Wiring Yaskawa Motoman UP6 XRC on a Rotary Phase Converter

Overview

Re-energizing a vintage Yaskawa Motoman UP6 six-axis robot arm paired with an XRC controller from single-phase residential service is a recurring retrofit scenario in small shops, training labs, and home workshops. The XRC cabinet ships configured for multi-voltage industrial service with an internal power transformer that has primary taps for 240 V, 480 V, and 560 V three-phase input and a secondary that can be re-strapped for 240 V or 208 V output. When the controller is instead fed from a home-built Rotary Phase Converter (RPC), three problems surface that the factory never had to solve:

1. Voltage imbalance between the two utility-derived legs and the RPC "manufactured" leg.
2. Elevated phase-to-ground voltage on one or more legs because the RPC output is not a true wye and does not provide a stable neutral.
3. Grounding and neutral bonding questions because the original three-phase service entrance may have used a bonded neutral that has since been disconnected.

This reference walks through the electrical decisions required to bring a UP6/XRC online from a 220 V single-phase service through a small (3 hp class) RPC, drawing on the published Motoman product documentation set and field-proven servo-drive behavior.

Yaskawa Motoman UP6 and XRC Controller Power Specifications

The UP6 is a six-axis articulated robot with a payload of approximately 6 kg. The matching XRC (eXpert Robot Controller) is a stand-alone cabinet containing the teach pendant, CPU board, I/O boards, servo amplifier stack, and an autotransformer with selectable taps. According to the manufacturer product documentation portal, the controller is multi-voltage by design and can be ordered or re-tapped for 200 V class, 400 V class, or 560 V class three-phase input. The exact input configuration is identified by the part number suffix and the wiring diagram glued inside the cabinet door.

Reference documentation is available at the official Motoman Product Documentation portal. The site lists the operator's manual, maintenance manual, and controller wiring diagram for the XRC, all of which must be on the bench before any rewire.

Rotary Phase Converter Fundamentals for Robot Loads

An RPC synthesizes a third phase (the "manufactured" leg) by spinning an idler motor with single-phase power and using the back-EMF of that motor to generate a voltage on the third terminal. Under no load the three output terminals read:

• L1–L2: utility leg-to-leg, typically 220–240 V
• L1–L3: utility-to-manufactured, often 230–250 V
• L2–L3: utility-to-manufactured, often 230–250 V

When the load draws current, the manufactured leg sags more than the two utility legs because it has no rotating source behind it on the utility side. With a small RPC (3 hp idler) and a small load (1.5 kVA UP6), the imbalance is usually 3–8% line-to-line and 5–15% line-to-neutral. For a robot whose only load is six servo drives and their DC bus capacitors, this imbalance is normally absorbed by the front-end rectifier without damage, but the imbalance must be measured, not assumed away.

Voltage Tolerance and the Manufactured Leg Problem

Field measurements from the UP6 installation are reproduced below. Readings were taken at the controller's main disconnect with the cabinet door closed and no load on the controller (the disconnect was open for safety, so the readings reflect the RPC output alone).

The +10% / -15% tolerance window for a 240 V nameplate is 264 V upper and 204 V lower. All line-to-line values are in spec. The problem is the line-to-ground distribution: a 240 V three-phase wye with a bonded neutral would yield 120/120/120 V phase-to-ground, but the RPC has no neutral and the XRC's internal transformer was originally wired as a delta or high-leg delta. With the transformer bypassed, L2–GND rose to 172 V, which is well above the 150 V peak that the input filter capacitors and Y-capacitors on the EMC filter are typically rated to withstand continuously.

Why the Manufactured Leg Reads High to Ground

An RPC idler motor develops its generated leg by acting as an induction generator once the run capacitors excite the auxiliary winding. The voltage on the manufactured leg floats with respect to ground because the RPC provides no intentional neutral bond. If the load is unbalanced (as it almost always is in a single-phase-supplied robot), the neutral point of the load drifts away from earth, and one phase-to-ground voltage rises while another falls. This is the same mechanism that creates the "wild leg" or "high leg" in a center-tapped 240 V delta service, except in the RPC case the imbalance is load-dependent and time-varying.

