The CMA7 is an electromechanical motor drive unit for on-load tap changers (OLTCs) in power and distribution transformers. Mounted on the transformer tank sidewall, it drives the tap changer through a horizontal drive shaft, bevel gear box, and vertical drive shaft. Each tap change executes on the step-by-step (inching) principle — a single control signal initiates one complete position change without requiring external timing or coordination. The 750 W three-phase motor delivers 18 Nm at 1400 rpm, completing a full switching operation in approximately 5 seconds across up to 107 positions.

What safety features protect against over-travel?

Dual electrical and mechanical limit switches at both end positions prevent over-travel beyond the tap changer's terminal positions. The electrical limit switch cuts motor power before the mechanical stop engages; the mechanical limit provides a hard backup. An integrated 50 W anti-condensation heater maintains safe internal humidity levels inside the IP66 cast aluminum enclosure, which is designed for outdoor transformer-sidewall mounting at altitudes up to 2500 meters.

What operating modes are supported?

The CMA7 can be operated locally via push-buttons on the unit, remotely from a control room, or manually using the hand crank — 33 crank revolutions equal one tap change. Step-by-step and continuous overrun modes are both supported. A mechanical position indicator displays the current tap position through a viewing window in the enclosure door. Companion controllers include the HMK-2A automatic voltage regulator and the HMC-3C digital position indicator with BCD code output.

Which tap changers is the CMA7 compatible with?

The CMA7 pairs with Huaming's CM, CMD, CV, and SV series on-load tap changers for transformers rated up to 252 kV. It accommodates up to 107 operating positions and is rated for 800,000 mechanical operations — equivalent to decades of service in utility and industrial transformer applications. Control circuit and heater voltages are 220 VAC; aviation sockets on the enclosure bottom provide weatherproof cable entry.