phase angle will deviate.

Do You Need to Derate? Probably, but maybe not drastically.

This is the big question, and the answer is nuanced: Motor Derating is generally recommended, but the extent depends on the quality of the RPC and the motor's design. A well-designed RPC will minimize the voltage imbalance, reducing the need for heavy derating. Voltage Imbalance Matters Most

Voltage imbalance is the primary reason for motor derating with RPCs. Even a small voltage imbalance can cause significant current imbalance, leading to overheating and reduced motor life.

Consider the Application

A sander is a relatively constant-load application. This is good. If it were a heavily cycling load (like a hydraulic press) the stress on the motor would be much greater.

Factors to Consider and How to Mitigate Problems

Here's a checklist of things to consider:1. RPC Quality



Brand/Reputation

Some RPC manufacturers are better than others. Look for reputable brands that are known for producing balanced output.

Capacitor Tuning

Properly tuned capacitors on the RPC are CRITICAL for minimizing voltage imbalance. Some RPCs have adjustable capacitors to fine-tune the output for a specific load. Check the RPC's manual for tuning procedures.2.

Measure the Voltages (CRITICAL)



Line-to-Line Voltages

Use a good-quality multimeter to measure the voltage between each pair of legs (L1-L2, L2-L3, L3-L1) under load. This is the most important measurement. Do it at the motor.

Calculate Voltage Imbalance

Use the following formula:`Voltage Imbalance (%) = (Maximum Deviation from Average Voltage / Average Voltage) 100`Where: `Average Voltage = (L1-L2 + L2-L3 + L3-L1) / 3` `Maximum Deviation = Largest difference between any individual voltage and the Average Voltage.`

Example

L1-L2 = 230V L2-L3 = 200V L3-L1 = 220V Average Voltage = (230 + 200 + 220) / 3 = 216.67V Maximum Deviation = |200 - 216.67| = 16.67V Voltage Imbalance = (16.67 / 216.67) 100 = 7.69%

Acceptable Limits

NEMA MG 1-2016 specifies that polyphase motors should not operate continuously at nameplate horsepower if the voltage imbalance exceeds 1%. Above 1%, derating is generally recommended. Above 5%, motor damage is highly likely.

If the Voltage Imbalance is more than 5% you'll need to adjust the RPC capacitors or replace the converter.3. Motor Temperature Monitoring



Infrared Thermometer

After the sander has been running for a while under a typical load, use an infrared thermometer to check the temperature of the motor windings (if possible). Compare the temperature to the motor's nameplate temperature rating. If it's running hotter than expected, you definitely need to address the imbalance.

Listen for unusual humming.4. Derating Guidelines (if needed)

If the voltage imbalance is between 1% and 5%, consider derating the motor. A common rule of thumb is to reduce the motor's horsepower rating by 10% for every 1% above 1% imbalance. For example, at 3% imbalance, derate the 15 HP motor to roughly 12 HP.5.

Consider a Digital Phase Converter (DPC)

If voltage balance is critical (and you have the budget), consider a digital phase converter. DPCs use solid-state electronics to create a much cleaner and more balanced 3-phase output than an RPC. They are significantly more expensive, though.

Recommendations for your Customer


Stress the importance of a properly tuned RPC

The RPC manual should provide instructions on how to adjust the capacitors to optimize voltage balance for the sander's load.2.

Insist on voltage measurements under load

This is the only way to know the true voltage imbalance.3.

Monitor motor temperature

If the motor is running hot, the customer must address the imbalance.4.

Be conservative with derating (if needed)

It's better to be safe than sorry. A slightly oversized motor running at a derated horsepower will last longer than a borderline motor running near its full capacity with imbalanced voltage.5.

Don't overload the sander

Overloading any motor, especially one running on an RPC, will shorten its life.

In summary: measure the voltage imbalance under load and address the imbalance if it exceeds acceptable limits. If you can't reduce the voltage imbalance below 5% (ideally below 1%), you will need to de-rate the motor to avoid early failure. Flag for review