Electrical Machines, Transformers, and Motors
Electrical machines, transformers and motors covers the magnetic and mechanical side of the trade: how a transformer or motor behaves, how it is started, and how it is protected.
Transformers
- EMF equation E = 4.44·f·N·Φ; turns ratio a = N1/N2 = U1/U2, so an 11 kV/400 V transformer has a ≈ 27.5.
- Losses are copper (I²R, load-dependent) and iron (constant). Efficiency peaks near full load, and the per-unit impedance sets the short-circuit current: Isc ≈ In/Z%.
- Vector groups such as Dyn11 and Yyn0 determine phase shift and harmonic behaviour; a neutral is available only where the winding is star-connected.
Motors
- Synchronous speed ns = 60f/p. Slip s = (ns − n)/ns; a 4-pole 50 Hz motor at 4% slip runs at 1440 rpm.
- DOL starting draws about 6–8× full-load current at full torque. Star-delta reduces starting current and torque to one third but needs a motor rated for delta on 400 V with six terminals.
- Soft starters and VFDs limit starting current; a VFD holds torque at low speed only with adequate cooling, because the shaft-mounted fan slows with the rotor.
- Protection: thermal overload relay set to nameplate full-load current (adjusted for service factor and ambient), class 10 for standard motors, thermistors or PT100 for large machines, and single-phasing protection.
Worked example
A 15 kW, 400 V motor has a nameplate full-load current of 28 A. In delta run the line current is 28 A while each winding carries 28/√3 = 16.2 A. The overload relay must be set according to whether it is installed in the line or in the winding circuit, or it will trip in service.
Traps
- Connecting star-delta to a motor wound for star only.
- Ignoring the one-third torque and trying to accelerate a loaded conveyor.
- Setting overloads to the measured current of an unloaded motor.