A Cost Controller's Checklist for Specifying Siemens Motors – 7 Steps to Avoid Hidden Costs

If you're responsible for buying Siemens motors (or any industrial drive), you've probably been told to "compare prices and pick the best value." That sounds obvious – until you realize that half the costs are buried in things you never thought to ask about.

I've managed motor procurement for a 120-person automation integrator for about 6 years (maybe 7, I'd have to check). We spend roughly $180,000 annually on motors, drives, and related components. In that time I've compared quotes from 15+ distributors, taught myself to read Siemens drawing sheets, and made some expensive mistakes that taught me what actually matters.

This checklist is for anyone who needs to buy a Siemens motor – whether it's a standard induction motor, a blower motor, a gear motor with integrated reducer, or a brushless AC servo. It's also for the person who ends up asking: "What size VFD do I need for a 5 HP motor?" (Spoiler: it's not always the same as the motor rating.)

When to Use This Checklist

Use this when you're evaluating a new motor purchase – especially if you're sourcing multiple quotes, replacing an existing motor, or building a new system from scratch. It's designed to surface hidden costs before you commit, not after.

There are 7 steps. Stick to the order – I've arranged them so each step builds on the previous one.

Step 1: Lock Down the Operating Context

Before you even look at a motor catalog, you need to know exactly what the motor will do. Most buyers focus on horsepower and speed, but miss duty cycle, ambient temperature, altitude, and enclosure type.

Here's what I include in every spec request:

• Load type: constant torque (conveyor), variable torque (fan/pump), or constant power (machine tool spindle)
• Duty cycle: continuous (S1), short-time (S2), or intermittent periodic (S3-S8) – this drastically affects thermal sizing
• Environment: indoor/outdoor, washdown (IP55+), corrosive atmosphere, hazardous area (ATEX/IECEx)
• Power supply: voltage, frequency, phase – and whether you'll use a VFD

Checkpoint: If the end user can't answer all of these, push back. It's better to delay a quote than to buy the wrong motor and pay for return shipping – we learned that the hard way when a "standard" fan motor died in 8 months because the plant humidity exceeded the enclosure rating.

Step 2: Match the Motor Type to the Application

Siemens makes a huge range: induction (AC), synchronous reluctance, permanent magnet, servo, stepper, and DC. The rookie mistake is assuming one induction motor fits all. It doesn't.

For example, a gear motor (integrated reducer) saves space and assembly time, but if you need high dynamic response, a servo with separate gearbox might be a better choice. Brushless AC motors (like Siemens SIMOTICS S-1FL6) offer higher efficiency and longer life than induction motors for servo applications, but they require a matching drive – you can't just hook them to a VFD.

The question everyone asks is "which one is cheaper?" The question they should ask is "which one has the lowest total cost over 5 years?" (I'll come back to that in Step 5.)

Checkpoint: Confirm whether the motor needs a separate brake (servo vs. induction holding brake), encoder feedback (and what type), and the gear ratio if it's a gear motor.

Step 3: Get the Siemens Motor Drawings – Don't Assume Dimensions

This is the step most people skip, and it costs them. I've seen buyers order a motor that doesn't fit the existing mounting plate because they used the wrong frame size. Siemens publishes detailed motor drawings (outline and dimension sheets) for every model – you can find them on the Siemens Industry Online Support site. But you have to look at the specific winding variant, because a 5 HP, 1800 rpm motor in a 184T frame vs. a 213T frame has completely different mounting holes.

When I request drawings, I always ask for:

• Overall length, width, height (including terminal box and fan cover)
• Shaft diameter, keyway, and extension length
• Mounting hole pattern and bolt size
• Terminal box orientation (top, side) and conduit entry size (like 1/2" NPT vs. M20)

Checkpoint: Cross-reference the drawing with your mechanical interface. If you're replacing an old motor and the drawing shows a different bolt pattern, you'll need an adapter plate – that's an extra $200 (as of Q1 2025, at least).

Step 4: Specify the Right VFD – Especially for 5 HP and Up

The key phrase "what size VFD for 5 HP motor" comes up constantly. The simple answer is: a 5 HP VFD for a 5 HP motor – if both are general-purpose, inverter-rated, and you're not overloading. But reality is messier.

• If the motor is a standard NEMA Design B induction motor (non-inverter duty), you may need to oversize the VFD by one frame size to handle the extra current ripple – or better, buy an inverter-duty motor like Siemens SIMOTICS GP.
• If the load is a high-inertia fan or a heavily loaded conveyor at startup, you might need a 7.5 HP VFD to handle the surge current without tripping.
• If you're using a servo or stepper, you need a matching servo drive, not a VFD. (Yes, people have asked me – don't laugh.)

