Why should you care about the total cost of owning an electric motor?
Did you know that the cost of electricity is 90% of the total cost of owning an electric motor for a period of 20 years? The remaining 10% is the cost of the motor itself (1%), downtime (5%) and re-build costs (4%) as estimated by the U.S. Department of Energy.
So here is where variable frequency drives (VFDs) come in: Using a VFD to drive an electric motor can save energy, usually 30-50% or more, especially in applications with varying loads. With VFDs, motors do not need to run at full speed but can be run at the most efficient speed replacing louvers, dampers and valves.
For example, even a 20% reduction in fan speed can reduce energy consumption by nearly 50%. In fact, reducing a fan's speed by half cuts the horsepower needed to run it by a factor of 8! So you can easily see that adding a VFD to a system can save a tremendous amount of energy and therefore money. But what if there are other hidden costs?
What if the energy efficiency increases, but uptime and reliability suffer? Then the energy savings vanish! Shaft voltages and bearing currents induced by VFDs can wreak havoc with the motor's bearings, dramatically shortening motor life and causing costly repairs, often repeatedly, during the life cycle of the motor. To mitigate these currents and realize the full potential of VFDs, a reliable method of shaft grounding is essential. We will therefore compare three Total Cost of Ownership (TCO) scenarios:
- Line driven motor (no VFD)
- VFD driven motor with no shaft grounding ring (bearing protection) installed
- VFD driven motor with a shaft grounding ring installed
So here is an example: Let's say you pay $10,000 for a 200 HP (150kW) motor and operate it for 20 years, with and without a VFD. Your Total Cost of Ownership (TCO) is as follows:
Line Driven Motor (no VFD)
- Electricity: $855,000
- Down time from repair (1 x in the 20 year period): $47,500
- Rebuild costs (1 x in 20 years): $38,000 (removal, repair, replacement, alignment, labor)
- Cost of the new motor: $10,000
Total cost of ownership over 20 years: $950,500
Now lets compare this to a motor with a VFD but no shaft grounding ring
- Electricity: $570,000 (VFD saves 30% in energy costs)
- Down time costs: $190,000 (4 x motor failure from fluted bearings)
- Rebuild costs: $152,000 (removal, repair, replacement, alignment, labor)
- Cost of Motor + VFD: $20,000
Total cost of ownership over 20 years: $932,000
Last, let's compare the VFD driven motor that has a shaft grounding ring installed.
- Electricity: $570,000 (you still get the savings!)
- Down time costs: $47,500 (1 x in 20 years - nothing lasts forever!)
- Rebuild costs: $38,000 (1 x in 20 years - you eliminated the frequent bearing failures!)
- Cost of the Motor + VFD + Bearing Protection: $22,000
Total cost of ownership over 20 years: $677,500: You save $273,000 on this 200 HP (150kW) motor
The savings are proportional to the HP (kW) rating of the motor, so now ask yourself:
- How many motors do I have in my facility?
- How many are on VFDs?
- How many will be operated by VFDs in the next 10 years?
- Do all my motors have bearing protection (shaft grounding rings) installed?
- Does my motor and drive specification require shaft grounding rings on the motors?
And last but not least, if all of your motors had bearing protection,
How much money would I save?
How much more profitable would my operations be?
Can I really afford NOT to specify motors with shaft grounding rings installed?
The trick is to select a motor which already has a shaft grounding ring installed by the manufacturer whenever a new motor is purchased. There are many options for this from most motor manufacturers such as ABB Motors and Mechanical (formerly Baldor); WEG Electric; Marathon; LEESON; TECO-Westinghouse and GE. For motors already in service, whenever a motor is re-wound or repaired by a motor repair shop, a shaft grounding ring should be installed at the same time.