High-Leg Delta Topology Considerations

The factory three-phase supply most likely to feed a Yaskawa cabinet in a North American shop is one of the following:

1. 120/208 V wye (three phases + neutral, all phases 120 V to neutral)
2. 120/240 V high-leg delta (two phases 120 V to neutral, one phase 208 V to neutral — the "wild leg")
3. 240 V corner-grounded delta (one phase bonded to ground, the other two 240 V phase-to-phase and 240/240/0 V to ground)

An RPC emulates none of these perfectly. It is closer to an isolated delta, where the phase-to-ground voltages depend entirely on the load. The XRC's internal transformer was designed for any of the three topologies, so the controller will tolerate the imbalance as long as the line-to-line voltages stay in the +10% / -15% window. The line-to-ground voltages, however, are not specified by Yaskawa because the XRC's power supply is referenced to chassis ground through a Y-capacitor network that bleeds a small amount of 50/60 Hz to the chassis. This is normal.

Transformer Tap Configuration and Bypass Decisions

The autotransformer inside the XRC has primary taps for 240, 480, and 560 V input and a secondary strap point that selects between 240 V and 208 V output to the servo amplifiers. In a UP6/XRC retrofit fed from a 220 V single-phase RPC, the relevant questions are:

1. Should the autotransformer be used at all, or should the RPC output be wired directly to the servo amplifier input?
2. If the transformer is retained, which primary tap and which secondary strap apply?

For a 240 V class controller, the answer to the first question depends on the RPC's voltage regulation. If the RPC produces 230–240 V line-to-line with the load connected, the autotransformer adds nothing except cost and can be bypassed. If the RPC sags below 210 V under load, the autotransformer can be re-strapped to buck the 240 V utility feed down to 208 V for the secondary, which compensates for the sag on the manufactured leg by reducing the voltage the controller actually sees.

Field procedure for the bypass:

1. Lock out and tag out the main disconnect.
2. Open the cabinet and photograph the existing transformer wiring.
3. Mark and disconnect the three primary leads (H1, H2, H3) and the three secondary leads (X1, X2, X3).
4. Using insulated ring lugs, land the RPC output directly on the terminals that the secondary was feeding (the servo amplifier input contactor).
5. Re-verify line-to-line and line-to-ground voltages at the contactor line side before re-energizing.

Ground and Neutral Bonding for Retrofit Installations

The original installation in question used a 3-wire 220 V single-phase feed (hot, hot, neutral) with the neutral and ground bonded at the main breaker box. Modern NEC practice since the 2008 cycle requires the neutral and ground to be bonded only at the service disconnect or at the first disconnecting means. Sub-panels and downstream equipment must keep the neutral isolated from the equipment grounding conductor.

For the robot retrofit, the correct topology is:

1. Service entrance: neutral bus bonded to ground bus, single bonding screw or strap installed.
2. Sub-panel for the robot: neutral bus isolated from ground bus, four-wire feeder (L1, L2, N, G) brought in.
3. RPC: chassis bonded to the sub-panel ground bar; the RPC output has no neutral terminal at all.
4. XRC cabinet: chassis bonded to the sub-panel ground bar; the XRC's internal PE terminal is tied to the cabinet chassis. Do not bond the XRC neutral terminal to chassis — there is no neutral feed.

If the existing junction box still has the neutral and ground jumpered together on the load side of the sub-panel, the jumper must be removed. Leaving the bond in place places the neutral current on the equipment grounding conductor, which is a NEC 250.142(B) violation for any new load.

Servo Drive Tolerance to Phase Imbalance

Modern three-phase servo amplifiers, including the Yaskawa Sigma-series amplifiers used in the XRC, contain an input rectifier that converts the three-phase AC to a DC bus. The rectifier is followed by a large electrolytic capacitor bank and a soft-charge circuit. With a phase imbalance of up to about 5%, the DC bus ripple is well within the amplifier's tolerance. Above 5%, the ripple frequency becomes dominated by the lowest phase and the bus voltage droops on each cycle of the missing phase.

