Coolant pump motors on deep hole drilling machines need properly sized starters and drives to operate reliably. A pump that starts and stops frequently under high pressure draws significant current during startup, and the starter must handle this without tripping or welding its contacts. I have seen coolant pump motors fail prematurely because the electrical system was undersized or improperly configured.
Motor and Starter Sizing Guidelines
The motor full-load current is listed on the motor nameplate, and this is the starting point for sizing the starter and protective devices. I follow standard electrical engineering practice with some specific adjustments for coolant pump applications.
| Motor Power | Full Load Current (480V) | Starter Size (across-line) | Overload Setting | Wire Size (copper) |
|---|---|---|---|---|
| 5 kW (7.5 HP) | 11 A | NEMA Size 1 | 13.8 A (125%) | 14 AWG |
| 10 kW (15 HP) | 21 A | NEMA Size 2 | 26.3 A (125%) | 10 AWG |
| 15 kW (20 HP) | 27 A | NEMA Size 2 | 33.8 A (125%) | 10 AWG |
| 22 kW (30 HP) | 40 A | NEMA Size 3 | 50.0 A (125%) | 8 AWG |
| 30 kW (40 HP) | 52 A | NEMA Size 4 | 65.0 A (125%) | 6 AWG |
| 45 kW (60 HP) | 77 A | NEMA Size 4 | 96.3 A (125%) | 4 AWG |
The starter should be sized for 125 percent of the full-load current for across-the-line starting. This oversizing accounts for the high inrush current during startup and provides a safety margin for motor overloads caused by pump wear or partial blockages.
I calculate the full-load current from the motor nameplate rather than from the motor power rating. Motor manufacturers have different efficiency ratings and the actual full-load current varies. The nameplate current is the guaranteed value that the motor will draw at full load.
For VFD-driven pumps, the drive should be sized for 100 percent of the motor full-load current. The VFD limits the inrush current through its soft-start feature, so the oversizing needed for across-the-line starting is not required. I size the VFD input fusing and wiring for the drive full-load current rating, which is listed on the drive nameplate.
Soft Start and VFD Selection
I use VFDs on coolant pumps whenever possible. The VFD provides soft starting, which reduces the electrical stress on the motor and the mechanical stress on the pump and piping. The VFD also allows adjustable pressure control, which provides the energy savings I discussed in the pump optimization article.
| Feature | Across-Line Starter | Soft Starter | VFD |
|---|---|---|---|
| Starting current | 600% of full load | 200-300% of full load | 100-150% of full load |
| Mechanical stress on pump | High | Medium | Low |
| Pressure control | Fixed (pump speed fixed) | Fixed | Variable (adjustable) |
| Energy savings during idle | None | None | 80% reduction at 50% speed |
| Cost (15 kW size) | $300 - $600 | $600 - $1,200 | $1,500 - $3,000 |
The VFD cost premium over a simple starter is offset by the energy savings and the improved process control. I have installed VFDs on several machines and the payback period was under 18 months in every case.
When selecting a VFD for a coolant pump, I look for a drive with constant torque rating. Coolant pumps are constant torque loads — the torque required to turn the pump is roughly constant across the speed range. Some VFDs are rated for variable torque loads like fans and are not suitable for pump applications.
The VFD should include a DC link choke or input line reactor to reduce harmonic distortion. Coolant pump VFDs often share a power distribution panel with CNC controls and servo drives. The harmonics from a VFD can interfere with the CNC and cause intermittent faults. A line reactor reduces the harmonics to acceptable levels.
Overload Protection Settings
The motor overload protection should be set correctly to protect the motor without causing nuisance tripping. I set the overload protection based on the starting method and the motor service factor.
For across-the-line starting, the overload is set to 125 percent of the motor full-load current. This setting provides motor protection while allowing the high starting current to pass without tripping. The overload has a time delay characteristic that allows the motor to draw high current for the brief startup period.
For VFD operation, the overload is set to 115 percent of the motor full-load current. The VFD limits the starting current so the overload does not need the extra margin. The lower setting provides better motor protection during normal operation.
Motors with a 1.15 service factor can be overloaded by 15 percent continuously without damage. I use the 1.15 multiplier when setting the overload for motors that run at high ambient temperatures or that are subject to frequent overload conditions from pump wear.
The overload protection should be tested annually by running the motor at full load and verifying that the overload trips within the expected time at a specified overcurrent level. I test the overload by clamping an ammeter on the motor leads and reading the current while the pump is at full pressure.
Wiring and Connection Requirements
The power wiring to the coolant pump motor is sized for the motor current at the rated voltage. I use copper wire sized for 125 percent of the motor full-load current. Undersized wiring causes voltage drop that reduces pump performance and can cause the motor to overheat.
| Distance from Panel to Motor | Voltage Drop at Full Load (10 AWG, 27A) | Voltage Drop at Full Load (8 AWG, 27A) | Recommendation |
|---|---|---|---|
| 10 meters | 0.5% | 0.3% | 10 AWG is adequate |
| 25 meters | 1.3% | 0.8% | 8 AWG recommended |
| 50 meters | 2.6% | 1.6% | 8 AWG required |
| 100 meters | 5.2% | 3.2% | 6 AWG required |
I keep voltage drop under 3 percent at the motor terminals. Higher voltage drop reduces the motor torque and increases the current draw, which causes the motor to run hotter. I have seen motors fail prematurely on long wire runs where the voltage drop was over 5 percent.
The motor connection should be a grounded wye (star) configuration for 480 VAC systems. The motor leads should be connected in the correct phase sequence so the pump rotates in the correct direction. I verify the rotation direction by jogging the motor and checking the pump rotation arrow before putting the motor into service.
Common Failure Modes From Improper Sizing
I have seen several coolant pump motor failures that were caused by improper starter or drive sizing. The most common failure is welded starter contacts from high inrush current. The starter contacts weld closed and the motor runs continuously until the pump bearings fail from overheating.
Another common failure is motor winding burnout from undervoltage. The motor draws higher current when the voltage is low, which generates more heat in the windings. If the voltage drop exceeds 5 percent, the motor can overheat and fail within months.
I have also seen VFD failures from improper sizing. A VFD that is undersized for the motor current runs hot and the internal cooling fan cannot keep up. The VFD shuts down on overtemperature, causing unplanned downtime. Replacing the VFD with a correctly sized unit solved the problem.
Key Takeaways
- Motor starters should be sized for 125 percent of the nameplate full-load current for across-the-line starting, while VFDs should be sized for 100 percent of the motor current.
- VFDs provide soft starting with 100-150 percent of full-load current compared to 600 percent for across-line starting, reducing electrical and mechanical stress on the system.
- Overload protection settings are 125 percent of full-load current for across-line starting and 115 percent for VFD operation.
- Power wiring must be sized to keep voltage drop under 3 percent at the motor terminals for a 480 VAC system to prevent motor overheating and premature failure.
- The VFD cost premium over a simple starter ($1,500-$3,000 vs $300-$600 for a 15 kW motor) pays for itself in under 18 months through energy savings from idle speed reduction.
- I include a DC link choke or input line reactor with every VFD installation to reduce harmonic interference with CNC controls and servo drives.