In electrical engineering low voltage is a relative term, the definition varying by context. Different definitions are used in electric power transmission and distribution, and electrical safety codes define “low voltage” circuits that are exempt from the protection required at higher voltages. These definitions vary by country and specific codes or regulations.In this context, low voltage motors refers to motors that operate at voltages less than 1 000 V and produce a maximum power of 1 000 kW.
Three definitions low voltage motors in three countries
By IEC is defines supply system low voltage motors as voltage in the range 50 to 1000 V AC or 120 to 1500 V DC. In electrical power systems low voltage most commonly refers to the mains voltages as used by domestic and light industrial and commercial consumers. “Low voltage” in this context still presents a risk of electric shock, but only a minor risk of electric arcs through the air. United Kingdom by British Norma BS 7671:2008 defines supply system low voltage motores as 50 to 1000 VAC or 120 to 1500 V ripple-free DC between conductors and 50 to 600 VAC or 120 to 900 V ripple-free DC between conductors and Earth. In electrical power distribution, the United States 2005 National Electrical Code (NEC) defines low (distribution system) voltage as 0 to 49 volts.
Slip ring low voltage motors
The slip ring or wound rotor motor is an induction machine where the rotor comprises a set of coils that are terminated in slip rings to which external impedances can be connected. By changing the impedance connected to the rotor circuit, the speed/current and speed/torque curves can be altered. The slip ring motor is used primarily to start a high inertia load or a load that requires a very high starting torque across the full speed range.
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