Cyndi Nyberg 
Former EASA Technical Support Specialist 

There are many applications where it is necessary for a low-voltage, single-phase AC or DC power supply to be available for auxiliary equipment such as brakes, clutches, lamps, etc., used along with a three-phase motor. The single-phase voltage can be supplied by tapping the stator winding at the correct place. DC voltage can be produced by tapping the single-phase or three-phase voltage from the three-phase winding and rectifying it to DC. Determining where to tap the winding is fairly straightforward. 

A three-phase stator winding, when energized, will have a certain number of volts per turn. That is, if you know the number of turns in each phase, and you know the phase voltage, you can determine the volts per turn. Knowing the number of volts per turn, and the required voltage supply for the auxiliary equipment, you can calculate pre­cisely where to tap the winding. 

Volts/turn = (phase voltage x number of circuits x 3) / (total coils x turns per coil). 

Determinations to make 
It is necessary to know whether the auxiliary equipment operates on single-phase, three-phase, or DC power. If it operates on DC, you will also need to know how the voltage is rectified, full wave or half wave, and whether it rectifies three-phase or single-phase voltage. The reason: the ratio of AC in to DC out is affected by each of these items. 

Two lines are required for a single-phase supply. Very often, one of the line leads or one of the neutral points is used as one side of the tap, although it doesn’t have to be. It is usually just easier that way. If the auxiliary voltage differs from line voltage, three taps are necessary for a three-phase supply. These may come from a point within each of the phases. 

Note: A tap can be made on a single-phase machine winding — just remember that the phase voltage is the same as the line voltage. How­ever, in this article, three-phase machine windings will be considered, although the rules for determining the placement of the taps are the same. 

Before considering the exact location of a tap on a three-phase winding, the volts per turn must be calculated. First, determine the volts per phase. This is best illustrated with examples: 

For a delta connected winding, volts/phase = Line voltage. 

Assume a 575v, 1-Delta connected winding. This is a 4-pole, 36-slot motor, with 12 groups of 3 coils. Each coil has 24 turns. 
Line voltage = 575v 
Volts per phase = 575v 

There are 4 groups per phase, 3 coils per group, and 24 turns per coil: 
First calculate the turns per group: 
3 coils per group x 24 turns per coil = 72 turns per group. 

Next calculate the turns per phase: 72 turns per group x 4 groups per 
phase = 288 turns per phase. Finally, calculate the volts per turn: 575 volts per phase / 288 turns per 
phase = 2 volts per turn. See Figure 1. 

Now, suppose we need a single-phase supply of 24v to operate a relay on the piece of equipment. Two taps are required for a single-phase supply. 

Bring one “tap” off one of the line leads. To calculate the position of the tap within the winding, divide the volts per turn into the number of volts needed: 
24 volts / 2 volts per turn = 12 turns. 

The tap will be located 12 turns away from the line lead selected of the same phase as the chosen lead. In this example, with 24 turns per coil, the tap will be located halfway through the 1st coil of the 1st group. (Figure 2). 

For a wye connected winding, volts/phase = line voltage x 0.577. 

Assume a 230v, 1-wye connected winding. This is a 2 pole, 24-slot motor, with 6 groups of 4 coils.  Each coil has 50 turns. 

Line voltage = 230v Volts per phase = 230 x 0.577 = 132.7v There are 2 groups per phase, 4 coils per group, and 50 turns per coil: First, calculate the turns per group: 4 coils per group x 50 turns per coil = 200 turns per group. Next calculate the turns per phase: 200 turns per group x 2 groups per phase = 400 turns per phase. See Figure 3. 

Finally, calculate the volts per turn: 132.7 volts per phase / 400 turns per phase = 0.332 volts per turn. 

Now, suppose we again need a single phase supply of 24v. Use either one line lead, or one neutral point as one tap. To determine how many turns from the first tap the second tap should be located, divide the volts per turn into the number of volts needed: 
24 volts / 0.332 volts per turn = 72.29 or 72 turns. 

Since we have 50 turns per coil, the tap should be located after the 22nd turn (72-50=22) of the second coil in a group. If the other tap was from the lead, then it will be the 22nd turn of the 2nd coil of the 1st group. If the other tap is from the neutral, then the tap will be located at the 28th (50­22=28) turn of the 3rd coil of the 2nd group of that phase. See Figure 4. 

If a winding has paralleled circuits, the calculations are the same. The volts per phase are identical for each leg of a parallel connection. It is a good practice to double-check to ensure that the taps are correctly located. 

Other types of auxiliary equipment taps 
What about different power supplies? You will not always need a single phase supply – some auxiliary equipment runs on three phase, and very often, DC power. For three-phase, you will typically make taps in three identical places within each phase. 

A common application for a motor with taps is for a DC brake. With DC, it is important to know how the AC supply is rectified, since the resulting voltage will not be equal to the supply voltage. 

There are 6 possible combinations for supplied power to auxiliary equipment: 

• Single-phase tap for single-phase supply – use straight v/t calculation 
• Single-phase tap for DC supply, half-wave rectified, v_AC = 2.22 v_DC 
• Single-phase tap for DC supply, full-wave rectified, v_AC = 1.11 v_DC
• Three-phase taps for three phase supply, use straight v/t calculation 
• Three-phase taps for DC supply, half-wave rectified, v_AC = 1.64 v_DC
• Three-phase taps for DC supply, full-wave rectified, v_AC= 0.82 v_DC

As an example, if we use the same data as the first example, but instead of single phase, 24v, we need 24v DC, full-wave rectified, to supply a brake coil: 
Line voltage = 575v, Volts per phase = 575v

There are 4 groups per phase, 3 coils per group, and 24 turns per coil: 72 turns per group. 288 turns per phase 2 volts per turn For single-phase, full-wave rectified,

v_AC = 1.11v_DC 

1.11 x 24 = 26.64v AC 

Use one of the line leads as one of the taps. To calculate the position of the tap within the winding, divide the volts per turn into the number of volts needed: 
26.64 volts / 2 volts per turn = 13.3 turns, or 13 whole turns. 

The tap will be located after the 13th turn of the same phase as the chosen lead. In this example, with 24 turns per coil, the tap will be located at the 13th turn of the 1st coil of the 1st group. 

Categories: Winding connections

Tags: Winding connections

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