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A motor can have star and delta connection ratings without being suitable for star-delta starting on your supply. Before ordering a replacement or approving a starter, separate two questions: how the motor must run at the available voltage, and how the machine will accelerate.
Read the voltage pair as two connection ratings
Consider a nameplate marked 230/400 V, Δ/Y. It identifies delta operation at 230 V and star operation at 400 V. A 400/690 V, Δ/Y motor instead has its delta rating at 400 V. These examples explain the labels; they do not replace the connection diagram supplied with the particular motor.
For conventional star-delta starting, the motor must be suitable for continuous delta operation at the supply voltage, with the necessary winding ends accessible. Six terminals alone do not establish voltage compatibility.
Do not confuse reduced starting current with rated current
For the same motor at the same supply voltage, the ideal star-start line current and starting torque are approximately one third of their direct-delta starting values. They are not one third of rated running current or rated running torque. The actual acceleration still depends on the motor and driven load.
Three questions to resolve before comparing quotes
- Supply: What voltage and frequency will be present at the starter?
- Motor: What are the exact delta and star voltage ratings, rated current, duty and terminal arrangement?
- Machine: What torque is required during acceleration, and how frequently must it start?
A quote that lists only power and frame size leaves these interfaces unresolved. Keep a photograph of the existing nameplate and obtain the new supplier’s connection drawing before approval.
Ask the panel builder to confirm the complete starting arrangement
The motor supplier and panel builder should agree on starting method, protection, transition arrangement and commissioning responsibilities. A universal timer value or contactor size taken from an unrelated motor is not an acceptance criterion. Electrical installation and verification belong to qualified personnel working from the approved drawings.
RFQ wording buyers can use
Please confirm the running connection at our stated supply voltage, the accessible winding terminals, rated current and suitability for the proposed starting method. Identify the load assumptions and all exclusions for starter selection and commissioning.
Send Dongchun the nameplate, machine type, supply and starting method when requesting a three-phase motor quotation. We can then discuss the exact model and documents required for that application.
Technical reference: Fuji Electric: motor starting applications and selection. The procurement checklists above are our suggested purchasing workflow.
