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.
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:
-
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.
-
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.
Related information
Please follow these links to related and useful information:
About this author
Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing articles such as this.