Single-Phase or Three-Phase?

Single-Phase Starters
Having established the rating and f.l.c. required, it is essential to consider the type of duty which the starter will be required to perform. The following guide also includes reference to the pages on which typical kW/hp and current range figures are given for Eaton MEM starters used in single-phase applications.

NOTE THAT THESE FIGURES DIFFER SIGNIFICANTLY FROM THE VALUES APPLICABLE TO 3-PHASE USE.

The current taken by single-phase motors of a given kW rating is considerably higher than for a three phase motor of the same kW rating, simply because power is supplied by only one live conductor instead of three. Therefore, the current for 240V 1-phase is at least three times higher than for 415V 3-phase.

These types may be switched directly with maual or automatic starters. Single-phase reversing starters and starters with more complex windings (e.g. series/parallel connection) can be produced by Eaton MEMs Specials Department. In these cases, it is important to provide confirmation of the motor winding configuration, or at least the motor manufacturer's type reference, since there is a wide variety of single-phase motor types.

Small capacitor squirrel cage motors have a centrifugal switch for opening the starting winding or capacitor and a standard direct-on-line starter is suitable for this function. Care must be taken to ensure the starter is connected in accordance with the instructions for single phase applications.

On larger motors, series parallel switching is the most common and a special starter is required, which can be either manual or automatic. In addition there are split-phase motors having various switching configurations and a connection diagram is often necessary to ensure the correct starter is supplied.

Three-Phase Starters
Again, in addition to kW rating, voltage and frequency, you need to establish the actual motor rated full load current to select a starter with the appropriate overload device. If f.l.c. is not known, use the kW (hp) motor rating to establish a typical figure.

You also need to know:

A)	What type of duty is the starter required to perform?
B)	Is there a requirement for reduced voltage starting?
C)	Is the starter intended for local or automatic control?

Let us take these in turn.

A) Type of Duty
As in single-phase operation, a manual starter used in three-phase applications is suitable for infrequent or intermittent starting of small motors. For frequent duty requiring high mechanical durability, long electrical life and the added facility of remote control operation (as outlined in (c) below), a counter starter is required. As an indication of the mechanical performance levels to which they are tested, an ADS7 contactor must be able to achieve at least 5 x 106 - or 5,000,000 - mechanical operations.

The most commonly used starter is the direct-on-line (d.o.l.) type where its three mains outgoing cables are connected directly to the motor terminals. Any restrictions demanding reduced voltage starting are discussed below.

In addition to the conventional d.o.l. surface-mounting pattern of contactor starter, the ADS7 range incorporates varients which cater for specific requirements. The flush mounting starter is designed to fit into a suitably-sized recess to give a minimum projection control point for machinery. Two-direction and reversing starters incorporate two contactors enabling the direction of motor rotation to be changed. For Two-direction with intermediate stop the AC-3 rating is applicable whereas for rapid reversal without intermediate stop the AC4 rating is applied. Two direction starters are suitable for such applications as roller-shutter doors and small hoists. Starter-isolator starters incorporate a built-in, padlockable switch disconnector providing isolation facilities within a single compact unit.

Two-speed starters may be required for equipment designed to operate at more than one fixed speed, e.g. mixers, fans, certain machine tools. Two-speed starters are of two types: to control either dual-wound motors or tapped-wound (alternatively known as pole change) motors including PAM type.

Dual-wound motors have six or twelve terminals and two sets of stator windings. They are in effect two motors in one, each of equal power, and can be designed to deliver any two speeds from the normal motor r.p.m. range. Although usually arranged for direct-on-line, they can be star-delta operated. Starters for dual-wound motors incorporate two contactors and two overload relays to cater for the two sets of stator windings.

Dual-Wound Motor

Tapped-wound or pole change motors have six terminals. They use the principle that induction motor speed at given frequency is determined by the number of pairs of stator poles fitted. Motor speed is thus varied by switching in or out an equal number of stator poles to give 2 or 4, 4 or 8, 6 or 12 poles and so on.

The higher speed is always twice that of the lower with this arrangement. Starters for tapped wound motors incorporate three contactors plus two overload relays, one for each speed.

Tapped Wound Motor

ADS7 starters suitable for either form of 2-speed operation are produced to order by Eaton MEMs Specials Department. When specifying, supply the usual kW and f.l.c. information, but ensure that this is given for both operating speeds.

The approximate no-load motor speed can be determined by using the formula:

N =
N =	Synchronous speed (theoretical speed of motor with no load and no losses).
F =	Frequency (Hz).
P =	Number of pairs of stator poles.

Approx motor speeds
based on formula

No of Poles 2 4 6 8 10 12 16

Rpm at 50Hz 3000 1500 1000 750 600 500 375

Example Synchronous speed of 4-pole 50Hz motor

Note: Induction motors will run at approximately 3-5% below the synchronous speed, known as 'slip', according to motor design.

B) Reduced Voltage Starting
The most common methods of starting a three-phase squirrel cage motor are direct-on-line and star-delta.

The starting current of a standard squirrel cage motor when switched directly on the supply (direct-on-line is approximately 6 to 8 times full load current) may develop up to 150% full load torque. This method of starting is not always permissible, particularly on larger machines owing to the following:

a) Limitations of switching peaks by supply authority or back up circuit breaker.
b) Starting peak will cause volt drop which can result in overheating of motor and supply cables.
c) High starting torque can under certain load conditions cause excessive mechanical wear.
In these conditions reduced voltage starters must be used and the most common is the star-delta starter. This method of starting restricts the starting current to 1/3 of direct switching i.e. 2 to 3 times FLC with a corresponding drop in the starting torque.

An alternative is the auto-transformer starter, which is normally used where a higher starting torque is required to accelerate the driver or the motor only has three terminals. The starting current and torque are determined by the auto-transformer tapping used.

The table below gives the appropriate starting current and torque likely to be obtained.

C) Local or Automatic Control?
Starters fitted with push buttons are used in applications which require local control of the motor by an operative, as in drilling machine or lathe. They may be manual or contactor, d.o.l. or star-delta, as appropriate, depending on the motor rating, frequency of switching operations involved and the local electricity supply authority regulations.

Automatic - usually '2 wire' - control is applied to motors which are required to operate automatically as conditions dictate, e.g. when the drive for a fan or compressor is activated by a thermostat or pressure switch.

All ADS7 starters are suitable for either local or automatic control. Overload relays are readily adjustable between HAND and AUTO/RESET via a simple knob control. In the case of 2-wire control, it is important to set to HAND reset.