In a previous blog, we examined why AC drives continue to be applied at a rate of approximately 20:1 even in situations when soft starters are sufficient to do the job.
The reason: AC drives will work in almost all applications, but soft starters will not. This does not mean that AC drives are easier to specify, install and commission - it simply means that AC drives work in a wider variety of applications.
So why choose a soft starter? Substantial savings can be realized when using a soft starter instead of an AC drive in the proper applications. These savings can come in:
- Physical size / volume
- Energy and efficiency
In order to reap the benefits of these savings, application characteristics are very important when choosing a soft starter or an AC drive. Items such as torque requirements, speed control and power distribution concerns must be carefully considered.
For soft starter applications, some general rules of thumb include:
- Lower starting torque applications
- Lightly or moderately loaded applications
- Full speed operation
- Reduction of mechanical wear and damage to system
- Limiting current is prime reason for starting method
Significant cost savings can be realized when using a soft starter instead of an AC drive. A soft starter has silicon-controlled rectifiers (SCRs) that connect the AC input line to the motor. In contrast, AC drives have many more components – typically a diode front end, DC bus and insulated-gate bipolar transistors (IGBTs). More components simply means more cost.
Typical cost ratios of AC drives versus soft starters are given below (cost AC drive / cost soft starter)
- Below 5Hp (3.7kW) 1.5 to 2.0
- 5Hp (3.7kW) to 50 Hp (37kW) 2.0 to 4.0
- 50Hp (37kW) to 250 Hp (185kW) 1.5 to 4.0
- 250 Hp (185 kW) to 500 Hp (375kW) 3.0 to 4.5
- Above 500 Hp (375kw) 2.5 to 7.0