For a UP6 whose peak per-axis power is approximately 1200 W and whose total continuous load is approximately 1.5 kVA, a 3 hp RPC provides roughly 2.2 kW of mechanical generation capacity. The idler runs at roughly 50% of its nameplate rating when the robot is at rest and briefly peaks at full load during acceleration. This headroom is what allows the bus voltage to stay above the undervoltage threshold during the manufactured-leg sag.

Capacitor Sizing for RPC Voltage Balance

The voltage on the manufactured leg is set by the capacitance on the idler motor's start or auxiliary winding. For an oil-filled motor-run capacitor, the approximate sizing rule for a 3 hp (2.2 kW) RPC idler running unloaded is:

C (μF) ≈ 4 × HP of idler

For a 3 hp idler this gives roughly 12 μF per phase, typically implemented as two 24 μF capacitors in series (or one 12 μF unit if available). For a 5 hp idler the rule yields 20 μF. For a 7.5 hp idler the rule yields 30 μF. These values are starting points only; the actual capacitance must be tuned by measuring the manufactured-leg voltage under load and adding or subtracting capacitance until the leg-to-leg voltages are within 2% of each other at full load.

To measure the manufactured-leg voltage under load, bring up the RPC with the idler running, connect a clamp-on ammeter on the manufactured-leg conductor, and command a slow move of the largest servo axis from the teach pendant. Watch the manufactured-leg voltage on a true-RMS multimeter; if it sags more than 5% line-to-line, add 2 μF at a time until the sag is within tolerance.

Step-by-Step Commissioning Procedure

1. Confirm single-phase service is 220–240 V AC, two hot legs and a neutral, with a separate equipment grounding conductor back to the main panel.
2. Verify the RPC is wired per its build sheet, with the idler motor rotation in the correct direction (reversing L1 and L2 on the idler will reverse the manufactured leg's phase rotation).
3. Open the XRC cabinet main disconnect and verify zero energy with a known-good voltage tester on all three input terminals.
4. Bypass the autotransformer if the line-to-line voltage under load is already within the 204–264 V window for a 240 V class controller.
5. Land L1, L2, L3 from the RPC output to the XRC input contactor or fuse block. Land the cabinet chassis to the sub-panel ground bar with a dedicated bonding conductor. Do not bond neutral.
6. Verify phase rotation at the XRC input terminals with a phase rotation meter. Yaskawa motors rotate in a specific direction; if the meter shows a negative sequence, swap any two of the three phase conductors at the XRC input.
7. Close the main disconnect and power the cabinet. Confirm the controller boots and the teach pendant initializes without an alarm related to input voltage, encoder battery, or absolute encoder position.
8. From the pendant, enter the diagnostic screen and read the DC bus voltage for each servo amplifier. The expected reading for a 240 V class amplifier is approximately 340 V DC at no load and 320 V DC under full acceleration.
9. Command a slow, single-axis jog of the largest axis (typically the L-axis on a UP6). Monitor the DC bus voltage and the AC line-to-line voltages on the manufactured leg.
10. If the line-to-line imbalance exceeds 5%, add or remove run capacitance on the RPC idler until the imbalance is within tolerance.
11. Run a multi-axis coordinated move through the full envelope of the robot. Monitor all six servo DC bus voltages and the input line currents on the RPC.
12. Capture all measurements, photograph the final wiring, and update the cabinet wiring diagram with any deviations from the factory drawing.

Verification Checklist

Troubleshooting Matrix

Field Notes and Long-Term Operation

An RPC feeding a small six-axis robot is a viable long-term solution, but it requires more attention than a true three-phase service. Schedule the following checks at six-month intervals:

• Measure the three line-to-line voltages under full-load jog and compare against the commissioning record.
• Inspect the run capacitors for bulging, oil leakage, or discoloration. Replace any capacitor that has lost more than 10% of its rated capacitance.
• Check the absolute encoder battery on the robot arm. The XRC uses a 3.6 V lithium thionyl chloride cell; replace it on a 5-year cycle or whenever the controller has been de-energized for more than a week.
• Verify the cabinet-to-sub-panel ground conductor is mechanically sound and free of corrosion, especially in a shop environment with cutting fluid or salt air.
• Re-torque the power terminal lugs on the RPC output contactor and the XRC input contactor to the manufacturer-specified torque.