I always calculate the full-load amps from the motor data sheet, then size the VFD to at least 120% of that. Here's a rule of thumb I use: 1 HP ≈ 746 W, but for VFD sizing, go by amps, not horsepower. A 5 HP, 230 V motor draws about 15 A full load – a 5 HP VFD rated for 17 A continuous is fine, but check the short-time overload rating.

Oh, and don't forget braking resistors or DC injection for fast stopping – that's an extra cost (around $150–$300 depending on size, circa 2025).

Checkpoint: Confirm the VFD model supports the motor's feedback device (encoder or resolver) if you need closed-loop control.

Step 5: Calculate Total Cost of Ownership (TCO) – Not Just Price

Here's where the cost controller's heart lives. The cheapest quote upfront is rarely the cheapest over the motor's life.

I use this formula:

TCO = Purchase Price + Installation (labor + mounting hardware) + Energy Costs (over expected life) + Maintenance (bearing regreasing, seal replacement) + Spare Parts + Downtime Risk

Let me give you a real example from Q2 2024. We compared two quotes for a Siemens 5 HP blower motor:

• Vendor A: $1,850 – including motor data sheets, free shipping, 18-month warranty
• Vendor B: $1,620 – no drawings included, $220 shipping, 12-month warranty

Vendor A looked 14% more expensive. But when I added the cost of having our engineer track down drawings ($90 for 2 hours), shipping ($220 instead of free), and the risk of early failure (we had a similar motor fail in month 14 under a 12-month warranty – that cost us $1,200 in downtime), Vendor A was actually about $390 cheaper over 3 years. And that's not even counting the time I spent processing the return.

So my advice: treat warranty length, included documentation, and lead-time reliability like line items in your comparison spreadsheet.

Checkpoint: Ask every vendor for an itemized breakdown – not just line item price, but shipping, packaging, documentation, and any optional services (like special testing or modified shaft). If they can't provide it, that's a red flag.

Step 6: Verify the 'Ecosystem' – Motor + Drive + Protection

Siemens motors aren't standalone. They need a starter or VFD, possibly a soft-starter, motor protection circuit breaker (MPCB), overload relay, and sometimes a brake rectifier or filter. If you buy a motor from one source and the drive from another, you risk compatibility headaches – especially with servo or high-efficiency motors like the SIMOTICS SD.

I prefer bundled packages when they make sense. For instance, a Siemens SIRIUS protection system matched with a SIMOTICS motor and SINAMICS drive gives you coordinated tripping characteristics and simpler wiring diagrams. The upfront price might be slightly higher than mixing brands, but the troubleshooting time saved (no finger-pointing between suppliers) is real.

Checkpoint: Ask your distributor for a complete BOM: motor, drive, control, cables, terminations, and any filters/brakes. Then ask them what's not included – like mounting hardware, or line-side fuses.

Step 7: Test Your 'What If' Scenarios

Before you send the PO, run through two or three scenarios:

• If the motor fails early: Is the warranty clear on what's covered (labor? shipping? downtime)? I once had a gearbox seal leak at month 11, and the vendor said the warranty didn't cover seals because they were "consumables." (Should mention: we changed vendors after that.)
• If you need a replacement fast: Which vendor can deliver in 2 weeks, not 8? That matters more than a 5% price difference for critical spares.
• If the application changes: Can you reuse the motor with a different VFD? For brushless AC servos, you're usually locked into the same drive brand – that's a big hidden cost if you switch control platforms.

Checkpoint: Get all these responses in writing. Verbal commitments vanish when a new sales rep takes over.

Common Mistakes and How to Avoid Them

Here are three traps I've fallen into (or watched colleagues fall into):

1. Ignoring the motor nameplate. A used or surplus Siemens motor might look identical to your existing one, but the winding code could be different – meaning different torque curve, different voltage tolerance. Always get the full type designation (e.g., 1LA2074-4... something like that) and cross-reference with the data sheet.
2. Oversizing the motor to be safe. A 10 HP motor running at 30% load is far less efficient than a 5 HP motor at 60% load. You pay for that inefficiency every month in electricity. For continuous operations, I aim for 75–90% load.
3. Assuming 'off-the-shelf' means 'in stock'. Many distributors list parts as 'standard' but still backorder them. Ask for the current lead time before you negotiate price. A 16-week lead time can kill your project schedule, no matter how good the price.

One last thing: the cheapest price I ever got on a Siemens servo motor turned into a nightmare because the drawing didn't match my mounting plate. I saved $220 on the unit and spent $450 on an adaptor and extra labor. The numbers said go with the cheaper option. My gut said stick with the distributor that sends proper documentation. I went with the numbers. I was wrong. So now I always ask for the drawing before the quote.

(As of April 2025, prices have changed a bit – but the principle hasn't.)