When customers compare motors, efficiency usually ends up as one line in a spec table. 90 % on one datasheet, 92 % on the other. The price column gets far more attention.
I understand that. At Turncircles I run operations and build our business cases, and the purchase price is the number that hits a budget first. A motor runs for years, though, and the cost of running it shows up on someone's books every single month. This post walks through how I translate efficiency into money, so you can run the same math for your own application.
Look at the losses first
The efficiency figure hides the part that matters. Take a motor delivering 15 kW at the shaft:
- At 90 % efficiency it draws 16.7 kW and turns 1.7 kW into heat.
- At 92 % efficiency it draws 16.3 kW and turns 1.3 kW into heat.
Two points on the datasheet look small. In losses, they mean about 22 % less heat. Keep that number in mind, because heat costs you twice: once on the electricity bill, and again in everything you build to get rid of it.
The energy bill
Say the motor runs 8,000 hours a year (a machine in continuous production) and you pay €0.20 per kWh. The difference in input power is 0.36 kW. Over a year that adds up to roughly 2,900 kWh, or about €580. Over ten years you are at €5,800 for a single motor, before any price increases.
If you want to plug in your own numbers, the formula is short:
Annual saving = shaft power × operating hours × energy price × (1/η₁ − 1/η₂)
Multiply by the number of motors in your plant or your fleet, and the gap between two quotes often looks very different.
Heat costs you a second time
Every watt of loss leaves the motor as heat, and you pay to remove it. With less heat you can sometimes size a smaller fan, drop a liquid cooling loop, or fit the motor into a tighter enclosure. Each of these saves money on the system around the motor, and that saving rarely shows up when people compare motor prices.
Temperature also affects lifetime. A common rule of thumb for winding insulation says that every 10 °C of extra temperature roughly halves its life. A motor that runs cooler needs replacing less often, and in many applications the downtime for a replacement costs more than the motor.
This is one reason we build coreless axial-flux motors. Without iron in the stator, there are no iron core losses in the stator to turn into heat, and the machine keeps the torque advantage of the axial topology.
In battery-powered machines, efficiency buys range
For robots, drones, and other mobile machines, the math changes. You don't pay for kilowatt-hours from the grid. You pay for battery capacity, and the battery is often the heaviest and most expensive part of the system.
If most of your stored energy goes into the motors, a few points of motor efficiency translate into a few percent more range, or a battery a few percent smaller for the same range. A smaller battery weighs less, which means the motors work less hard, which saves energy again. Engineers on mobile platforms know this loop well, and it's why they ask about efficiency before they ask about price.
Ask where the motor actually runs
The peak efficiency on a datasheet describes one operating point. Your machine probably spends most of its life somewhere else: at partial load, at low speed, or cycling between the two.
A motor with a great peak number can sit 5 to 10 points lower at the point where your application spends most of its hours. So when you compare motors, ask for the efficiency map, or at least the efficiency at your main operating points. Then weight each point by how many hours you spend there. That weighted number is the one that belongs in your business case.
Designing for a specific duty cycle is the main reason to go custom. We can put the efficiency sweet spot where your machine runs, instead of where it looks best in a brochure.
Putting it together
When I build a total cost of ownership comparison, I include:
- the purchase price
- energy cost over the expected lifetime, weighted by duty cycle
- cooling hardware and its running cost
- battery size or range, for mobile applications
- expected replacement intervals and the downtime they cause
The cheapest motor on the quote is often the most expensive one on this list.
If you'd like to go through these numbers for your own application, send us your duty cycle and operating conditions. I'm happy to work through the business case with you.