DigiKey is a distributor of the Siemens SMF, MMS, and MRS fractional horsepower manual motor controllers. An example is the SMFFF1P as pictured in Figure 1.
This article is part of the DigiKey Field Guide for Industrial Automation
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Author: Aaron Dahlen | MSEE | Senior Applications Engineer, DigiKey
Last update: 09 Mar 2026
Figure 1: Image of a Siemens SMF single-phase toggle starter with an integral pilot lamp.
Classification of the Siemens SMF, MMS, and MRS Motor Controllers
Siemens SMF, MMS, and MRS controllers often have similar appearances. They are available in NEMA Type 1 enclosures as shown in Figure 1 or heavy-duty NEMA type 4 enclosures. The primary difference is the type of motor protection:
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SMF Starter: The SMF starter provides motor overload protection via a thermal overload heater (sold separately). They are available in single or two poles configurations with the capacity to control a single-phase 1 hp motor on a 277 VAC system.
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MMS and MRS switch: The Siemens MMS and MRS series are local motor starting switches. Options are available in single, two, or three poles with the capacity to control a 10 hp three-phase motor on a 600 VAC system. MMS and MRS units do not provide overload protection. A separate magnetic starter or other appropriate protection is recommended.
Tech Tip: The SMF unit will not operate without a heater such as the one shown in Figure 2. Heaters are selected based on the full rated current of the associated motor. Refer to the Siemens catalog for a complete listing.
DO NOT bypass this safety feature with a jumper wire. An overload, or a brown out condition could destroy your expensive motor.
Figure 2: Image of the thermal overload heater used in the Siemens SMF series.
Locating Part Numbers for Motor Starting Switches
There are over 100 unique part numbers for the Siemens SMF, MMS, and MRS switches. This reflects the wide variety of switch options including:
- Number of poles
- With and without enclosure
- Pilot lamp (red is default – add a G to the part number for a green pilot lamp)
- NEMA 1 and NEMA 4
- Inclusion of an Auto-Off-Hand selector switch
- Two speed starters
- Reversing switch (MMS series)
The best way to locate the parts is to download the latest Siemens catalog. The catalog provides a brief description of each motor starter series, tables containing all part numbers, wire diagrams, and tables for the SMF heaters.
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About This Author
Aaron Dahlen, LCDR USCG (Ret.), is a Senior Applications Engineer at DigiKey in Thief River Falls. His background in electronics and industrial automation was shaped by a 27-year military career as both technician and engineer, followed by over a decade of teaching.
Dahlen holds an MSEE from Minnesota State University, Mankato. He has taught in an ABET-accredited electrical engineering program, served as coordinator of an electronic engineering technology program, and instructed military technicians in component-level repair.
Today, he has returned to his home in northern Minnesota, completing a decades-long journey that began with a search for capacitors. Read his story here.